END-GROUP FUNCTIONALIZATION OF ANIONICALLY SYNTHESIZED
POLYMERS VIA HYDROSILATION REACTIONS
A Dissertation
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
Hoon Kim
May, 2006
END-GROUP FUNCTIONALIZATION OF ANIONICALLY SYNTHESIZED
POLYMERS VIA HYDROSILATION REACTIONS
Hoon Kim
Dissertation
Approved: Accepted:
Advisor Department Chair Dr. Roderic P. Quirk Dr. Mark D. Foster
Committee Member Dean of the College Dr. Joseph P. Kennedy Dr. Frank N. Kelley
Committee Member Dean of the Graduate School Dr. William Brittain Dr. George R. Newkome
Committee Member Date Dr. Coleen Pugh
Committee Member Dr. Chrys Wesdemiotis
ii ABSTRACT
One of the unique features of living, alkyllithium-initiated, anionic polymerization
is the ability to produce a stable carbanionic chain end after complete monomer
consumption, which can be followed by reaction with electrophiles to form various end-
functionalized polymers. Although a variety of functional polymers have been
synthesized in the last few decades, each specific functionalization has had to be designed
and optimized individually. Consequently, the development of general functionalization
methodologies has drawn recent interest. However, even these general functionalization
methods require the use of protecting groups, and the complexity in synthetic routes and
the thermal/moisture instability of many protected functional agents have restricted their
practical application. This thesis describes a new, general functionalization methodology,
combining well-defined, living anionic polymerization with efficient and highly selective,
platinum-catalyzed hydrosilation reactions with functionalized alkenes.
Well-defined, Si-H functionalized polymers (P-SiH) have been synthesized by sec- butyllithium-initiated, living anionic polymerization in benzene followed by termination with dimethylchlorosilane. Even though the Si-H bond is polar and labile, it is stable with respect to reactions with organolithium compounds in hydrocarbon solvents. These Si-H bonds are also stable to oxygen and moisture in the atmosphere so that
iii Si-H functionalized polymers can be isolated and handled in air. These silyl hydride-
functionalized polymers were isolated simply by precipitation into methanol, in which they are also stable. Silyl hydride-functionalized polystyrenes and polyisoprenes have been prepared and characterized by SEC, 1H, 13C and 29Si NMR spectroscopy, FT-IR
spectroscopy and MALDI-TOF mass spectrometry. For quantitative analysis, the ratio of
the integration area of 1H NMR resonances for the six methyl protons of the dimethylsilane unit at δ –0.1 ppm to the other six methyl protons from the sec-butyl end group from the initiator at about δ 0.7 and 0.9 ppm for the silyl hydride-functionalized polystyrene and polyisoprene, respectively has been used. The MALDI-TOF mass analysis of silyl hydride-functionalized polystyrene produces the corresponding Si-OH functionalized polymer from oxidation of Si-H functional group by the silver cation as the cationizing agent. Reliable mass spectra were obtained using sodium ion as the cationizing agent. The utility of this method for the preparation of ω-functionalized polymers was demonstrated by the hydrosilation of both a protected allylamine
derivative, 3-[N,N–bis(trimethylsilyl)amino]-1-propene, and allyamine itself with a silyl
hydride-functionalized polystyrene in the presence of Karstedt’s catalyst, 1,3-
divinyltetramethldisiloxane-platinum. Quantitative functionalization for each of these
functionalizations was proven by 1H NMR and TLC analyses as well as end group
titration, and further supported by 13C, 29Si and 13C DEPT NMR, FT-IR and mass spectral
analyses. The usefulness of this general functionalization methodology was also
demonstrated by phenol functionalization of polystyrene by the hydrosilation reaction of
the unprotected phenol, 2-allylphenol, with silane-functionalized polystyrene. The
hydrosilation product was characterized by SEC, TLC, NMR (1H, 13C and 13C DEPT),
iv FT-IR and mass spectral analyses. The resulting data indicated the successful preparation of phenol-functionalized polystyrene. In addition, epoxy-functionalized polystyrene was synthesized by hydrosilation of 1,2-epoxy-5-hexene with silane-functionalized polystyrene without protection, and characterized by SEC, 1H and 13C NMR spectroscopy, FT-IR spectroscopy and MALDI-TOF mass spectrometry. All characterization results were consistent with an efficient incorporation of epoxy functionality at the chain end. Similarly to other functionalizations, perfluoroalkyl functionalization of polystyrene was effected by hydrosilation of 1H,1H,2H-perfluoro-1- octene with silane-functionalized polystyrene without protection. The perfluoroalkyl- functionalized polymer was characterized by SEC, NMR (1H, 13C and 19F), FT-IR and
MALDI-TOF mass spectral analyses. No evidence for any side reactions was found.
The applicability of this functionalization methodology to the preparation of well-
defined star-branched polymers was investigated by the reaction of octavinyl-T8-
silsesquioxane with silane-functionalized polystyrene in the presence of Karstedt’s
catalyst. A novel POSS (Polyhedral Oligomeric Silsesquioxane) cored, 8-arm, star-
branched polystyrene was isolated by fractionation in a methanol/toluene mixture. The
successful formation of POSS core was evidenced by SEC, 1H and 13C NMR, and FT-IR
spectral analyses. However, the results from MALDI-TOF mass spectrometric analysis
were not consistent with the predominant formation of the 8-arm, star polymer. The
thermal properties of this polymer were also investigated by TGA and DSC analyses, and
the star-branched polymer exhibited improved thermal stability compared to a linear
standard polymer with similar molecular weight.
v One limitation of this method is the use with polydienes such as polybutadiene and polyisoprene because of competing self-induced inter- or intramolecular hydrosilation of silane-functionalized polydienes during functionalization with external, substituted alkenes. Along this line, the functionalization of silyl hydride-functionalized polyisoprene has been examined in detail. Amine functionalization of polyisoprene was effected by the hydrosilation of both a protected allylamine derivative, 3-[N,N–
bis(trimethylsilyl)amino]-1-propene, and allyamine itself with a silyl hydride-
functionalized polyisoprene in the presence of Karstedt’s catalyst. The hydrosilation
product was characterized by SEC, NMR (1H, 13C and 1H-1H COSY) and FT-IR spectral
analyses. Surprisingly, the results indicated the successful preparation of amine-
functionalized polyisoprene (protected and non-protected) using this method. No
evidence for self-induced inter- or intramolecular hydrosilation reactions was found.
vi
ACKNOWLEDGEMENTS
First of all, I thank my advisor, Professor Roderic P. Quirk, with all my heart for his
endless encouragement, precious advice and financial support throughout my degree. I
also thank all my group members for their co-operation and discussions. Especially, I
thank Michael Olechnowicz for his friendship that I will not forget during my life.
I truly wish to thank my wife and daughter for their great patience and the sacrifices
they have made through my course of study. Their loving support truly made this possible.
To my mother having lived alone in my hometown since my father’s sudden death, I
really appreciate her great patience, warmhearted regard and financial support throughout
my study.
I thank my father-in-law for his warm encouragement and generous financial
support.
With my heart and soul, I apologize to my late father for my absence at the last
moment of his life and thank him for his constant enthusiasm and unlimited support for
me even in his period of struggle against lung cancer before he passed away.
Finally, I thank God, my Lord, in heaven.
vii
TABLE OF CONTENTS Page
LIST OF TABLES .………...…………………………………………………….…..…xiv
LIST OF FIGURES ……...…………………………………………………………...…xv
LIST OF SCHEMES .………………………………………………………….………..xx
CHAPTER
I. INTRODUCTION ...………...………………………………….…...………………..1
1.1 Living anionic polymerization……..……..……...……………………....….…….1
1.1.1 General aspects……….……………..……………………………...……….1
1.1.2 Living polymerization……………………...………………...……………...2
1.1.3 Monomers………………………………………………...……..………..…8
1.1.4 The nature and stability of organolithium compounds..…………...……....10
1.1.5 Reaction media…..…….……….…...…………………...……….………...13
1.1.6 Initiation…………….…………………………..……….…………………14
1.1.7 Propagation.…………………………………………….………………….17
1.1.8 Lewis bases.…...……………………………….…………………………..18
1.1.9 Alkali metal alkoxides….………………………………………………….19
1.1.10 Lithium halides……………………………………..………….……..…..20
1.2 End-group functionalization…….……………….……………………………...…21
1.2.1 General aspects…. …….……………...…………………….………...…...21
1.2.2 Amination………………….…...…………………………….…………....25
viii
1.2.3 Fluorination…………..……...…………….……..………………...…....…28
1.2.4 Epoxidation…………………...…..…………………………………...…...31
1.2.5 Phenol functionalization………….……………...……….……………..…33
1.3 Hydrosilation……………..………………………………………………….…...34
1.3.1 General aspects……….………….………….…………...…...……...….....34
1.3.2 Transition metal catalyzed hydrosilation………..…………………………35
1.3.3 Development of a new general functionalization methodology by the combination of anionic polymerization with hydrosilation reactions….…..37
II. EXPERIMENTAL….…….…………….....…………..…………….….….…….....42
2.1 Inert atmosphere techniques ……..……..……...………………….………….…42
2.1.1 High vacuum techniques…….……………………….…..……………….. 42
2.1.2 Schlenk line techniques…………………….……….....…..……………….44
2.1.3 Dry box techniques……….………….……………………..…………….45
2.2 Purification…….…………………………………………………...…………….47
2.2.1. Solvents………………………………………………………..…………..47
2.2.1.1 Benzene………….……..…………………………………..……..….47
2.2.1.2 Toluene…………………………………………..…………………. 48
2.2.1.3 Tetrahydrofuran……………………………………..……………….48
2.2.1.4 Cyclohexane………………………………………..………………..49
2.2.1.5 Methanol………………………..………………………..…….…….50
2.2.1.6 Hexamethyldisilazane…………..……………………...…………….50
2.2.1.7 Deuterated benzene…………….……………………….……...…….50
2.2.2 Monomers………………………………………………………………….51
ix
2.2.2.1 Styrene………………….……………………………………………51
2.2.2.2 Isoprene…………………………………………………..….………52
2.2.3 Initiators……………………………………………………………….…...52
2.2.3.1 sec-Butyllithium…………..……………………………..…….…..…52
2.2.3.2 n-Butyllithium…………..……………………………………..….….53
2.2.3.3 Dibutylmagnesium………………………………………………..….53
2.2.4 Functionalizing agents……………………………….………………….…54
2.2.4.1 Chlorodimethylsilane……………….………...…………….…….….54
2.2.4.2 Allyamine………………………………………………….…………54
2.2.4.3 2-Allyphenol…………………………………………………………54
2.2.4.4 3,3,4,4,5,5,6,6,7,7,8,8,8,-Tridecafluoro-1-octene………...……….…55
2.2.4.5 2-Epoxy-5-hexene…………………………….……………………...55
2.2.5 Catalysts and other reagents…………………………….………………….55
2.2.5.1 1,3-Divinyltetramethyldisiloxane-platinum…………………..…...…55
2.2.5.2 Hydrogen hexachloroplatinate(IV) hydrate…………...……………..55
2.2.5.3 Ally bromide…………………..…………………..…………………55
2.2.5.4 Octavinyl-T8-silsesquioxane………..………..………………………56
2.2.5.5 Potassium N,N-bis(trimethylsilyl)amide……..………………..…….56
2.2.5.6 Tetrabutylammonium fluoride……..……..………………………….56
2.3 Synthesis…………………….………………..……………..…………….….….57
2.3.1 Polymerization………………..…………...……………………….………57
2.3.2 Hydrosilation…………………………..…………..………………………60
x
2.3.3 Preparation of reagents……..……………………………………...………61
2.3.3.1 3-[N,N-bis(trimethylsilyl)amino] –1-propene….……………..….…61
2.3.3.2 1-( Chlorodimethylsilyl)-3-[N,N-bis(trimethylsilyl)amino] propane…….……...………………………………..…………....….63
2.3.4 Functionalization by termination with electrophiles…………...…………..64
2.3.4.1 Functionalization of poly(styryl)lithium with dimethylchlorosilane...64
2.3.4.2 Functionalization of poly(styryl)lithium with 1-(chloro dimethylsilyl)-3-[N,N-bis(trimethylsilyl)amino]propane….……..…65
2.3.4.3 Functionalization of poly(isoprenyl)lithium with Dimethylchlorosilane.………………………………………………..67
2.3.4.4 Functionalization of poly(isoprenyl)lithium with 1- (chlorodimethylsilyl)-3-[N,N-bis(trimethylsilyl)amino]propane……68
2.3.5 Functionalization by hydrosilation reactions...……………………...……..69
2.3.5.1 Protected amine functionalization of polystyrene……………………69
2.3.5.2 Amine functionalization of polystyrene without protection…….…...70
2.3.5.3 Protected amine functionalization of polyisoprene…………….….…71
2.3.5.4 Amine functionalization of polyisoprene without protection…….….72
2.3.5.5 Perfluoroalkyl functionalization of polystyrene……………………..73
2.3.5.6 Epoxide functionalization of polystyrene……………....……………75
2.3.5.7 Phenol functionalization of polystyrene……………………...……..76
2.3.5.8 Synthesis of star-branched polystyrene with POSS (Polyhedral Oligomeric Silsesquioxane) core……………………...……………..77
2.4 Characterization……………………………………..…………………………...79
2.4.1 Size exclusion chromatography………………………..…………………..79
2.4.2 Static light scattering measurement…………………………..……………80
xi
2.4.3 NMR spectroscopy………………………...……………………………….80
2.4.4 FT-IR spectroscopy………………..……………………………………….81
2.4.5 MALDI-TOF mass spectrometry………………..…………………………81
2.4.6 DSC (Differential scanning calorimetry)……………………………..……82
2.4.7 TGA (Thermogravimetric analysis)….…….………………..……………..82
2.4.8 TLC (Thin-layer chromatography)…………………………..…………….82
2.4.9 Column chromatography………………………….……………………….82
2.4.10 End-group titration for amine functionality……………………..…..……83
III. RESULTS AND DISCUSSION...………….…...…………………………………85
3.1 Primary amine functionalization with pre-synthesized terminating agent…..…..85
3.1.1 Amine functionalization of poly(styryl)lithium……...…………………….86
3.1.1.1 Synthesis of terminating agents………………………………...……86
3.1.1.2 Stability of terminating agents……………………………….………87
3.1.1.3 Amine functionalization………..……………………………..……...89
3.1.2 Amine functionalization of poly(isoprenyl)lithium……………..…………96
3.2 Primary amine functionalization by hydrosilation reactions…..……..………….99
3.2.1 Amine functionalization of polystyrene…………………………..………100
3.2.1.1 Preparation of silyl hydride-functionalized polystyrene…………....100
3.2.1.2 Preparation of protected amine-functionalized polystyrene by hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with silane-functionalized polystyrene……………………………..106
3.2.1.3 Preparation of amine-functionalized polystyrene by hydrosilation of allylamine with silane-functionalized polystyrene without protection………………………………………………………...…115
xii
3.2.2 Amine functionalization of polyisoprene…………………..……………..125
3.2.2.1 Preparation of silyl hydride-functionalized polyisoprene………..…125
3.2.2.2 Self-induced hydrosilation of silyl hydride-functionalized polyisoprene…………….…………………………………………..130
3.2.2.3 Preparation of protected amine-functionalized polyisoprene by hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with silane-functionalized polyisoprene………………………...….134
3.2.2.4 Preparation of amine-functionalized polyisoprene by hydrosilation of allylamine with silane-functionalized polyisoprene without protection……………………………………….……………..……141
3.3 Phenol functionalization of polystyrene by hydrosilation of 2- allylphenol with silane-functionalized polystyrene without protection………………...…...147
3.4 Epoxide functionalization of polystyrene by hydrosilation of 1,2-epoxy- 5-hexene with silane-functionalized polystyrene without protection………..…153
3.5 Perfluoroalkyl functionalization of polystyrene by hydrosilation of 1H,1H,2H-perfluoro-1-octene with silane-functionalized polystyrene without protection………………………………………..……………………….…….159
3.6 Synthesis of POSS (Polyhedral Oligomeric Silsesquioxane) cored, 8-arm, star-branched polystyrene by hydrosilation of octavinyl-T8-silsesquioxane with silane functionalized polystyrene……………………..………………..…167
IV.SUMMARY…………………………………………………………………….…176
REFERENCES…………………………………………………………...…………..181
xiii
LIST OF TABLES
Table Page
1.1. pKa values of the conjugate acids of the propagating anions arising from common monomers in DMSO…………………………………...………………………...….10
1.2. Degree of aggregation for common alkyllithium initiators in hydrocarbon solvents (benzene and cyclohexane) at room temperature……………...……………….……16
1.3. Degree of aggregation for common alkyllithium initiators in polar solvents (diethyl ether and tetrahydrofuran) at room temperature.………………………………...….16
1.4. pKa values (estimated) in DMSO……………………………………………….…..39
2.1. Standarization of HClO4………………………………………………………….…83
2.2. Titration of amine-functionalized polystyrene with HClO4…………………….…...84
xiv
LIST OF FIGURES
Figure Page
1.1. Reaction of poly(styryl)lithium with oxygen………………………………………..11
1.2. Reaction of polymeric organolithium compounds with carbon dioxide…………….12
1.3. Decomposition of butyllithium in the presence of tetrahydrofuran…………………12
1.4. Tapered block microstructure……………………………………………………….14
1.5. Proposed µ-complex structure of organolithium compounds cross-associated with lithium bromide…………………………………………………………………..….21
1.6. Various reactions of organolithium chain ends with ω-halo-α-functional alkanes: A, end functionalization; B, proton transfer followed by elimination; C, metal- halogen exchange followed by dimer formation...... 23
1.7. Functionalization of organolithium chain ends with 1,1-diphenylethylene derivatives…………………………………………………………………………...24
1.8. Functionalization of organolithium chain ends with chlorosilane derivatives pre- synthesized by platinum-catalyzed hydrosilation…………………………………...25
1.9. Hydrosilation mechanism in homogeneous transition metal catalyzed system……..36
1.10. Reactivity of alkenes for platinum-catalyzed hydrosilation reactions……………..37
2.1. The typical construction of a high vacuum line for anionic polymerization………..42
2.2. Reactor design for anionic polymerization………………………….………………57
3.1. 1H NMR analysis for the decomposition of amine-functionalizing agent…………..88
3.2. FT-IR spectrum for the decomposition products…………………………………....89
3.3. TLC analysis before and after silica gel chromatography…………………………..90
xv
1 3.4. CH3 region of H NMR spectrum for the amine-functionalized polystyrenes..….…91
1 3.5. CH3 region of H NMR spectrum for non-polar polymer mixture………………….91
3.6. FT-IR analysis before and after silica gel column chromatography: A, base polystyrene; B, after functionalization (mixture); C, amine-functionalized polystyrene (separated with THF/toluene); D, non-polar polymer mixture (separated with toluene). Solid line, CH3 deformation; Dashed line, CH3 rocking…92
3.7. MALDI-TOF mass spectrum for non-polar polymer mixture (separated with toluene) with silver trifluoroacetate as a cationizing agent……………..…………..93
3.8. MALDI-TOF mass spectrum for amine-functionalized polystyrene (separated with THF/toluene) with silver trifluoroacetate as a cationizing agent……………...94
3.9. Possible reactions for the formation of trimethylsilyl-functionalized polystyrene and amine-functionalized polystyrene……………………..………………………..96
3.10. 1H NMR for different deprotection reactions: A, base polyisoprene; B, methanol treatment; C, dilute HCl treatment; D, TBAF treatment………………………….97
3.11. SEC chromatogram for the silyl hydride-functionalized polystyrene…………....101
3.12. 1H NMR spectrum for silyl hydride-functionalized polystyrene………………....102
3.13. 13C NMR spectrum for silyl hydride-functionalized polystyrene………………...103
3.14. FT-IR spectrum for silyl hydride-functionalized polystyrene………………...….103
3.15. MALDI-TOF mass spectrum for silyl hydride-functionalized polystyrene with silver trifluoroacetate as a cationizing agent………...……………...…………….104
3.16. MALDI-TOF mass spectrum for silyl hydride-functionalized polystyrene with sodium trifluoroacetate as a cationizing agent……………………………………105
3.17. SEC chromatogram for the protected amine-functionalized polystyrene……..….108
3.18. 1H NMR spectrum for protected amine-functionalized polystyrene…………..…109
3.19. 13C NMR spectrum for protected amine-functionalized polystyrene…………….110
3.20. FT-IR spectrum for protected amine-functionalized polystyrene………………...111
xvi
3.21. MALDI-TOF mass spectrum for protected amine-functionalized polystyrene with silver trifluoroacetate as a cationizing agent………………………………...112
3.22. The structure of dithranol…………………………………………………………113
3.23. MALDI-TOF mass spectrum for protected amine-functionalized polystyrene without silver trifluoroacetate………………………...……………………….….114
3.24. SEC chromatogram for amine-functionalized polystyrene…………………….…117
3.25. Zimm plot for amine-functional polystyrene without protection groups…………118
3.26. 1H NMR spectrum for amine-functionalized polystyrene without protection…....119
3.27. DEPT-135 13C NMR spectrum for amine-functionalized polystyrene…………...120
3.28. 29Si NMR spectra for silane- and amine-functionalized polystyrene…………….121
3.29. FT-IR spectrum for amine-functionalized polystyrene without protection………121
3.30. MALDI-TOF mass spectrum for amine-functionalized polystyrene with silver trifluoroacetate as a cationizing agent……………………………………………123
3.31. MALDI-TOF mass spectrum for amine-functionalized polystyrene without silver trifluoroacetate……………………………………………………..………124
3.32. SEC chromatogram for the silyl hydride-functionalized polyisoprene…………..126
3.33. 1H NMR spectrum for the silyl hydride-functionalized polyisoprene…………....126
3.34. 13C NMR spectrum for the silyl hydride-functionalized polyisoprene…………...127
3.35. 1H-1H COSY spectrum for the silyl hydride-functionalized polyisoprene……….128
3.36. 29Si NMR spectrum for the silyl hydride-functionalized polyisoprene…………..129
3.37. FT-IR spectrum for the silyl hydride-functionalized polyisoprene………………130
3.38. Self-induced hydrosilation of silyl hydride-functionalized polyisoprene…….…..131
3.39. Possible 1H-1H correlation caused by self-induced hydrosilation…………….….132
xvii
3.40. 1H-1H COSY spectrum for the silyl hydride-functionalized polyisoprene in the presence of Pt(0) catalyst after 4 days: a, Si(CH3)2; b, SiCH2C(CH3)=CHCH2; c, SiH.………………………………………………………………………..…...132
3.41. 1H NMR analysis for the silyl hydride-functionalized polyisoprene in the presence of Pt(0) catalyst for 10 days………………………….…….………..….133
3.42. SEC chromatogram for the protected amine-functionalized polyisoprene…….....136
3.43. 1H NMR analysis for the protected, amine-functionalized polyisoprene….…..…137
3.44. 13C NMR analysis for the protected, amine-functionalized polyisoprene………..138
3.45. 1H-1H COSY spectrum for the protected, amine-functionalized polyisoprene…..139
3.46. FT-IR spectrum for the protected, amine-functionalized polyisoprene…………..140
3.47. SEC chromatogram for the amine-functionalized polyisoprene………………….142
3.48. 1H NMR spectrum for the amine-functionalized polyisoprene…………………..143
3.49. 13C NMR spectrum for the amine-functionalized polyisoprene………………….144
3.50. 1H-1H COSY NMR spectrum for the amine-functionalized polyisoprene……….145
3.51. FT-IR spectrum for the amine-functionalized polyisoprene……………………..146
3.52. SEC chromatogram for the phenol-functionalized polystyrene………………….148
3.53. 1H NMR spectrum for the phenol-functionalized polystyrene…………………...149
3.54. 1H NMR spectrum for the phenol-functionalized polystyrene after addition of D2O into CDCl3 NMR solution…………………………………………………..149
3.55. DEPT 13C NMR spectrum for the phenol-functionalized polystyrene…………...150
3.56. FT-IR spectrum for the phenol-functionalized polystyrene………………………151
3.57. MALDI-TOF mass spectrum for the phenol-functionalized polystyrene……..….152
3.58. SEC chromatogram for the epoxide-functionalized polystyrene…………………155
3.59. 1H NMR spectrum for the epoxide-functionalized polystyrene………………….155
xviii
3.60. 13C NMR spectrum for the epoxide-functionalized polystyrene…………………156
3.61. FT-IR spectrum for the epoxide-functionalized polystyrene………………….….157
3.62. MALDI-TOF mass spectrum for the epoxide-functionalized polystyrene……….159
3.63. SEC chromatogram for the perfluoroalkyl-functionalized polystyrene……….….161
3.64. 1H NMR spectrum for the perfluoroalkyl-functionalized polystyrene…………...162
3.65. 19F NMR spectrum for the perfluoroalkyl-functionalized polystyrene………...…163
3.66. 13C NMR spectrum for the perfluoroalkyl-functionalized polystyrene…………..164
3.67. FT-IR spectrum for the perfluoroalkyl-functionalized polystyrene………………164
3.68. MALDI-TOF mass spectrum for the perfluoroalkyl-functionalized polystyrene...165
3.69. SEC chromatogram for the POSS-cored, 8-arm, star-branched polystyrene: A, before fractionation; B, after fractionation.……………………………….…..169
3.70. 1H NMR spectrum for the POSS-cored, 8-arm, star-branched polystyrene……...170
3.71. 13C NMR spectrum for the POSS-cored, 8-arm, star-branched polystyrene……..171
3.72. FT-IR spectrum for the POSS-cored, 8-arm, star-branched polystyrene…………171
3.73. MALDI-TOF mass spectrum for the POSS-cored, star-branched polystyrene after fractionation …………………………………………………………….…..172
3.74. TGA analysis for the POSS-cored, star-branched polystyrene…………………...173
3.75. DSC curve for the POSS-cored, star-branched polystyrene……………………...174
4.1. The combination of living anionic polymerization with hydrosilation..…….…….176
4.2. Versatility of the new general chain-end functionalization methodology…………177
xix
LIST OF SCHEMES
Scheme Page
1.1. Winstein-type spectrum of ionic species...………………………………………….13
1.2. Synthesis of silyl-protected aminating agents: [N,N-bis(trimethylsilyl)amino]- butylchlorodimethylsilane…………………………………………………………...28
1.3. Introduction of perfluorooctyl functionalities via diphenylethylene derivatives bearing a silyl-protected, phenol functional group on the aromatic ring……………31
1.4. New general end-functionalization methodology developed by the combination of anionic polymerization with hydrosilation reactions...……………………………...39
1.5. Synthesis of epoxy-functionalized polyolefin macromonomers………………….…40
3.1. Primary amine functionalization with a chlorosilane derivative……………………86
3.2. Base-catalyzed hydrolysis of the amine-functionalizing agent……………………..88
3.3. New general functionalization methodology………………………………………..99
xx CHAPTER I
INTRODUCTION
1.1. Living anionic polymerization
1.1.1 General aspects
The most important feature of alkyllithium-initiated living anionic polymerization is
the ability to proceed in the absence of chain transfer and termination during the whole
course of polymerization reaction (Eqs 1.1, 1.2).1-5 Accordingly, the molecular weight of
Initiation : R Li+ M R-M Li (Eq 1.1)
Propagation : Pi Li+ M Pi-M Li Pi+1 Li(Eq 1.2)
the resulting polymer can be readily controlled by the stoichiometry of the reaction and
degree of conversion. Based on the ratio of the amount of monomer to the moles of
initiator, the desired number average molecular weight (Mn) at complete monomer
consumption can be calculated (Eq 1.3).1
Mn = g of monomer consumed / moles of initiator (Eq 1.3)
Generally, using the living anionic polymerization method, a narrow molecular weight distribution (Mw/Mn ≤ 1.1), the so-called Poisson distribution, can be achieved
6,7 when the initiation rate (Ri) is comparable to or faster than propagation rate (Rp). In
other words, all chains should start at almost the same time and grow for the same period
of time to obtain polymers with narrow molecular weight distributions. Theoretically, for 1 living polymerization systems, polydispersity can be expressed in terms of degree of
polymerization as shown in Eq 1.4.8 The use of initiators with less reactivity,9 a mixture
2 Xw/Xn = 1 + [Xn/(Xn + 1) ] ≅ 1 + (1/Xn) (Eq 1.4)
of initiators,10 or addition of initiator in a continuous process during polymerization11-16
can result in broader molecular weight distributions.
Another advantage of living anionic polymerization is the unique ability for all
chains to retain their active chain ends even after complete monomer consumption (Eq
1.5).1 Therefore, versatile post-polymerization reactions of the anionic chain ends provide
R Li+ nM R-[M]n-1M Li (Eq 1.5)
useful methodologies to produce block copolymers by sequential monomer addition,17 diverse in-chain and chain-end functional polymers by nucleophilic substitution or addition reactions with electrophiles,18 and branched polymers by linking reactions with
multi-functional linking agents.19,20 Consequently, the living nature of anionic
polymerization provides methods to achieve well-defined structure, microstructure and
architecture, which affect the ultimate properties of the resulting polymers.
1.1.2 Living polymerizations
A living polymerization is a chain-growth reaction proceeding in the absence of the
kinetic steps of termination and chain transfer as described by Szwarc,2,3 which means
that active chain ends remain during the whole course of polymerization unless
terminating or chain transfer agents are deliberately added to deactivate living chain ends.
As previously mentioned, living polymerizations make it possible to produce tailor-made
polymers with well-defined structures and low degrees of compositional heterogeneity by
2 controlling important parameters such as molecular weight, molecular weight
distribution, copolymer composition, microstructure, stereochemistry, branching, and
chain-end functionality, which can be used to vary and control the desired properties of
resulting polymers.6,17,21-26 Since the important breakthrough of the living concept for
anionic polymerization in the mid 1950’s,2,3 various kinds of other living polymerization techniques have been designed differing in mechanistic intermediates which involve a cation,27 radical26,28 or transition metal complex.29 In addition, a number of “living” concepts have been introduced to explain the different “living” natures for various living
polymerization techniques. Those concepts include quasi-living,30 pseudo-living,28 immortal31 and truly living.17 Accordingly, it is very important to properly classify
polymerization systems of interest as living in a systematic manner. In line with this
issue, Quirk and coworkers1,10 not only reviewed most major criteria that have been
reported so far, but also thoroughly investigated the usefulness of each criterion as the tool to judge “livingness” for a certain polymerization system. On the basis of their
research, nine experimental criteria were proposed to diagnose reaction systems with
respect to their living character as follows.
The first and oldest criterion is that the polymerization proceeds until the complete
consumption of all of the monomer and further addition of monomer results in continued
polymerization.3 The complete monomer consumption doesn’t indicate the absence of chain transfer as well as termination at all. On the other hand, the continuous polymer growth with additional monomer feeding implies no termination to a certain extent, even if it doesn’t necessarily mean there is no chain transfer. Consequently, if we add a
3 proviso that all of the chains continue to grow when additional monomer has been added,
this criterion can be used as a useful and rigorous one.
The second criterion widely used in recent years is that the molecular weight
increases linearly with conversion. In other words, the number average molecular weight,
Mn, is a linear function of conversion. This can be useful to test absence of chain transfer.
However, for sec-butyllithium-initiated styrene polymerization, an experiment involving
incremental monomer addition with deliberate termination proved that a linear plot
between Mn and conversion is not sensitive to termination reactions, where the linearity
turned out to still remain even after sequential termination of 15 % of the polymer
chains.10 In this experiment, 5 % of termination per each monomer addition was effected
during the whole course of polymerization. Accordingly, this criterion by itself is not a
rigorous one to examine livingness for a given polymerization system.10,32
The third criterion is that the number of polymer molecules remains constant
throughout the polymerization and is independent of conversion. This is an efficient requirement to detect chain transfer during polymerization, but it is not sensitive to termination reactions that don’t cause any change in the total number of chains. Therefore
this criterion is not in itself a rigorous determinative test for a living system.
From a practical point of view, another useful and frequently utilized criterion is
that for a living polymerization system, the number average molecular weight can be
controlled by the stoichiometry of the reaction and degree of conversion. The most
important requirement to satisfy this criterion is quantitative initiator efficiency and
utilization before complete monomer consumption. Accordingly, this criterion is
sensitive to impurities in the reaction system, which can cause the deactivation of
4 initiators. Particularly, it should be noted that this criterion is always limited by the error
limits of the methods used to determine the number average molecular weight.
Of all the criteria, the most confusing one is that living polymerizations produce
polymers with narrow molecular weight distributions (Mw/Mn ≤ 1.1). As mentioned
previously, the essential proposition to attain a narrow molecular weight distribution is
the fact that the rate of initiation should be competitive with the rate of propagation.
Other fundamental requirements to prepare a polymer with a Poission molecular weight
distribution, which were proposed by Flory 8,33 and Henderson and Szwarc,34 include: (a)
each polymer chain must grow exclusively by consecutive addition of monomers to an
active chain end; (b) all of the propagating chain ends must have equal opportunity to
react with monomer during the whole course of polymerization; (c) all reactive
intermediates must be generated at the same time in the beginning of the polymerization;
(d) polymerization must proceed in the absence of chain transfer and termination
reactions; and (e) propagation must advance irreversibly. Although this criterion is very
important to produce well-defined polymers, it turns out that a narrow molecular weight
distribution by itself should not be used as a definitive criterion as illustrated by the fact
that a living polymerization using a less reactive initiator, such as n-butyllithium or t- butyllithium for styrene polymerization, produces polymers with broad molecular weight distributions. Furthermore ironically, even non-living polymerizations can provide
35 narrow molecular weight distributions. In addition, for mixtures of two (Mn = 19,000
and 34,500 g/mol) and three (Mn = 19,000, 34,500 and 48,000 g/mol) monodisperse
polystyrene standards in equimolar amounts, SEC analysis resulted in relatively narrow molecular weight distributions (Mw/Mn = 1.11 and 1.15) which corresponded, however,
5 to bimodal and trimodal distributions, respectively.10 Consequently, a narrow molecular
weight distribution per se cannot be used as a rigorous criterion to determine livingness for a given polymerization system.
One of the most practical advantages of living polymerization systems is their
unique ability to retain all active chain ends after complete monomer consumption. In
accordance with this concept, the ability to prepare block copolymers by sequential
monomer addition could be a useful criterion to judge whether or not a given
polymerization system is living, because this criterion is fairly sensitive to both chain
transfer and termination reactions. However, the functionalization of all the active chain
ends with functionalizing agents can only be also a good criterion when the
functionalization reaction is quantitative. From the practical point of view, however, this
criterion is limited by the fact that most functionalization reactions do not proceed in a
quantitative manner because they generate various side-products due to the high
reactivity of active chain ends.18,36 In addition, this method has a critical drawback of
difficulty in characterization of the end-groups, especially when the molecular weight
increases.
Another criterion widely used for determination of livingness for a polymerization
of interest is linearity of a kinetic plot of rate of propagation as a function of time (Eq
1.6). The basis of this criterion originated from the assumption that for a
ln ([M]0/[M]) = kobs t (Eq 1.6)
living polymerization system, the overall concentration of active propagating
intermediates is constant throughout the polymerization as implied in Eq 1.7. Based on
Rp = -d[M]/dt = kp[p*][M] = kobs [M] in which p* = propagating species (Eq 1.7)
6 Eq 1.7, this criterion is clearly sensitive to termination reactions, but not to chain transfer
because chain transfer doesn’t change the number of active propagating species.
Accordingly, the linear kinetic plot between propagation rate and time itself cannot be
used as a criterion to judge livingness, even if this can be a useful criterion when used in
combination of the second criterion, the linear increase of molecular weight with
conversion, which is sensitive to chain transfer.
As mentioned above, the combination of the second and eighth criteria is sensitive
to chain transfer as well as to termination. As a result, the last criterion, linearity of a
kinetic plot of the logarithmic function in Eq 1.8 versus time, was proposed as a practical
experimental method for determination of living polymerization (Eq 1.8).32
ln (1 – {([I0]/[M0])(DPn)}) = - kp[I0]t (Eq 1.8)
On the basis of the aforesaid investigation on the usefulness and restrictions of each
experimental test as a criterion for living polymerization, it can concluded that none of
various criteria is satisfactory alone to judge “livingness”, due to the inherent differences
from one another in terms of sensitivity to chain transfer and termination reactions.
Consequently, a proper combination of several criteria is the key point to attain a correct determination of livingness for a polymerization of interest.
Of all the living polymerization methodologies existing today, none matches
alkyllithium-initiated anionic polymerization of vinyl monomers with regard to its unique
living nature. Not all alkyllithium-initiated anionic polymerizations, however, have the
unique features of living polymerization, unless careful control of experimental procedures is maintained throughout the polymerization reaction. In general, a successful
7 living polymerization can be achieved by the careful selection of monomer, solvent,
temperature and reaction time.1
1.1.3 Monomers
Monomers that can be anionically polymerizable are classified into two categories,
in which one is the class of unsaturated monomers bearing one or more double bonds such as vinyl, diene and carbonyl-type monomers; the other is the class of heterocyclic
monomers that undergo ring-opening polymerization with nucleophiles.1 From a thermodynamic point of view, anionic polymerization of vinyl monomers is always exothermic and thermodynamically favorable, since strong σ-bonds are formed at the cost of weak π-bonds after the addition of monomers to active chain ends. In principle, thermodynamics of a certain reaction can predict whether or not the reaction can take place. However, it doesn’t say anything about the reaction rate that mainly depends on activation energy of the reaction. Therefore, it can take forever for the reaction to be done, even if the reaction is thermodynamically favorable with a negative free energy.
Consequently, in order for a given polymerization reaction to be effected in a reasonable experimental time scale, a kinetically suitable pathway must exist.37 In keeping with this kinetic requirement, in general, there must be electron-withdrawing or electron- delocalizing substituents to stabilize the negative charge in the transition state in the case of anionic polymerization of vinyl monomers. In order to minimize undesirable side- product formation arising from the inherent high reactivity of active anionic species in the course of polymerization, some monomers containing acidic protons or reactive groups in the molecule must be converted to derivatives by using suitable protecting
8 groups. The list of types of anionically polymerizable monomers not requiring protecting groups includes styrenes, dienes, methacrylates, epoxides, episulfides, cyclic siloxanes, and lactones.
For anionic polymerization, in general, the reactivity of the monomer can be deduced based on the pKa value, which is associated with the stability of the corresponding propagating anionic species as shown in Eq 1.9; in general, the larger the pKa value of the conjugate acid, the less stable the corresponding anionic species is.
C H C: + H Eq 1.9
In connection with this matter, care must be taken in choosing an initiator. When the pKa value of the conjugate acid of the initiator is too high compared to that of the conjugate acid corresponding to the anion formed from the monomer, there is the possibility of undesirable side-reactions. On the other hand, when it is too low, the rate of initiation becomes too slow, eventually causing broad molecular weight distributions for the resulting polymers. Therefore, for the purpose of a controlled polymerization, it is very important to consider the pKa values of both the conjugate acid of the initiator and that of the conjugate acid of the propagating species formed from the monomer before choosing the appropriate initiator for a polymerization. Some pKa values for the conjugated acids of the anions formed from monomers most widely used in anionic polymerization are listed in Table 1.1.38-40 In general, the initiator and propagating species have similar reactivity with similar pKa values for a controlled anionic polymerization without other additives.
9
Table 1.1. pKa values of the conjugate acids of the propagating anions arising from
common monomers in DMSO.38-40
Monomer pKa (DMSO)
Ethylene 56
Styrene 44
Diene 43
Alkyl Methacrylate 30- 31
Oxirane 29- 32
1,1-Diphenylethylene 32
1.1.4 The nature and stability of organolithium compounds
Of all alkali metals, lithium is unique in that it exhibits the highest electro- negativity and the smallest covalent and ionic bond radii, along with low-lying, unoccupied p-orbitals available for bonding.41 In the same way, unlike the organic derivatives of other alkali metals such as Na, K, and Cs, which are clearly ionic compounds, organolithium compounds possess their unique characteristics because of the nature of the C-Li bond which exhibits the dual properties of both covalent and ionic bonds.41 Although it is well known that most organoalkali metal compounds tend to aggregate in solution as well as in the solid state,1 the aggregation of organolithium
10 compounds is different from that of other organoalkali metal compounds in terms of their unique solubility in hydrocarbon solutions such as cyclohexane, benzene, and toluene.42
In addition, most polymeric organolithium compounds in solution exhibit characteristic colors, observable by UV-visible spectroscopy, which change or disappear with the elapse of time or upon addition of terminating agents. The color change or disappearance can be attributed to the destruction of the active anionic chain ends. Frequently, the characteristic UV-visible absorbance is used to identify the polymeric organolithium compounds of interest in the reaction system. A gradual and slow color change during initiation or during the reaction can be easily monitored using UV-visible spectroscopy.43
It has been found that most alkyllithium compounds are not thermally stable and usually undergo a series of decomposition processes as the temperature increases.42-44
Alkyllithium compounds are also very reactive toward protic and polar impurities such as oxygen, carbon dioxide, water and alcohols.42 Accordingly, methanol is practically employed as the most common terminating agent for anionic polymerization of styrene and diene monomers.
In the case of contact with oxygen, poly(styryl)lithium generates four different products as shown in Figure 1.1, in which P stands for the polymer chain.45,46
ROH PLi + O2 P-P+ P-O-O-P ++P-O-O-H P-O-H
18-22% 3-18% 2-9% 58-69%
Figure 1.1. Reaction of poly(styryl)lithium with oxygen.
In addition, polymeric organolithium compounds readily undergo carbonation47-51 in the presence of carbon dioxide in hydrocarbon solution in which the final products contain dimeric ketone and trimeric alcohol in addition to the carboxylated polymers (Figure 1.2). 11 H3O PLi + CO2 PCO2HP+ 2CO + P3COH 27-66% 23-27% 7-50%
Figure 1.2. Reaction of polymeric organolithium compounds with carbon dioxide.
In order to get rid of trace amounts of oxygen, water and carbon dioxide from the polymerization reactor, high vacuum techniques52 are used for the controlled anionic polymerization of vinyl monomers.
Reactions with polar compounds and polar solvents can be a problem. For example, side reactions occur for alkyllithium compound with tetrahydrofuran (THF) which is one of the most commonly used solvents for various reactions of alkyllithium compounds.
Hydrogen abstraction from the reactive methylene unit next to oxygen on the THF ring forms butane and an intermediate metalated ring, which further decomposes by cyclo- elimination to yield ethylene and the lithium enolate of acetaldehyde as shown in Figure
1.3.53-56
BuLi + + O H O Li
CH2 CH2 CH2 + LiO
Figure 1.3. Decomposition of butyllithium in the presence of tetrahydrofuran.
12 1.1.5 Reaction media
In essence, the unique nature of the C-Li bond in organolithium compounds can be ascribed to its dual nature; i.e., it exhibits properties of both covalent and ionic bonds depending on the structure of carbanion and solvent. Consequently, the choice of solvent is one of the most important issues for controlled anionic polymerization, and particularly, this has a great influence on the nature of the active organolithium species as illustrated by equilibria in the Winstein-spectrum of ionic species (Scheme 1.1).1,57 In general, the use of a polar solvent like THF as the reaction medium
Aggregated Unaggregated Tight Loose Free Ions Ion Pairs species species Ion Pairs
R + Li (RLi)n n RLi R , Li R // Li
k [M] k1[M] k2[M] k3[M] k4[M] 5
Scheme 1.1. Winstein-type spectrum of ionic species. causes the nature of the C-Li bond to shift toward more ionic character in the equilibria illustrated in Winstein spectrum,58 which increases the rate of initiation as well as propagation for alkyllithium-initiated anionic polymerization.
In addition to polymerization kinetics, the stereochemistry of the final polymers, especially the microstructure of polydienes, is also greatly influenced by the choice of solvent. For example, the use of polar solvents for the polymerization of diene monomers results in high 1,2- and 3,4-microstructure for butadiene and isoprene, respectively,59,60 which reduce the ultimate elastomeric properties of polydienes.61,62 Accordingly, in order to preserve the characteristic high 1,4-stereospecificity of polydienes polymerized using
13 alkyllithium-initiators, non-polar hydrocarbon media should be chosen such as cyclohexane, hexane, and benzene.
Regarding the microstructure of copolymers, the change of the compositional microstructure of a copolymer is another important aspect affected by the reaction medium. In hydrocarbon solvent, alkyllithium-initiated copolymerization of styrene and butadiene monomers provides tapered (or graded) block copolymers due to a large
1,63 difference in monomer reactivity ratios (rs= 0.04, rb= 10.8). Butadiene is preferentially incorporated into the polymer chains generating butadiene-rich tapered segments first, followed by styrene incorporation forming styrene-rich tapered terminal segments. In contrast, in polar solvents such as THF, the copolymerization of styrene and butadiene monomers reveals an opposite behavior with the inverted monomer reactivity ratios producing styrene-rich tapered segments first followed by butadiene-rich segments later as shown in Figure 1.4 in which I stands for initiator, [D] is diene segment, and [S] corresponds to styrene segment. 1,63 Furthermore, the polydiene microstructure in the copolymer changes from high 1,4-enchainment in hydrocarbon solvent to high 1,2- microstructure in polar solvents such as THF.1
In hydrocarbon solvent In polar solvent
I[D][D/S] [S] or I[S] [S/D] [D]
Figure 1.4. Tapered block microstructure.
1.1.6 Initiation
In general, the initiation of anionic polymerization of styrene and diene monomers is effected with alkyllithium compounds such as sec-butyllithium and n-butyllithium. For the purpose of the preparation of tailor-made polymers with narrow molecular weight 14 distributions, the control of the initiation rate is very important where the rate of initiation must be at least competitive with that of propagation.
Most organolithium compounds are stabilized by aggregation in hydrocarbon solvents, and even in polar solvents in most cases. With regard to this, a general trend has been found that the rate of initiation decreases as the degree of aggregation increases. In other words, the reactivity of alkyllithium initiator is strongly dominated by the degree of aggregation, which depends on the structure of the organolithium compound, solvent, concentration and temperature. In connection with the effect of the carbanion structure, the tendency of aggregation decreases with an increase of the steric bulkiness of the organolithium compounds as shown in Table 1.2 for non-polar solvents and in Table 1.3 for polar solvents.64-73 As a result of the aggregation phenomena, the kinetics of the initiation for anionic polymerization with alkyllithium initiators exhibits a fractional order dependence on the initiator concentration74,75 The fractional kinetic order dependence is attributed to the presence of both aggregated and dissociated alkyllithium species during initiation where, in general, the dissociated species has enough reactivity to initiate polymerization while the aggregated species must undergo a dissociation process prior to initiation. However, for initiation in aliphatic hydrocarbon solutions, first order dependencies on alkyllithium concentration are observed suggesting that the intact aggregates are initiating polymerization.76,77 The fractional kinetic order dependence varies with initiator. For instance, when n-butyllithium is used as the initiator for styrene polymerization in benzene, the kinetic order of initiation for n-butyllithium concentration turns out to be approximately one-sixth while it is approximately one-fourth when the initiator is sec-butyllithium.1,75
15 Table 1.2. Degree of aggregation for common alkyllithium initiators in hydrocarbon
solvents (benzene and cyclohexane) at room temperature.64-69
Initiator Degree of Aggregation Benzene Cyclohexane
ethyllithium 6 6
n-butyllithium 6 6
sec-butyllithium 4 4
tert-butyllithium 4 4
benzyllithium 2 -
Table 1.3. Degree of aggregation for common alkyllithium initiators in polar
solvents (diethyl ether and tetrahydrofuran) at room temperature.70-73
Initiator Degree of Aggregation Diethyl ether Tetrahydrofuran
ethyllithium 4 -
n-butyllithium 4a 2.4 a (D/T = 81/19) sec-butyllithium - 1.1 a (M/D = 88/12) tert-butyllithium 1 -
benzyllithium 1 -
a –108 °C, M = monomer, D = dimer, T = trimer
16
1.1.7 Propagation
For alkyllithium-initiated living anionic polymerization, the kinetic aspects of propagation are similar to initiation. The kinetics is complicated as a result of the aggregation of propagating anionic chain ends.1
The kinetics of the propagation can be expressed by the rate of monomer consumption as a function of time as shown in Eq 1.10, in which [M] is the monomer concentration at a specific time, [PLi] is the living chain end concentration, and kp is the rate constant of the propagation.
d[M] Rp = - = kp[PLi][M] Eq 1.10 dt
The rate of propagation for the polymerization of both styrene and diene monomers with alkylithium initiators in hydrocarbon solvents including heptane, cyclohexane, benzene, and toluene exhibits a first order kinetic dependence with respect to monomer concentration, which diminishes with the reaction progress.
Analogous to the kinetic effect of aggregation of organolithium initiators, the kinetics of propagation also exhibits a fractional order dependence on the concentration of the active chain ends due to their aggregation in solution. For styrene monomer, a one- half order kinetic dependence is observed,74 while the kinetic order for the propagation of poly(dienyl)lithium chain ends is generally less than that of poly(styryl)lithium chain ends, e.g., approximately one-fourth.
Although the rates of propagation for styrene and diene monomers are known to be in the order of styrene > isoprene > butadiene, the relative reactivity of styrene versus
17 dienes depends on the chain-end concentration.1 In fact, in the case of low chain-end concentration, it is possible for isoprene to propagate faster than styrene in hydrocarbon solution. Copolymerization is a different situation. It is governed by the crossover rates not the relative propagation rates of styrene versus diene.
1.1.8 Lewis bases
Lewis bases are commonly used as modifiers in anionic polymerization. The most widely used Lewis bases can be grouped into two categories in which the one is the class of amines including triethyl amine and N,N,N’,N’-tetramethylethylene diamine
(TMEDA), and the other one is the class of ethers such as diethyl ether and tetrahydrofuran.
Generally, the presence of Lewis bases increases both the rates of initiation and propagation for alkyllithium-initiated polymerization by promoting the dissociation of the organolithium aggregates by coordination with the lithium ions in the aggregates.78-80
From a practical point of view, however, Lewis bases must be used in minimum amount to prevent the unnecessary decomposition of organolithium compounds. For example, the reactive organolithium compounds are unstable in the presence of polar additives,43 as exemplified by the facile decomposition of alkyllithium compounds in the presence of
THF. Usually, the optimal amount of Lewis bases used ranges from 2-30 molar equivalents with respect to the amount of initiator, i.e. [Lewis base]/[Li] = 2-30. Lewis bases alter the microstructure of the resulting polydienes increasing the amount of 1,2- and 3,4-addition products for butadienes and isoprenes, respectively.81
18 With regard to the copolymerization behavior of styrenes and dienes in hydrocarbon solvents, the presence of Lewis bases increases the incorporation of styrenes yielding a more random copolymer composition along the polymer chain in contrast to the tapered block structure in hydrocarbon solution.1,22 The use of bidentate bases like TMEDA exerts much stronger effects on polydiene microstructure as well as on copolymerization behavior of styrenes and dienes.82
1.1.9 Alkali metal alkoxides
The general trend for the influence of lithium alkoxides on the kinetics of alkyllithium-initiated styrene polymerization is to decrease both initiation and propagation rates.83,84 On the other hand, for isoprene polymerization, the presence of lithium alkoxides significantly accelerates the rate of the initiation effected with alkyllithium compounds.84 No noticeable effect of lithium alkoxides on copolymerization behavior is observed in alkyllithium-initiated copolymerization of styrenes and dienes, which produces tapered block copolymers. It should be noted, however, that a depression of copolymerization rate is found, when a small amount of lithium alkoxide is present during the copolymerization process.85 The use of other alkali metal alkoxides such as sodium and potassium alkoxides forms copolymers with more random incorporation of the comonomers along the polymer back-bone.
With respect to the effect of lithium alkoxides on the microstructure of anionic polymerization of dienes, the presence of lithium sec-butoxide in equivalent ratios relative to initiator concentration appears to increase the vinyl contents in the polydiene chain. However, the effect of lithium alkoxide addition on the stereochemistry of the
19 resulting polydiene turns out to be small83,86 compared to the effects of other alkali metal alkoxides which cause a significant increase in vinyl microstructure, comparable to the amount of the vinyl content observed in polymerizations effected by the corresponding alkali metal counterions.
1.1.10 Lithium halides
Lithium halides affect the kinetics of propagation, initiation, and even functionalization of organolithium anionic species by modification of their reactivity toward monomers and functionalization agents, i.e. electrophiles.87-91 In general, lithium halides appear to reduce the reactivity and promote the selectivity of organolithium compounds simultaneously. A representative example is the alkyllithium-initiated polymerization of methyl methacrylates, which is usually effected using a less reactive initiator like diphenylhexyllithium in THF at –78 °C. This polymerization can be further controlled by the addition of lithium chloride which decreases the rate of polymerization resulting in polymers with narrower molecular weight distributions even for t-butyl acrylate.92-95 As another example, in the study of the influence of lithium halides on the reactivity of phenyllithium compounds, the presence of one equivalent of lithium halides was found to reduce the reactivity of phenyllithium down to one half compared to that in the absence of lithium salts, and the reactivity changed by varying the ratio of lithium halide to phenyllithium.87
In the presence of lithium bromide, alkyllithium compounds have been shown to form 1:1 complexes with μ-type structure as shown in Figure 1.5.96-98 The addition of lithium halides has proven to be particularly useful for the polymerization of relatively
20 reactive polar monomers, due to the characteristic ability of lithium halides to control the reactivity and selectivity of organolithium compounds by cross-association to form aggregates that result in lower degrees of chain end aggregation and rapid exchange of anionic groups.
Li
RC Br Li
Figure 1.5. Proposed μ-complex structure of organolithium compounds cross-
associated with lithium bromide.
1.2. End-group functionalization
1.2.1 General aspects
As mentioned, in principle, alkyllithium-initiated living anionic polymerization proceeds in the absence of chain transfer and termination reactions. The resulting polymers synthesized in this living system can have the characteristic advantages of controlled molecular weights as well as narrow molecular weight distributions.
Another practical feature of living anionic polymerization is the unique ability to generate stable carbanions at the end of the chain reaction such that all of the chain ends retain their active centers even after complete monomer consumption. Subsequent controlled post-polymerization reactions such as addition or substitution reactions with electrophiles provide useful tools to prepare various well-defined, end-functionalized polymers.1,18,36,99 The resulting functional polymers can undergo further reactions such as initiation of polymerization of other polar monomers,100-102 coupling with the functional
21 groups of other polymers103-106 and linking with multi-functional reagents to form star- branched polymers as well as chain-extended and cross-linked products.107,108 For example, coupling of well-defined synthetic polymers with naturally occurring biopolymers has been used to form diverse hybrid biomaterials.106
In addition to the use of post-polymerization reactions of anionic chain ends with electrophilic functionalization agents, functional polymers can also be prepared using functional initiators with which quantitative functionalizations of all of the chains can be accomplished. However, the practical applicability of this technique has been limited by the lack of available functional initiators, the requirement of suitable protecting groups due to the high reactivity of carbanions toward most polar functional groups, and their limited solubility in hydrocarbon solvent.1,109,110
Even though a variety of functional polymers have been synthesized by anionic functionalization methods in the last few decades, progress has been slow because each specific functionalization reaction must be developed and optimized for each different functional group. this restricts their practical application due to the complexity of the synthetic routes and often the lack of adequate characterization methods for the reaction products. Therefore, the development of new general functionalization methodologies has drawn considerable recent interest. Three general functionalization techniques have been developed and widely used so far. A characteristic common to all three methods is that they employed the post-polymerization reactions of anionic chain ends with reagents containing reactive electrophilic units and also various functional groups or their protected analogs.
22 One of them utilizes ω-halo-α-functional alkanes.111,112 This approach has drawbacks because of side reactions such as proton transfer, electron transfer followed by metal-halogen exchange and dimer formation,113 as shown in Figure 1.6 in which P is a polymer chain. For example, for the reaction of poly(styryl)lithium with 3- dimethylaminopropyl chloride, only a 67 % yield of amine functional polymer was obtained; the other products corresponded to the dimer (23 %) and the unfunctionalized polymer (10 %).112
A : P Li + Cl X P X + LiCl
H X B : P Li + Cl X PH + + LiCl
P Li Cl X Li C : P Li + PCl + X Dimer Figure 1.6. Various reactions of organolithium chain ends with ω-halo-α-functional
alkanes: A, end functionalization; B, proton transfer followed by elimination; C,
metal-halogen exchange followed by dimer formation.
Fortunately, it was found that additives such as lithium chloride can improve functionalization efficiency by complexing with the organolithium chain ends to reduce their reactivity and to increase their selectivity.91 For example, essentially quantitative amine functionalization has been effected by addition of lithium chloride in the functionalization of polymeric organolithiums with 3-dimethylaminopropyl chloride.
Another useful way to obtain end-functionalized polymers involves the use of substituted 1,1-diphenylethylene derivatives as functional agents where the
23 functionalization reactions with living chain ends can afford practical and versatile tools to prepare a diverse array of in-chain as well as chain-end functionalized polymers in quantitative yields in most cases as shown in Figure 1.7 where P is a polymer chain, and
X, Y represent functional groups.100,114 This approach has proven to be simple,
X X
P Li + PCH2C Li
Y Y
Figure 1.7. Functionalization of organolithium chain ends with 1,1-
diphenylethylene derivatives. stoichiometric and quantitative, and to form only the corresponding monoaddition product. It should be noted that the reaction progress in this method can be readily monitored using UV-vis spectroscopy due to the characteristic absorption band (λmax) of
1,1-diphenylethyllithium at around 428 nm. However, protection of most polar functional groups is still required to prevent the side-reactions between the anionic chain ends and the functional groups.
As another general anionic functionalization method, DeSimone and his coworkers115,116 first utilized the substitution reactions of living anionic chain ends with chlorosilane derivatives as functionalization agents; the substituted chlorosilanes were pre-synthesized by platinum-catalyzed hydrosilation reactions as shown in Figure
1.8.115,116 Since silyl halides don’t undergo the side reactions described for alkyl halides
24 with living anionic chains,112 this technique is preferentially used to introduce a variety of functional groups to chain ends, and it has turned out to be simple and quantitative.
However, due to the high susceptibility of the chlorosilane moiety to hydrolysis, the functionalization agents are unstable in air and are even thermally unstable in some cases.
Accordingly, they need to be stored under an inert gas in a refrigerator. Another restriction in using this method is the requirement of the use of protecting groups, for the same reasons as described for the two other general functionalization methods mentioned previously.
Pt Si X Cl Si Cl H + X
X + LiCl P Li + Cl Si X P Si
Figure 1.8. Functionalization of organolithium chain ends with chlorosilane
derivatives pre-synthesized by platinum-catalyzed hydrosilation.
1.2.2 Amination
Among a number of different kinds of functional groups, the introduction of the primary amine group to living anionic chain ends has been quite challenging due to the high acidity of the protons on the nitrogen atom.117 Amine-functionalized polymers are of particular importance in view of their industrial and biological applications. For instance, rubber compounds containing amine-functionalized elastomers have been found to exhibit far better mechanical properties compared to ordinary elastomers without amine
25 functional groups.118,119 These functionalized polymers improve dispersibility of the elastomers and organic additives with reinforcing fillers such as carbon black and silica in rubber compounds during the mixing process. Recently, there has been growing interest and research on the combination of biologically-produced polycondensates with well-defined, end-functionalized synthetic polymers which are produced via living anionic polymerization.102 Of course, primary amine-functionalized polystyrenes and polydienes are in the midst of the greatest concern among biomaterial scientists due to their ability to initiate ring-opening polymerization of N-carboxyanhydrides, generating polypeptide block copolymers known as molecular chimeras, as well as simply to couple with hydroxyl groups of enzymatic macromers through condensation.106 Accordingly, among various other functional polymers, well-defined, primary amine-functionalized polymers appear to be one of the most useful classes.
Schulz and Halasa109 introduced an elegant protection technique utilizing trimethylsilyl groups to prepare primary amine-functionalized polydienes in which the polymerization was effected by protected amine functional initiators. In order to dissolve the functional initiator, however, a dual solvent system (hexane and diethyl ether) was adopted for the polymerization, which resulted in low 1,4-microstructures and broad molecular weight distributions for the resulting polydienes. Some time later, Nakahama et al.120,121 applied this novel protection technique to post-polymerization functionalization reactions using a series of benzylidene(trimethylsilyl)amine and 2- bromoethylamine derivatives as the terminating agents in cyclohexane and THF to synthesize polystyrenes and polyisoprenes bearing primary amino end groups. In spite of the great improvement with respect to formation of polymers with low degrees of
26 compositional heterogeneity, controlled structure, narrow molecular weight distributions and high 1,4-microstructures in hexane, several side reactions were observed including formation of dimer in addition to H-terminated unfunctionalized polymers.122 A well- defined, very efficient method for preparation of primary aromatic amine end- functionalized polymers was developed by Quirk et al, 114,123 called “living functionalization reactions” due to its unique ability to re-initiate polymerization with additional monomer even after the functionalization reactions. In this approach, they utilized protected 1-(4-aminophenyl)-1-phenylethylene as an aminating agent, which was synthesized by Wittig reaction of 4-aminobenzophenone followed by a subsequent protection reaction with chlorotrimethylsilane.123 Quantitative functionalities were obtained for the resulting primary amine-functionalized polystyrenes while no detectable side reactions were observed. As a general quantitative functionalization, DeSimone and co-workers116 designed a new synthetic methodology utilizing chlorosilane derivatives as functionalization agents and applied this method for the preparation of primary aliphatic amine end-functionalized polystyrenes. In this study, initially the protected aminating agents were synthesized by the hydrosilation reaction of silyl-protected aminobutene with chlorodimethylsilane in the presence of catalytic amount of chloroplatinic acid as shown in Scheme 1.2. Subsequent nucleophilic substitution of living polystyrene chain ends at the chlorosilane portion of this novel aminating agent, followed by a deprotection reaction, afforded the primary aliphatic amine-functionalized polystyrene in quantitative yield.
27 (TMS)2N Li + ClCH2OCH3 (TMS)2NCH2OCH3 + LiCl
MgBr CH2N(TMS)2 CH OMgBr (TMS)2NCH2OCH3 + + 3
CH3 CH3 H2PtCl6 6H2O CH N(TMS) + Si H Si Cl 2 2 Cl (TMS)2N(CH2)4 CH3 CH3
Scheme 1.2. Synthesis of silyl-protected aminating agents : [N,N-
bis(trimethylsilyl)amino]-butylchlorodimethylsilane.
1.2.3 Fluorination
Other useful materials of recent interest are fluoropolymers such as poly(tetrafluoroethylene) (PTFE), poly(perfluoroalkyl acrylates), fluoroelastomers, and fluoroplastics. Fluoropolymers have been drawing great attention due to their unique characteristic properties including low surface free energy, low friction coefficient, low dielectric constant, excellent flame resistance, and both solvent and chemical resistance.124-126 Consequently, their practical applications in industrial fields are widespread such that they can be used for high temperature sealing materials in contact with chemicals and oils, lubricants in powder form for engine parts, cable coatings, and pistons rings. In terms of the synthesis and processing of perfluorinated polymers, however, severe reaction or processing conditions such as high temperature and pressure are required due to their unique properties. In addition to their difficulty in preparation and processing, the applicability of perfluorinated materials has been limited to a large extent by their inherent incompatibility with hydrocarbon-based materials.127 As a means of improving on these fundamental problems, recently the incorporation of perfluoroalkyl
28 groups into conventional organic polymers has been an active field of research.
Analogous to perfluorinated polymers, perfluoroalkyl functional groups attached to the hydrocarbon polymers are surface active due to their low surface energy compared to hydrocarbon segment in the molecule and tend to be segregated to enrich more at polymer surfaces and air-polymer interfaces than in the bulk of the polymers, forming both hydrophobic and lipophobic surfaces or interfaces.128,129 Accordingly, the development of efficient synthetic methodologies for the preparation of perfluoroalkyl- functionalized polymers has attracted growing interest to modify various polymer surfaces.
DeSimone and co-workers115 utilized the reaction of anionic chain ends with
(tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-dimethylchlorosilane as the perfluoroalkyl- functionalization agent to synthesize well-defined, perfluoroalkyl end-capped polysyrenes and polydienes as shown in Eq 1.11.
F2 F2 P Li + Cl Si C P Si C + LiCl C F C F F2 F2 (Eq 1.11) 3 3 In this study, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was adopted for the determination of absolute molar mass as well as the functionality of perfluoroakyl-functionalized polymers; the results confirmed the efficient introduction of fluorine functionalities at the polymer chain ends. In order to investigate the characteristic segregation phenomenon of perfluoroalkyl functional groups to air-polymer interfaces, they utilized the angle-dependent, X-ray, photo-electron spectroscopy
(ADXPS) technique where the results clearly indicated that the fluorine content decreased, as the depth of sampling increased. 29 Nakahama et al.130 successfully prepared well-defined, perfluoroalkyl-terminated polystyrene by the reaction of living anionic chain ends with 2-(perfluorobutyl)ethyl
19 iodide, C4F9(CH2)2I, in quantitative yield which was confirmed by F NMR spectroscopy. However, the use of THF as solvent and the low reaction temperature of –
78 ºC limit the practical usefulness of this procedure.
Recently, Hirao and co-workers131 have designed and reported a novel synthetic methodology to prepare various well-defined, perfluorooctyl-functionalized polystyrenes with different numbers of functional groups in which the functionalization is effected by the addition reaction of poly(styryl)lithium to diphenylethylene derivatives bearing one or two silyl-protected, phenol functional groups on the aromatic rings. After the functionalization and deprotection of the phenol group, a Williamson reaction between 3-
(perfluorooctyl)propyl bromide and the phenol functional groups of the polymer chain ends was effected as shown in Scheme 1.3.
In addition to chain end functionalization, this method has proven to be very efficient for in-chain functionalization as well. Sometime later, Hirao and co-workers further extended this method and they successfully prepared various polystyrenes carrying perfluoroalkyl functional groups with highly branched dendritic structures at the polymer chain ends.132
30 CH3OH P Li + PCH2CH
OSiMe But t 2 OSiMe2Bu
Bu4NF 1.NaH PCH2CH PCH2CH 2.C8F17(CH2)3Br
t OH O(CH2)3C8F17
Scheme 1.3. Introduction of perfluorooctyl functionalities via diphenylethylene
derivatives bearing a silyl-protected, phenol functional group on the aromatic ring.
1.2.4 Epoxidation
Similar to vinyl end-functionalized polymers, oxirane- or epoxy-functionalized polymers can serve as macromonomers in which epoxy functionalities at the polymer chain ends are further polymerized or copolymerized to prepare comb-type, graft copolymers having side-chains composed of the macromonomer residues.133,134 These kind of graft copolymers have found extensive use as polymer blend compatibilizers, emulsifiers, surface-modifiers, coating materials, and laminate adhesives for combining polar polymers with nonpolar polymers. Epoxy-functionalized polymers can also be used to form dihydroxy-functionalized macromonomers by acid treatment, and these groups can further participate in condensation polymerization with other polar monomers.135,136 31 However, the preparation of well-defined, epoxy-functionalized macromonomers through alkyllithium-initiated living anionic polymerization has been restricted due to the high reactivity of the epoxy ring toward anionic chain ends. In fact there has been no effective way to introduce epoxy functionalities directly onto anionic chain ends.
Xie and Sun137 utilized epichlorohydrin as the terminating agents for the preparation of epoxy-functionalized polystyrenes as shown in Eq 1.12. These functionalized polymers were copolymerized with ethylene oxide to form unique hydrophilic- hydrophobic graft copolymers. However very low functionality of the resulting macromonomers (less than 35 %) along with considerable dimer formation (10-15 %) was reported using this method.
O O CH3OH (Eq 1.12) P Li + ClCH2 PCH2
Takenaka, Hirao, and Nakahama138 reported a quantitative epoxy functionalization reaction of carbanionic chain ends utilizing 2-3 equivalents of (2-bromoethyl)oxirane as the terminating agent at –78 ºC in THF. However, the practical application of this technique has been restricted by the requirements of THF and low reaction temperature.
For the preparation of well-defined, epoxy-functionalized macromonomers, the functionalization reactions of poly(styryl)lithium and poly(butadienyl)lithium with epichlorohydrin were thoroughly reinvestigated and optimized by Quirk and Zhuo.139
They found that this functionalization reaction appeared to be complicated and sensitive to many reaction variables including solvent, Lewis base, mode of addition, end-capping with 1,1-diphenylethylene or ethylene oxide, and stoichiometry. However, 91-97 % yields could be obtained under optimized conditions.
32 1.2.5 Phenol functionalization
Phenol functional groups attached to polymer chain ends possess the unique ability to effect chain extension with dichloromethane, COCl2 or formaldehyde as well as to provide block copolymers composed of non-polar and polar blocks by condensation-type reactions. In addition, anionically synthesized, diphenol-functionalized polystyrenes and polydienes can serve as bifunctional condensation macromonomers having well-defined structures to afford comb-type, graft copolymers by step-growth copolymerization with other polar monomers.101 Therefore, the preparation of well-defined, phenol- functionalized polymers is of growing current interest. Analogous to the case of primary amine functionality, the introduction of phenol functionality into anionic chain ends is difficult due to the high acidity of the hydroxyl groups. As a result, most phenol functionalizations require suitable protection prior to reaction with living anionic chain ends.
Nakahama and co-workers140,141 investigated the living anionic polymerization of several 4-hydroxystyrene derivatives with different trialkylsilyl-protecting groups, and found that only the tert-butyldimethylsilyl-protecting group proved effective for the successful anionic polymerization of hydroxyl-functionalized styrene monomers. In the case of the protection of the hydroxyl functionalities with trimethylsilyl groups, considerable side reactions were observed between anionic chain ends and the protecting groups.
Quirk and Zhu100 reported the development of a novel general anionic functionalization method using the quantitative reaction of anionic chain ends with 1,1- diphenylethylene derivatives and first utilized this technique for the preparation of well-
33 defined, phenol-functionalized polystyrenes.114 In this study, in order to circumvent the undesirable termination reaction of anionic chain ends with the trimethylsilyl protecting group as previously reported by Nakahama co-workers,140,141 tert-butyldimethylsilyl groups were adopted as the protecting groups of the phenol functionalities as shown in
Eq 1.13. The reaction progress was readily monitored by UV-visible spectroscopy.
CH3OH 1% HCl (Eq 1.13) P Li + PCH2CH
t OSiMe2Bu OH
They were also successful in extending the use of 1,1-diphenylethylene derivatives as functionalization agents for the preparation of well-defined, in-chain and chain-end bis(phenol)-functionalized polymers which usefully serve as condensation macromonomers.101
1.3. Hydrosilation
1.3.1 General aspects
Since the first report by Sommer et al.142 utilizing acetyl peroxide as the catalyst, hydrosilation has been extensively studied and is well described as the addition reaction of silicon hydride groups into multiple bonds, particularly carbon-carbon double bonds, to form new Si-C and C-H bonds as shown in Eq 1.14.
Catalyst Si H + CH2 CHR Si CH2 CH2R + Si CHR CH3
(Eq 1.14) 34 Hydrosilations have been found to proceed via either a radical mechanism or, mostly, a polar mechanism depending on the reaction conditions. Hydrosilation can be effected by various kinds of catalysts including peroxides, nuleophiles, Lewis acids, and numerous transition metal complexes. Among them, transition metal catalyzed hydrosilations have proven to be the most efficient and versatile for both research and industrial synthetic applications.143-146
1.3.2 Transition metal catalyzed hydrosilation
In addition to the diverse synthetic versatility, the most important feature of transition metal catalyzed hydrosilation reactions is their unique tolerance to various functional groups such as RNO2 and RNH2 under mild conditions. As a result, usually most functional groups in the reactant molecule don’t interrupt hydrosilation.147
From a mechanistic point of view, for the homogeneous transition metal catalysis system, two important reaction pathways have been proposed: the one is Chalk-Harrod mechanism and the other is Modified Chalk-Harrod mechanism, as shown in Figure
1.9.148, 149 Both involve initial oxidative addition, alkene coordination followed by alkene insertion, and the final reductive elimination to form the desired hydrosilation products.
35
Figure 1.9. Hydrosilation mechanism in homogeneous transition metal catalyzed system.
In the case of heterogeneous systems, Lewis and Lewis150 first observed the formation of metal colloid in the beginning of the hydrosilation reaction, described as an induction period. They proposed that the metal colloid was the true active species that effected the hydrosilation reaction as depicted in Eq 1.15. (Eq 1.15) ++ Me2(EtO)SiH + MtClx H2 Silicon productsMtcolloid
Numerous transition metal containing catalysts have been developed and used to activate hydrosilation. The relative activities have appeared to depend on the type of transition metal employed: the general trend of reactivity is in the order of Pt > Rh > Ir =
Ru > Os > Pd.150,151 The most commonly used and commercially available hydrosilation catalysts include Speier’s catalyst (H2PtCl6), Karstedt’s catalyst (Pt{[Me2(vinyl)Si]2O}1.5)
36 and Wilkinson’s catalyst (ClRh(PPh3)3). Although a wide range of transition metal based hydrosilation catalysts have been designed, developed, and used, platinum-based catalysts such as Speier’s and Karstedt’s catalysts have the best catalytic activities.152-155
In addition, platinum-catalysed hydrosilations exhibit the characteristic regiospecific behavior, exclusively forming the anti-Markovnikov addition products as illustrated in Eq
1.16.143-145
Pt (Eq 1.16) Si H + CH2 CHR Si CH2 CH2R
Another important aspect in utilizing platinum complexes for hydrosilation is the unique selectivity of the reaction toward different alkenes where the more substituted alkenes exhibit less reactivity as shown in Figure 1.10.143,144
CH2 CHCH2 >>CH2 CRCH2 CHRCRCH2
Figure 1.10. Reactivity of alkenes for platinum-catalyzed hydrosilation reactions.
1.3.3 Development of a new general functionalization methodology by the combination of anionic polymerization with hydrosilation reactions
Alkyllithium-initiated anionic polymerization has proven to be one of the most useful methodologies for the synthesis of polymers with controlled molecular weight, low degrees of compositional heterogeneity, and fine-turned structure and morphology.1 As already mentioned in an earlier section, one of the unique features of living polymerizations is the ability to prepare chain-end functionalized polymers by termination of the living ends with appropriate reagents.1,17,18 Under appropriate conditions, alkyllithium-initiated polymerizations of styrenes, dienes and (meth)acrylates are living and form stable anionic chain ends when all of the monomer has been 37 consumed. These anionic chain ends can react with various electrophiles to form a diverse array of chain-end functionalized polymers. In spite of the potential of this methodology, there are relatively few well-characterized, efficient chain-end functionalization reactions due to the high reactivity of anionic chain ends toward most polar functional groups.18
On the other hand, transition metal catalyzed hydrosilation reactions have unique tolerance toward various polar functional groups and proceed under mild reaction conditions.144,147 Furthermore, even though the silicon-hydride bond (79 kcal/mol) is weak and labile compared to the carbon-hydrogen bond (96 kcal/mol), it is stable in air and in polar media even in the presence of weak bases.147 The acidities of organosilanes of interest are estimated to be comparable to the hydrocarbon analogues (Table 1.4).156
Accordingly, hydrosilation of various alkene-containing functionalizing agents with anionically-synthesized, silyl hydride-functionalized polymers can provide versatile tools for diverse end-functionalizations. Along this line, a general functionalization methodology has been developed. In this approach, first the quantitative termination of living polymeric organolithium compounds with dimethylchlorosilane readily forms the corresponding ω-silyl hydride-functionalized polymer. The resulting silyl hydride- functionalized polymer can then react with a variety of readily available, substituted alkenes to form the corresponding chain-end substituted polymers via efficient, regioselective, transition metal catalyzed hydrosilation reactions (Scheme 1.4).
38 156 Table 1.4. pKa values (estimated) in DMSO
Compound pKa
PhCH3 43.0
CH2=CH-CH3 44.0
PhSi(CH3)2H 41.2
CH2=CH-Si(CH3)2H 42.3
CH3 CH3
PLi + ClSiH P-Si-H
CH3 CH3
CH3 CH3
P-Si-H + CH2=CH(CH2)n-X P-Si-CH2CH2(CH2)n-X
CH CH3 3
Scheme 1.4. New general end-functionalization methodology developed by the
combination of anionic polymerization with hydrosilation reactions.
Riffle and coworkers157 first described this methodology to prepare an epoxide- functionalized polymer by reacting poly(butadienyl)lithium (prepared in
THF/cyclohexane) with chlorodimethylsilane, hydrogenating the double bonds, and then effecting hydrosilation with allyl glycidyl ether as shown in Scheme 1.5. Quantitative
39 hydrosilation was confirmed by 1H NMR spectroscopy. This technique provided a very efficient route to well-defined, epoxy-functionalized macromonomers.
n Li Li y-1 C6H12/THF x = 58% x y = 42%
CH3 CH3 Si Cl H Si H CH3 y CH3
x
CH3
Si H H2 2y Ni catalyst CH3
x
CH3 O O O O Si 2y CH3 Pt catalyst x
Scheme 1.5. Synthesis of epoxy-functionalized polyolefin macromonomers.
Recently Loos and Müller106 prepared maltoheptaose-block-polystyrene by the hydrosilation reaction of a silyl hydride-functionalized polystyrene (prepared in THF at -
78 ºC) with trieicosaacetyl-N-allylmaltoheptaonamide. Prior to the hydrosilation, the hydroxyl functionalities of the alkene-containing maltoheptaose derivative were protected by peracetylation to prevent the facile oxidation of silyl hydride functional group at 40 polystyrene chain end in the presence of hydrosilation catalyst. They observed more than
90% conversion within 6 hours during the hydrosilation for a silyl hydride-functionalized polystyrene with molecular weight of 3000 g/mol. The hydroxyl functionalities were then deprotected by KCN in methanol after hydrosilation.158
One of the most important advantages of this methodology is that the hydrosilation reaction is relatively insensitive to anionically reactive functional groups such as carboxyl, phenol and nitro, as well as primary and secondary amine groups; i.e. no protecting groups are required. In this thesis, the utility of this methodology will be discussed by its application to the facile synthesis of various end-functionalized polystyrenes and polyisoprenes for which few other simple, efficient anionic functionalization procedures are available.
41
CHAPTER II
EXPERIMENTAL
2.1 Inert atmosphere techniques
2.1.1 High vacuum techniques52,159
In order to protect reaction systems from undesirable contact with oxygen, carbon
dioxide and water in air, high vacuum techniques were employed using a specially designed high vacuum line that was built from Pyrex® glass tubing carrying several
Teflon Rotoflo® or vacuum grease stopcock valves as shown in Figure 2.1.
Figure 2.1. The typical construction of a high vacuum line for anionic polymerization.
42 The vacuum line was composed of two horizontal manifolds in which the main one
was made from 50 mm, medium-walled tubing and the other from 15 mm, medium-
walled tubing, that were connected to each other through 10 mm, medium-walled glass
tubing. The main manifold was connected to a diffusion pump (Chemglass, Air free)
filled with silicone oil (Dow Corning 704 Diffusion Fluid) through a two-piece, bent top
stem, vacuum trap (Chemglass, Air free) in a liquid-nitrogen filled Dewar flask. The
diffusion pump was finally coupled to a mechanical vacuum pump (Welch DuoSeal,
Model 1405). The performance of the high vacuum line was evaluated using Tesla coils
which convert electricity into high frequency, high voltage current. In the case of the
presence of air or gas flow in the vacuum line, the Tesla coil creates purple-colored
electrical discharge along with noise when placed close to the line surface. The absence
of electrical discharge and noise indicates that the vacuum state in the system reaches less
than approximately 10-3 torr. All glass reactors and ampoules were attached to the high
vacuum line using glass blowing techniques, followed by flame-drying under high
vacuum to remove any trace amounts of air and moisture before the reaction. All living anionic polymerizations and the purifications of monomers and solvents were performed
on the high vacuum line, where the purified monomers and solvents were vacuum
transferred into ampoules and reactors by cooling with a dry ice/isopropyl alcohol (IPA)
bath (-78 °C). For the introduction of a small quantity of air sensitive initiators into
reaction system using a syringe technique, high purity nitrogen gas (4.8 grade, Praxair)
was flowed through the system and the pressure was controlled by a Rotoflo® valve
connected to an oil bubbler.
43 2.1.2 Schlenk line techniques160
Schlenk techniques were utilized for most synthetic work with air/moisture sensitive compounds, hydrosilation reactions, removal of solvents, drying of products and handling of the vacuum distillation apparatus. The Schlenk line used consisted of a glass manifold containing two-way taps, which enable the manipulation of the vacuum and inert gases within reaction vessels linked to the line. In order to protect the pump from harmful vapors, the vacuum part of the Schlenk line was connected to a mechanical vacuum pump via a low-temperature trap (two-piece, bent top stem) submerged in a
Dewar flask filled with liquid nitrogen or an IPA/dry ice bath at all times when the pump was under operation. Prior to filling the Dewar flask with liquid nitrogen, the line was closed to prevent the condensation of air which can cause violent explosions. Once the cooling source is removed, the liquefied air can vaporize and expand very fast creating an overpressure in the reaction system. Because nitrogen has a lower boiling point than oxygen, the condensed oxygen exists in liquid form and reacts vigorously with any organic chemicals collected in the cold trap. The inert gas part of the Schlenk line was connected to a cylinder of argon gas (technical grade, Praxair). Between the line and argon cylinder, a catalyst tower was installed and filled with the mixture of alumina and deoxo catalysts (Alfa Aesar, De-Ox) pre-activated by heating to 250 oC under reduced pressure to remove adventitious water and oxygen. At the end of the line, a pressure release oil bubbler was attached to monitor the pressure in the inert gas line. Care was taken to keep a positive pressure in the line during the manipulation of this part of the line. When a very sensitive material was handled, it was ensured that the line was air/water free using a rubber septum sealed vial containing a tiny drop of diethyl zinc
44 solution as a indicator in which the absence of fumes indicates that the oxygen and
humidity level in the line is in an acceptable range.160 In general, the Schlenk line was
flame-dried prior to usage.
2.1.3 Dry box techniques
An inert atmosphere glovebox (also referred to as a ‘dry box’) is a completely
enclosed chamber ideal for the manipulation of air/water-sensitive materials. The inert
gas could be nitrogen, argon or helium to achieve a preferred atmosphere depending on
the purpose. For example, nitrogen is the cheapest among them, but sometimes reacts
with the materials under study. In addition, the use of nitrogen as inert gas is more prone
to static problems than are argon or helium; this causes difficulty in weighing without an
anti-static device.
All air-sensitive compounds were weighed and stored in a glove box (Vacuum
Atmospheres Co., HE-43-2 Dri-Lab Model) filled with technical grade of argon gas
(Praxair). Titration of initiators, some hydrosilation reactions, and filtration of air- sensitive products were also performed in the dry box, which consisted of a large chamber with one window having two ports for a pair of arm-length gloves allowing the manipulation of equipment and chemicals inside.
In order to achieve a desired low level of moisture, the dry box chamber was continuously purged by re-circulation of argon gas through three sequential glass catalyst columns wrapped with heating jackets (Glas-Col®) where the re-circulation was effected using a mechanical circulating pump (Robin Myers). The argon gas first flowed through two catalyst columns packed with a mixture of zeolites (Molecular sieves 4 Å and 13 X,
45 Union Carbide) with vermiculite (JP Austin) in which 4 Å and 13 X sieves served as
water and solvent scavengers, respectively. Then the gas was passed through a de-
oxygenation catalyst column filled with Cu catalyst (BASF, R3–11) and vermiculite.
These catalysts were regenerated to remove bound oxygen and water approximately
every six months or sooner as needed. Molecular sieves were regenerated by heating-up
to 350 oC using heating jakets under vacuum for several hours. The regeneration of the
Cu de-oxygenation catalyst was effected by purging the column with hydrogen gas (5 %
o H2 in N2, Praxair) at a temperature of 250 C under vacuum. All water driven from the catalyst columns during a regeneration cycle was collected in a liquid nitrogen trap (-196
oC) installed between the column and the vacuum pump.
The atmosphere in the dry box should be always kept free of water, oxygen and
even polar solvents. Several simple tests were used to determine condition of the
atmosphere. In the light-bulb test, a small hole was made on a standard 40W incandescent
light-bulb using an extremely hot glass rod heated over a Bunsen-burner flame until the
glass rod turned soft. It was confirmed that the filament was unbroken by viewing the
filament through the hole. It is recommended to introduce several of these light bulbs into
the dry box before carrying out the tests as the first few may burn out. A trace amount of
oxygen and moisture inside of dry box will oxidize the hot filament and burn it out.
However, a good atmosphere in optimal condition will permit the bulb to glow for up to
one week. Usually the observed burn time was more than 24 hours indicating an
acceptable atmosphere in the dry box. Liquid indicator solution was prepared by simple
mixing of bis(cyclopentadienyl)titanium dichloride (Aldrich) with excess zinc dust in
benzene, and this solution was frequently used to check the oxygen level in the dry box
46 atmosphere.161 The color of the indicator changed from green through yellow to orange as the oxygen content increased with time. With an optimal oxygen level of less than 5 ppm,
the green color was maintained for more than one month. Diethyl zinc solution was also
employed to test the condition of the dry box atmosphere; it visibly fumes in the presence
of water and oxygen by simply opening a bottle containing it.160 In order to prevent
deterioration of the inert atmosphere, all glassware and chemicals were completely dried
in an oven before being brought into the dry box antechamber.
2.2 Purification
2.2.1 Solvents
2.2.1.1 Benzene
Benzene (Fisher Scientific, Certified ACS) was stirred over concentrated sulfuric
acid for seven days, separated from the acid layer by simply decanting into a separatory
funnel with a Teflon stopcock, washed with de-ionized water passed through a purifier
(Sybron Barnstead, NANOpure II), neutralized with a saturated aqueous sodium
bicarbonate solution and washed again several times with de-ionized water. The removal
of residual water in benzene was performed by stirring over anhydrous magnesium
sulfate (Fisher Scientific, 99.5 %). The solution was then filtered into a long-necked,
round-bottom flask containing fleshly crushed calcium hydride (Acros Organics, 93 %)
and dried by vigorously stirring under an inert nitrogen atmosphere overnight on the high vacuum line. It was then stirred for an additional 24 hours, frozen in an IPA/dry ice bath
(-78 oC), degassed, and thawed repeatedly to remove hydrogen gas generated during the
process. In order to remove even trace amounts of water and other protic polar impurities
47 from the resulting solution, a further drying process continued with sodium (Aldrich,
lump in kerosene, 99 %). Benzene was vacuum transferred into a sliced-sodium-
containing, round-bottom flask immersed in an IPA/dry ice bath, and stirred for three
days with frequent degassing as described before. Finally, the dried benzene was vacuum
transferred into a reservoir flask with a Teflon Rotoflo® stopcock which contained
oligomeric poly(styryl)lithium. The characteristic red color was immediately observed
indicating the dryness and purity level of the benzene. The finally purified benzene was
stored at room temperature and directly used as a solvent for polymerization by vacuum
transfer into the reaction flask.
2.2.1.2 Toluene
Toluene (Fisher Scientific, Certified ACS) was stirred over fleshly ground calcium
hydride for 24 hours with periodic degassing on the high vacuum line. After that the
solution was vacuum transferred onto sodium dispersion, further purified by stirring
overnight, and finally stored over molecular sieves (4 Å) by vacuum transfer where the
zeolites were activated on the Schlenk line at 300 oC under reduced pressure (10-3 mmHg). The purified toluene was used as a solvent for azeotropic removal of water using a Dean Stark trap, some hydrosilation reactions and re-precipitation of functionalized polymers.
2.2.1.3 Tetrahydrofuran
Tetrahydrofuran (THF) (Fisher Scientific, Certified ACS) was stirred over fleshly ground calcium hydride for three days with frequent degassing on the high vacuum line, and then vacuum transferred into a 2-L storage flask fitted with a Teflon Rotoflo®
48 stopcock which contained sodium dispersion and benzophenone. The dryness of THF
was confirmed by the characteristic purple color arising from the formation of the ketyl
radical anion. The purified THF was vacuum transferred into other flasks as needed, and the BHT-free THF was directly used as an eluent for column chromatography to separate
primary amine-functionalized polymers.
2.2.1.4 Cyclohexane
The purification of cyclohexane was analogous to that of benzene. Cyclohexane
(Fisher Scientific, Certified ACS) was stirred over concentrated sulfuric acid for seven
days, separated from the acid layer, washed with de-ionized water, neutralized with a
saturated aqueous sodium bicarbonate solution and washed again several times with de-
ionized water. The residual water in cyclohexane was removed by stirring over
anhydrous magnesium sulfate. The resulting solution was then filtered, and subsequently
stirred over fleshly ground calcium hydride overnight on the high vacuum line. The solution was further stirred for an additional 24 hours with periodic degassing. The
removal of adventitious water and other protic polar impurities was effected by stirring
over sliced-sodium dispersion for three days with frequent degassing. The purified
cyclohexane was finally vacuum transferred and stored in a reservoir flask fitted with a
Teflon Rotoflo® stopcock which contained oligomeric poly(styryl)lithium. The
appearance of characteristic orange-red color indicated the complete dryness of the
cyclohexane. The final cyclohexane was vacuum transferred into another flask with a
Teflon Rotoflo® stopcock as needed, brought into the dry box and used as a solvent for
Gilman double titration of initiators.
49
2.2.1.5 Methanol
Methanol (EM Science, GR) was dried by stirring over fleshly ground calcium
hydride overnight under nitrogen flow, and then degassed by five repetitions of the
freezing-evacuation-thawing process on the high vacuum line. The dried methanol was
then vacuum transferred into a 250-mL flask fitted with a Teflon Rotoflo® stopcock and
stored over the activated molecular sieves (4 Å). The anhydrous methanol was also stored
in break-seals and used for the termination of living chain ends. The methanol breakseals
were made by vacuum transfer of anhydrous methanol from the reservoir flask into the
several breakseals attached to a single body manifold connected to the high vacuum line
using glass blowing techniques. After degassing using a liquid-nitrogen filled Dewar
flask (-196 oC), each methanol breakseal was flame-sealed off under vacuum, stored at
room temperature, and directly used as needed.
2.2.1.6 Hexamethyldisilazane
Hexamethyldisilazane (HMDS) (Aldrich, 99 %) was received in a sure-seal bottle
under inert nitrogen, stored in the dry box, and used without further purification.
2.2.1.7 Deuterated benzene
Benzene-d6 (Aldrich, 99 %) was put into a 100-mL, round-bottom reaction flask which was connected to a distilling apparatus (short path) equipped with distributing adaptor and three, 50-mL receiver flasks one of which was fitted with a Teflon Rotoflo® stopcock, and then several pieces of sliced sodium and benzophenone were quickly added
to the reaction flask under a positive argon pressure. The deuterated benzene solution was
50 then heated under reflux under an anhydrous argon atmosphere on the Schlenk line. A
series of color changes were observed during the reflux from green through dark blue
eventually to the characteristic purple color which confirmed the complete dryness of the
solvent. The anhydrous benzene was distilled and the middle fraction was collected in the
receiver flask fitted with a Teflon Rotoflo® stopcock, which was then disjointed from the
distilling apparatus, brought into dry box, transferred, and stored in a dried amber bottle
containing activated molecular sieves (4 Å). The anhydrous deuterated benzene was used
for monitoring hydrosilation reactions by NMR spectroscopy.
2.2.2 Monomers
2.2.2.1 Styrene
Styrene (Aldrich, 99 %) was stirred over fleshly ground calcium hydride for 24
hours on high vacuum line with periodic degassing. A 250-mL, round-bottom flask fitted
with a Teflon Rotoflo® stopcock was then connected to the high vacuum line, and
evacuated for 24 hours while occasionally flame-dried using a Bunsen burner where the
Teflon stopcock was temporarily separated, and the stopcock part was capped with a
rubber septum and further sealed by tightly taping with electrical tape. After that dibutyl
magnesium solution (FMC Lithium, 12 % in cyclohexane) was injected into the dried
storage flask under a nitrogen atmosphere using a gas-tight syringe followed by quick
replacement of the rubber septum with the Teflon Rotoflo® stopcock plug under a
positive nitrogen pressure. After the removal of cyclohexane solvent by evaporation
under reduced pressure, the dried styrene monomer was vacuum transferred onto the dibutylmagnesium, and stored in a refrigerator. The purified styrene solution with a light
51 yellow-green color was then used for polymerization after vacuum transferred into a
calibrated ampoule which was flame-sealed and then attached to the reactor using glass
blowing techniques.
2.2.2.2 Isoprene
Isoprene (Goodyear Chemical, 99 %) was stirred over fleshly ground calcium
hydride for 24 hours on the high vacuum line while periodically degassed. Then the
isoprene was vacuum transferred at –78 oC into a reservoir flask fitted with a Teflon
Rotoflo® stopcock and which contained neat dibutylmagnesium, and then the flask was
stored in a refrigerator as described previously in the case of styrene. Prior to polymerization, the isoprene was further purified by vacuum transferring into a flask
containing neat n-butyllithium and stirring for 30 min at 0 oC. Analogous to styrene, the
isoprene was finally vacuum transferred into an ampoule that was then flame-sealed, and
attached to the polymerization reactor using glass blowing techniques.
2.2.3 Initiators
2.2.3.1 sec-Butyllithium
sec-Butyllithium (FMC Lithium Division, 12 wt % in cyclohexane) was used as
received after double titration with allyl bromide to determine the concentration of active
anionic species. The Gilman double titration162 was performed to determine the
concentration of both the total base and the free base where the total base titration is
effective for both carbanionic and non-carbanionic lithium compounds, while the free
base titration is effective for only non-carbanionic lithium. For total base titration, 4 mL
of sec-butyllithium solution was injected into three dried, crimp-sealed bottles containing
52 15 mL of purified cyclohexane in the dry box using a syringe (10 mL). The solutions
were taken out of the dry box, quenched by addition of 20 mL of de-ionized water, and
finally titrated with 1.00 N HCl. The end point was determined using a couple of drops of
phenolphthalein indicator solution. The titration of free bases was carried out in a similar
manner to that of the total base, in which 4 mL of sec-butyllithium solution was added
into 15 mL of purified cyclohexane followed by the injection of 4 mL of allyl bromide.
The precipitation of lithium bromide was immediately observed after the addition of allyl
bromide. The solution was quenched with 20 mL of de-ionized water outside of the dry box, and then titrated using 0.100 N HCl with phenolphthalein as indicator. In general, the concentration of active carbanionic species ranged between 1.45 M and 1.55 M, which was calculated by the subtraction of free base concentration from the total base
concentration.
2.2.3.2 n-Butyllithium
The received n-butyllithium (FMC Lithium Division, 12 wt % in cyclohexane) was
divided into several crimp-sealed bottles in the dry box, and stored in a refrigerator. It
was used without further purification as needed for the purification of isoprene monomer.
2.2.3.3 Dibutylmagnesium
Similarly to n-butyllithium, dibutylmagnesium (FMC Lithium Division, 12 wt % in
cyclohexane) was used as received for the purification of both styrene and isoprene
monomers.
53
2.2.4 Functionalizing agents
2.2.4.1 Chlorodimethylsilane
Chlorodimethylsilane (98 % Aldrich) was stirred over fleshly ground calcium
hydride for 24 hours on the high vacuum line with frequent degassing, vacuum
transferred into a storage flask fitted with a Teflon Rotoflo® stopcock and containing
activated 4 Å molecular sieves, and stored in the refrigerator. The purified
chlorodimethylsilane was then vacuum transferred as needed into calibrated ampoules
that were flame-sealed.
2.2.4.2 Allylamine
Allylamine (Aldrich, 99 %) was heated under reflux (53 oC)163 under an anhydrous
argon atmosphere on the Schlenk line over calcium chloride which had been activated in
a vacuum oven at elevated temperature. It was then fractionally distilled using a distilling
apparatus (short path) composed of a distilling head, a thermometer, a distributing
adaptor, and three 50-mL receiver flasks; the middle one was fitted with a Teflon
Rotoflo® stopcock. Finally, the purified allylamine was stored in the refrigerator, and used, as needed.
2.2.4.3 2-Allylphenol
2-Allylphenol (Aldrich, 98 %) was dried for 24 hours in the dry box with
intermittent shaking over fleshly activated molecular sieves (4 Å), transferred into another dried amber bottle containing fleshly activated molecular sieves (4 Å), and then
used without further purification.
54 2.2.4.4 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluoro-1-octene
Analogous to allylphenol, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octene (Aldrich,
99 %) was dried over fleshly activated molecular sieves (4 Å) for 24 hours with
intermittent shaking, decanted, and stored in an amber bottle containing fleshly activated
molecular sieves (4 Å).
2.2.4.5 1,2-Epoxy-5-hexene
1,2-Epoxy-5-hexene (Aldrich, 97 %, containing 3 % of benzene) was dried over
fleshly activated molecular sieves (4 Å) in a dry vial in the dry box. The solution was
kept in the dry box for 24 hours with intermittent shaking, decanted, and finally stored in
an amber bottle containing fleshly activated molecular sieves (4 Å).
2.2.5 Catalysts and other reagents
2.2.5.1 1,3-Divinyltetramethyldisiloxane-platinum
Karstedt’s catalyst, 1,3-divinyltetramethyldisiloxane-platinum, (Gelest, 2.1-2.4 %
Pt conc in xylene) was stored in the dry box, and used without further purification.
2.2.5.2 Hydrogen hexachloroplatinate(IV) hydrate
Spier’s catalyst, hydrogen hexachloroplatinate(IV) hydrate (Aldrich, 99.9 %, Pt 38-
40 %), was stored in the dry box, and used as received.
2.2.5.3 Allyl bromide
Allyl bromide (Aldrich, 97 %) was stirred for 24 hours over calcium chloride
activated in a vacuum oven at elevated temperature, degassed, and vacuum transferred
into a round-bottom flask containing phosphorus pentoxide that had been dried under
55 reduced pressure for 24 hours on the high vacuum line. Then the solution was
continuously stirred for an additional 24 hours while intermittently degassed. After that,
the purified allyl bromide was vacuum transferred into a reservoir flask fitted with a
Teflon Rotoflo® stopcock, brought into dry box, and transferred into a brown amber bottle containing fleshly activated molecular sieves (4 Å). The allyl bromide was stored in the dry box, and used as needed.
2.2.5.4 Octavinyl-T8-silsesquioxane
Octavinyl-T8-silsesquioxane (Gelest) was purified by azeotropic vacuum distillation in which the octavinyl-T8-silsesquioxane was added to dried toluene, stirred overnight in
the dry box, and evacuated with continuous stirring for azeotropic removal of water. The
purified octavinyl-T8-silsesquioxane was then stored in a dried brown amber bottle in the
dry box until needed.
2.2.5.5 Potassium N,N-bis(trimethylsilyl)amide
Potassium N,N-bis(trimethylsilyl)amide (Aldrich, 95 %) was used after being dried
under high vacuum.
2.2.5.6 Tetrabutylammonium fluoride
Tetrabutylammonium fluoride (Aldrich, 1.0 M in tetrahydrofuran) was received in a
sure-seal bottle under inert nitrogen, and used without further purification.
56 2.3 Syntheses
2.3.1 Polymerization
Polymerizations were effected in benzene at room temperature using sec- butyllithium as initiator in all-glass, sealed reactors with breakseals and standard high- vacuum techniques.52,159 The reactors for anionic polymerization were designed and built using glass blowing techniques. A typical polymerization reactor is shown in Figure 2.2; it is composed of a reaction flask, a series of ampoules, and a sidearm.
Figure 2.2. Reactor design for anionic polymerization.
Prior to polymerization, the main body of the reactor and all ampoules were placed in the annealing oven, heated to 800 oF, and slowly cooled to room temperature. The purified monomers were vacuum transferred into calibrated, annealed ampoules fitted with annealed breakseals, flame-sealed under reduced pressure using a hand torch on high vacuum line, and then stored in the refrigerator until needed. Functionalizing agents were transferred into annealed ampoules fitted with both Teflon Rotoflo® stopcocks and breakseals under an inert argon atmosphere in the dry box, put on high vacuum line,
57 degassed several times at -78 oC, and then flame-sealed. After the reaction flask was
washed with a small amount of ammonium hydroxide to reduce the acidity of the glass
surface, rinsed with de-ionized water, and dried, all ampoules (A, B, D, E) containing
reagents were then attached to the main body of the reactor one by one using glass
blowing techniques. An empty ampoule (F) fitted with a methanol breakseal was also
attached for removal of a base polymer sample. After the C part of the reactor was flame-
connected to high vacuum line using glass blowing techniques, the absence of pinholes
was confirmed under reduced pressure using a Tesla coil. The reactor was then further
evacuated overnight with periodic flame-heating using a Bunsen burner. Then since the G
part was closed with a rubber septum, careful attention was needed to prevent an
accidental burning of the rubber septum during flame-heating. After confirmation of
complete dryness by no noise when tested using a Tesla coil, the reactor was purged with
purified nitrogen gas (Praxair, 4.8 grade). Prior to the introduction of initiator into the
reactor, the required amount of initiator was precisely calculated based on Eq 1.3, and
then injected through the injection arm (G) under a positive nitrogen pressure at –78 oC
using a gas-tight syringe which was dried overnight in the oven, and flushed with
nitrogen gas right before use. In order to prevent an accidental contamination of the injection arm surface with residual initiator solution on the tip of syringe needle, a small glass tube was inserted into the arm during the introduction of initiator. After completion of initiator injection, the needle tip was pulled up around middle of the glass tube, and then both the glass tube and the syringe were slowly taken out of the injection arm simultaneously under a positive nitrogen pressure. Immediately after the removal of the gas-tight syringe, the sidearm was quickly closed again with a clean rubber septum. The
58 residual hexane was evaporated by opening the vacuum after shutting off the nitrogen
gas, and then the sidearm was flame-cut under reduced pressure using a hand torch. The
closed reaction flask was put in an IPA/dry ice bath and cooled under vacuum for the introduction of solvent. After degassing several times under vacuum at -78 oC, the solvent
was vacuum transferred from the reservoir flask into the reactor, where the amount of
solvent was determined on the basis of the solvent to monomer ratio which was generally
about 10:1 in vol %. After all of the ampoules were wrapped with wet paper towels and
the reaction flask was still kept in an IPA/dry ice bath, then the entire polymerization
reactor system was separated from high vacuum line by flame sealing tube (C). After the
removal of the cold bath and equilibration at room temperature, the monomer was
discharged into the reaction flask by cracking the monomer breakseal (A) with a hammer.
The initiation of polymerization was confirmed by the appearance of characteristic color
(red for styrene, pale yellow for isoprene) whose intensity was dependent of the
concentration of the active chain ends. The reaction mixture was then vigorously stirred
overnight. After completion of the polymerization reaction, some of polymer solution
was collected into the ampoule (F) for a sample by tilting the reactor. The sampling
ampoule was put in a small Dewar flask filled with IPA/dry ice, flame-cut using a hand
torch, and then quenched by hammering the methanol breakseal attached the ampoule.
After confirmation of the desired molecular weight and molecular weight distribution by
GPC analysis of the base polymers, functionalizing agents were introduced by breaking
the appropriate ampoule (E), and the mixture was further stirred overnight until the functionalization reaction was completed. Finally, a methanol breakseal was cracked to quench possible residual, unreacted active chain ends.
59
2.3.2 Hydrosilation
Most hydrosilations were performed in benzene at room temperature using Pt catalysts under an inert atmosphere on a Schlenk line or in the dry box. Glassware was
flamed dried or dried in an oven overnight at 140 °C prior to use. A two-necked, round-
bottom flask (25 mL) containing a stir bar, a condenser, and a inner joint adaptor fitted
with a glass stopcock was thoroughly cleaned, dried in an oven for 24 hours, and then
quickly brought into the antechamber of the dry box. After storing overnight under
vacuum, it was then brought into the dry box. Anhydrous benzene was put into the two- necked, round-bottom flask as needed, and then silane-functionalized polymers and an excess of vinyl-containing functionalizing agents were introduced into the benzene solution. A condenser fitted with the inner joint adaptor on the top was set into one of the necks of the reaction flask, and then the other neck of the reaction flask was tightly closed with a new rubber septum. The mixture was stirred until the polymer was completely dissolved in the dry box. The whole reaction set was taken out of the dry box and then connected to a Schlenk line. The reaction flask was purged with argon gas by opening the glass joint stopcock of the adaptor on the top of condenser. The argon gas was purified by passing through a column filled with a mixture of alumina and deoxygenation catalyst (Alfa Aesar, De-Ox). To the mixture was added 2-3 drops of
Karstedt’s catalyst using a gas-tight syringe with vigorous stirring under an argon atmosphere. The resulting solution was stirred at room temperature until the hydrosilation reaction was completed depending on the reactivities of the double bonds of the different functionalizing agents. In the case that either the reaction rate was very slow, or the
60 reaction progress was monitored by NMR spectroscopy with time, the hydrosilation
reaction was performed in the dry box where all reagents and solvent were put together
into a one-necked, round-bottom flask, and stirred until uniformly mixed. After addition
of catalytic amounts of the Pt catalyst, the reaction mixture was stirred until the
hydrosilation was completed. The reaction flask was tightly closed with a new rubber
septum during the reaction and intermittently opened as needed for the monitoring of the
reaction progress.
2.3.3 Preparation of reagents
2.3.3.1 3-[N,N-Bis(trimethylsilyl)amino]-1-propene164
A solution of allyl bromide (8.9 g, 45 mmol) and 5 mL of hexamethyldisilazane
(HMDS) was put into a two-necked, round-bottom flask in the dry box where one neck of
the flask was fitted with an inner joint adaptor with a glass stopcock while the other one
was tightly sealed with a clean rubber septum. To another two-necked, reaction flask was
added a suspension of potassium N,N–bis(trimethylsilyl)amide (5.1 g, 42 mmol) in 50
mL of HMDS in the dry box. After fitting a condenser into one neck followed by putting
an inner joint adaptor with a glass stopcock into the top of the condenser, the reaction flask was closed by sealing the other neck with a clean rubber septum. Both reaction flasks were taken out of the dry box through the antechamber, connected to the Schlenk line, and then purged with purified argon gas by opening the respective glass stopcocks.
The suspension of potassium N,N–bis(trimethylsilyl)amide in HMDS was cooled in an ice bath. To the suspension was slowly cannulated the mixture solution of allyl bromide and HMDS under the dry argon atmosphere at 0 oC where the canulation rate was
61 controlled using two separated oil bubblers in which one was installed before the mixture
solution flask (allyl bromide and HMDS) while the other containing less oil than the
previous one was set after the suspension flask [potassium N,N–bis(trimethylsilyl)amide
in HMDS]. After the completion of the cannulation the resulting reaction mixture was
vigorously stirred for 1 h under the dry argon atmosphere at 0 oC, slowly warmed to room
temperature by removal of the ice bath, and then further stirred overnight at room
temperature. The whole reaction set was then transferred into the dry box, and opened. A
Buchner filter funnel with vacuum adapter was fitted into an Erlenmeyer flask in the dry
box, and connected to a vacuum pump through a pressure relief valve inside of the dry
box. The resulting slurry was then filtered through a bed of Celite 545 on the filter to
remove the potassium bromide. For the separation from HMDS and purification of the
final product, fractional vacuum distillation was performed on the Schlenk line under
reduced pressure set by a manostat, using a vacuum distillation apparatus which was
composed of a reaction flask, a distilling head, thermometer with taper-ground joint, a cold finger condenser, a distributing adapter, and receiving flasks. Finally, 5.2 g (26 mmol, 62 % yield) of the pure silyl-protected allylamine was obtained as a colorless, clear liquid (bp 85 oC at 35 mbar; lit164 bp 82 ºC at 30 mbar). 1H NMR: δ 0.11 (s, 18H,
13 Si(CH3)3), 3.46 (d, 2H, CH2N), 5.06 (dd, 2H, CH2=), and 5.79 ppm (m, 1H, =CH). C
NMR: δ 2.15 (Si(CH3)3), 47.50 (CH2N), 113.50 (CH2=), and 141.45 ppm (=CH).
62 2.3.3.2 1-(Chlorodimethylsilyl)-3-[N,N-bis(trimethylsilyl)amino]propane164
3-[N,N–Bis(trimethylsilyl)amino]-1-propene (4.3 g, 21 mmol) and
chlorodimethylsilane (4.1 g, 43 mmol) were mixed into a two-necked, round-bottom flask
equipped with a condenser and a clean rubber septum in the dry box. The entire reaction
set was then brought out of the dry box, and attached on Schlenk line. To the reaction
mixture was added just two drops of Karstedt’s catalyst, 1,3-
divinyltetramethyldisiloxane-platinum complex, at room temperature under a dry argon atmosphere using a gas-tight syringe. Immediately after injection of a drop of Karstedt’s catalyst, an exotherm was observed with a color change of the solution to light yellow.
While further stirring the solution for 12 hours, a continuous color change was observed from light yellow through deep yellow to light brown. After the completion of the hydrosilation reaction, the resulting solution was brought into the dry box, and transferred to a reaction flask with a Teflon Rotoflo® stopcock. The reaction flask was then taken out of the dry box, and fitted into a vacuum distillation apparatus which had been set on the Schlenk line and flame-dried in advance. 1-(Chlorodimethylsilyl)-3-[N,N–
bis(trimethylsilyl)amino]-propane was finally isolated in 81 % yield (4.9 g, 17 mmol) by
fractional vacuum distillation as a colorless, clear liquid (bp 115 oC at 1 mbar; lit164 bp
104 ºC at 1 mbar). This material turned out to be very sensitive to hydrolysis even in the
presence of just a trace amount of moisture. Accordingly, special care was needed in the
whole course of handling the compound to prevent contact with any adventitious
moisture. This compound was stored under an inert atmosphere in a refrigerator. 1H
NMR: δ 0.11 (s, 18H, Si(CH3)3), 0.42 (s, 6H, ClSiCH3), 0.71 (m, 2H, ClSiCH2), 1.44 (m,
63 13 2H, CH2), and 2.77 ppm (m, 2H, NCH2). C NMR: δ 1.83 (ClSi(CH3)3), 2.31
(NSi(CH3)3), 16.57 (CH2), 28.78 (ClSi(CH3)2), and 48.83 ppm (NCH2).
2.3.4 Functionalization by termination with electrophiles
2.3.4.1 Functionalization of poly(styryl)lithium with dimethylchlorosilane165,166,167
Purified styrene (24.9 mL, d = 0.9016,168 22.4 g, 0.215 mol) stored over dibutylmagnesium was vacuum transferred into a calibrated ampoule fitted with a breakseal, and the ampoule was flame-sealed using a hand torch. Dimethylchlorosilane
(2.5 mL) previously dried over calcium hydride was diluted in benzene, transferred into an ampoule fitted with both a Teflon Rotoflo® stopcock and a breakseal in the dry box, and flame-sealed on the vacuum line. As described in section 2.3.1 in detail, a polymerization reactor was built by attaching three ampoules containing styrene, dimethylchlorosilane, and methanol, respectively, along with one empty ampoule for sampling. The reactor was then attached on the high vacuum line, and dried overnight under vacuum. After injection of sec-BuLi initiator (7.7 mL, 11.2 mmol) using a gas-tight syringe and vacuum transfer of the purified benzene (250 mL) from the solvent reservoir flask into the reactor, the entire reactor was flame-cut from the high vacuum line, and the polymerization was effected under vacuum by cracking the styrene ampoule. The characteristic red color was observed immediately after introduction of the styrene. After completion of polymerization by vigorous stirring for 12 hours, the functionalization of
3 poly(styryl)lithium (22.6 g, 9.8 mmol, Mn = 2.1 × 10 g/mol, Mw/Mn = 1.04) was effected directly in the polymerization reactor by smashing the breakseal for the ampoule
64 containing 2.3 molar equivalents of chlorodimethylsilane (2.5 mL, d = 0.852,168 2.13 g,
22.5 mmol) in benzene at room temperature. The disappearance of the red color along with salt precipitation (LiCl) was observed almost immediately. After further stirring for additional 11 hours, the resulting solution was precipitated into an 8-fold excess of anhydrous methanol, and the suspension was stirred for 1 hour. After filtration and
3 washing with methanol, the silane-functionalized polystyrene (Mn = 2.2 × 10 g/mol,
Mw/Mn = 1.04) was obtained as a white solid. The functional polymer was dried in a vacuum oven overnight, followed by further intensive drying on the high vacuum line for
48 hours. Finally, the silane end-functionalized polystyrene was isolated in 98 % yield.
4 Silane-functionalized polystyrene with different molecular weight (Mn = 1.44 × 10
1 g/mol, Mw/Mn = 1.01) was also successfully prepared in the same way. H NMR: δ –0.1
13 (CH3-Si-C), and 3.8 ppm (H-Si). C NMR: δ –5.5 ppm (CH3-Si-C). FT-IR: 882 (Si-C),
-1 1249 (CH3-Si), and 2111 cm (Si-H). MALDI-TOF MS: for C4H9-(C8H8)18-
+ SiH(CH3)2·Na , calcd monoisotopic mass = 2012.17 Da; observed m/z = 2012.15.
2.3.4.2 Functionalization of poly(styryl)lithium with 1-(chlorodimethylsilyl)-
3-[N,N-bis(trimethylsilyl)amino]propane165
The polymerization of styrene (10.5 mL, d = 0.9016,168 9.47 g, 90.9 mmol) with sec-BuLi (0.65 mL, 1.46 M, 0.95 mmol) was performed in benzene (styrene:benzene = 1:
10, vol/vol) in the exactly same way described in section 2.3.4.1 to form
3 poly(styryl)lithium (Mn = 11 × 10 g/mol, Mw/Mn = 1.01). For the preparation of
protected amine-functionalized polystyrene, the functionalization was effected by the
3 reaction between poly(styryl)lithium (9.5 g, 0.86 mmol, Mn = 11 × 10 g/mol, Mw/Mn =
65 1.01) and 1.8 molar equivalents of 1-(chlorodimethylsilyl)-3-[N,N– bis(trimethylsilyl)amino]propane (0.46 g, 1.55 mmol) in benzene at room temperature for
15 h until the unique red color of poly(styryl)lithium completely disappeared. The resulting polymers were then deprotected and precipitated by slow addition into dried methanol over 1 hour followed by vigorous stirring at room temperature for 7 hours.
After filtration and drying in a vacuum oven overnight, a small portion of the polymer was dissolved in toluene, and the solution was spotted on the a TLC plate using a capillary followed by development of the air dried spot using toluene as a eluent. TLC analysis exhibited two distinct spots, one on the bottom and the other on the top of the
TLC plate. These two spots were attributed to amine functional polymer and the mixture of unfunctional polymer and by-product, respectively. On the basis of the TLC results, the pure amine functional polymer was isolated from the crude polymer mixture by silica gel column chromatography with a column size of 2 feet in length and 2 inches in diameter where the column was prepared by wet packing in toluene. Onto the sea sand bed on top of the column was slowly loaded 2.3 g of polymer mixture in 5 mL of toluene.
A yield of 0.78 g (34 %) of unfunctionalized polystyrene including by-products was obtained from the first chromatography fraction with toluene as the eluent. After careful switching the eluents from toluene to a THF/toluene mixture (75/25, vol/vol) by slow increments, 1.4 g (61 %) of pure amine-functionalized polystyrene was isolated from the column from this second fraction.
66 2.3.4.3 Functionalization of poly(isoprenyl)lithium with dimethylchlorosilane
Freshly purified isoprene (9.8 mL, d = 0.681,168 6.67 g, 98 mmol) was vacuum
transferred into a calibrated ampoule fitted with a breakseal, and flame-sealed using a
hand torch. Pure dimethylchlorosilane was diluted in benzene, and transferred into an ampoule fitted with both a Teflon Rotoflo® stopcock and a breakseal in the dry box. The ampoule was taken out of the dry box, and flame-sealed on the vacuum line. The rest of the polymerization procedure using sec-BuLi (2.28 mL, 1.46 M, 3.3 mmol) as initiator
followed that described for styrene polymerization in section 2.3.4.1 to generate
3 poly(isoprenyl)lithium (Mn = 2.1 × 10 g/mol, Mw/Mn = 1.03) where the pale yellow
color was sustained during the polymerization. The polymerization was carried out for 12
hours with vigorous stirring. The functionalization of poly(isoprenyl)lithium (6.7 g, 3.2
3 mmol, Mn = 2.1 × 10 g/mol, Mw/Mn = 1.03) was then effected by cracking the ampoule
containing 1.8 molar equivalents of chlorodimethylsilane (0.65 mL, d=0.852,168 0.55 g,
5.8 mmol) in benzene at room temperature. Analogous to poly(styryl)lithium
functionalization, the loss of the pale yellow color and lithium chloride precipitation were
observed immediately after addition of chlorodimethylsilane. The reaction mixture was
further stirred for 12 hours, and then precipitated into an 8-fold excess of anhydrous
methanol followed by filtration and subsequent washing with methanol. The resulting
silane-functionalized polyisoprene was then dried in a vacuum oven overnight, followed
by further intensive drying on the high vacuum line for 5 days with moderate stirring.
3 Finally, the pure silane-functionalized polyisoprene (Mn = 2.2 × 10 g/mol, Mw/Mn =
1.03) was obtained in clear viscous liquid form (6.6 g, 94 % yield). The functional
1 polymer was then stored in the dry box at room temperature. H NMR: δ –0.1 (CH3-Si-
67 13 C), and 3.9 ppm (H-Si). C NMR: δ –3.8 ppm (CH3-Si-C). FT-IR: 1249 (CH3-Si), and
2116 cm-1 (Si-H).
2.3.4.4 Functionalization of poly(isoprenyl)lithium with 1-
(chlorodimethylsilyl)-3-[N,N-bis(trimethylsilyl)amino]propane
The polymerization of isoprene monomer (9.3 mL, d = 0.681,168 6.33 g, 93 mmol)
initiated with sec-BuLi (0.11 mL, 1.46 M, 0.16 mmol) was conducted in the exactly same
way as described in previous section 2.3.4.3, and produced poly(isoprenyl)lithium (Mn =
4 4.3 × 10 g/mol, Mw/Mn = 1.02). The protected amine functionalization of
4 poly(isoprenyl)lithium (6.3 g, 0.15 mmol, Mn = 4.3 × 10 g/mol, Mw/Mn = 1.02) was
performed by the termination reaction with 2.3 molar equivalents of the fleshly-distilled,
diluted 1-(chlorodimethylsilyl)-3-[N,N–bis(trimethylsilyl)amino]propane (0.10 g, 0.34
mmol) in benzene. The characteristic pale yellow color of poly(isoprenyl)lithium
completely disappeared within 12 hours without any extra addition of THF. The resulting
polymers were then precipitated into dried methanol, and dried on the vacuum line over
several days with gentle stirring. The dried protected amine-functionalized polyisoprene
was obtained in clear viscous liquid form in 92 % yield, and stored in dry box after
addition of 0.2 wt % of BHT as stabilizer. The deprotection of the functional
polyisoprene was performed by treatment with tetrabutylammonium fluoride (TBAF, 1.0
M in THF). TBAF was injected into the polymer solution in THF (TBAF: polymer = 5:1,
mol/mol) under a dry argon atmosphere at room temperature and stirred for 2 hours. The
deprotected amine-functionalized polyisoprene was isolated after precipitation into dried
methanol.
68 2.3.5 Functionalization by hydrosilation reactions
2.3.5.1 Protected amine functionalization of polystyrene 165,166
The detailed procedure of hydrosilation was well described in section 2.3.2. The
pure silane-functionalized polystyrene (3.0 g, 1.3 mmol, Mn = 2,200 g/mol) was
dissolved in dry benzene (12 mL) in a two-necked flask (25 mL) by stirring for 10 min in the dry box. To the polymer solution was added 3.1 molar equivalents of 3-[N,N–
bis(trimethylsilyl)amino]-1-propene (1 mL, d = 0.816, 0.8 g, 4.0 mmol, MW = 201.5
g/mol). The entire reaction set was then taken out of the dry box, and connected to a
Schlenk line. The hydrosilation was effected by injection of 1-2 drops of Karstedt’s
catalyst, 1,3-divinyltetramethyldisiloxane-platinum complex, into the reaction mixture
using a gas-tight syringe under a dry argon atmosphere at room temperature followed by
further stirring overnight. Within 10 min after the injection of platinum catalyst, the so-
called “induction period”, a light yellow color appeared indicating the onset of the
hydrosilation reaction with formation of active species in colloidal form, and the color gradually turned to brown in the course of hydrosilation which arose from precipitation of platinum by catalyst deactivation. After completion of the hydrosilation reaction, the brown solution was precipitated into a 7-fold excess of anhydrous methanol (80 mL), and stirred for 20 min. The precipitation procedure was repeated three times after subsequent filtration and quick drying in a vacuum oven (15 min). The resulting polymer was then re-dissolved in dry benezene, and freeze-dried on the high vacuum line at 0 ºC in an ice water bath followed by further drying for three days with mild heating at 55 ºC for the complete removal of excess aminopropene functionalizing agent and trace amounts of xylene from Karstedt’s catalyst. Pure silyl-protected, amine-functionalized polystyrene
69 3 (Mn = 3.0 × 10 g/mol, Mw/Mn = 1.06) was finally obtained in a good yield (3.1 g, 95 %)
1 without any further purification process. H NMR: δ -0.2 (CH3-Si-C), 0.05 (18H, CH3-Si-
13 N), 0.2 (CH2-Si-C), and 2.6 ppm (CH2-N-Si2). C NMR: δ -4.7 (CH3-Si-C), 2.3 (6C,
CH3-Si-N), 11.3 (CH2-Si-C), and 49.4 ppm (CH2-N-Si2). FT-IR: 875 (Si-C), 1250 (s-
-1 CH3-Si), and 1409 cm (a-CH3-Si). MALDI-TOF MS: for C4H9-(C8H8)17-
+ SiC5H12N(SiC3H9)2·H , calcd. monoisotopic mass = 2087.34 Da; observed m/z =
2087.31.
2.3.5.2 Amine functionalization of polystyrene without protection165,166
The primary amine functionalization of polystyrene in the absence of protecting
groups was performed by Pt-catalyzed hydrosilation similar to the protected amine functionalization as described in the previous section. A two-necked flask (25 mL) containing a mixture of silane-functionalized polystyrene (3.0 g, 1.3 mmol, Mn = 2,200 g/mol), 3.1 molar equivalents of allylamine (0.3 mL, d = 0.761,168 0.23 g, 4.0 mmol, MW
= 57.10 g/mol) and dry benzene (12 mL) was prepared in the dry box, and then connected
to the Schlenk line. To the mixture solution was added 2 drops of Karstedt’s catalyst with
vigorous stirring under an argon atmosphere at room temperature to initiate hydrosilation.
The resulting solution was then further stirred for a total of 72 hours. The very light
yellow color appeared very slowly over several hours. In addition, a further color change
to light brown appeared much slower than observed for the protected amine
functionalization. After confirmation of completion of the hydrosilation by TLC analysis
where only one spot was observed on the bottom of silica gel plate, the brown solution
was precipitated into a 7-fold excess of anhydrous methanol followed by filtration and
70 freeze-drying in benzene at 0 ºC in an ice water bath. In order to remove any residual
allylamine, the resulting amine-functionalized polystyrene underwent a further intensive
drying process under high vacuum for 2 days, and the final pure functionalized polymer
1 was obtained in the yield of 98 % (3.0 g, 1.3 mmol). H NMR: δ -0.1 (CH3-Si-C), 0.4
13 (CH2-Si-C), and 2.6 ppm (CH2-NH2). C NMR: δ -4.7 (CH3-Si-C), 10.9 (CH2-Si-C),
28.3 (C-CH2-C), and 45.8 ppm (CH2-NH2). FT-IR: 1248 (s-CH3-Si), 1412 (a-CH3-Si),
-1 3320 (a-NH2), and 3380 cm (s-NH2). MALDI-TOF MS: for C4H9-(C8H8)17-
+ SiC5H12NH2·Ag , calcd. monoisotopic mass = 2049.11 Da; observed m/z = 2049.08.
2.3.5.3 Protected amine functionalization of polyisoprene
The functionalization of polyisoprene by hydrosilation of silane-functionalized
polyisoprene was conducted in the same way as that described in the previous sections
for silane-functionalized polystyrene. The pure silane-functionalized polyisoprene (1.5 g,
3 0.7 mmol, Mw = 2.2 × 10 g/mol, Mw/Mn = 1.03) was mixed with 2.9 molar equivalents
of fleshly vacuum distilled 3-[N,N–bis(trimethylsilyl)amino]-1-propene (0.5 mL, d =
0.816, 0.4 g, 2.0 mmol, MW = 201.5 g/mol) in anhydrous benzene (10 mL) in a pear- shaped, two-necked flask (25 mL) in the dry box. The reaction system was then brought out of the dry box, and quickly connected to the Schlenk line. Then 2 drops of Karstedt’s catalyst, 1,3-divinyltetramethyldisiloxane-platinum complex, was added into the reaction mixture using a gas-tight syringe with vigorous stirring under a dry argon atmosphere at both room temperature and 13 ºC. The reaction mixture was further stirred for 36 hours.
Analogous to polystyrene functionalization, the characteristic color change from yellow to brown was observed during the hydrosilation progress. After completion of the
71 hydrosilation reaction which was confirmed by the disappearance of Si-H proton
resonance in the 1H NMR spectrum, the resulting solution was precipitated dropwise into
a 7-fold excess of anhydrous methanol (70 mL) containing a small amount of BHT (0.2
wt %). After decantation of the methanol solution, the resulting viscous polymer was
placed into the vacuum oven, and dried overnight. The complete removal of excess
aminopropene functionalizing agents and trace amounts of xylene from Karstedt’s
catalyst was performed by subsequent intensive drying under high vacuum for three days
with stirring at 50 ºC. The silyl-protected, amine-functionalized polyisoprene (Mn = 2.8 ×
3 10 g/mol, Mw/Mn = 1.07) was finally obtained in a good yield (1.5 g, 91 %) without any
1 further purification processes. H NMR: δ 0.0 (CH3-Si-C), 0.1 (18H, CH3-Si-N), 0.4
13 (CH2-Si-C), and 2.7 ppm (CH2-N-Si2). C NMR: δ -2.8 (CH3-Si-C), 2.3 (6C, CH3-Si-N),
13.3 (CH2-Si-C), 28.5 (C-CH2-C), and 49.5 ppm (CH2-N-Si2). FT-IR: 875 (Si-C), 911
-1 and 1186 (N-Si2), and 1250 cm (CH3-Si).
2.3.5.4 Amine functionalization of polyisoprene without protection
The unprotected primary amine-functionalized polyisoprene was successfully
synthesized by Pt-catalyzed hydrosilation between silane-functionalized polyisoprene and
allylamine. The experimental process followed that described for the protected amine
functionalization as in the previous section. To a pear-shaped, two-necked flask (25 mL)
containing 10 mL of anhydrous benzene were added the pure silane-functionalized
3 polyisoprene (1.5 g, 0.7 mmol, Mn = 2.2 × 10 g/mol, Mw/Mn = 1.03) and 3.7 molar
equivalents of dried allylamine (0.2 mL, d = 0.761,168 0.15g, 2.6 mmol, MW = 57.10
g/mol) in the dry box. After that, the whole reaction set was brought out through the
72 antechamber and connected to the Schlenk line. For initiation of the hydrosilation, 2 drops of Karstedt’s catalyst was injected into the reaction mixture using a gas-tight syringe under a dry argon atmosphere at room temperature. The resulting solution was then further stirred for a total of 72 hours. A relatively weak and slow color change phenomenon from very light yellow to very light brown was observed in a similar pattern to that observed for polystyrene functionalization. The completion of the hydrosilation was confirmed by absence of the Si-H resonance peak in proton NMR analysis. The light brown solution was then precipitated into a 7-fold excess of anhydrous methanol (70 mL) with addition of a small amount of BHT (0.2 wt %) followed by decantation of the methanol solution. The resulting viscous polymer was first dried in a vacuum oven without heating, and then, for removal of residual allylamine and xylene, subjected to high vacuum with mild heating at 50 ºC for three days. The final unprotected amine- functionalized polyisoprene (1.4 g) was then obtained in a good yield (93 %). 1H NMR: δ
13 0.0 (CH3-Si-C), 0.5 (CH2-Si-C), and 2.7 ppm (CH2-NH2). C NMR: δ -2.8 (CH3-Si-C),
12.8 (CH2-Si-C), 28.5 (C-CH2-C), and 45.9 ppm (CH2-NH2). FT-IR: 1250 (CH3-Si), 3321
-1 (a-NH2), and 3386 cm (s-NH2).
2.3.5.5 Perfluoroalkyl functionalization of polystyrene167
The preparation of perfluoroalkyl-functionalized polystyrene was successfully
performed by Pt-catalyzed hydrosilation of 1H,1H,2H-perfluoro-1-octene with silane-
functionalized polystyrene using an experimental procedure similar to that described for
the amine functionalization. The pure silane-functionalized polystyrene (1.5 g, 0.7 mmol,
Mn = 2,200 g/mol), 2.9 molar equivalents of 1H,1H,2H-perfluoro-1-octene (0.44 mL, d =
73 1.520, 0.67 g, 1.9 mmol, MW = 346.10 g/mol), and dry benzene (15 mL) were mixed
together in a two-necked flask (25 mL) in the dry box. The reaction flask was removed
from the dry box, and then attached to the Schlenk line. To the mixture solution was
introduced 2 drops of Karstedt’s catalyst under vigorous stirring using a gas-tight syringe through one neck of the reaction flask which was sealed with a clean rubber septum. A fast initiation of hydrosilation was confirmed by the appearance of yellow color within a couple of minutes. The resulting yellow solution was further stirred for 24 hours under a dry argon atmosphere at room temperature during which time the yellow color turned to brown as the reaction progressed. A small amount of the solution was sampled out using a gas-tight syringe, and the completion of the hydrosilation was confirmed by the absence of the characteristic Si-H resonance of silane-functionalized polystyrene using 1H NMR spectroscopy. Then the brown solution was precipitated into a 7-fold excess of anhydrous methanol, and the methanol was decanted. The precipitate was stirred in 15 mL of dry benzene until it dissolved completely, and precipitated into methanol again. After filtration, the resulting functional polymer was freeze-dried in benzene at 0 ºC in an ice water bath followed by further intensive drying under high vacuum with mild heating at
50 ºC for three days to remove residual 1H,1H,2H-perfluoro-1-octene. Finally, the
3 perfluoroalkyl-functionalized polystyrene (1.7 g, Mn = 2.5 × 10 g/mol, Mw/Mn = 1.04)
1 was obtained in a yield of 98 %. H NMR: δ -0.03 (CH3-Si-C), and 0.16-0.17 ppm (CH2-
13 Si-C). C NMR: δ -5.0 (CH3-Si-C), 3.5 (CH2-Si-C), 25.7 (CH2-CF2), and 111.0-119.0
-1 ppm (-CF2-). FT-IR: 1119, 1144, 1209, 1240, 1365 (CF3 and CF2), and 1253 cm (CH3-
+ Si). MALDI-TOF MS: for C4H9-(C8H8)16-SiC10H10F13·Ag , calcd. monoisotopic mass =
2234.01 Da; observed m/z = 2234.12.
74
2.3.5.6 Epoxide functionalization of polystyrene
Similarly to other functionalizations as described in previous sections, the epoxy functionalization of polystyrene was effected by Pt-catalyzed hydrosilation between pre- synthesized, silane-functionalized polystyrene and 1,2-epoxy-5-hexene at room temperature. In this reaction, 1.5 g of silane-functionalized polystyrene (0.7 mmol, Mn =
2,200 g/mol) was dissolved in a pear-shaped, two-necked flask (25 mL) containing 10 mL of dry benzene followed by addition of 3.0 molar equivalents of 1,2-epoxy-5-hexene
(0.22 mL, d = 0.870, 0.19 g, 1.9 mmol, MW = 98.2 g/mol) in the dry box. The entire reaction system was taken out of the dry box, and connected to the Schlenk line. To the mixture solution was then added 2 drops of Karstedt’s catalyst using a gas-tight syringe under vigorous stirring at room temperature to initiate hydrosilation. The characteristic yellow color appeared within a couple of minutes after the Pt catalyst injection. After that, the resulting yellow solution was further stirred for a total 24 hours during which time a continuous color change to brown was observed. After the confirmation of completion of the hydrosilation by 1H NMR analysis where the disappearance of the characteristic Si-H resonance near 3.8 ppm was observed after 24 hours of reaction, the mixture solution was precipitated into a 7-fold excess of anhydrous methanol (70 mL).
After filtration and freeze-drying in benzene at 0 ºC in an ice water bath overnight, the epoxy-functionalized polystyrene was intensively dried on the high vacuum line for 3 days without heating until no noise occurred when the vacuum line was tested using a
3 Tesla coil. Finally, the purified epoxy-functionalized polystyrene (1.5 g, Mn = 2.3 × 10
1 g/mol, Mw/Mn = 1.04) was obtained in 97 % yield. H NMR: δ -0.03 (CH3-Si-C), 2.53 (t-
75 13 CH2-O-C), 2.82 (c-CH2-O-C), and 2.96 ppm (CH-O-C). C NMR: δ -4.6 (CH3-Si-C),
47.1 (CH2-O-C), and 52.3 ppm (CH-O-C). FT-IR: 855 (C-O-C), 1249 (s-CH3-Si), and
-1 + 1409 cm (a-CH3-Si). MALDI-TOF MS: for C4H9-(C8H8)16-SiC6H14OC2H3·Ag , calcd. monoisotopic mass = 1986.07 Da; observed m/z = 1986.05.
2.3.5.7 Phenol functionalization of polystyrene
The phenol-functionalized polystyrene was successfully prepared by Pt-catalyzed hydrosilation of 2-allylphenol with the pure silane-functionalized polystyrene which was
3 pre-synthesized by controlled termination reaction of poly(styryl)lithium (Mn = 2.1 × 10 g/mol, Mw/Mn = 1.04) with dimethylchlorosilane. An experimental procedure similar to
that described for amine functionalization was used. Into a two-necked flask (25 mL)
were added the silane-functionalized polystyrene (1.5 g, 0.7 mmol, Mn = 2,200 g/mol),
3.0 molar equivalents of 2-allylphenol (0.26 mL, d = 1.028, 0.27g, 2.0 mmol, MW =
134.2 g/mol), and dry benzene (10 mL) in the dry box followed by gentle stirring until the solution became homogeneous. The whole reactor set was transferred through the antechamber out of the dry box, and connected to the Schlenk line followed by dry argon purging. For initiation of the hydrosilation, 2 drops of Karstedt’s catalyst was injected through the rubber septum on the neck of the reaction flask using a gas-tight syringe while the mixture solution was stirred under a dry argon atmosphere. A few minutes after the addition of Pt catalyst, the gradual appearance of yellow color was clearly observed.
The resulting yellow solution was then stirred further for 5 days at room temperature during which time the characteristic yellow color continuously turned to brown as the reaction proceeded. No detectable amount of Si-H functionality was found in the 1H
76 NMR spectrum indicating the completion of the hydrosilation. The resulting brown
solution was then precipitated slowly into a 7-fold excess of anhydrous methanol (70 mL)
over 20 min followed by filtration and quick drying in a vacuum oven. After re-
dissolving the functional polymer in 10 mL of dry benzene, the series of precipitation,
filtration, and drying processes was repeated in the same way. For the purpose of removal
of 2-allyphenol and the small amount of xylene arising from Karstedt’s catalyst, the
resulting polymer was freeze-dried in benzene at 0 ºC followed by a further intensive
drying process under high vacuum with mild heating at 50 ºC for 4 days. Finally, the
3 phenol-functionalized polystyrene (1.5 g, Mn = 2.3 × 10 g/mol, Mw/Mn = 1.04) was
1 obtained in a good yield of 93 %. H NMR: δ -0.1~0.4 (CH3-Si-C), 2.7 (CH2-Ph-OH),
13 and 4.4~4.5 ppm (Ph-OH). C NMR: δ -4.7 (CH3-Si-C), 14.1 (CH2-Si-C), 24.3 (C-CH2-
C), 34.1 (CH2-Ph-OH), and 115~131 ppm (Ph-OH). FT-IR: 1251 (s-CH3-Si), 1260 (C-
-1 O), 1409 (a-CH3-Si), and 3300 cm (O-H). MALDI-TOF MS: for C4H9-(C8H8)16-
+ SiC5H12PhOH·Ag , calcd. monoisotopic mass = 2021.85 Da; observed m/z = 2022.0.
2.3.5.8 Synthesis of star-branched polystyrene with POSS (Polyhedral
Oligomeric Silsesquioxane) core
An 8-arm, star-branched polystyrene with a POSS core was successfully
synthesized by the linking reaction of silane-functionalized polystyrene with octavinyl-
-4 T8-silsesquioxane. Octavinyl-T8-silsesquioxane (69.0 mg, 1.1 × 10 mol, MW = 633.04
g/mol) was dissolved in a one-necked, round-bottom flask (50 mL) containing 20 mL of benzene-d6 which was previously distilled over sodium and benzophenone. Silane-
-4 functionalized polystyrene (2.0 g, 8.7 × 10 mol, Mw = 2,300 g/mol, 8.0 molar
77 equivalents) was then mixed into the POSS solution in dry box. Prior to introduction of
Pt catalyst, the mixture solution was gently stirred at room temperature for 1 hour to
make the solution homogeneous. After that, 2 drops of Karstedt’s catalyst was added into
the reaction mixture under vigorous stirring. The reaction system was closed by tightly
sealing with a rubber septum, and further stirred at room temperature in the dry box where the reaction progress was periodically monitored by 1H NMR spectroscopy. After
1 week, a considerable amount of Si-H functionality was detected in the 1H NMR spectrum. In order to speed up the reaction rate, 2 drops of Pt catalyst and an additional
2.0 molar equivalents of silane-functionalized polystyrene (0.5 g) were added. The reaction mixture was then stirred for an additional 2 weeks for completion of the reaction which was assumed based on the further decrease of Si-H signal in 1H NMR spectrum. A very weak color change (light yellow to light brown) was observed during the whole course of the linking reaction. The resulting mixture solution was then slowly precipitated into a 7-fold excess of anhydrous methanol (140 mL) followed by filtration and drying in a vacuum oven overnight. The 8-arm, star-branched polystyrene was successfully separated from the mixture by fractionation using a toluene/methanol solvent system where about 2.6 g of the crude polymer mixture was dissolved in 800 mL of toluene, and then methanol was slowly added dropwise into the toluene solution until the polymer solution became just cloudy; after that, the cloudy solution was gently heated until the solution became clear. The solution was then cooled to room temperature while the Erlenmeyer flask was kept tilted for easier collection of the oil-like precipitate on the bottom. The first fraction was discarded (0.15g) because of the presence of a higher molecular weight component after SEC analysis. More methanol was slowly added to the
78 main toluene/methanol solution until the solution became cloudy, and then it was gently
heated until the solution became clear. After cooling, the resulting polymer fraction on the bottom of the flask was removed. This procedure was repeated six times and the collected fractions were precipitated into methanol followed by drying. Finally, the
4 POSS-cored, 8-arm star-branched polymer (1.6 g, Mn = 1.9 × 10 g/mol, Mw/Mn = 1.04) was obtained in 77 % yield after intensive drying under high vacuum. 1H NMR: δ -0.37
13 (CH3-Si-C), and 0.03 ppm (CH2-Si-C & CH2-Si-O). C NMR: δ –5.5 (CH3-Si-C), 4.1
-1 (CH2-Si-C), and 27.1 ppm (CH2-Si-O). FT-IR: 1120 (Si-O-Si), and 1250 cm (CH3-Si).
2.4 Characterization
2.4.1 Size exclusion chromatography
Size exclusion chromatographic analyses (SEC) for the base polymers and various functionalized polymers were performed using a Waters 150-C Plus instrument equipped with a Viscotek model 301 triple detector system and three Waters high resolution columns in THF as eluent at a flow rate of 1.0 mL/min at 30 oC. The detector system
consisted of a differential refractometer (Waters 410), a differential viscometer (Viscotek
100) and a laser light scattering detector (Wyatt Technology, DAWN EOS, λ = 670 nm).
The column system consisted of Waters High Resolution 1 (100 Å), Waters High
Resolution 4E (500, 103, 105 Å) and Waters High Resolution 5E (103, 104, 106 Å). The
sample preparation (Mn = 2100 ~ 90000 g/mol) was performed with concentrations
ranging from 0.5 to 8.0 mg/mL in THF. Prior to injection, the samples were filtered
through a Teflon® filter with a 0.45 µm pore size.
79 2.4.2 Static light scattering measurement
Static light scattering measurements for molecular weight determination of the
amine-functionalized polystyrene without a protecting group were performed using a
Brookhaven Laser Light Scattering System composed of computer-controlled BI-200SM
goniometer, Melles Griot 35mW He-Ne laser (632.8 nm) and EMI-9863 photomutiplier.
Five sample solutions with concentrations of 0.903, 1.718, 3.403, 4.968, and 6.681
mg/mL in THF were individually put into 27 mm diameter, cylinderical scattering cells
that were placed in the center of a thermostatted bath filled with decahydronapthalene for refractive index matching. Absolute molecular weight was determined from Zimm plots from light scattering measurements at nine different scattering angles (30, 50, 60, 70, 80,
90, 105, 120 and 130˚).
2.4.3 NMR spectroscopy
All 1H (300 MHz), 13C (75 MHz), 29Si (60 MHz), 19F (280 MHz), DEPT (θ = 135°),
1 1 and H- H Cosy NMR spectra were acquired in CDCl3 (Aldrich, 99.8 % D) or C6D6
(Aldrich, 99 %) using Varian Mercury 300 or Gemini 300 NMR spectrometers. The 1H
NMR spectra were referenced to the residual proton impurities in the deuterated solvents and for 13C NMR spectra relative to the deuterated solvent, while 29Si NMR spectra were
referenced to hexamethyldisiloxane (Aldrich, 99.5 %) as internal standard. For
1 identification of the OH resonance in H NMR spectra, D2O (Cambridge Isotope
Laboratories, 99.9 %) was used to effect the proton exchange reaction which caused the
disappearance of the OH signal. Most 1H and 19F NMR samples were prepared in 5 mm
13 29 NMR tubes with approximately 5 mg of polymer in 0.5 mL of CDCl3 while C and Si
80 NMR samples were prepared with approximately 100 mg of polymer in 0.5 mL of
CDCl3.
2.4.4 FT-IR spectroscopy
Infrared spectra were recorded on an Excalibur Series FT-IR spectrometer
(DIGILAB, Randolph, MA, USA) by casting polymer films on KBr plates from THF solutions with subsequent drying at 40-50 oC for 5 min.
2.4.5 MALDI-TOF mass spectrometry169
Matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-
TOF) were recorded on a Bruker Reflex-III TOF mass spectrometer (Bruker Daltonics,
Billerica, MA). The instrument was equipped with an LSI model VSL-337ND pulsed
337 nm nitrogen laser (3 nm pulse width), a single-stage pulsed ion extraction source and a two-stage gridless reflector. Solutions of dithranol (20 mg/mL) (Fluka, 1,8,9- anthracenetriol, 99 %), polymer sample (10 mg/mL), and silver or sodium trifluoroacetate
(10 mg/mL) (Aldrich, 98 %) were prepared in THF (Aldrich, 99.9 %). These solutions were mixed in the ratio of matrix:cationizing salt:polymer (10:1:2), and 0.5 μL of the mixture was applied to the MALDI sample target and allowed to dry. In order to minimize undesirable polymer fragmentation and to achieve optimal intensity, the intensity of the nitrogen laser pulses was frequently attenuated and adjusted. Mass spectra were measured in the linear and reflecton modes, and the mass scale was calibrated externally using the peaks of a polystyrene standard at the molecular weight under consideration.
81 2.4.6 DSC (Differential scanning calorimetry)
Measurement of glass transition temperatures (Tg) was carried out using a DuPont
Instruments DSC 2910 differential scanning calorimeter. After loading a polymer sample
(5~10 mg) the heat flow was measured under N2 purging with equal heating and cooling rates of 10 ºC/min. In order to remove thermal hysteresis the data were obtained only from the second heating cycle.
2.4.7 TGA (Thermogravimetric analysis)
Decomposition temperatures were measured using a TA Instruments Hi-Res TGA
2095 thermogravimetric analyzer. The analyses were performed under N2 purging with
5~10 mg of polymer sample loading. The weight loss was recorded as a function of temperature, and the decomposition temperature was determined at 5.0 % weight loss.
2.4.8 TLC (Thin-layer chromatography)
Thin-layer chromatographic analyses (TLC) were carried out on the functionalized polymers by spotting and developing polymer samples on flexible silica gel plates
(Selecto Scientific, Silica Gel 60, F-254 with fluorescent indicator) which were activated in an oven at 140 ºC using toluene as eluent, in general. In some cases, TLC analysis was performed on basic alumina plates (Selecto Scientific, 200 µm, 254 nm).
2.4.9 Column chromatography170
Separation of functionalized polymers from the unfunctionalized polymers was effected by quantitative column chromatography using silica gel (EM Science, Silica Gel
60) with particle size of 0.040-0.063 mm (230-400 mesh). Toluene or toluene/hexane
82 (50/50, vol/vol) was employed to separate unfunctionalized or protected, functionalized polymers, and then THF/toluene (75/25, vol/vol) was used to extract the remaining functionalized polymers from the column.
2.4.10 End-group titration for amine functionality171
The amine group functionality for the ω-aminopolystyrene (Mn = 2200 g/mol) prepared by hydrosilation with allylamine was determined by perchloric acid titration in glacial acetic acid/chloroform (1/1, vol/vol) using methyl violet as indicator. For the purpose of the standardization of perchloric acid (HClO4) in glacial acetic acid, potassium hydrogen phthalate (0.5008 g) was dissolved in a mixture of chloroform and glacial acetic acid (40 mL, 1.0/1.0, vol/vol). To the mixture was added 3 drops of aqueous methyl violet (0.02 %) using a disposable pipette followed by titration with perchloric acid in glacial acetic acid (Table 2.1). The end-point was determined by the characteristic color change from violet to blue.
Table 2.1. Standardization of HClO4.
a b KHP (g) KHP (mmol) HClO4 (mL) HClO4 (N)
1 0.5008 2.452 24.55 0.0999
2 0.5101 2.498 25.10 0.0995
a Potassium hydrogen phthalate: MW = 204.23 g/mol. b 0.1014N Labelled standard perchloric acid in glacial acetic acid.
83 Table 2.2. Titration of amine-functionalized polystyrene with HClO4.
a b c PS-NH2 (g) PS-NH2 (mmol) HClO4 (mL) HClO4 (mmol) Mn,exp/Mn,tit
1 0.501 0.218 2.10 0.209 1.043
2 0.487 0.212 2.05 0.204 1.039 a, c Mn(silane) = 2240 g/mol, Mn,exp = Mn(silane) + 57.10 g/mol = 2297.10 g/mol. b 0.0997N Standard perchloric acid in glacial acetic acid.
In order to determine the amine functionality, 0.501g of amine-functionalized polystyrene was dissolved in 40 mL of a mixture of chloroform and glacial acetic acid
(1.0/1.0, vol/vol). To the mixture was added 5 drops of aqueous methyl violet (0.02 %) using a disposable pipette followed by titration with perchloric acid (0.0997 N) in glacial acetic acid (Table 2.2). The end-point was determined by the characteristic color change from violet to blue.
84 CHAPTER III
RESULTS AND DISCUSSION
3.1 Primary amine functionalization with pre-synthesized terminating agent
In the search for a general anionic methodology for the synthesis of primary amine- functionalized polymers, the procedure of DeSimone and coworkers115,116 utilizing the termination of polymeric organolithium compounds with a chlorosilane-functionalized bis(trimethylsilyl)amino group was examined. Scheme 3.1 illustrates this method including an improved synthesis of the silylamine-functionalized silyl chloride recently reported by Schlaad and coworkers.164 It was found that the chlorosilane derivative did not store well in a sealed flask in the dry box and that the desired amine-functionalized polymer was obtained in only 61 % yield; it was contaminated with trimethylsilyl- functionalized polymer and non-functional polymer.
One of the main advantages of this method is the fact that the reaction of polymeric organolithium compounds with silyl chlorides is a relatively facile reaction that proceeds without significant side reactions.172-174 As shown in Scheme 3.1, this reaction does require the use of protecting groups and these protecting groups must be removed to recover the desired functional end group.
85 [(CH3)3Si]2NK + CH2=CHCH2Br [(CH3)3Si]2NCH2CH=CH2 CH3
[(CH3)3Si]2NCH2CH=CH2 + (CH3)2SiClH [(CH3)3Si]2NCH2CH2CH2Si-Cl Pt catalyst
CH3 CH3 CH3
PLi + [(CH3)3Si]2NCH2CH2CH2Si-Cl [(CH3)3Si]2NCH2CH2CH2Si-P
CH3 CH3
CH3 CH3 1) HCl [(CH3)3Si]2NCH2CH2CH2Si-P H2NCH2CH2CH2Si-P 2)NaOH
CH3 CH3
Scheme 3.1. Primary amine functionalization with a chlorosilane derivative.
3.1.1 Amine functionalization of poly(styryl)lithium
3.1.1.1 Synthesis of terminating agent
For the preparation of the protected, primary aliphatic amine-functionalizing agent,
the two-step synthetic procedure designed by Schlaad and his coworkers164 was adopted.
In this approach, the precursor, 3-[N,N–bis(trimethylsilyl)amino]-1-propene, was
synthesized by nucleophilic substitution between allyl bromide and potassium N,N–
bis(trimethylsilyl)amide under a dry argon atmosphere at 0 oC in an ice water bath, as
described in Eq 3.1.164 (Eq 3.1) 0°C [(CH3)3Si]2NK + CH2=CHCH2Br [(CH3)3Si]2NCH2CH=CH2 + KBr HMDS
While the allyl bromide solution in HMDS was added dropwise into the potassium
N,N–bis(trimethylsilyl)amide suspension in HMDS, a color change of the suspension was observed from white to dirty light yellow. After filtration, a clear yellow solution was 86 obtained. The pure protected allylamine derivative as separated in 62 % yield as a
colorless, clear liquid by fractional vacuum distillation.
The desired trimethylsilyl-protected, primary aliphatic amine-functionalizing agent,
1-(chlorodimethylsilyl)-3-[N,N–bis(trimethylsilyl)amino]propane, was then prepared by
the platinum-catalyzed hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with
chlorodimethylsilane under a dry argon atmosphere at room temperature as shown in Eq
3.2.164 (Eq 3.2) CH3 Pt (0) [(CH3)3Si]2NCH2CH=CH2 + (CH3)2SiClH [(CH3)3Si]2NCH2CH2CH2Si-Cl Toluene, RT
CH3 The injection of a drop of Karstedt’s catalyst into the reaction mixture caused an
exotherm with appearance of light yellow color implying the initiation of hydrosilation.
During the course of the reaction, the light yellow color continued to change from deep
yellow to light brown arising from gradual deactivation of the active catalysts.147 After fractional vacuum distillation, the 1-(chlorodimethylsilyl)-3-[N,N–bis(trimethylsilyl) amino]propane was separated as a colorless, clear liquid in a yield of 81 %.
3.1.1.2 Stability of terminating agents
The stability of the final aminating agents was investigated by 1H NMR
spectroscopy as shown in Figure 3.1. While the intermediate silylamino propene
derivative, 3-[N,N–bis(trimethylsilyl)amino]-1-propene, turned out to be very stable in an
argon atmosphere at room temperature over the period of several months, the
chlorosilane containing aminating agent, 1-(chlorodimethylsilyl)-3-[N,N–
bis(trimethylsilyl)amino]propane, appeared to be unstable in an argon atmosphere at 87 room temperature, and underwent more than 60 % decomposition within 3 months. It is assumed that the decomposition proceeds by the hydrolysis of the chlorosilane moiety by adventitious moisture to form the dimer species as a side product where the hydrolysis is speeded up by the basic silyl amine in the other side of molecule (Scheme 3.2).
Si Si H2O Cl Si N HO Si N + HCl Si Si (a)
Si Si 2 (a) N Si O Si N + H2O Si Si
Scheme 3.2. Base-catalyzed hydrolysis of the amine-functionalizing agent.
Decomposition
Si Cl Si N Si
Si N Si
Figure 3.1. 1H NMR analysis for the decomposition of amine-functionalizing agent.
88
Figure 3.2. FT-IR spectrum for the decomposition products.
The siloxane dimer formation was supported by FT-IR analysis after
decomposition; a strong Si-O-Si stretching band was observed at 1050 cm-1 along with
-1 characteristic CH3-Si absorption bands at around 800-840 cm (Si-C stretching) and
-1 1256 cm (CH3 stretching), as shown in Figure 3.2.
3.1.1.3 Amine functionalization
Primary amine-functionalized polystyrene was prepared by the reaction between
3 poly(styryl)lithium (Mn = 11 × 10 g/mol, Mw/Mn = 1.01) and 1-(chlorodimethylsilyl)-3-
[N,N–bis(trimethylsilyl)amino]propane (1.8 molar equivalents) in benzene at room temperature for 15 hours as shown in Eq 3.3.
NH2 Cl Si N[Si(CH3)3]2 PS Si CH OH 61% PS Li 3 (Eq 3.3)
C6H6, at RT, 15h + by-products 34%
89 The completion of the reaction was confirmed by the disappearance of the red color
arising from poly(styryl)lithium. The crude polymer was obtained after a series of
deprotection, precipitation, filtration and drying. TLC analysis of the polymer products
with toluene indicated the presence of a considerable amount of non-polar polymer by
displaying two spots. Therefore, in order to determine the amount of desired, amine-
functionalized polymer in the mixture, quantitative silica gel chromatography was performed where the non-polar portion of the mixture was first separated by elution with
toluene, and then the amine-functionalized polystyrene was eluted using a THF/toluene
mixture (75/25, vol/vol). From the separation, the pure amine-functionalized polystyrene
was obtained in 61 % yield. Nevertheless, TLC analysis showed only one spot at the
bottom of the plate for the functional polymer (Figure 3.3)
Figure 3.3.TLC analysis before and after silica gel chromatography.
The 1H NMR spectrum for the amine-functionalized polystyrenes further evidenced
the introduction of amine functionality at the polystyrene chain ends resulting in a 1.7:1.8
integration ratio for the two CH3 units from the silylamine functional group to the two
CH3 units from the sec-butyl initiator portion of the polymers, as shown in Figure 3.4.
90
1 Figure 3.4. CH3 region of H NMR spectrum for the amine-functionalized polystyrenes.
1H NMR analysis for the non-polar polymer mixture exhibited quite different
results from that of the amine-functionalized polystyrene (Figure 3.4). A Si-CH3 group was detected at δ –0.10 ppm. The integration ratio of the Si-CH3 unit to the C-CH3 unit originating from the sec-butyl initiator residue turned out to be 2.8:2.0. Furthermore, contrary to amine-functionalized polystyrene case, a characteristic broad peak at δ 0.3 ppm attributed to Si-CH2 unit was not detected (Figure 3.5).
1 Figure 3.5. CH3 region of H NMR spectrum for non-polar polymer mixture.
FT-IR studies provide useful information about the various functional groups
present in macromolecules. The IR spectra overlay shows a sharp absoption band at 1250
-1 cm corresponding to CH3 symmetric deformation of a Si-CH3 unit for both the non-
polar polymer mixture and the amine-functionalized polymers indicating that they both
91 -1 have Si-CH3 units in their molecules. Interestingly, a distinct absorption band at 864 cm arising from a CH3 rocking mode was observed only for the non-polar polymer mixture
implying that there may be a Si-CH3 unit relatively free from steric hinderance compared
to that of the amine-functionalized polystyrene (Figure 3.6).
Figure 3.6. FT-IR analysis before and after silica gel column chromatography:
A, base polystyrene; B, after functionalization (mixture); C, amine-functionalized
polystyrene (separated with THF/toluene); D, non-polar polymer mixture (separated with
toluene). Solid line, CH3 deformation; Dashed line, CH3 rocking.
Accordingly, the data of FT-IR and 1H NMR spectroscopic analyses strongly
suggest that the majority of the non-polar polymer mixture consisted of trimethylsilyl-,
Si(CH3)3, functionalized polystyrenes.
MALDI-TOF mass spectra were obtained using the silver trifluoroacetate/dithranol
system as the cationizing agent/matrix system for end-functionalized polystyrenes.169 The
92
Figure 3.7. MALDI-TOF mass spectrum for non-polar polymer mixture (separated with
toluene) with silver trifluoroacetate as a cationizing agent. resulting mass spectrum (Figure 3.7) for the non-polar polymer mixture provided further evidence for Si(CH3)3 end-functionalized polystyrene as the major species.The mass peak for the polystyrene 96-mer with one butyl and a silver-complexed trimethylsilyl end
+ group, C4H9-(C8H8)96-Si(CH3)3·Ag , was detected at m/z = 10242.0 within the error limit
93 for this molecular weight region (± 8.00 for polystyrene with Mn of 10,000 g/mol);
Figure 3.8. MALDI-TOF mass spectrum for amine-functionalized polystyrene (separated
with THF/toluene) with silver trifluoroacetate as a cationizing agent. the calculated average-isotopic mass is {57.12 (C4H9) + 96 x 104.15 [(C8H8)96] + 73.1
+ [Si(CH3)3] + 107.87 (Ag )} = 10236.49 Daltons. The other minor series at 40-45 m/z
units above the main series could arise from residual protected amine-functionalized
polystyrene, which is not separable from unfunctionalized polystyrenes by silica gel
column chromatography using THF/Toluene. The protected amine-functionalized
94 polystyrene is easily deprotected by dithranol in the presence of Ag+ during MALDI-TOF mass analysis (see Figure 3.21). In the case of the amine-functionalized polystyrene, the major distibution turned out to be the desired product, and the mass peak for the polystyrene 95-mer with one butyl and a silver-complexed amine end group, C4H9-
+ (C8H8)95-Si(CH3)2(CH2)3NH2·Ag , was observed at m/z = 10181.7; the calculated average-isotopic mass is {57.12 (C4H9) + 95 x 104.15 [(C8H8)17] + 116.2 [SiC5H12NH2] +
107.87 (Ag+)} = 10175.44 Daltons (Figure 3.8). Two additional series are observed at 18 m/z units below and 28 m/z units above the main series, respectively. These peaks could not be identified.
In summary, the relatively low yield (61 %) of the amine-functionalized polystyrene compared to that obtained by Schlaad and his co-workers164 (> 90 %) is thought to arise mostly from the side reaction of poly(styryl)lithium compounds with impurities in amine-functionalizing agent. The major impurity seems to be the siloxane type of dimer formed by the hydrolysis of the amine-functionalizing agent during storage in the dry box at room temperature for about one and half months (Figure 3.9). Therefore, special care is needed for the storage of this amine-functionalizing agent to avoid the undesirable decomposition arising from the inherent instability of chlorosilane group in the presence of amine functionality in the same molecule. It is highly recommended that this amine-functionalizing agent be used right after it is freshly distilled.
95 Si Cl Si N Si
Si PS CH2 CH Li Si Cl Si N O Si N Si Si 2
CH3OH CH3OH
CH 3 CH3
Si Si NH2 PS CH3 PS CH 3 CH3
Figure 3.9. Possible reactions for the formation of trimethylsilyl-functionalized
polystyrene and amine-functionalized polystyrene.
3.1.2 Amine functionalization of poly(isoprenyl)lithium
Protected amine-functionalized polyisoprene was prepared by the termination
4 reaction of poly(isoprenyl)lithium (Mn = 4.3 × 10 g/mol, Mw/Mn = 1.02) with fleshly
vacuum distilled 1-(chlorodimethylsilyl)-3-[N,N–bis(trimethylsilyl)amino]propane (2.3
molar equivalents) in benzene at room temperature for 12 hours (Eq 3.4).
Cl Si N[Si(CH3)3]2
PIP Li PIP Si N[Si(CH3)3]2 C6H6, at RT, 12h (Eq 3.4)
96 The completion of functionalization was confirmed by the disappearance of the unique light yellow color from living anionic chain ends of poly(isoprenyl)lithium compounds. After precipitation into methanol followed by drying, the protected amine-
functionalized polyisoprene was obtained as a colorless gel in 92 % yield.
With regard to the use of the trimethylsilyl-protected amine-functionalizing agent
for end functionalization, one of the most important aspects could be the establishment of
optimal deprotection methodology.175 The three most widely used deprotection methods
for trimethylsilyl-protecting groups were investigated using 1H NMR spectroscopy
(Figure 3.10).
Figure 3.10. 1H NMR for different deprotection reactions: A, base polyisoprene; B,
methanol treatment; C, dilute HCl treatment; D, TBAF treatment.
The mildest one involves precipitaion into methanol. The protected amine-
functionalized polyisoprene solution in benzene (70 mL; isoprene:benzene = 1:10,
vol/vol) was slowly precipitated dropwise into a 7-fold excess of methanol over 45
97 minutes with vigorous stirring followed by further stirring at room temperature for 2 hours. However, 1H NMR analysis after the methanol treatment for 2 hours indicated the
incompleteness of the deprotection by showing residual protecting group resonances (B
in Figure 3.10). Accordingly, it seems to take a longer time to achieve the complete
deprotection, since methanol treatment for 7 hours resulted in residual protecting groups
in the case of protected amine-functionalized polystyrenes. Nonetheless, this method is
considered to be the safest way to preserve chain end functionalities from undesirable
attack at the α-silicon attached to polymer chain end as observed in other methods.116
As another alternative, dilute acid treatment has been known as one of the most
effective and common methods for the deprotection of trimethylsilyl-protecting
groups.175 In this approach, the protected amine-functionalized polyisoprene was treated
in dilute acid solution [THF:1N HCl (aq) = 9:1, vol/vol] at room temperature for 2 hours followed by neutralization with a dilute NaOH solution. Surprisingly, no detectable amount of end functional groups were observed in the 1H NMR spectrum after the 2 hour
treatment indicating allylic carbon-silicon bond breaking caused by attack at the α-silicon
(C in Figure 3.10).
Another popular method for deprotection is the use of tetrabutylammonium fluoride
(TBAF) in THF which is prone to form a strong Si-F bond by the reaction with
trimethylsilyl-protection group. In this study, TBAF solution (1.0 M in THF) was
injected into the polymer solution in THF (TBAF: polymer = 5:1, mol/mol) using a gas
tight syringe under a dry argon atmosphere at room temperature, and the resulting
solution was stirred at room temperature for 2 hours without heating. Interestingly, only
60-70 % of deprotected amine-functionalized polyisoprene was obtained based on 1H
98 NMR integration results indicating that partial allylic carbon-silicon bond cleavage
occured as well as deprotection during the reaction.
In conclusion, for the use of trimethylsilyl-protected amine-functionalizing agents, a prudent choice of the optimal deprotection method must be carried out in order to achieve
an effective chain end functionalizaton.
3.2 Primary amine functionalization by hydrosilation reactions
Upon careful examination of the DeSimone functionalization methodology115,116 outlined in Scheme 3.1, it can be seen that the two key reactions are chlorosilane functionalization of polymeric organolithium compounds and hydrosilation of an alkene for functionalization. This recognition, in turn, suggested that one could slightly modify these steps and develop a general functionalization methodology as illustrated in Scheme
3.3 for primary amine group functionalization.
CH3 CH3
PSLi + ClSiH PS-Si-H
CH3 CH3
CH3 CH3
PS-Si-H + CH2=CHCH2N[Si(CH3)3]2 PS-Si-CH2CH2CH2N[Si(CH3)3]2 Pt CH3 Catalyst CH3
Scheme 3.3. New general functionalization methodology.
The first step is the formation of a silyl hydride-functionalized polymer by reaction of poly(styryl)lithium (or other polymeric organolithiums) with dimethylchlorosilane, a reactive, readily available reagent. The second step, hydrosilation, is a reaction that proceeds readily and efficiently with a variety of substituted alkenes to form the product
99 from anti-Markovnikov addition of the silyl hydride to the alkene using typical platinum-
based catalysts.144,147 In order to evaluate the usefulness of this methodology, the
primary amine chain-end functionalization of poly(styryl)lithium was investigated.165,166
3.2.1 Amine functionalization of polystyrene
3.2.1.1 Preparation of silyl hydride-functionalized polystyrene
As the precursor for further functionalization, the silyl hydride-functionalized polystyrene was synthesized by the controlled termination reaction of poly(styryl)lithium 3 (Mn = 2.1 × 10 g/mol, Mw/Mn = 1.04) with 2.3 molar equivalents of chlorodimethylsilane in benzene at room temperature as shown in Eq 3.5.
CH3 CH3 Si H Li Cl Si H CH3 CH3 C H 6 6 (Eq 3.5) n-2 n-2
Precipitate formation was observed immediately after smashing the breakseal
containing chlorodimethylsilane; a color change from red to colorless occurred within a
few minutes, indicating a very fast reaction rate. The silyl hydride-functionalized
polystyrene was isolated in 98 % yield.
The molecular weight and molecular weight distribution were determined by GPC
analysis where the SEC chromatogram for the silane-functionalized polystyrene (Mn =
3 2.2 × 10 g/mol, Mw/Mn = 1.04) exhibited a narrow, monomodal curve along with Mn
3 consistent with the expected value (Mn, calc = 2.0 × 10 g/mol) as shown in Figure 3.11.
100
Figure 3.11. SEC chromatogram for the silyl hydride-functionalized polystyrene.
The 1H NMR spectrum of this polymer displayed characteristic resonances for the
silane proton at δ 3.8 ppm157 and for the silicon-bonded methyl groups at δ –0.1 ppm
indicating the efficient incorporation of silyl hydride functional groups at the chain ends
(Figure 3.12). From the integration ratio of the six methyl protons of the dimethylsilane unit to the other six methyl protons from the sec-butyl end group from the initiator, the silane functionality of this polymer was readily calculated (100 %).
101
Figure 3.12. 1H NMR spectrum for silyl hydride-functionalized polystyrene.
The 13C NMR spectrum further supported the introduction of silyl hydride
functionality at the chain end by showing distinct resonances for the silicon-bonded
methyl carbons at δ –5.5 ppm (Figure 3.13). It is also noteworthy that no resonance was observed at δ 33.6 ppm which would correspond to the terminal benzyl group in
unfunctionalized polystyrene.176
FT-IR spectroscopic analysis was found to be a useful tool for confirmation of the
incorporation of silyl hydride functionality. A strong absorption band corresponding to an
-1 157 Si-H streching vibration mode was observed at 2111 cm along with CH3 deformation
-1 -1 and CH3 rocking absorptions of CH3-Si at 1249 cm and 882 cm , respectively (Figure
3.14).177,178
102
Figure 3.13. 13C NMR spectrum for silyl hydride-functionalized polystyrene.
Figure 3.14. FT-IR spectrum for silyl hydride-functionalized polystyrene.
103 It has been shown that MALDI-TOF mass spectrometry is a powerful tool for the
analysis of anionically-prepared, chain-end functionalized polymers.179-183 The MALDI-
TOF mass spectrum of the silyl hydride-functionalized polystyrene using silver
trifluoroacetate as cationizing agent is shown in Figure 3.15.
Figure 3.15. MALDI-TOF mass spectrum for silyl hydride-functionalized polystyrene
with silver trifluoroacetate as a cationizing agent.
Although it is a monomodal distribution as expected, the peaks do not correspond to the expected structure. For example, as shown in the expanded region between m/z 2000
+ and 2200, the peak at m/z 2008.0 corresponds to C4H9-(C8H8)18-Si(CH3)2OH·Ag ; the
104 calculated monoisotopic mass is 57.07 (C4H9) + 17 x 104.06 [(C8H8)17] + 75.027
+ [Si(CH3)2OH] + 106.90 (Ag ) = 2008.02 Daltons. The unexpected formation of the silanol chain end can be explained by the oxidation of the silyl hydride functionality by silver ion in the presence of dithranol.184
Figure 3.16. MALDI-TOF mass spectrum for silyl hydride-functionalized polystyrene
with sodium trifluoroacetate as a cationizing agent.
When sodium was used as the cationizing agent, the mass spectrum of the silyl hydride-functionalized polystyrene was observed (Figure 3.16). The expanded trace of the mass spectrum exhibited two different series of peaks. The main series corresponds to 105 the silyl hydride-functionalized polymers as shown in the expanded region between m/z
+ 2000 and 2200. The peak at m/z = 2012.2 corresponds to C4H9-(C8H8)18-SiH(CH3)2·Na ;
the calculated monoisotopic mass is 57.07 (C4H9) + 18 x 104.06 [(C8H8)18] + 59.03
+ [Si(CH3)2H] +22.99 (Na ) = 2012.17 Daltons. The minor series, which appears 40 m/z
units above the main distribution, has not been identified; it is probably formed during the analysis, although the degradation mechanism is not obvious.185-187
In summary, the reaction of poly(styryl)lithium with dimethylchlorosilane in
hydrocarbon solution at room temperature results in the quantitative formation of the
corresponding ω-silyl hydride-functionalized polystyrene, based on NMR, FT-IR and
MALDI-TOF MS analyses.
3.2.1.2 Preparation of protected amine-functionalized polystyrene by
hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with silane-functionalized
polystyrene
The second step in development of the proposed general functionalization
procedure involves the reaction of silyl hydride-functionalized polystyrene with substituted alkenes (see Scheme 3.3). For the synthesis of the primary amine- functionalized polystyrene, the hydrosilation of silyl hydride-functionalized polystyrene with 3-(N,N-bis(trimethylsilyl)amino]-1-propene was investigated (Eq 3.6). This protected amine was the precursor for the silyl chloride used in the previous amine functionalization studies of DeSimone and coworkers115,116 and Schlaad and coworkers164
(see Scheme 3.1).
106 CH 3 Si(CH3)3 Si H N Si(CH3)3 CH3
Pt(0), C6H6 n-2 (Eq 3.6)
CH3 Si(CH3)3 Si N Si(CH3)3 CH3
n-2
Hydrosilation can be catalyzed by a variety of transition metal complexes, including those of Pt, Rh, Ru, Ir, Os and Pd.144 Among these complexes, Pt catalysts exhibit the
150,151 highest activity. Spier’s original soluble Pt(IV) catalyst, H2PtCl6 in 2-propanol, and
Karstedt’s Pt (0) catalyst, Pt2[CH2=CSi(CH3)2OSi(CH3)2CH=CH2]3, are two of the most
commonly used catalysts for hydrosilation.144 In this study, however, Karstedt’s catalyst
was chosen because it is soluble in hydrocarbon solvents; thus, hydrosilations were easily
1 monitored by H NMR spectroscopy in C6D6. It was also considered that use of the
Karstedt’s catalyst would minimize undesirable hydrolysis of Si-H with adventious water
or alcohol impurities which are more likely in polar media. Karstedt’s catalyst is also an
extremely efficient hydrosilation catalyst; i.e. concentrations of less than 1 ppm are
sufficient to complete the reaction.147
For the hydrosilation reaction, the silane-functionalized polystyrene (Mn = 2,200 g/mol) was stirred overnight with 3.1 molar equivalents of 3-[N,N– bis(trimethylsilyl)amino]-1-propene and one drop of Karstedt’s catalyst in dry benzene
107 under a dry argon atmosphere. A few minutes after injection of the platinum catalyst, a
light yellow color appeared indicating formation of the active colloidal species;188 the color gradually turned to brown during the course of hydrosilation. Finally, the ω-N,N- trimethylsilylamino-functionalized polystyrene was isolated in good yield (95 %) after standard work-up.
The polymer exhibited a monomodal SEC chromatogram (Figure 3.17) as expected with no evidence of dimer formation. The SEC results also revealed a narrow molecular weight distribution (Mw/Mn = 1.06) along with a little bit higher molecular weight (Mn =
3 3 3.0 × 10 g/mol) than we expected (Mn, calc = 2.4 × 10 g/mol).
Figure 3.17. SEC chromatogram for the protected amine-functionalized polystyrene.
The 1H NMR spectrum for the protected amine-functionalized polystyrene (Figure
3.18) exhibited distinct resonances from the 18 methyl protons of trimethylsilyl-
protecting groups at ca. δ 0.05 ppm and from the 6 methyl protons connected to the
108 silicon from dimethylchlorosilane functionalization near δ –0.2 ppm with a relative integration ratio of 2.5/1 which is lower than expected value of 3/1. This discrepancy was
Figure 3.18. 1H NMR spectrum for protected amine-functionalized polystyrene.
attributed to partial deprotection of the N,N-bis(trimethylsilyl)-protecting groups when
the polymer was recovered from solution by precipitation into methanol. Two new,
discrete methylene resonances appeared at δ 2.6 and 0.2 ppm corresponding to CH2N and
CH2Si groups, respectively. The absence of a peak corresponding to the silyl hydride at δ
3.8 ppm is consistent with efficient functionalization of the chain end.
In the 13C NMR spectrum for the protected, amine-functionalized polystyrene
(Figure 3.19), two different sets of resonances corresponding to methyl carbons attached to silicon atoms were observed at δ – 4.7 ppm (CH3Si-C) and 2.3 ppm (CH3Si-N), as well
as resonances for two characteristic methylene carbons corresponding to CH2Si and
109 CH2N at δ 11.3 and 49.4 ppm, respectively, indicating the successful incorporation of protected amine functional groups at the chain ends.
Figure 3.19. 13C NMR spectrum for protected amine-functionalized polystyrene.
The FTIR spectrum (Figure 3.20) of the protected amine-functionalized polystyrene
exhibited a high intensity absorption for a (CH3)2Si deformation band (symmetric
bending mode) at 1249 cm-1 along with a new weak absorption at 1413 cm-1 coming from
asymmetric methyl bending modes due to the introduction of six (CH3)3Si groups into the
polymer molecule.177,178 The absence of the unique Si-H stretching band (2111 cm-1) is consistent with efficient chain-end functionalization.
110
Figure 3.20. FT-IR spectrum for protected amine-functionalized polystyrene.
The nature of the end groups from this protected amine functionalization was provided by MALDI-TOF MS. Unexpected results were obtained using the silver trifluoroacetate/dithranol system, the most commonly used cationizing agent/matrix system for polystyrenes. The MALDI-TOF mass spectrum (Figure 3.21) for the protected, amine-functionalized polystyrene exhibited two overlapping series of peaks.
For the lower m/z series, the peaks are assigned to the deprotected, protonated, amine-
+ functionalized polymers, C4H9-(C8H8)n-SiC5H12NH3 . As shown in the expanded
spectrum, the peak at m/z 2047.1 has the exact mass for the polystyrene 18-mer with
+ butyl and protonated amine end groups, C4H9-(C8H8)18-SiC5H12NH3 ; the calculated
monoisotopic mass is {57.07 (C4H9) + 18 x 104.06 [(C8H8)18] + 116.99 [SiC5H12NH3]} =
2047.14 Daltons. The peak at m/z 2151.2 corresponds to the 19-mer in this series. For the
111
Figure 3.21. MALDI-TOF mass spectrum for protected amine-functionalized polystyrene
with silver trifluoroacetate as a cationizing agent. higher m/z series, the peaks are assigned to the deprotected, silver-complexed, amine-
+ functionalized polymers, C4H9-(C8H8)n-SiC5H12NH2Ag . As shown in the expanded spectrum, the peak at m/z 2049.0 has the exact mass for the polystyrene 17-mer with one
. + butyl and a silver-complexed amine end group, C4H9-(C8H8)17-SiC5H12NH2 Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 17 x 104.06 [(C8H8)17] + 115.8
+ [SiC5H12NH2] + 106.905 (Ag )} = 2049.04 Daltons. The smaller series of peaks is
112 + assigned to the unsaturated chain end, C4H9-(C8H8)n-CH=CH(C6H5)·Ag arising from
elimination of chain end functionality during the analysis. Some unfunctionalized PS
+ chains, C4H9-(C8H8)n-H·Ag , may also be present in this distribution.
It was surprising to find no MS evidence for the protected amine-functionalized polymer. This unusual phenomenon may arise from cleavage of the Si-N bonds by the slightly acidic phenolic hydroxy groups of dithranol (Figure 3.22) in the presence of Ag+, generating the NH2 functional group.
OH O OH
Figure 3.22. The structure of dithranol.
Amine-terminated PS chains can ionize by Ag+ addition as well as by protonation
because of the high intrinsic basicity of the amine group. As a result, overlapping isotope
clusters are observed in Figure 3.21 for the same oligomers. This finding suggested that it
might be possible to obtain the MALDI-TOF MS spectrum of the protected amine-
functionalized polystyrene in the absence of Ag+ ion, simply by using the protons of
dithranol as cationizing agents. When MALDI-TOF MS analysis was carried out using
dithranol as cationizing agent as well as matrix, it was possible to record the mass
spectrum of the amine-functionalized polymer with the protecting groups intact (see
Figure 3.23).
113
Figure 3.23. MALDI-TOF mass spectrum for protected amine-functionalized polystyrene
without silver trifluoroacetate.
In the expanded portion of the spectrum between m/z 2000 and 2200, the peak at
m/z 2087.3 corresponds to the functionalized 17 mer, C4H9-(C8H8)17-
+ SiC5H12N(SiC3H9)2·H ; the calculated monoisotopic mass is {57.07 (C4H9) + 17 x 104.06
[(C8H8)18] + 100.071 [SiC5H12] + 161.13 [N(SiC3H9)2·H]} = 2087.22 Daltons.
+ Deprotected, protonated amine-functionalized polystyrene, C4H9-(C8H8)19-SiC5H12NH3 , was also observed at m/z 2151.4 as the second major species implying that partial hydrolysis still occurred even in the absence of Ag+ ion; the calculated monoisotopic
114 mass is {57.07 (C4H9) + 19 x 104.06 [(C8H8)19] + 116.99 [SiC5H12NH3]} = 2151.20
Daltons. Two additional series of peaks were detected, which were not observed with
Ag+ ion. One series of peaks corresponds to the dehydrogenation product of the half-
deprotected amine-functionalized 17-mer with only one trimethylsilyl group {e.g. m/z
2191.41-72.04 [Si(CH3)3-H] = 2119.37}. The other, minor series could not be identified.
Based on the NMR, FT-IR and mass spectral analyses of the hydrosilation product,
it is concluded that the hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with silyl hydride-functionalized polystyrene can be performed successfully and efficiently without any detectable side reactions using Karstedt’s catalyst in benzene solution at room temperature.
3.2.1.3 Preparation of amine-functionalized polystyrene by hydrosilation of
allylamine with silane-functionalized polystyrene without protection
The proposed general functionalization procedure has a powerful advantage over
most direct anionic functionalization reactions that involve reaction of electrophiles with
polymeric organolithium compounds. These reactions must utilize protecting groups that
are stable to the anionic chain end. However, the platinum-catalyzed hydrosilation
reaction does not require the use of protecting groups for many functional groups of
interest.144,147 Platinum catalysts can tolerate a wide variety of functional groups
including nitro, cyano, amine, sulfonate, phenol, ester, ether, thioether, isocyanate, phenylthio, epoxide and perfluoroalkyl, i.e. these groups do not interfere with the hydrosilation reaction.144,147 Functional groups that do react directly with silyl hydride
groups and platinum catalysts include hydroxyl, thiol, ketone, aldehydes and acids.144,147
115 In order to demonstrate the versatility and simplicity of the silyl hydride-based
functionalization procedure, the hydrosilations of ω-silyl hydride-functionalized
polystyrenes (Mn = 2,200 and 14,400 g/mol, Mw/Mn = 1.04 and 1.01, respectively) with
allylamine were investigated (Eq 3.7).
CH3 Si H NH2 CH3
Pt(0), C6H6 n-2 (Eq 3.7)
CH3 Si NH2 CH3
n-2
The silane-functionalized polystyrene (Mn = 2,200 g/mol) was reacted with 3.1
molar equivalents of allylamine and Karstedt’s catalyst. Contrary to the corresponding
reaction with the silyl-protected amine, the characteristic light yellow color appeared very
slowly over several hours, indicating a relatively long induction period; in addition,
further color changes to brown appeared much slower than for the corresponding protected amine. The reduced reaction rate has been ascribed to the ligand coordination of the amine group to the active platinum center, thus competing with the vinyl
groups.188,189 Amine derivatives are currently used industrially as retarders to control
hydrosilation reactions.190-192
116 A total of seventy-two hours was required to complete this hydrosilation reaction, compared with less than 12 hours for the analogous reaction with the protected amine.
The ω-aminopropyl-functionalized polystyrene was isolated in high yield (98 %) after standard work-up. TLC analysis of the resulting polymer exhibited only one spot at the bottom of silica gel plate, consistent with a high yield of amine-functionalized polymer (≥
98 %).123 The high yield of amine functionalization (96 %) was also confirmed by end group titration of the amine groups with perchloric acid in glacial acetic acid.171
The SEC chromatograms for both amine-functionalized polystyrenes [Mn (PS-SiH)
= 2,200 and 14,400 g/mol] exhibited very broad, unsymmetrical curves with very low intensity (see Figure 3.24 for the polymer with Mn (PS-SiH) = 14,400 g/mol).
Figure 3.24. SEC chromatogram for amine-functionalized polystyrene.
Previous investigations of amine-functionalized polystyrenes reported a similar phenomenon and it was proposed that it results from physical adsorption of the primary
117 amine onto the column packing during the elution process.116,193 Consequently, light
scattering analysis was used to determine the molecular weight for the amine-
functionalized polystyrene with Mn (PS-SiH) = 14,400 g/mol). The data were collected
for the polymer solutions in THF at five different concentrations and at nine different
scattering angles (see Figure 3.25). The weight average molecular weight determined
Figure 3.25. Zimm plot for amine-functional polystyrene without protection groups. from the Zimm plot (Mw = 15,300 g/mol) was in good agreement with the value expected
(Mw, calc = 14,557 g/mol) based on the silyl-hydride functionalized base polymer sample;
Mw (PS-SiH), 14,500 g/mol + MW (allylamine), 57.10 g/mol = Mw, calc, 14,557 g/mol.
The 1H NMR spectrum for the lower molecular weight, ω-amine-functionalized
polystyrene (Figure 3.26) exhibited characteristic resonances for the -CH2NH2 protons at
δ 2.6 ppm and for the –Si(CH3)2CH2 protons at δ 0.4 ppm along with the Si(CH3)2 protons near δ –0.1 ppm. The functionality (> 95 %) was determined from the integration
118 ratio of the two –Si(CH3)2CH2 protons compared to six Si(CH3)2 protons. No resonances
corresponding to vinyl or silyl hydride residues were detected. The amine-functionalized
polystyrene proved to be rather stable in CDCl3 in air at room temperature; no detectable
oxidation or decomposition products were observed by 1H NMR spectroscopy over a 2
week period.
Figure 3.26. 1H NMR spectrum for amine-functionalized polystyrene without protection.
A DEPT-135 13C NMR spectrum (Figure 3.27) distinctly exhibited three negative
signals at δ 10.9 (-SiCH2-CH2-CH2N-), 28.3 (SiCH2-CH2-CH2N) and 45.8 ppm SiCH2-
CH2-CH2N), respectively, arising from the three different CH2 units between the silyl
group and the amine group at the terminal chain end of the functional polystyrene (see Eq
3.7). Charateristic positive signals for the (CH3)2Si group were observed at δ -4.7 ppm. It is noteworthy that no negative signal at δ 33.6 ppm corresponding to the terminal
benzylic carbon of unfunctionalized polystyrene was detected.176 119
Figure 3.27. DEPT-135 13C NMR spectrum for amine-functionalized polystyrene.
29Si NMR spectra (Figure 3.28) further evidenced the efficient functionalization at the chain ends by clearly exhibiting only one negative signal for both silane- and amine- functionalized polystyrenes at δ –8.3 (Si-H) and 3.7 (SiCH2-) ppm, respectively, where
hexamethylsiloxane was used as the reference material (δ 6.9 ppm).177
120
Figure 3.28. 29Si NMR spectra for silane- and amine-functionalized polystyrene.
FT-IR spectroscopic analysis provided further supporting evidence for the incorporation of amine functionality at the terminal polystyrene chain end (Figure 3.29).
Figure 3.29. FT-IR spectrum for amine-functionalized polystyrene without protection.
121 Characteristic absorption bands for symmetric and asymmetric N-H stretching
modes were observed at 3380 and 3320 cm-1, respectively.178 In contrast to the
bis(trimethylsilyl)-protected amine-functionalized polystyrene, only a relatively low
-1 intensity CH3 deformation absorption band at 1249 cm is observed. The completion of
hydrosilation reaction was evidenced by the lack of the characteristic Si-H stretching
band (2111 cm-1).
MALDI-TOF MS analysis of the ω-primary amine-functionalized polystyrene was
performed using silver trifluoroacetate/dithranol system (Figure 3.30). The resulting MS
spectrum presented two series of peaks where the main series comprised two overlapping
+ isotope clusters originating from C4H9-(C8H8)n-SiC5H12NH2·Ag and C4H9-(C8H8)n-
+ SiC5H12NH2·H as observed also for the bis(trimethylsilyl)-protected amine-
functionalized polystyrene. Contrary to the protected amine case, however, the peaks for
the protonated (H+) species were less abundant and hence, not well resolved. As shown
in the expanded spectrum for m/z between 2000 and 2200, the main series of peaks
corresponds to the silver-cationated, amine-functionalized polystyrene, C4H9-(C8H8)n-
+ SiC5H12NH2·Ag . Thus, the peak at m/z 2049.1 has the exact mass for the polystyrene
17-mer with one butyl and a silver-complexed amine end group, C4H9-(C8H8)17-
. + SiC5H12NH2 Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 17 x 104.06
+ [(C8H8)17] + 115.8 [SiC5H12NH2] +106.905 (Ag )} = 2049.04 Daltons. Similarly, the
+ peak at m/z 2047.0 corresponds to C4H9-(C8H8)18-SiC5H12NH3 . The other minor series is
observed at 70 m/z units above the main series and also appears to be composed of
overlapping Ag+- and H+-cationized oligomers; this series could not be identified.
122 Nevertheless, the predominant products observed are the desired primary amine-
functionalized PS oligomers.
Figure 3.30. MALDI-TOF mass spectrum for amine-functionalized polystyrene with
silver trifluoroacetate as a cationizing agent.
Because of the appearance of this other series of peaks observed for the ω-amine- functionalized polystyrene using silver as cationizing agent, a complementary MALDI-
TOF MS experiment was conducted where the amine-functionalized polystyrene was analyzed without Ag+, i.e. using H+ as a cationizing agent. Interestingly, in the resulting
+ MS spectrum (Figure 3.31), the distribution C4H9-(C8H8)n-SiC5H12NH3 (18-mer at m/z 123 2047.0) is less abundant than that corresponding to the by-product observed at 70 m/z
units higher (e.g. at m/z 2117.0). The origin of the latter product is currently under investigation.
Figure 3.31. MALDI-TOF mass spectrum for amine-functionalized polystyrene without
silver trifluoroacetate.
Based on the NMR, FT-IR and mass spectral analyses, it is concluded that the
hydrosilation reaction of the unprotected allylamine with silyl hydride-functionalized
polystyrene can be performed successfully and efficiently using Karstedt’s catalyst in
benzene solution at room temperature.
124 3.2.2 Amine functionalization of polyisoprene
3.2.2.1 Preparation of silyl hydride-functionalized polyisoprene
For the preparation of the silyl hydride-functionalized polyisoprene as the precursor
3 for other functionilzations, poly(isoprenyl)lithium (Mn = 2.1 × 10 g/mol, Mw/Mn = 1.03) was terminated with 1.8 molar equivalents of anhydrous chlorodimethylsilane in benzene at room temperature as shown in Eq 3.8.
CH3 Cl Si H CH3 CH3 Si H Li C6H6 n-1 n CH3
(Eq 3.8)
Analogous to the reaction with poly(styryl)lithium, precipitation of LiCl salt was observed immediately after the addition of chlorodimethylsilane along with the disappearance of the characteristic pale yellow color originating from poly(isoprenyl)lithium compounds within a few minutes, implying a very fast termination in terms of the reaction rate. Finally, the silyl hydride-functionalized polyisoprene was obtained in 94 % yield after the standard work-up procedure.
For the silane-functionalized polyisoprene, the molecular weight and molecular weight distribution were determined by GPC analysis. The resulting SEC chromatogram
3 revealed a monomodal curve along with the expected Mn (2.2 × 10 g/mol) and narrow
molecular weight distribution (Mw/Mn = 1.03) as shown in Figure 3.32.
125
Figure 3.32. SEC chromatogram for the silyl hydride-functionalized polyisoprene.
Figure 3.33. 1H NMR spectrum for the silyl hydride-functionalized polyisoprene.
126 The 1H NMR spectrum of this polymer exhibited characteristic resonances for the
silane proton at δ 3.9 ppm157,177 and for the silicon-bonded methyl groups at δ 0.1 ppm.
The silane functionality of this polymer (97 %) was determined from the integration ratio
of the six methyl protons of the dimethylsilane unit to the other six methyl protons from
the sec-butyl end group at δ 0.90 ppm indicating the successful introduction of silyl hydride functional groups at the chain ends (Figure 3.33).
The 13C NMR result was consistent with that of 1H NMR by displaying distinct
resonances for the silicon-bonded methyl carbons at δ –3.8 ppm (Figure 3.34) that further supported the introduction of silyl hydride functionality at the chain end.
Figure 3.34. 13C NMR spectrum for the silyl hydride-functionalized polyisoprene.
1H-1H COSY NMR analysis has turned out to be a very useful characterization
method for the confirmation of new bond formation after various reactions including
hydrosilation.178 The 1H-1H COSY spectrum (Figure 3.35) of this polymer exhibited the 127 characteristic correlation of the silane proton, SiH, at δ 3.9 ppm (c) with the silicon- bonded methyl protons, Si(CH3)2, at δ –0.1 ppm (a) and methylene protons,
SiCH2C(CH3)=CHCH2, at about δ 2.0 ppm (b) confirming the silyl hydride functionalities at the polymer chain ends.
Figure 3.35. 1H-1H COSY spectrum for the silyl hydride-functionalized polyisoprene.
Unlike the polystyrene analogue, the 29Si NMR spectrum (Figure 3.36) for the silyl hydride-functionalized polyisoprene displayed two separate peaks at δ –10.9 and -15.2
128 ppm, and it is thought that the peak separation is attributed to the stereoisomeric trans-
and cis-microstructural units of polyisoprene.177
Figure 3.36. 29Si NMR spectrum for the silyl hydride-functionalized polyisoprene.
FT-IR spectroscopic analysis provided further evidence for the successful
incorporation of silyl hydride functionality at the terminal polyisoprene chain end (Figure
3.37). In the resulting FT-IR spectrum, a sharp and strong Si-H stretching band was
-1157 observed at 2116 cm along with characteristic CH3 deformation absorption band of
-1 177,178 -1 CH3-Si at 1249 cm (lit 1245-1275 cm ) in a very similar manner to the polystyrene analogue.
129
Figure 3.37. FT-IR spectrum for the silyl hydride-functionalized polyisoprene.
It can be summarized that the controlled termination of poly(isoprenyl)lithium with dimethylchlorosilane in hydrocarbon solution at room temperature successfully produces
ω-silyl hydride-functionalized polyisoprene, based on various NMR and FTIR analyses.
In spite of several attempts, MALDI-TOF MS analysis of the silyl hydride-functionalized polyisoprene was not successful due to the difficulty in finding a suitable cationizing agent. In contrast to polystyrene, polyisoprene doesn’t have aromatic rings to form
complexes with the cationizing agent, and the simple double bonds in the molecule are
not sufficient to form a strong complex with most common cationizing agent such as
silver trifluoroacetate during the MS analysis.
3.2.2.2 Self-induced hydrosilation of silyl hydride-functionalized polyisoprene
One of the most important features of organolithium compounds is the unique
ability to provide polydienes with high 1,4-microstructure. In general, living anionic
polymerization of isoprene produces more than 90% of 1,4-microstructure along with 5-
130 8% of 3,4-microstructure in hydrocarbon media at room temperature. Accordingly, it
should be noted that the pendent vinyl groups can undergo both inter and intramolecular
hydrosilation with silyl hydride groups at the chain ends competing with the vinyl-
containing functionalizing agents during hydrosilation as shown in Figure 3.38.195-197
Intermolecular Dimerization, Hydrosilation Trimerization etc.
Si H n 5-8%
Si H
n 5-8%
Cyclic Products Intramolecular Hydrosilation
Figure 3.38. Self-induced hydrosilation of silyl hydride-functionalized polyisoprene.
Prior to further functionalization, the self-induced hydrosilation of silyl hydride-
functionalized polyisoprene was investigated to confirm the formation of either chain extended or cyclic products. In this study, the silyl hydride-functionalized polyisoprene
(1.0 g) was stirred with 1 drop of Karstedt’s catalyst in dry C6D6 under a dry argon
atmosphere at room temperature, and the reaction progress was monitored by 1H-1H
COSY NMR spectroscopy where the formation of inter- and intramolecular hydrosilation products can be readily confirmed by appearance of the characteristic new 1H-1H correlation peaks between the silicon-bonded methylene protons, SiCH2, at around δ 0.3-
131 0.4 ppm and the methine proton, SiCH2CH, at around δ 1.2-1.4 ppm as depicted in Figure
3.39.
Figure 3.39. Possible 1H-1H correlation caused by self-induced hydrosilation.
Figure 3.40. 1H-1H COSY spectrum for the silyl hydride-functionalized polyisoprene in the presence of Pt(0) catalyst after 4 days: a, Si(CH3)2; b, SiCH2C(CH3)=CHCH2; c, SiH.
132
Figure 3.41. 1H NMR analysis for the silyl hydride-functionalized polyisoprene in the
presence of Pt(0) catalyst for 10 days.
133 Surprisingly, however, no such kinds of correlation peaks were found after 4 days in 1H-
1H COSY spectrum (Figure 3.40) implying that no self-induced hydrosilation products were formed within 4 days within the detection limits of NMR; for a 300 MHz pulsed instrument, it is possible to obtain the signals for 1 µg of a sample in a microtube
(volume 185 µl).178
The hydrosilation reaction was also investigated by 1H NMR spectroscopy. The 1H
NMR analysis results were in good agreement with the 1H-1H COSY results (Figure 3.41)
where no noticeable change in the spectra with time was found even after 10 days. It
should be noted that appearance of a new resonance was detected immediately after
addition of Pt(0) catalyst at about δ 0.1 ppm along with some reduction of the resonance for the silicon-bonded methyl protons, Si(CH3)2, at δ 0.0 ppm. This seems to arise from
the coordination or oxidative addition of silyl hydride groups to a platinum metal center.
Furthermore, the integration ratio of backbone vinyl groups versus pendant ones turned out to be constant for 10 days.
In summary, self-induced inter- or intramolecular hydrosilation are not likely to occur within a couple of days at room temperature, and if it occurs at all, the rate seems to be slow based on 1H-1H COSY and 1H NMR spectroscopic data.
3.2.2.3 Preparation of protected amine-functionalized polyisoprene by
hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with silane-functionalized
polyisoprene
For the preparation of the primary amine-functionalized polyisoprene, the hydrosilation of 3-(N,N-bis(trimethylsilyl)amino]-1-propene with silyl hydride-
134 functionalized polyisoprene was carried out (Eq 3.9). The protected allylamine, 3-[N,N– bis(trimethylsilyl)amino]-1-propene, was hydrosilated with the silyl hydride- functionalized polyisoprene (Mn = 2,200 g/mol) for 36 hours in the presence of a catalytic amount of Karstedt’s catalyst in dry benzene under a dry argon atmosphere at room temperature.
Si(CH3)3 N
Si(CH3)3 Si H n Pt(0), C6H6 (Eq 3.9)
Si(CH3)3 Si N Si(CH ) n 3 3
Similarly to the hydrosilation with polystyrene analogue, the appearance of the characteristic light yellow color was observed within a few minutes after injection of the platinum catalyst where the color intensity was relatively low compared to that of polystyrene analogue. A further color change to brown slowly occurred during the course of hydrosilation. The ω-N,N-trimethylsilylamino-functionalized polyisoprene was isolated in good yield (91 %) after standard work-up.
In addition to the main curve corresponding to the desired product, the SEC chromatogram of the products (Figure 3.42) exhibited an extra, broad shoulder peak which appears to correspond to dimer formation (less than 5 %) during hydrosilation or work-up. To reduce dimer formation, hydrosilation was conducted at reduced temperatures (13 ºC), but the SEC chromatogram didn’t exhibit any noticeable difference. 135 Nevertheless, the SEC results indicated the formation of a narrow molecular weight
3 distribution product (Mw/Mn = 1.07). The obtained molecular weight (Mn = 2.8 × 10
3 g/mol) turned out to be a little higher than expected (Mn, calc = 2.4 × 10 g/mol).
Figure 3.42. SEC chromatogram for the protected amine-functionalized polyisoprene.
In the 1H NMR spectrum (Figure 3.43), the protected amine-functionalized polyisoprene exhibited distinct resonances corresponding to the 18 methyl protons of trimethylsilyl-protecting group at ca. δ 0.1 ppm and corresponding to the 6 methyl protons connected to the silicon from dimethylchlorosilane functionalization near δ 0.0 ppm with a relative integration ratio of 2.7/1 which is lower than expected value of 3/1.
This discrepancy was ascribed to partial deprotection of the N,N-bis(trimethylsilyl)- protecting groups during work-up in methanol. The characteristic methylene resonance corresponding to CH2N appeared at δ 2.7 ppm with an expected integration ratio of 3/1 to the 6 methyl protons originating from the sec-butyl end group from the initiator. In
136 addition, another new methylene resonance was observed at 0.4 ppm corresponding to
CH2Si groups. The absence of the silyl hydride resonace at δ 3.8 ppm well matched with efficient functionalization of the chain end. Any other additional peaks arising from side reactions were not found within the NMR detection limits.
Figure 3.43. 1H NMR analysis for the protected, amine-functionalized polyisoprene.
137
Figure 3.44. 13C NMR analysis for the protected, amine-functionalized polyisoprene.
Analogous to the protected, amine-functionalized polystyrene, the 13C NMR spectrum (Figure 3.44) further supported the successful incorporation of protected amine functional groups at the polyisoprene chain ends by displaying two different sets of resonances corresponding to methyl carbons attached to silicon atoms at δ – 2.8 ppm
(CH3Si-C) and 2.3 ppm (CH3Si-N), as well as three new separate resonances for the characteristic methylene carbons corresponding to CH2Si, CH2CH2Si and CH2N at δ 13.3
28.5 and 49.4 ppm, respectively.
138
Figure 3.45. 1H-1H COSY spectrum for the protected, amine-functionalized polyisoprene.
1H-1H COSY NMR analysis was performed to confirm new bond formation after hydrosilation reaction (Figure 3.45). The 1H-1H COSY spectrum of this polymer exhibited the characteristic correlation among the three new characteristic methylene protons corresponding to CH2Si, CH2CH2Si and CH2N at δ 0.4 (a), 1.3 (b) and 2.7 (c) ppm, respectively.
The FT-IR spectrum (Figure 3.46) for the protected amine-functionalized polyisoprene further evidenced the efficient chain-end functionalization by exhibiting a 139 -1 strong absorption for (CH3)2Si deformation band (symmetric bending mode) at 1250 cm along with new absorption bands at 875 cm-1 coming from Si-C stretching mode, and at
911 and 1186 cm-1 corresponding to characteristic N-Si stretching modes.177,178 The completion of hydrosilation was confirmed by the absence of the unique Si-H stretching band (2116 cm-1).
Figure 3.46. FT-IR spectrum for the protected, amine-functionalized polyisoprene.
Even though a small amount (less than 5 %) of dimer species was detected in the
SEC chromatogram of the products, it is concluded that the Pt(0)-catalyzed hydrosilation reaction of 3-[N, N–bis(trimethylsilyl)amino]-1-propene with silyl hydride-functionalized polyisoprene can be an effective way to prepare amine-functionalized polyisoprene based on the various NMR and FT-IR spectral analyses of the hydrosilation product.
140 3.2.2.4 Preparation of amine-functionalized polyisoprene by hydrosilation of allylamine with silane-functionalized polyisoprene without protection
As previously mentioned, hydrosilation has a unique tolerance toward many polar functional groups.144,147 Therefore, the primary amine-functionalized polyisoprenes were successfully synthesized by hydrosilations of allylamine with ω-silyl hydride- functionalized polyisoprenes (Mn = 2,200 g/mol, Mw/Mn = 1.03) in the absence of protection groups, as shown in Eq 3.10.
NH2 Si H n Pt(0), C6H6 (Eq 3.10)
Si NH2 n
In this study, the silane-functionalized polyisoprene (Mn = 2,200 g/mol) was reacted with 3.7 molar equivalents of allylamine in the presence of 2 drops of Karstedt’s catalyst for 72 hours at room temperature. Similary to the corresponding reaction with the silyl hydride-functionalized polystyrene, the characteristic light yellow color appeared very slowly, and further color changes to light brown was very weak compared to the corresponding protected amine. The final ω-aminopropyl-functionalized polyisoprene was isolated in good yield (93 %) after standard work-up.
In spite of several tries, the GPC analysis turned out to be unsuccessful due to the interaction between primary amine functional groups and column materials.116,193
141 Analogous to amine-functionalized polystyrenes, the SEC chromatograms (Figure 3.47) of the polymer [Mn (PS-SiH) = 2,200 g/mol] exhibited very broad, unsymmetrical curves with low intensity.
Figure 3.47. SEC chromatogram for the amine-functionalized polyisoprene.
The 1H NMR spectrum for ω-amine-functionalized polyisoprene (Figure 3.48) revealed characteristic resonances for the CH2NH2 protons at δ 2.7 ppm and the
Si(CH3)2CH2 protons at δ 0.5 ppm along with the Si(CH3)2 protons at about δ 0.0 ppm.
The integration ratio of the two Si(CH3)2CH2 protons and the other two CH2NH2 protons compared to six Si(CH3)2 protons indicated an efficient incorporation of amine functional groups at the chain ends resulting in 92 and 96 % of functionalities, respectively. No resonances corresponding to vinyl or silyl hydride residues were detected.
142
Figure 3.48. 1H NMR spectrum for the amine-functionalized polyisoprene.
13C NMR analysis (Figure 3.49) results agreed with 1H NMR analysis clearly showing three new signals at δ 12.8 (-SiCH2-CH2-CH2N-), 28.5 (SiCH2-CH2-CH2N) and
45.9 ppm SiCH2-CH2-CH2N), respectively, arising from the three different CH2 units between the silyl group and the amine group at the terminal chain end of the functional polyisoprene. Charateristic methyl proton resonances corresponding to the (CH3)2Si group were detected at δ –2.8 ppm indicating the effective amine functionalization of the polymer.
143
Figure 3.49. 13C NMR spectrum for the amine-functionalized polyisoprene.
1H-1H COSY NMR analysis for ω-amine-functionalized polyisoprene further supported the efficient amine functionalization at the chain ends (Figure 3.50) where the resulting spectrum of this polymer clearly exhibited the characteristic correlations among three new characteristic methylene protons corresponding to CH2Si, CH2CH2Si and
CH2N at δ 0.5 (a), 1.4 (b) and 2.7 (c) ppm, respectively. No noticeable side products were detected based on the correlation spectrum.
144 Figure 3.50. 1H-1H COSY NMR spectrum for the amine-functionalized polyisoprene.
FT-IR spectroscopic analysis (Figure 3.51) provided another supporting evidence for the successful introduction of amine functionality at the terminal polyisoprene chain end. Characteristic amine absorption bands were observed at 3380 and 3320 cm-1 corresponding to symmetric and asymmetric N-H stretching modes, respectively. In addition, a unique methyl deformation absorption band for Si(CH3)2 was observed at
1250 cm-1 with a relatively low intensity compared to the silyl-protected amine-
145 functionalized polyisoprene due to the absence of trimethylsilyl-protecting groups in that case.177,178 The completion of hydrosilation reaction was confirmed by the lack of the characteristic Si-H stretching band (2116 cm-1) indicating a good efficiency in functionalization of the polymer.
Figure 3.51. FT-IR spectrum for the amine-functionalized polyisoprene.
In summary, based on the various NMR (1H, 13C and 1H-1H COSY) and FT-IR spectral analyses, the Pt-catalyzed hydrosilation of allylamine with silyl hydride- functionalized polyisoprene turned out to be an efficient way for the preparation of amine-functionalized polyisoprene even in the absence of protection groups.
Consequently, it is concluded that even though there is a possibility of self- induced hydrosilation reaction during functionalization, the application of hydrosilation for end-functionalization of polyisoprene seems very effective because the undesirable self-induced hydrosilation is slow and can even be further reduced by using excess vinyl- containing functionalizing agents.
146 3.3 Phenol functionalization of polystyrene by hydrosilation of 2-allylphenol with silane- functionalized polystyrene without protection
Even though alcoholic hydroxyl functional groups react fast with silyl hydride groups in the presence of Pt-catalysts to form Si-O-C bond by dehydrogenative coupling,144,147 interestingly, the phenol group does not interfere with hydrosilation.144
Accordingly, phenol-functionalized polystyrene was successfully prepared by Pt catalyzed hydrosilation of 2-allyphenol with silane-functionalized polystyrene in the absence of protecting groups (Eq 3.11).
CH3 HO Si H
CH3 Pt(0), C6H6 n-2 (Eq 3.11)
HO CH3 Si CH3
n-2
Similarly to amine functionalization, 2-allyphenol (3.1 molar equivalents) was hydrosilated with silyl hydride-functionalized polystyrene (Mn = 2,200 g/mol) in the presence of Karstedt’s catalyst for 5 days until the Si-H resonance completely disappeared in the 1H NMR spectrum. A deep yellow color appeared within 15 minutes after the injection of the Pt(0) catalyst, and further color changes continued to dark brown. The phenol-functionalized polystyrene was isolated in good yield (93 %) after standard work-up. TLC analysis was performed using a toluene/hexane (50/50, vol/vol) 147 mixture and the resulting polymer exhibited only one spot at the bottom of silica gel plate implying efficient functionalization.
The molecular weight and molecular weight distribution were determined by GPC analysis, and the resulting SEC chromatogram of the phenol-functionalized polystyrene showed a narrow, monomodal curve (Mw/Mn = 1.04) along with the expected molecular
3 weight (Mn = 2.3 × 10 g/mol) as shown in Figure 3.52.
Figure 3.52. SEC chromatogram for the phenol-functionalized polystyrene.
The 1H NMR spectrum of the phenol-functionalized polystyrene (Figure 3.53 exhibited characteristic resonances for the hydroxyl proton (HO-Ph) at δ 4.5 ppm,100,101 as well as for the benzylic protons (CH2-Ph) at δ 2.7 ppm from the phenol functional group of the chain ends. The hydroxyl proton resonance was identified by the disappearance of O-H peak after addition of D2O into CDCl3 NMR solution due to the fast proton-deuterium exchange, as shown in Figure 3.54.100,101 Interestingly, peak
148 broadening of methyl protons (Si(CH3)2) was observed at about δ 0.0 ppm. It is thought
Figure 3.53. 1H NMR spectrum for the phenol-functionalized polystyrene.
Figure 3.54. 1H NMR spectrum for the phenol-functionalized polystyrene after addition
of D2O into CDCl3 NMR solution.
149 to arise from the increased steric hinderance at the chain end by introduction of the phenol group.
The DEPT-135 13C NMR spectrum (Figure 3.55) clearly revealed three separate negative signals at δ 14.1 (SiCH2-CH2-CH2Ph), 24.3 (SiCH2-CH2-CH2Ph) and 34.1 ppm
(SiCH2-CH2-CH2Ph), respectively, arising from the three different CH2 units between the silyl and the phenol group at the chain end of the functional polystyrene. Charateristic aromatic carbon signals of the phenol group were observed between δ 115-135 ppm as well as positive signals at δ –4.7 ppm for the (CH3)2Si group. It is should be noted that a negative signal at 33.6 ppm176 corresponding to the terminal benzylic carbon of unfunctionalized polystyrene was not detected, indicating effective phenol functionalization at the chain end of the polymer.
Figure 3.55. DEPT 13C NMR spectrum for the phenol-functionalized polystyrene.
150 FT-IR analysis further supported the successful incorporation of the functional group by showing a characteristic broad absorption band for O-H stretching modes at about 3300 cm-1 (Figure 3.56). The completion of the hydrosilation reaction was confirmed by the lack of the characteristic Si-H stretching band (2111 cm-1).177,178
Figure 3.56. FT-IR spectrum for the phenol-functionalized polystyrene.
MALDI-TOF MS analysis of the phenol-functionalized polystyrene was performed using the silver trifluoroacetate/dithranol system (Figure 3.57). The resulting MS spectrum was complicated by peaks in addition to the desired peaks, exhibiting three
+ distinct series of peaks which correspond to C4H9-(C8H8)n-SiC5H12C6H5OH·Ag , C4H9-
+ + (C8H8)n-H·Ag and C4H9-(C8H8)n-SiC2H6OH·Ag . Nevertheless, the main series of peaks turned out to be the silver-cationated, phenol-functionalized polystyrene, C4H9-(C8H8)n-
+ SiC5H12C6H5OH·Ag in which the peak at m/z 2022.0 has the exact mass for the polystyrene 16-mer with one butyl and a silver-complexed phenol end group, C4H9-
+ (C8H8)16-SiC5H12C6H5OH·Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 16 x
104.06 [(C8H8)17] + 193.1 [SiC5H12C6H4OH] +106.905 (Ag+)} = 2022.04 Daltons. 151
Figure 3.57. MALDI-TOF mass spectrum for the phenol-functionalized polystyrene.
Interestingly, the second most predominant species turned out to be the silverated
+ unfunctional polystyrene, C4H9-(C8H8)n-H·Ag . The peak at m/z 2038.1 has the mass for
. + the silver-complexed polystyrene 18-mer with one butyl, C4H9-(C8H8)18-H Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 18 x 104.06 [(C8H8)17] + 1.01 (H) +
106.905 (Ag+)} = 2038.07 Daltons. The third most predominant species is assigned to the 152 + silanol-functionalized polystyrene, C4H9-(C8H8)n-SiC2H6OH·Ag . The peak at m/z 2008.1 has the mass for the polystyrene 17-mer with one butyl and a silver-complexed silanol
. + end group, C4H9-(C8H8)17-SiC2H6OH Ag ; the calculated monoisotopic mass is {57.07
+ (C4H9) + 17 x 104.06 [(C8H8)17] + 75.02 [SiC2H6OH] + 106.905 (Ag )} = 2008.02
Daltons. These additional peaks are assumed to arise from fragmentation processes during analysis.185-187
Based on the TLC, NMR, FT-IR and mass spectral analyses of the hydrosilation product, it is concluded that the phenol-functionalized polystyrene can be successfully prepared by hydrosilation of 2-allyphenol with silyl hydride-functionalized polystyrene with a catalytic amount of Karstedt’s catalyst in benzene solution at room temperature even in the absence of protecting groups.
3.4 Epoxide functionalization of polystyrene by hydrosilation of 1,2-epoxy-5-hexene with silane-functionalized polystyrene without protection
While most organolithium compounds are very reactive to epoxy groups,137 platinum catalysts are known to tolerate epoxy groups.157 Consequently, epoxide functionalization of polystyrene was performed by Pt-catalyzed hydrosilation of 1,2-epoxy-5-hexene with silane-functionalized polystyrene in the absence of protecting groups (Eq 3.12).
153 CH3 Si H O CH3
Pt(0), C6H6 n-2
(Eq 3.12)
CH3 Si CH3 O
n-2
In this study, the silane-functionalized polystyrene (Mn = 2,200 g/mol) was reacted with 3.0 molar equivalents of 1,2-epoxy-5-hexene and Karstedt’s catalyst in dry benzene under an argon atmosphere at room temperature. The characteristic light yellow color was observed within 10 minutes after addition of the Pt(0) catalyst indicating a relatively fast activation of hydrosilation, and further color changes to brown continued throughout the hydrosilation. A total of twenty-four hours was taken to complete this hydrosilation reaction. The final epoxide-functionalized polystyrene was obtained in high yield (97 %) after standard work-up.
The SEC analysis for the epoxide-functionalized polystyrene resulted in a monodisperse narrow molecular weight distribution (Mw/Mn = 1.04) as well as a
3 molecular weight (Mn = 2.3 × 10 g/mol) in good agreement with the expected value as shown in Figure 3.58.
154
Figure 3.58. SEC chromatogram for the epoxide-functionalized polystyrene.
Figure 3.59. 1H NMR spectrum for the epoxide-functionalized polystyrene.
155 The 1H NMR spectrum (Figure 3.59) of this polymer exhibited characteristic resonances for the silicon-bonded methyl groups [Si(CH3)2] at about δ –0.03 ppm, as well as three proton resonances at δ 2.53 (trans-CH2OCH), 2.82 (cis-CH2OCH), and 2.96
139 (CH2OCH) ppm, respectively, arising from epoxide ring at the chain end indicating the efficient functionalization.
The 13C NMR spectrum (Figure 3.60) supported the successful incorporation of epoxide functional groups at the polystyrene chain ends by displaying two distinct resonances corresponding to methylene and methine carbons of the epoxy ring at δ 47.1
139, 198 ppm (CH2OCH) and 52.3 ppm (CH2OCH), as well as the resonance of silicon- bonded methyl carbons at δ –4.6 ppm [Si(CH3)2]. It is noteworthy that no detectable signal at 33.6 ppm corresponding to the terminal benzylic carbon of unfunctionalized polystyrene was observed within the NMR detection limits.176
Figure 3.60. 13C NMR spectrum for the epoxide-functionalized polystyrene. 156 The FT-IR spectrum for the epoxide-functionalized polystyrene revealed a weak absorption band corresponding to an epoxy ring stretching vibration mode at 855 cm-1
-1 along with a CH3 deformation absorption band of CH3-Si at 1249 cm supporting epoxy functional groups at the chain ends (Figure 3.61).177,178 The lack of the characteristic Si-H absorption band confirmed the complete hydrosilation indicating an efficient chain-end functionalization of the polymer.
Figure 3.61. FT-IR spectrum for the epoxide-functionalized polystyrene.
As previously mentioned, MALDI-TOF MS spectrometry is one of the most useful tools to identify relatively small end-functional groups compared to the entire macromolecule due the extremely high sensitivity of this method. The MALDI-TOF MS spectrum for the epoxide-functionalized polystyrene was obtained using the silver trifluoroacetate/dithranol system (Figure 3.62). The resulting MS spectrum presented the main series of peaks corresponding to be the silver-cationated, epoxide-functionalized
+ polystyrene, C4H9-(C8H8)n-SiC6H14OC2H3·Ag , where the peak at m/z 1986.1 has the exact mass for the polystyrene 16-mer with one butyl and a silver-complexed epoxide 157 + end group, C4H9-(C8H8)16-SiC6H14OC2H3·Ag ; the calculated monoisotopic mass is
+ {57.07 (C4H9) + 16 x 104.06 [(C8H8)16] + 157.1 [SiC6H14OC2H3] +106.905 (Ag )} =
1986.03 Daltons. The minor series of peaks is assigned to the unsaturated chain end,
+ C4H9-(C8H8)n-CH=CH(C6H5)·Ag , arising from elimination of chain end functionality during the analysis185-187 in which the peak at m/z 2036.1 has the exact mass for the polystyrene 17-mer with one butyl and a silver-complexed vinylbenzene end group,
. + C4H9-(C8H8)17-CH=CH(C6H5) Ag ; the calculated monoisotopic mass is {57.07 (C4H9) +
+ 17 x 104.06 [(C8H8)17] + 103.05 [CH=CH(C6H5)] + 106.905 (Ag )} = 2036.05 Daltons.
. + No unfunctionalized PS chains, C4H9-(C8H8)n-H Ag , was detected in this distribution.
In conclusion, the epoxide functionalization of polystyrene chain end was efficiently performed by Pt(0)-catalyzed hydrosilation of 1,2-epoxy-5-hexene with silyl hydride-functionalized polystyrene in benzene solution at room temperature even in the absence of protecting groups. NMR, FT-IR and mass spectral analyses of the hydrosilation product indicated the presence of the epoxide functionality at the polymer chain end.
158
Figure 3.62. MALDI-TOF mass spectrum for the epoxide-functionalized polystyrene.
3.5 Perfluoroalkyl functionalization of polystyrene by hydrosilation of 1H,1H,2H- perfluoro-1-octene with silane-functionalized polystyrene without protection
Introduction of fluorine functionality into polymers has been of great interest lately because fluorinated polymers have enormous applicabilities in both industrial and academic fields due to their characteristic low surface free energy124-126 and other unique
159 properties. Since hydrosilation has been found to be tolerant toward various fluorine functional groups,115 perfluoroalkyl end-functionalized polystyrene was successfully prepared by Pt-catalyzed hydrosilation of 1H,1H,2H-perfluoro-1-octene with silane- functionalized polystyrene in the absence of protecting groups (Eq 3.13).
CH3 F Si H F F F F CH3 5
Pt(0), C6H6 n-2 (Eq 3.13)
CH3 Si F CH F 3 F F F 5
n-2
For the hydrosilation, the silane-functionalized polystyrene (Mn = 2,200 g/mol), 2.9 molar equivalents of 1H,1H,2H-perfluoro-1-octene and 2 drops of Karstedt’s catalyst were reacted together in dry benzene for 24 hours under an argon atmosphere at room temperature. Analogous to previous epoxide functionalization, the characteristic yellow color appeared within a couple of minutes after addition of Pt(0) catalyst followed by continuous color changes to dark brown at the end of the reaction. The final perfluoroalkyl-functionalized polystyrene was isolated in 98 % yield after standard work- up.
This polymer presented a monomodal SEC curve indicating no dimer formation during the hydrosilation, and analysis of the SEC chromatogram resulted in a narrow molecular weight distribution (Mw/Mn = 1.04) along with the molecular weight (Mn = 2.5 160 3 3 × 10 g/mol) well consistent with the expected value (Mn, calc = 2.6 × 10 g/mol) as shown in Figure 3.63.
Figure 3.63. SEC chromatogram for the perfluoroalkyl-functionalized polystyrene.
1H NMR analysis for the perfluoroalkyl-functionalized polystyrene was not very informative; however, the lack of the characteristic silyl hydride proton resonance of dimethylsilyl functional group at δ 3.8 ppm implied an efficient functionalization of the chain end (Figure 3.64). No resonances corresponding to vinyl residues of 1H,1H,2H- perfluoro-1-octene were detected.
161
Figure 3.64. 1H NMR spectrum for the perfluoroalkyl-functionalized polystyrene.
Due to the abundance of fluorine atoms of this polymer, 19F NMR analysis was employed to confirm that the perfluoroalkyl functionality was at the chain end where the
19F NMR spectrum of the polymer was compared with starting material (1H,1H,2H- perfluoro-1-octene) as shown in Figure 3.65. In the resulting spectrum, the polymer presented a strong resonance at δ –81.6 ppm corresponding to trifluoromethyl group
[(CF2)5-CF3], as well as an additional five characteristic resonances at δ –117.2, –122.8, –
123.7, –124.2 and –127.0 ppm, respectively, arising from the difluoromethylene groups
[(CF2)5-CF3] at the chain end. It is notable that the allylic fluorine resonance of
1H,1H,2H-perfluoro-1-octene ( CH=CH-CF2-) at δ –114.6 ppm disappeared along with the appearance of a new resonance at δ –117.2 ppm corresponding to the difluoromethylene groups attached to the nonfluoromethylene unit ( CH2-CH2-CF2-) after hydrosilation indicating an effective functionalization of the polymer chain end. 162
Figure 3.65. 19F NMR spectrum for the perfluoroalkyl-functionalized polystyrene.
The efficient introduction of perfluoroalkyl functionality at the polymer chain ends was further evidenced by 13C NMR analysis in which the resulting spectrum (Figure
3.66) clearly displayed two distinct resonances corresponding to new methylene carbons of the chain end after hydrosilation at δ 3.5 ppm (Si-CH2-CH2-CF2-) and 25.7 ppm (Si-
CH2-CH2-CF2-), respectively, along with the characteristic resonance of silicon-bonded
177 methyl carbons at δ –5.0 ppm [Si(CH3)2]. In addition, several finely divided resonances spread between 110 and 125 ppm due to the carbon-flurorine coupling supported a successful perfluoroalkyl functionalization at the chain ends.
The FT-IR spectrum for the perfluoroalkyl-functionalized polystyrene presented many characteristic carbon-fluorine stretching absoption bands at 1119, 1144, 1209,
1240, and 1365 cm-1 (Figure 3.67).178 The absence of characteristic Si-H absorption band
163 at 2111 cm-1 confirmed the completion of hydrosilation indicating an efficient chain end- functionalization of the polymer.177,178
Figure 3.66. 13C NMR spectrum for the perfluoroalkyl-functionalized polystyrene.
Figure 3.67. FT-IR spectrum for the perfluoroalkyl-functionalized polystyrene.
164 MALDI-TOF MS analysis provided additional evidence to identify the perfluoroalkyl functional group at the chain end of the polymer where the silver trifluoroacetate/dithranol system was employed to obtain the optimal spectrum of this polymer. The resulting MS spectrum (Figure 3.68) exhibited a main series of peaks corresponding to the silver-cationated, perfluoroalkyl-functionalized polystyrene, C4H9-
+ (C8H8)n-SiC10H10F13·Ag , in which the peak at m/z 2234.1 has precisely
Figure 3.68. MALDI-TOF mass spectrum for the perfluoroalkyl-functionalized
polystyrene.
165 the expected mass for the polystyrene 16-mer with one butyl and a silver-complexed
+ perfluoroalkyl end group, C4H9-(C8H8)n-SiC10H10F13·Ag ; the calculated monoisotopic
+ mass is {57.07 (C4H9) + 16 x 104.06 [(C8H8)16] + 405.03 [SiC10H10F13] + 106.91 (Ag )}
= 2234.0 Daltons. Two additional series of peaks were observed in addition to the main series where interestingly, one of them is assigned to the dimethylfluorosilane chain end,
+ C4H9-(C8H8)n-SiC2H6F·Ag , and the other is the unsaturated chain end, C4H9-(C8H8)n-
+ CH=CH(C6H5)·Ag . The peak at m/z 2218.3 has the exact mass for the polystyrene 19- mer with one butyl and a silver-complexed dimethylfluorosilane end group, C4H9-
. + (C8H8)19-SiC2H6F Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 19 x 104.06
+ [(C8H8)19] + 77.02 [SiC2H6F] + 106.905 (Ag )} = 2218.14 Daltons. The other additional peak at m/z 2244.3 has the mass for the polystyrene 19-mer with one butyl and a silver-
. + complexed vinylbenzene end group, C4H9-(C8H8)19-CH=CH(C6H5) Ag ; the calculated monoisotopic mass is {57.07 (C4H9) + 19 x 104.06 [(C8H8)19] + 103.05 [CH=CH(C6H5)]
+ 106.905 (Ag+)} = 2244.17 Daltons. Those are assumed to arise from undesirable fragmentations of the chain end during MS analysis.185-187 No unfunctionalized PS
+ chains, C4H9-(C8H8)n-H.Ag , were detected in this distribution.
On the basis of various spectral analyses including NMR, FT-IR and MALDI-TOF
MS, it is concluded that the perfluoroalkyl-functionalized polystyrene was successfully prepared by Pt(0)-catalyzed hydrosilation of 1H,1H,2H-perfluoro-1-octene with silyl hydride-functionalized polystyrene in benzene solution at room temperature in the absence of any detectable side reactions.
166 3.6 Synthesis of POSS (Polyhedral Oligomeric Silsesquioxane) cored, 8-arm, star- branched polystyrene by hydrosilation of octavinyl-T8-silsesquioxane with silane functionalized polystyrene
The use of silyl hydride-functionalized polymers as precursors for various functionalizations provides another unique potential for further application to the syntheses of a variety of branched polymers. Along this line, the linking reaction of the silane-functionalized polystyrene with octavinyl-T8-silsesquioxane was investigated in order to demonstrate the applicability of this approach for the preparation of novel POSS
(Polyhedral Oligomeric Silsesquioxane) cored, 8-arm star-branched polystyrenes where analogous to end-functionalization, hydrosilation was employed as the linking reaction
(Eq 3.14).
PS Si Si PS CH3 Si O Si Si O Si O O PS Si H PS Si O O Si PS O O O O Si O Si CH Si O Si Si O Si 3 Si O Si OO OO PS Si OO O Pt(0), C6H6 O Si PS Si O Si Si O Si PS Si Si PS (Eq 3.14)
In general, many POSS containing polymer systems have been found to exhibit improved mechanical and thermal properties, and those materials have attracted great
199-202 interest lately. In this study, octavinyl-T8-silsesquioxane without a hydrocabon tether between the POSS core and the eight vinyl groups was chosen in order to avoid the undesirable Pt catalyzed isomerization of double bonds during hydrosilation.203
167 For the linking reaction, octavinyl-T8-silsesquioxane was hydrosilated with 10 molar equivalents of silyl hydride-functionalized polystyrene (Mn = 2,200 g/mol) in the presence of Karstedt’s catalyst under an argon atmosphere for three weeks at room temperature. A light yellow color appeared very slowly after the injection of the Pt(0) catalyst, and continuous color changes to light brown was observed during the course of hydrosilation where the intensity of the color was very weak compared to that of most end-functionalizations. The final 8-arm, star-branched polystyrene was isolated in 77 % yield after standard work-up followed by fractionation204 in a methanol/toluene mixture.
The successful isolation of the 8-arm, star-branched polymer from the crude mixture was confirmed by SEC analysis (Figure 3.69), and the resulting SEC chromatogram of the fractionated branched polystyrene indicated a narrow molecular weight distribution
4 (Mw/Mn = 1.04) as well as a molecular weight (Mn = 1.85 × 10 g/mol) well matching
4 with the expected value (Mn, calc = 1.83 × 10 g/mol). The Mn was determined using a triple detector system consisting of a differential refractometer, a differential viscometer and a laser light scattering detector. The triple detector combination eliminates the requirement of SEC column calibration and accurately provides an absolute molecular weight including the molecular size information. Accordingly, this system is especially useful to determine the absolute molecular weight of star-branched polymer. It should be noted that the presence of a distinct dimer species in the crude mixture most likely arises from hydrolysis of excess silane-functionalized polystyrene with adventitious water impurities in toluene/methanol mixture in the presence of residual active Pt(0) during the fractionation procedure to form Si-O-Si bonds as shown in Figure 3.69.144, 147, 205, 206
168
Figure 3.69. SEC chromatogram for the POSS-cored, 8-arm, star-branched polystyrene:
A, before fractionation; B, after fractionation.
The 1H NMR spectrum for the 8-arm, star-branched polystyrene (Figure 3.70) exhibited a broad proton resonance arising from two methylene units between two silicon atoms, Si(CH3)2CH2 -CH2Si(OSi)3, at δ 0.03 ppm along with the characteristic resonance
177,178 for silicon-bonded methyl protons, Si(CH3)2, near δ –0.37 ppm. The peak broadening is ascribed to steric hinderance between the methyl groups and POSS molecule.178 No resonances corresponding to silyl hydride residues were detected.
169
Figure 3.70. 1H NMR spectrum for the POSS-cored, 8-arm, star-branched polystyrene.
The 13C NMR spectrum for the novel POSS-cored, 8-arm, star-branched polystyrene (Figure 3.71) supported a successful linking reaction presenting two separate resonances corresponding to two characteristic methylene carbons, Si(CH3)2CH2 and
CH2Si(OSi)3 at δ 4.1 and 27.1 ppm, respectively, as well as the silicon-bonded methyl carbons, Si(CH3)2, at δ –5.5 ppm.
This polymer clearly exhibited a characteristic siloxane absorption band in the FT-
IR spectrum at 1120 cm-1 corresponding to asymmetric Si-O-Si stretching vibration mode
-1 of the POSS cage along with a (CH3)2Si deformation band at 1250 cm (Figure
3.72).177,178 The absence of a characteristic Si-H absorption band at 2111 cm-1 confirmed the successful isolation of the branched polymer by fractionation.
170
Figure 3.71. 13C NMR spectrum for the POSS-cored, 8-arm, star-branched polystyrene.
Figure 3.72. FT-IR spectrum for the POSS-cored, 8-arm, star-branched polystyrene.
Unlike the SEC analysis results indicating a narrow molecular weight distribution
4 (Mw/Mn = 1.04) and the expected molecular weight (Mn = 1.85× 10 g/mol), the MALDI-
TOF MS spectrum of this polymer displayed a set of peaks with a broad molecular weight distribution in the region of interest, and it appeared to be composed of three 171 different series of peaks corresponding to 6-, 7- and 8-arm, star-branched polymers, respectively, indicating incomplete linking as shown in Figure 3.73. The reason for this discrepancy is that the peak intensity of MALDI-TOF MS spectrum is strongly dependent of desorption efficiency of the analyte molecules, i.e. the analyte molecules are fractionated based on their volatility in this method, while the peak intensity of the SEC curve determined using the triple detector system is proportional to the concentration of the analyte polymers with same absolute molecular weight. In accordance with this fact, base arm polystyrene and dimer species were detected in the MS spectrum with very high intensity compared to the branched polystyrene with much higher molecular weight, which is attributed to the difference in desorption efficiency strongly depending on molecular weight.169
Figure 3.73. MALDI-TOF mass spectrum for the POSS-cored, star-branched polystyrene
after fractionation.
The thermal properties of this polymer were investigated by TGA and DSC analyses. As expected, the TGA result (Figure 3.74) indicated improved thermal stability 172 of the POSS-cored, star-branched polystyrene compared to that of a linear standard polystyrene with similar molecular weight, where the thermal decompostion temperatures at 5 % weight loss were 383 and 363 ºC for the branched polymer and standard polymer, respectively. While the linear standard polystyrene produced no residue, the branched polystyrene resulted in 5.7 wt % of residue after decomposition consistent with the residue from the inorganic POSS-core in the polymer. The amount of residue was higher than the expected value of 4.7 wt % based on the molecular weight of the star-
4 branched polymer (Mn, calc = 1.83 × 10 g/mol); the expected amount of residue is 857.8 g/mole (Octavinyl-T8-silsesquioxane, 633.04 g/mole + eight silicon atoms, 8 x 28.1 g/mole).
Figure 3.74. TGA analysis for the POSS-cored, star-branched polystyrene.
173
Figure 3.75. DSC curve for the POSS-cored, star-branched polystyrene.
The glass transition temperature (Tg) of this polymer was determined by DSC analysis (Figure 3.75). The observed Tg was 85 ºC which is the same as that of a 4-arm,
4 star-branched ordinary polystyrene with similar molecular weight (Mn, star = 1.67 × 10
3 g/mol, Mn, arm = 4.1 × 10 g/mol, Mw/Mn = 1.01) but lower than a linear standard
4 207 polystyrene (Tg = 99 ºC, Mn = 1.67 × 10 g/mol, Mw/Mn = 1.01).
In conclusion, SEC analysis for the POSS-cored, star-branched polystyrene resulted in a narrow molecular weight distribution (Mw/Mn = 1.04) as well as the expected
4 absolute molecular weight (Mn = 1.85 × 10 g/mol) indicating an efficient incorporation of POSS core to form an 8-arm, star-branched polymer. Although the MALDI-TOF MS result implies incompletion of the desired linking reaction displaying a broad molecular weight distribution, it does not accurately reflect the extent of efficiency of the linking reaction due to the extremely high dependency of this method on the volatility of the
174 analyte molecules as previously mentioned. Based on 1H NMR, 13C NMR and FTIR spectroscopic data, the formation of the novel POSS-cored, star-branched polystyrene was successfully proven after hydrosilation of octavinyl-T8-silsesquioxane with silyl hydride-functionalized polystyrenes. The resulting polymer presented improved thermal stability compared to linear standard polymer with similar molecular weight.
175 CHAPTER IV
SUMMARY
A new general chain end-functionalization methodology has been developed by combining alkyllithium-initiated, living anionic polymerizations with platinum-catalyzed hydrosilations (Figure 4.1). In this approach, a general two-step, anionic functionalization method has been described as shown in Figure 4.2. The first step involves the
Figure 4.1. The combination of living anionic polymerization with hydrosilation.
176 quantitative silyl hydride functionalization of well-defined, polymeric organolithium
compounds with dimethylchlorosilane in hydrocarbon solution at room temperature. In
the second step, a variety of functionalized alkenes are hydrosilated with the silyl
hydride-functionalized polymer using a platinum catalyst. The versatility of this
methodology arises from the fact that the well-defined, silyl hydride-functionalized
CH3 PLi+ Cl Si H
CH3
- LiCl
F2 CH3 C CF3 PhOH P Si H 5 F2 PhOH C CH3 P Si CF3 P Si 5 NH2 With Pt(0) O
L NH P Si 2 L= POSS Cage
O P P P Si P P L P P P P
Figure 4.2. Versatility of the new general chain-end functionalization methodology. polymer can be reacted with a wide variety of substituted alkenes and hydrosilation is tolerant to a variety of functional groups of interest, thus eliminating the need for protection of the functional groups and subsequent deprotection. In contrast, the direct anionic functionalizations by electrophilic termination normally require protecting group due to the high reactivity of carbanionic chain end toward most polar functional groups.
The utility and versatility of this procedure has been illustrated by effecting various end- functionalizations of poly(isoprenyl)lithium (Mn = 2,100 g/mol) as well as 177 poly(styryl)lithium (Mn = 2,200 g/mol) including primary amine-, phenol-, epoxide-, and perfluoroalkyl end-functionalization. As the precursor for further functionalizations, silyl hydride-functionalized polystyrenes and polyisoprenes have been successfully prepared by controlled termination of the living anionic chain ends with dimethylchlorosilane. A very fast reaction rate was deduced from the immediate formation of a precipitate as well as the disappearance of the characteristic color arising from polymeric anionic chain ends, red for poly(styryl)lithium and light yellow for poly(isoprenyl)lithium, after smashing the breakseal containing chlorodimethylsilane. The silane functionalization turned out to be quantitative and easy to monitor based on various spectral analyses including 1H, 13C and 29Si NMR spectroscopy, FT-IR spectroscopy and MALDI-TOF mass spectrometry. Protected amine-functionalized polystyrene was successfully prepared by hydrosilation of 3-[N,N–bis(trimethylsilyl)amino]-1-propene with silyl hydride-functionalized polystyrene in the presence of Pt(0) catalyst in benzene at room temperature. NMR, FT-IR and MALDI-TOF mass spectral analyses of the hydrosilation product clearly indicated the presence of the amine functionality at the chain end. Amine functionalization was also performed by hydrosilation of allylamine with silane- functionalized polystyrene even in the absence of protecting group due to the unique tolerance of hydrosilation toward the amine functionality. However, a reduced reaction rate was observed compared to the corresponding reaction with the silyl-protected amine, which is ascribed to the ligand coordination of the amine group to the active platinum center, thus competing with the vinyl groups. TLC analyses and end group titration confirmed quantitative amination, and various spectral analyses including 1H, 13C, 29Si and 13C DEPT NMR, FT-IR and MALDI-TOP MS further evidenced quantitative
178 functionalization. Similarly to the hydrosilation with the polystyrene analogue, ω-amine-
functionalized polyisoprene was synthesized by hydrosilation of both 3-[N,N–
bis(trimethylsilyl)amino]-1-propene and allyamine with silane-functionalized
polyisoprene. In spite of the possibility of self-induced inter- or intramolecular
hydrosilation, the Pt(0)-catalyzed hydrosilation reaction of allylamine derivatives with
silyl hydride-functionalized polyisoprene turned out to be an effective way to prepare
amine-functionalized polyisoprene based on the various NMR (1H, 13C and 1H-1H COSY) and FT-IR spectral analyses of the hydrosilation product. Surprisingly, no evidence for self-induced inter- or intramolecular hydrosilation reactions was found. Despite the high acidity of the phenolic proton, phenol-functionalized polystyrene was successfully prepared by Pt-catalyzed hydrosilation of 2-allyphenol with silane-functionalized polystyrene in the absence of protecting groups. TLC, NMR, FT-IR and MALDI-TOP mass spectral analyses of the polymer indicated efficient incorporation of phenol functionality at the chain end. It is very difficult to prepare epoxy-functionalized polymers using direct anionic functionalization due to the high reactivity of anionic chain end toward the epoxy group. However, using this new general functionalization methodology, epoxide functionalization of polystyrene chain end was efficiently performed by Pt(0)-catalyzed hydrosilation of 1,2-epoxy-5-hexene with silyl hydride- terminated polystyrene. NMR, FT-IR and MALDI-TOP mass spectral analyses of the resulting polymer indicated the presence of the epoxide functionality at the polymer chain end. In addition, perfluoroalkyl-functionalized polystyrene was also successfully prepared by Pt(0)-catalyzed hydrosilation of 1H,1H,2H-perfluoro-1-octene with silyl hydride-functionalized polystyrene. All results from the analyses (NMR, FT-IR and
179 MALDI-TOF MS) of the resulting end-functionalized polymers were consistent with functionalization in high yield. The applicability of this technique was extended to the synthesis of a novel POSS-cored, star-branched polymers by the hydrosilation of octavinyl-T8-silsesquioxane with silyl hydride-functionalized polystyrenes. The successful incorporation of the POSS core was evidenced by various spectroscopic data, and the resulting polymer exhibited improved thermal properties.
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