SYNTHESIS OF MIDDLE-CHAIN CARBOXYL- AND
PRIMARY AMINE-FUNCTIONALIZED POLYSTYRENES USING ANIONIC
POLYMERIZATION TECHNIQUES
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Mustafa Y. Sen
December, 2005 SYNTHESIS OF MIDDLE-CHAIN CARBOXYL- AND
PRIMARY AMINE-FUNCTIONALIZED POLYSTYRENES USING ANIONIC
POLYMERIZATION TECHNIQUES
Mustafa Y. Sen
Thesis
Approved: Accepted:
______Advisor Dean of the College Dr. Roderic P. Quirk Dr. Frank N. Kelley
______Faculty Reader Dean of the Graduate School Dr. William J. Brittain Dr. George R. Newkome
______Department Chair Date Dr. Mark D. Foster
ii
ABSTRACT
Middle-chain, carboxyl-functionalized polystyrene was synthesized by the
coupling of poly(styryl)lithium with a PDDPE-polystyrene macromonomer, which was
prepared by the reaction of poly(styryl)lithium with PDDPE [1,4-bis(1- phenylethenyl)benzene], followed by the carbonation of living chain ends using gaseous carbon dioxide. The carboxyl functionality of the isolated product was determined by simple acid-base titration. The number average molecular weights from stoichiometric calculation, size exclusion chromatography and titration showed good agreement. The effect of chain-end structure on carbonation reactions under different reaction conditions was also investigated. It was found that the resulting carboxyl-functionalized polystyrene
was free of side products, i.e. dimeric ketone and trimeric alcohol, regardless of reaction
conditions and that quantitative carboxylation of these 1,1-diphenylalkyllithiums could be
accomplished due to the high steric hindrance around the living chain end.
Middle-chain, primary amine-functionalized polystyrenes were synthesized by the
chemical modification of anionically prepared, middle-chain, alkyl chloride-
functionalized polystyrenes. The chloro functionality was introduced into the middle of a
polystyrene chain by the reaction of 2-(chloroethyl)methyl dichlorosilane with two moles
of poly(styryl)lithium. It was determined by 1H NMR spectroscopy and SEC that the
living poly(styryl) anions reacted chemoselectively at the electrophilic silicon atom. The
alkyl chloride group on the polystyrene chains was transformed into an alkyl azide by
iii nucleophilic substitution with azide ion, and then the azide was reduced to the primary amine group. The presence of primary amine group was confirmed by MALDI-TOF mass spectroscopy. The amine functionality of the polymer (99.5 %) was determined by acid-base titration using perchloric acid. The polymer exhibited a relatively narrow molecular weight distribution (Mw/Mn=1.06) by SEC analyis after derivatizing the amine groups using trichloroacetyl isocyanate.
iv
ACKNOWLEDGMENTS
I would like to thank Professor Roderic P. Quirk for his guidance and financial sponsorship which made this academic study possible. I also thank Haci B. Erdem for his helpful discussions and suggestions about my research.
Finally, I would like to thank my family, especially my parents, Isa and Emel Sen for their kindness and support.
v
TABLE OF CONTENTS
Page
LIST OF TABLES ...…………………………………………………………………….. x
LIST OF FIGURES …………………………………………………………………….. xi
CHAPTER
I. INTRODUCTION ………………………………………………….…………………. 1
II. HISTORICAL BACKGROUND……………………………………………………... 3
2.1 General Features of Anionic Polymerization ………………………………... 3
2.1.1 Initiators …………………………………………………………… 6
2.1.2 Monomers …………………………………………………………. 7
2.1.3 Solvents ……………………………………………………………. 8
2.2 Anionic Polymerization Kinetics …………………………………………... 10
2.2.1 Kinetics of initiation ……………………………………………... 10
2.2.2 Kinetics of propagation ………………………………………...... 11
2.3 Carbonation of Organolithium Compounds ………………………………... 12
2.3.1 Carboxylated Polymers Made by Carbonation …………………... 14
2.3.2 Some Applications of Carboxyl Functionalized Polymers ………. 22
2.4 Primary Amine Functionalized Polymers ………………………………….. 23
III. EXPERIMENTAL …………………………………………………………………. 37
3.1 Handling of Air Sensitive Materials ……………………………………...... 37
vi
3.1.1 High Vacuum Apparatus …………………………………………. 37
3.1.2 Inert Atmosphere Dry Box ……………………………………….. 38
3.2 Purification of Solvents, Monomers and Reagents ………………………… 39
3.2.1 Benzene …………………………………………………………... 39
3.2.2 Tetrahydrofuran (THF) …………………………………………... 39
3.2.3 Styrene …………………………………………………………… 40
3.2.4 sec-Butyllithium ………………………………………………….. 40
3.2.5 Methanol …………………………………………………………. 41
3.2.6 2-Chloroethylmethyldichlorosilane ……………………...... 41
3.3 High Purity CO2 ……………………………………………………………. 42
3.4 Trichloroacetyl isocyanate …………………………………………………. 42
3.5 Synthesis and Purification of 1,4-bis(1-phenylethenyl) benzene (PDDPE) ………………………………. 42
3.5.1 1,4-Dibenzoylbenzene …………………………………………… 42
3.5.2 Methyltriphenylphosphonium ylide ……………………………… 44
3.5.3. 1,4-bis(1-phenylethenyl) benzene (PDDPE) …………………..... 44
3.6 Anionic Polymerization of Styrene ………………………………………… 46
3.7 Preparation of middle-chain carboxyl-functionalized polystyrene…………. 47
3.7.1 Preparation of PDDPE-polystyrene macromonomer …………….. 47
3.7.2 Coupling reaction of poly(styryl)lithium with the PDDPE-polystyrene macromonomer ………………………… 48
3.7.3 Carbonation of the adduct of poly(styryl)lithium with PDDPE-polystyrene macromonomer ………………………. 48
3.8 Preparation of middle-chain primary amine-functionalized polystyrene ...... 49
vii 3.8.1 Synthesis of middle-chain alkyl chloride-functionalized polystyrene …………………………...... 49
3.8.2 Synthesis of middle-chain azide-functionalized polystyrene ……. 50
3.8.3 Synthesis of middle-chain primary amine-functionalized polystyrene ………………………. 50
3.9 Characterization ……………………………………………………………. 51
3.9.1 Size Exclusion Chromatography (SEC)…………………………... 51
3.9.2 UV-visible Spectroscopy ……………………………………...... 51
3.9.3 Nuclear Magnetic Resonance Spectroscopy (NMR) …………...... 51
3.9.4 Fourier Transform Infrared (FTIR) Spectroscopy …………...... 52
3.9.5 Thin Layer Chromatography (TLC) ……………………………... 52
3.9.6 Column Chromatography ………………………………………… 52
3.9.7 End Group Titration ……………………………………………… 53
3.9.7.1 Carboxylic acid group titration ………………………… 53
3.9.7.2 Amine group titration …………………………………... 54
3.9.8 Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) ………………………….. 55
IV. RESULTS AND DISCUSSION …………………………………………………… 56
4.1 Anionic synthesis of middle-chain carboxyl-functionalized polystyrene ……………………………………….. 56
4.1.1 Preparation of PDDPE-polystyrene macromonomer …………….. 56
4.1.2 Coupling reaction of poly(styryl)lithium with the PDDPE-polystyrene macromonomer ……………………...... 62
4.1.3 Carbonation of the adduct of poly(styryl)lithium with PDDPE-polystyrene macromonomer …………………………….. 64
4.2 Preparation of middle-chain, primary amine-functionalized polystyrene …..71
viii 4.2.1 Synthesis of middle-chain, alkyl chloride-functionalized polystyrene ……………………………… 71
4.2.2 Synthesis of middle-chain, azide-functionalized polystyrene ….... 75
4.2.3 Synthesis of middle-chain, primary amine-functionalized polystyrene ………………………. 76
V. SUMMARY ………………………………………………………………………… 85
REFERENCES …………..…………………………………………………………….. 88
APPENDIX …………………………………………………………………………….. 95
ix LIST OF TABLES
Table Page
1. The pKa values for the conjugate acids of carbanions formed from some common monomers in dimethylsulfoxide ……………………. 8
2. The number average molecular weights of carboxylated polymer determined by different methods…………………………………………………. 66
3. The number average molecular weights of middle-chain primary amine-functionalized polystyrene determined by different methods………….. 82
x LIST OF FIGURES
Figure Page
3.1 1H NMR spectrum of 1,4-Dibenzoylbenzene ………………………………….. 43
3.2 13C NMR spectrum of 1,4-Dibenzoylbenzene …………………………………. 44
3.3 1H NMR spectrum of 1,4-bis(1-phenylethenyl) benzene (PDDPE) …………… 45
3.4 13C NMR spectrum of 1,4-bis(1-phenylethenyl) benzene (PDDPE) …………... 46
3 4.1 UV-visible spectra of (A) poly(styryl)lithium (Mn=2.2x10 g/mol) and (B) its addition product with PDDPE in benzene in the presence of THF ([THF]/[Li]=20) ………………………………………. 59
4.2 1H NMR spectra of (A) base polystyrene and 3 (B) PDDPE-polystyrene macromonomer (Mn=2.4x10 g/mol) ………………... 60
4.3 13C NMR spectra of (A) base polystyrene and 3 (B) PDDPE-polystyrene macromonomer (Mn=2.4x10 g/mol) ………………... 61
4.4 SEC chromatogram of PDDPE-polystyrene 3 macromonomer (Mn=2.4x10 g/mol) …………………………………………... 62
3 4.5 UV-visible spectra of (A) poly(styryl)lithium (Mn=2.2x10 g/mol) and (B) the coupling reaction product of PDDPE-polystyrene 3 macromonomer (Mn=2.4x10 g/mol) with poly(styryl)lithium in benzene …..... 64
4.6 TLC of the crude carbonation product (eq.35) using toluene as the eluent ……. 65
4.7 SEC chromatograms of middle-chain carboxyl-functionalized 3 polystyrene (Mn=4.2x10 g/mol) before and after fractionation by column chromatography ……………………………………………………. 66
4.8 FTIR spectra of (A) base polystyrene and (B) middle-chain 3 carboxyl-functionalized polystyrene (Mn=4.2x10 g/mol) …………………….. 68
4.9 13C NMR spectrum of middle-chain carboxyl-functionalized 3 polystyrene (Mn=4.2x10 g/mol) ………………………………………………. 69 xi 4.10 SEC chromatograms of the carboxylated polystyrenes with unreacted macromonomers at different reaction conditions …………………… 71
3 4.11 SEC chromatograms of (A) base polystyrene (Mn=2.2x10 g/mol) and (B) middle-chain, alkyl chloride-functionalized 3 polystyrene (Mn=4.5x10 g/mol) ………………………………………………. 73
4.12 1H NMR spectrum of middle-chain, chloride-functionalized 3 polystyrene (Mn=4.5x10 g/mol) ………………………………………………. 74
4.13 1H NMR spectrum of middle-chain, azide-functionalized 3 polystyrene (Mn=4.5x10 g/mol) ………………………………………………. 76
4.14 FTIR spectra of middle-chain (A) chloride- and 3 (B) azide-functionalized polystyrenes (Mn=4.5x10 g/mol) …………………… 77
4.15 FTIR spectrum of middle-chain, primary amine-functionalized 3 polystyrene (Mn=4.5x10 g/mol) ………...…………………………………….. 78
4.16 TLC of (a) alkyl chloride-functionalized polystyrene, (b) alkyl azide-functionalized polystyrene, (c) primary amine-functionalized polystyrene; eluent:toluene ……………….... 78
4.17 1H NMR spectrum of middle-chain, primary amine-functionalized 3 polystyrene (Mn=4.5x10 g/mol) ………………………………………………. 80
4.18 1H NMR spectrum of middle-chain, primary amine-functionalized polystyrene after the reaction with 3 trichloroacetyl isocyanate (Mn=4.5x10 g/mol) ………………………………... 80
4.19 SEC chromatogram of middle-chain, primary amine-functionalized 3 polystyrene (Mn=4.5x10 g/mol) ………………………………………………. 81
4.20 SEC chromatogram of middle-chain, primary amine-functionalized polystyrene derivatized with 3 trichloroacetyl isocyanate (Mn=4.5x10 g/mol) ………………………………... 82
4.21 MALDI-TOF mass spectrum for middle-chain, primary amine-functionalized polystyrene with silver trifluoroacetate 3 as cationizing agent [Mn(calc.)=4.2x10 g/mol] ……………………………..… 84
xii CHAPTER I
INTRODUCTION
The carbonation of polymeric carbanions using carbon dioxide is one of the most
widely-used carbonation functionalization reactions. However, carbonation does not
proceed as a simple, quantitative reaction and the resulting polymer has been reported to
be contaminated with dimeric ketone and trimeric alcohol,1 the formation of which was favored by the aggregation of chain ends in hydrocarbon solutions.2 The addition of
Lewis bases such as tetrahydrofuran (THF) or N,N,N’,N’-tetramethylethylenediamine
(TMEDA) prior to carbonation resulted in carboxylated polymers with essentially quantitative yields.2 Further studies of the carboxylation of polymeric organolithium
compounds revealed that quantitative yields could be obtained by end-capping of
poly(styryl)lithium chain ends with 1,1-diphenylethylene.3 In this study, the effect of
chain end structure on carbonation reactions was reinvestigated by using a polymeric
organolithium species having the anionic chain end in the middle of the chain which was
obtained by the coupling of poly(styryl)lithium with a PDDPE-polystyrene
macromonomer containing a 1,1-diphenylethylene functionality.
The direct synthesis of polymers with primary amine groups by anionic
polymerization is a challenge because the acidic amine protons are prone to react with
1 organolithium compounds. Therefore, the amine group has to be protected to avoid chain transfer or termination reactions and it can be deprotected after the polymerization is complete.4 In addition to protection-deprotection strategy, some other alternative methods in which the chain end is modified via multi-step reactions have been employed to prepare primary amine-functionalized polymers.5,6 In this research, middle-chain,
primary amine-functionalized polystyrenes were synthesized by the chemical
modification of anionically prepared, middle-chain, alkyl chloride-functionalized
polystyrenes.
2
CHAPTER II
HISTORICAL BACKGROUND
2.1 General Features of Anionic Polymerization
A living polymerization is a chain polymerization that proceeds in the absence of
the kinetic steps of termination and chain transfer.4 Using this method, it is possible to
synthesize polymers with predictable molecular weights and narrow molecular weight
distributions. In addition to anionic polymerization, a variety of other living
polymerization chemistries have been developed including cationic7, radical8, ring- opening metathesis9, and Ziegler-Natta.10 Just because a polymer is synthesized with
controlled molecular weight and low molecular weight distribution (≤1.1) does not
necessarily mean that the polymerization system is living. Whether a system is living can
be determined by the application of various experimental criteria as delineated by Quirk
and Lee.11
In living polymerizations, due to the absence of spontaneous termination or chain-
transfer reactions, the number average molecular weight M n )( is a function of the
stoichiometry and degree of conversion of the reaction. At complete conversion
M n )( can be calculated as shown in eq. 1.
grams monomer M n = (1) moles initiator
3
In the case of difunctional initiating species the predicted molecular weight is twice as high per mole of initiator (eq. 2).
grams monomer M n = (2) 1 moles initiator 2
At the intermediate degrees of conversion, the “grams monomer” in Eq. 1 can be replaced by “g monomer consumed”. When the rate of initiation is equal to or greater than the rate of propagation, all the chains grow for essentially the same period of time and a polymer with a narrow molecular weight distribution is obtained. Equation 3 shows the relationship between polydispersity and degree of polymerization:
X X 1 w 1+= n 1+≅ (3) X X + )1( 2 X n n n
where Xw and Xn represent weight average degree of polymerization and number average
degree of polymerization respectively; the second approximation is valid for high
molecular weights. It can be concluded from this equation that as the molecular weight
increases, the molecular weight distribution decreases. Living polymerization
methodologies also provide control over other structural parameters such as
stereochemistry and microstructure, and because of the living nature of chain ends, it is
possible to synthesize:
4
a) chain-end functional polymers with a diverse array of functional end groups by
terminating the living polymers with electrophilic or nucleophilic reagents having
functional groups or by using functionalized initiators12
b) block copolymers by introducing different monomers sequentially into the
system13
c) star branched polymers by terminating the living chain ends with multifunctional
linking agents or by using multifunctional initiators14
Alkyllithium initiated anionic polymerization is a very useful living
polymerization method. The polymerization proceeds through a carbanionic center for
monomers that have the ability to stabilize the negative charge which develops in the
transition state (eq.4).
+ X X X - δ -δ RCH C (4) R + H2C C R CH2 C 2 Y Y Y
The highly nucleophilic initiating and propagating species can react with any reactive impurities (e.g.O2, H2O, CO2) present in the reaction medium. Therefore, the removal of impurities is necessary in anionic polymerization. This can be accomplished by the use of inert atmosphere or high vacuum line techniques and careful purification of reagents. However, high vacuum lines are preferred more as they provide better control over reaction conditions.15
5
The high reactivity of initiating and propagating species also limits the range of
solvents used in anionic polymerization as some solvents can participate in chain transfer
or termination reactions.16 Therefore, a careful selection of solvent, initiator and
monomer is necessary to avoid side reactions which result in loss of molecular weight
control and broad molecular weight distribution.
2.1.1 Initiators
In general, an appropriate initiator is an anionic species that has a reactivity
similar to the propagating carbanionic species.17 The most frequently used anionic initiators are alkyllithium compounds since many of them are commercially available.
They are soluble in a variety of solvents, especially hydrocarbons, because the carbon-
lithium bond exhibits properties characteristic of both ionic and covalent bonds.18 The
unique nature of C-Li bond is also responsible for the ability of only lithium among other
alkali metals to polymerize 1,3-dienes to high 1,4-microstructure polydienes.12,16,19
The rates of initiation of hydrocarbon monomers in hydrocarbon solvents using alkyl lithium initiators depend on the monomer, the solvent, the initiator and the temperature4. It has been reported that the reactivity of initiators is generally inversely
related to the degree of association of the alkyllithium compound.20,21 The relative
reactivities of alkyllithium initiators for styrene and diene polymerizations with the
average degree of association in hydrocarbon solutions indicated in parenthesis are
shown below22:
Styrene polymerization: menthlylithium (2) > sec-BuLi (4) > i-PrLi (4-6) > i-BuLi > n-
BuLi (6) > t-BuLi (4)
6
Diene Polymerization: menthyllithium (2) > sec-BuLi (4) > i-PrLi (4-6) > t-BuLi (4) > i-
BuLi > n-BuLi (6)
The ability of an initiator to start the polymerization of a monomer depends on the pKa values of the corresponding acids of the initiator and the propagating chain end. A
higher value of pKa corresponds to a less stable anion. Therefore, the initiation reaction
can occur if the pKa of the initiator is greater than that of monomer since the resulting
anion will have a higher stability.
Alkyllithium initiators, mainly n-butyllithium and sec-butyllithium, are primarily
used for the polymerization of styrene and dienes, but are too reactive for alkyl
methacrylates. Controlled anionic polymerization of this type of monomer can be
effected by employing sterically hindered initiators, such as 1,1-diphenylhexylithium.23,24
2.1.2 Monomers
There are three general types of monomers involved in anionic polymerization:
monomers based on the carbon-carbon double bond, heterocyclic monomers, and
monomers based on the carbon-heteroatom double (or triple) bond.16 Among these,
styrenic, dienic and cyclic monomers are the most studied monomers.
One of the requirements for a monomer to polymerize anionically is to have
substituents which stabilize the negative charge, that develops during the transition state
at the monomer addition step. Stabilization can occur by charge withdrawal or
delocalization. In addition, the monomer should not have relatively acidic or strongly
electrophilic groups because of the high nucleophilicity and basicity of the initiators and
propagating chain ends. If the monomer has these kinds of group, they must be protected
7
by conversion to a suitable derivative which can be deprotected after polymerization.25 In anionic polymerization, monomer reactivity is related to the stability of the anions formed by nucleophilic addition or ring opening. The stability of anions formed is deduced from the pKa values of their conjugate acids where pKa is the negative log of the
equilibrium constant for proton ionization of a carbon acid. Thus, monomers that form
the least stable anions (i.e., have the largest values of pKa for the corresponding conjugate
acids) are the least reactive monomers in anionic polymerization and therefore require the
4 most reactive organometallic initiators. Table 1 shows the pKa values of the conjugate
acids of anionic propagating intermediates for some common monomers.
Table 1 The pKa values for the conjugate acids of carbanions formed from some common monomers in dimethylsulfoxide. Monomer pKa of conjugate acid (DMSO)
Ethylene26 56 Dienes26 44 Styrene26 43 Alkyl 30-31 methacrylates27
2.1.3 Solvents
The solvents used for the anionic polymerization are rather limited because of the
high reactivity of initiating and propagating chain ends. The solvents used for styrene and
diene monomers are: alkanes and cycloalkanes, aromatic hydrocarbons and ethers.12,15,16
Many solvents can participate in chain transfer or termination reactions due to the high reactivity of the living chain ends. The importance of these reactions increases with
8
increasing temperature and in the presence of polar additives such as ethers or amines.28-
31 For example, sec-butyllithium is not stable in THF at room temperature (eq.5)
O O Li sec-C H Li + 4 9 C4H10 +
(5)
H
H2C CH2 +H2C C OLi
This side reaction can be minimized by working at lower temperatures. In
addition to organometallic initiators, ethers can also react with polystyryl or polydienyl
anions to terminate the chain growth.28,32 Although, these kinds of reactions are
encountered more in polar solvents, hydrocarbon solvents such as benzene28 and toluene33 can also participate in chain transfer and metalation reactions under certain
conditions. The rates of initiation and propagation are much faster in polar solvents than
in hydrocarbon solvents due to the much greater solvating ability of polar solvents
through complex formation between the solvent and the lithium counterion which tends
to loosen or ionize the carbon-lithium bond leading to lower energy requirements in the
transition state.28,34 As previously noted, lithium-based initiators in hydrocarbon media
have a unique ability to polymerize 1,3-dienes to high 1,4-microstructure polydienes.
These polymers exhibit low glass transition temperatures and thus good elastomeric 9
properties. However, the high 1,4-stereospecificity of lithium is lost in the presence of
polar additives (e.g. THF) and polymers with larger amounts of 1,2-(vinyl) and 4,3- linkages are obtained.15,35
2.2 Anionic Polymerization Kinetics
2.2.1 Kinetics of initiation
The initiation kinetics of styrene and diene monomers in hydrocarbon solvents
using alkyllithium initiators has been extensively studied.36 The initiation reaction of n-
butyllithium with styrene in benzene leads to the following relationship:
1/6 Ri α [n-BuLi] [S]
This relation implies that, since n-butyllithium is aggregated predominantly into hexamers in hydrocarbon solvents, the species that reacts with the styrene monomer is the
unassociated form of the initiator produced by the following equilibrium:
(n-BuLi) 6 6 n-BuLi
However, direct dissociation to unassociated species is not necessarily a correct
interpretation because the experimental measurements of the energy of activation for the
initiation reaction appears to be too low to include the energy of complete dissociation of
the aggregates.37,38 Therefore, it has been proposed that there is an incomplete or stepwise
dissociation of the aggregate which leads to aggregates with lower degrees of
association.39 In the case of aliphatic solvents, kinetic orders close to unity are observed suggesting direct reaction between the aggregated species and the monomer to form cross-associated species (eq.6).40 10
(RLi)n + M [(RLi)n-1(RMLi)] (6)
It was reported that aromatic solvents which tend to decrease the average degree of aggregation lead to initiation rates 102-103 times faster than aliphatic solvents.22 The
rate of initiation can also be increased by the addition of Lewis bases which promote the
dissociation of aggregates.22
2.2.2 Kinetics of propagation
The kinetics of propagation of styrene polymerization with lithium as counterion
has been studied and the following relationship has been found independent of the
identity of the hydrocarbon solvent (i.e. aromatic or aliphatic).41,42
1/2 Rp α [PSLi] [S]
The reaction kinetics exhibit a first order dependence on monomer concentration and one-half order in chain end concentration, [PSLi]. The observed one-half order dependence on chain end concentration is due to the association of living polymer chains into dimers in hydrocarbon solvents. The reactive entity for monomer addition is unassociated poly(styryl)lithium and the dissociation of dimers into unaggragated chain ends is required.
The propagation kinetics of dienes is complicated by the lack of agreement with
respect to the aggregation of dienyllithium chain ends in hydrocarbon solvents. Many
controversial results have been reported in the literature. For instance, Bywater and
Worsfold43 reported that poly(dienyl)lithium chain ends are aggregated into tetramers,
whereas Morton and Fetters44 claimed that the degree of association for
11
poly(dienyl)lithiums is two. Lewis bases increase the rate of propagation as they decrease
the average degree of association of polymeric organolithium aggregates.45,46 In addition,
the alkali metal alkoxides, other than lithium, accelerate the rate of propagation.47
Lithium alkoxides, as additives or as impurities, generally decrease the propagation rate, although the effects are not large.40,48,49
2.3 Carbonation of Organolithium Compounds
The synthesis of aromatic and aliphatic carboxylic acids using organolithium
reagents and carbon dioxide is a general method known for more than a half
century.28,50,51 This reaction is almost universal for organolithium compounds, and is
widely used to characterize them. However, alternative products may be obtained if the
conditions required for a high yield of acid are not provided. An excess of carbon dioxide
must be maintained throughout the reaction and this condition is commonly achieved by
adding an organolithium compound solution to crushed carbon dioxide or to a slurry of
carbon dioxide in a solvent.50,52 If this procedure is not followed, little acid may be
obtained. For example, when gaseous carbon dioxide is introduced into a warmed
solution of phenyllithium, the main product is benzophenone and the yield of benzoic
acid is only 4-6%.53
A high concentration of CO2 favors the formation of RCO2Li (I) and removes RLi from the system. Thus, the probability of side reactions depicted below is lowered (eq.
7).1,28
12
O OLi RLi RLi + CO2 R C OLi R C R
(I) OLi (II)
H2O - Li2O (7) OH O
- H2O R C R R C R (III) OH
RLi
OLi
H2O R3COH R C R (IV) R
The side reactions involve the addition of the organolithium compound to the carboxylate salt leading to a dilithio intermediate (II) which forms a ketone (III) on hydrolysis or by elimination of lithium oxide. The ketone can also react with more organolithium compound to yield a tertiary alkoxide (IV) which becomes a tertiary alcohol after hydrolysis.
Although ketones are undesirable side products in the synthesis of acids, the above reaction leading from the carboxylate to the ketone is a simple method for the synthesis of symmetrical and unsymmetrical ketones.54 However, the reaction has often
appeared to be unreliable and unpredictable since sometimes large amounts of tertiary
13
alcohol form as by-product. On the other hand, it is now established that when
precautions are taken to avoid the simultaneous presence of organolithium compound and
proton donor, yields over 90% are obtainable.55
The possible factors responsible for the formation of alcohol are either the presence of moisture in the system50 or the decomposition of the dilithio intermediate (II)
to lithium oxide and ketone.28 According to the latter assumption, the alcohol formation
is an intrinsic feature of the carboxylation of organolithium compounds regardless of the
presence of moisture. However, irreproducible experimental results were obtained with
respect to the amount of alcohol formed and thus more systematic studies supporting this
mechanism are needed.
2.3.1 Carboxylated Polymers Made by Carbonation
Alkyllithium-initiated anionic polymerizations proceed without the incursion of
spontaneous termination or chain-transfer reactions and thus they generate stable
polymeric carbanions which can be converted into a diverse array of functional groups.12
The carbonation of polymeric carbanions using carbon dioxide is one of the most useful and widely used functionalization reactions.12 The preparation of carboxylated polymers
by reacting living anionic polymers with carbon dioxide was first mentioned by Szwarc
and his coworkers56 in 1958. It was reported in that publication that the viscosities of
tetrahydrofuran solutions of poly(styryl)lithium did not change by addition of trace amounts of H2O but increased markedly on addition of CO2 due to the ionic association
of the chain ends. However, no explanations were given about the possibility of side
reactions. Although the reaction of poly(styryl)lithium with carbon dioxide might be
14
expected to proceed as a simple quantitative reaction, it has been reported by Wyman and
co-workers1 that the resulting carboxylated polymer was contaminated with significant
amounts of corresponding ketone (dimer) and tertiary alcohol (trimer) as shown in eq.8.
O O OH
C6H6 PSLi PS C OH ++PS C PS PS C PS (8) + CO2(g) H3O PS
It was observed that when gaseous carbon dioxide was allowed to diffuse into the system, the yields of dimeric ketone and trimeric alcohol were 28% and 12% respectively, and that when the poly(styryl)lithium solution was poured onto crushed, solid dry ice, the total yield of ketone and tertiary alcohol was only 22%. Therefore, the authors concluded that the amount of those side products was dependent on the rate at which carbon dioxide was added to the system. The ultracentrifugal sedimentation patterns were given as the evidence for product composition which showed three definite profiles indicative of three different molecular weight species, and the molecular weights were in approximate ratios of 1:2:3.
The tertiary alcohol formation was attributed to the presence of adventitious moisture contamination during the workup process as shown below because the dilithium acetal salt (I) was believed to be inert toward further reaction (eq.9).54
15
O OLi PSLi PSLi + CO2 PS C OLi PS C PS
OLi (9) (I)
O H O PSLi H O (PS) C(OLi) 2 2 2 2 PS C PS (PS)3COLi (PS)3COH (I)
The carbonation of poly(styryl)lithium was reexamined by Mansson57 using meticulously purified carbon dioxide and high vacuum techniques. The amount of tertiary alcohol was even higher (38%) than that reported by Wyman and coworkers1, though the reactions were carried out under more vigorous conditions.
In addition, Mansson57 reported yields higher than 90% for the carboxylic acid
when a hydrocarbon solution of poly(styryl)lithium was poured onto granulated, solid carbon dioxide (eq. 10). This confirms the earlier work done by Wyman1 which stated
that a high local concentration of carbon dioxide was needed to avoid side reactions.
O O OH H3C / THF + H3O PSLi + CO2(s) PS C OH ++PS C PS PS C PS (10) 99.5 / 0.5 PS
> 90 % trace trace
The author also claimed that the ability of THF to dissociate dimeric into
monomeric species had no dramatic influence on the yield of carboxylic acid depending
on the results shown below (eq. 11): 16
O O OH H3C / THF + H3O PSLi + CO2(g) PS C OH ++PS C PS PS C PS (11) 99.5 / 0.5 PS
(43%) (19%) (38%)
Using high vacuum techniques and high-purity, gaseous carbon dioxide, Quirk
and Chen58 reported that carbonation of poly(styryl)lithium, poly(isoprenyl)lithium and
poly(styrene-b-isoprenyl)lithium in benzene produced carboxylic acid in about 60% yield
and the corresponding ketone (dimer) in about 40% yield, and in contrast to the previous
work of Wyman1 and Mansson57 no tertiary alcohol formation was reported. This result
supported the early assumption that the formation of tertiary alcohol was due to the
existence of moisture in the reaction system54, since moisture was excluded from the
system under those reaction conditions.
However, continuation of these earlier studies58 by Quirk, Yin and Fetters2 showed that even under anhydrous conditions the tertiary alcohol was still produced. The previous, erroneous conclusions58 were caused by the inability of the SEC column set consisting only of three columns to detect the presence of the trimeric alcohol. The trimer peak was clearly observed as a distinct shoulder on the dimer peak when six chromatographic columns were employed in the SEC2. This finding reveals the fact that
the dilithium acetal salt (1) is not stable under the reaction conditions and that (1)
decomposes to form the ketone along with Li2O or Li2CO3 as shown in the equation
below (eq. 12):
17
OLi O
CO2 P C P P C P + Li2O (or Li2CO3) (12)
OLi
The yield of trimeric alcohol was also dependent on several factors such as the
rate of stirring, the chain end concentration and the pressure of carbondioxide. For
example, when the polymer solution was stirred during carbonation, the yield of carboxylated polymer was reduced from >90% to 60% even in the presence of 25 vol %
THF in benzene. Similarly, the decrease of CO2 pressure favored the formation of the
trimeric adduct. At comparable values of monomer concentration, the yield of trimeric
alcohol was 51% and 12% when poly(styrl)lithium chain ends were terminated with
gaseous CO2 at pressures of 130 mm Hg and 760 mm Hg respectively. A higher chain- end concentration also promoted the formation of side products.2
Quirk and Chen58 also reported that the carbonation reactions of
poly(styryl)lithium, poly(isoprenyl)lithium and poly(styrene-b-isoprenyl)lithium in a
75/25 (v/v) mixture of benzene and tetrahydrofuran occurred quantitatively to form the
corresponding carboxylic acid chain ends. In contrast to the deduction of Mansson57, they concluded that the dissociation of polymeric organolithium species using Lewis bases4 such as tetrahydrofuran (THF) and N,N,N,N’-tetramethylethylenediamine (TMEDA) favored the formation of the carboxylic acid functionalized chains. The association of organolithium chain ends would promote coupling to form the ketone (dimer) since the probability of geminate interaction between the carboxylated chain (I) and the resulting monomeric poly(styryl)lithium (II) is higher when the chain ends are in the associated
18
form (eq. 13). The conflicting observation by Mansson57 was attributed to the use of
methylcyclohexane which was a Theta solvent for polystyrene (60-700C)59, and to the amount of tetrahydrofuran which might not have been sufficient for complete dissociation.
OLi
(PSLi)2 + CO2 (PSCO2Li) (PSLi) PS C PS (13) (II) (I) OLi
The inefficient of carbonation of poly(isoprenyl)lithium and
poly(butadienyl)lithium in comparison to poly(styryl)lithium is another observation3
supporting the assumption that association of organolithium chain ends favors the
formation of dimer (ketone) and trimer (tertiary alcohol) as side products since it is
known that the dienyllithium chain ends are either more highly associated than
styryllithium chain ends, or their association constant is larger4. Consistent with the
higher extent or strength of association of poly(dienyl)lithium versus poly(styryl)lithium
chain ends,4 it was observed in the same study3 that to eliminate the effects of chain end
association on carboxylation side reactions much larger amounts of N,N,N,N’-
tetramethylethylenediamine (TMEDA) as a Lewis base additive were required for dienyllithium chain ends ([TMEDA]/[Li]=46) compared to styryllithium chain ends
([TMEDA]/[Li]=12).
19
The effects of chain end structure (stability and steric requirements) was also investigated by reacting poly(styryl)lithium with 1,1-diphenylethylene.3 When the
carbonation of the adduct of poly(styryl)lithium and 1,1-diphenylethylene was effected in benzene at room temperature, the carboxylated polymer was isolated in more than 99% yield compared to only 47% yield for the analogous poly(styryl)lithium without end-
capping under the same conditions (eq.14).3
C6H5 C6H5 C6H5 1)CO2 PSLi + CH2 C PSCH2CLi PSCH2CCO2H (14) + 2)H3O C6H5 C6H5 C6H5
≥ 99%
The average degree of association of the polymeric 1,1-diphenylalkyllithium chain ends has been reported to be two in hydrocarbon solutions.60 Therefore, it can be
deduced from the observation stated above (eq. 14) that the steric and electronic nature of
the chain ends are also important variables and that the reaction leading to dimeric and
trimeric side-products is quite sensitive to the steric requirements of the chain end.
The solid state carbonation is another method employed for the efficient
preparation of carboxyl-functionalized polymers. This technique can be used to carbonate
poly(styryl)lithium and other living polymers with glass transition temperatures
significantly above room temperature. For example, carbonation was 98% efficient for
freeze-dried poly(styryl)lithium in the absence of polar additives and virtually
quantitative yields were obtained when the freeze-dried poly(styryl)lithium was
20
complexed with 2 molar equivalents of N,N,N,N’-tetramethylethylenediamine
(TMEDA).2 These results were not expected since higher degrees of chain end
association and thus larger amounts of ketone were predicted in the solid state compared
to solution. One possible explanation for this unexpected behavior was the limited
flexibility of poly(styryl)lithium chain ends in the solid state which might depress the
ability of poly(styryl)lithium to react with lithium carboxylated polystyrene61. In order to
evaluate this assumption, the poly(styryl)lithium chain ends were converted into
poly(styrene-b-isoprenyl)lithium chain ends which were more flexible than those of poly(styryl)lithium and the carbonation reactions were carried out in the solid state. The amount of dimer and trimer formation increased as expected and the acid yield was decreased from 98% to 84%.61 Another explanation for the mechanism of solid state
carbonation might be the much higher local concentration of carbon dioxide around
poly(styryl)lithium chain ends in the solid state compared to solution.61 A high concentration of CO2 might favor the formation of lithium carboxylated polystyrene and
remove poly(styryl)lithium from the system. As a result, the formation of dilithio
intermediate might be reduced and thus the probability of side reactions would be
lowered.
Another possible mechanism for the high yield of acid in the solid state was
suggested by Szwarc.62 He proposed that the spontaneous and reversible dissociation-
association of the paired poly(styryl)lithium end groups might still persist below the glass
transition temperature of poly(styryl)lithium and that the relatively long lifetime of
unassociated poly(styryl)lithium chains in the solid state allowed for the addition of two
21
consecutively arriving CO2 molecules to the ends of the nonaggregated polymers prior to
their reapproachment.
2.3.2 Some Applications of Carboxyl Functionalized Polymers
The carboxylated polymers can be utilized in many ways. Although the range of
applications is quite broad, only some of these will be given herein due to the limited
space availability.
The carboxylated polymers can be employed for the preparation of ionomers. The
word “ionomer” is used to describe generally the ionic polymers consisting of a
hydrocarbon backbone and pendant carboxylic acid groups which are neutralized either
+ ++ ++ 63 partially or completely with metal ions such as Na , Ba and Zn . Ionomers are
usually copolymers containing both non-ionic repeat units and a small amount of ionic
repeat units which make up less than 10% of the polymer.64
The strong ionic forces developed between ionized carboxyl groups and free
metallic cations and their aggregates result in improved physical properties in comparison
to the corresponding base polymer. For instance, increases in moduli, glass transition
temperatures (Tg) and viscosities were observed for the ionomers prepared from ethylene- methacrylic acid copolymers63 and α,ω-carboxylpolybutadiene.65 In addition, ionomers
are truly thermoplastic materials since the ionic bonds are labile at elevated temperatures.
Other useful features of ionomers include transparency, insolubility in all conventional
solvents, unusual oil resistance, outstanding toughness and impact resistance. 66
Due to the presence of carboxyl functionality, the carboxylated polymers can be
grafted onto suitable polymers to modify the properties of the backbone polymer. For
22
example, when carboxylated polystyrene was grafted onto isobutylene-p-
bromomethylstyrene-p-methylstyrene copolymer and the resulting polymer was
compounded with tackifiers, it formed an adhesive that exhibited enhanced shear thinning
for spray application of the adhesive onto a substrate.67
Narayan et al.68 grafted poly(styryl)carboxylate anions onto a mesylated cellulose
acetate backbone. The highly reactive poly(styryl) carbanions were first converted into
less reactive poly(styryl)carboxylate anions, since the use of strongly basic poly(styryl)
carbanions required the protection of the cellulose hydroxyl groups, and then the
carboxylate anions was grafted onto the mesylated cellulose acetate by an SN2-type
displacement of the poly(styryl)carboxylate anions with mesylate groups. It was found that the reaction was complete within 20 h at 75ºC and that the grafting yields were
limited by the efficiency of the carboxylation of the polystyrene.
The curing of a liquid carboxy-telechelic polybutadiene with a polyaziridinyl
curing agent resulted in network structures having superior physical properties in
comparison to non-telechelic polybutadiene analogues since the carboxyl chain ends were
incorporated into the network structure in the case of telechelic polymer and this resulted
in fewer number of free chain ends, thus fewer imperfections.69
2.4 Primary Amine Functionalized Polymers
Primary amines have the ability of sharing or giving up protons and of interacting
with electron deficient compounds through their unshared electron pairs. Because of
these features, they can undergo a wide variety of chemical reactions including
condensation, addition, ring opening and quaternization types of chemistry.70 There are
23
many examples reported in the literature where these reactions have been used for the
applications of polymers with primary amine functionality.71-76
In anionic polymerization direct synthesis of primary amine functionalized polymers is a challenge because acidic amine protons undergo chain transfer and termination reactions with the carbanionic chain ends.77,78 Therefore, the amination of living ends require indirect methods such as the use of protecting groups.79
Higginson and Wooding77 were the first investigators who prepared amine terminated polystyrene using anionic polymerization. The study involved the initiation of
styrene polymerization by potassium amide in liquid ammonia and consequent generation
of terminal primary amine groups (eq. 15). The elemental analysis of nitrogen by the
Kjeldahl method implied that the product possessed one primary amine group per chain.
However, this technique has not found general use as a way of preparing primary amine
terminated polymers since it has many limitations such as low reaction temperatures (-
0 33 C), limited solubility of the polymer in the solvent (NH3), and chain transfer to
solvent.70
24
H
+ - NH3 (liq) - + K NH2 + H C CH H2N CH2 C K 2 -330C
H H
- + H - + H2N CH2 C K + n H2C CH H2N CH2 C CH2 C K (15) n
H H H H - + + - + CH C H H2N CH2 C CH2 C K + NH3 K NH2 H2N CH2 C 2 n n
Amine-terminated polymers were also prepared by employing some functional groups that reacted with the carbanionic chain end much more rapidly than an amine or
other nitrogen containing functionality. An early attempt to prepare amine-terminated
polymers without a protected amine functional group involved the reaction of
poly(styryl)sodium with ethyl p-aminobenzoate (eq.16).80
O O THF - + - + PS Na + EtO C NH2 PS C NH2 + EtO Na (16) -700C
25
The number average molecular weights of the polymers were calculated via the
absorption intensity of aromatic amine chromophore in ultraviolet spectrum, and the
weight average molecular weights were determined by light scattering. The results
indicated that polymers with a narrow molecular weight distribution and high degrees of
functionality were obtained and that there was no competition between the
functionalization reaction (eq.16) and proton transfer (from the amine group to the living chain end). However, it has been reported that esters were effective coupling agents for living anionic polymers81 and therefore dimeric species could also form as side products
(eq.17). Thus, a complete characterization of the resulting polymers is necessary to
confirm their structure.
O O- Na+ - + PS C NH2 + PS Na PS C NH2 (17)
PS
Beak and Kokko82 reported that the direct stoichiometric amination of simple
organolithium compounds could be achieved in high yields by methoxyamine and
methyllithium in hexane-ether (eq.18).
1) CH3ONH2 / CH3Li RLi RNH2 (18) 2) H2O
26
This procedure was applied to living polystyrene systems and a 92% yield of poly(styryl)amine was obtained by using twofold excess of the methoxyamine/methyllithium reagent at -78ºC in a mixture of THF/benzene/hexane.83 It
was proposed that the reaction proceeded via a nucleophilic displacement of the initially
formed alkoxyamide intermediate (A) (eq.19). The pure-amine functionalized polymers
were isolated using silica gel chromatography.
CH3Li - + PSLi CH3ONH2 CH3ONH Li PSNHLi + CH3OLi (19) THF,-780C (A)
An alternative route to prepare amine functionalized polymers is to employ
initiators with protected amine functionality. It was reported by Walton84 that N,N- bis(trimethylsilyl)amine groups were stable toward organometallic reagents and could be
readily hydrolyzed under mildly acidic conditions to recover the corresponding amines.
Based on these observations, Schulz and Halasa79 polymerized 1,3-dienes using a
protected primary amine-functionalized organolithium initiator, i.e. p-lithio-N,N-
bis(trimethylsilyl)aniline (eq.20) and then they converted the resulting α-N,N-
bis(trimethylsilyl)amine terminated polymers to primary-amine ended macromolecules
by simple acid hydrolysis.
(H3C)3Si N Li (20) (H3C)3Si
27
This organolithium initiator was soluble in diethyl ether but it was found to be insoluble in toluene. The solution of the initiator in diethyl ether and the dispersion of the
initiator in toluene were used to initiate the anionic polymerization of isoprene and
butadiene in hexane. Polymers with low polydispersity (1.06 to 1.25) but with 40 to 50%
vinyl content were obtained when polymerization was carried out homogenously in
diethylether/hexane mixtures. For heterogeneously initiated polymerizations without
polar media, polymers with lower (10%) vinyl content were obtained. However, the
control over molecular weight was lost since initiation was a heterogeneous reaction. The
close correspondence between theoretical and empirical molecular weights for polymers
prepared homogenously in diethyl ether/ hexane solvent proved that the protected amine
did not react with the carbanionic chain end. The presence of amine functionality in
hydrolyzed polymers was verified by infrared spectroscopy and end-group titration.
Titrimetric amine contents approaching the theoretical values were observed.
Dickenstein and Lillya85 reported that a protected primary amine initiator could be
generated by the reaction of sec-butyllithium with p-[bis-(trimethylsilyl)amino]styrene
(B) in benzene or cyclohexane at 25ºC (eq. 21).
Li Li C6H6 / C6H12 + [(H3C)3Si]2N (21) 250C
(B)
N[Si(CH3)3]2
28
The combination of gas chromatography and NMR analysis of the acetic acid-
quenched product indicated that there was no oligomer formation and thus the initiator
had a single amine functionality. Poly(dimethylsiloxane) with >95% amine functionality
was obtained using this initiator.
It is noteworthy that this initiator was soluble in hydrocarbon solvents unlike most
protected functional initiators. Therefore, it is potentially applicable to the preparation of
amine-functionalized polydienes with high 1,4-microstructure.
In contrast to the methods suggested by Schulz and Halasa79 and Dickenstein and
Lillya85 where an initiator with a protected primary amine was used, Hirao et al.86 terminated the living polymers with a protected imine. They reported that high yields (96-
100%) of primary amine-functionalized polymers could be obtained by the reaction of poly(styryl)lithium with 1.5-2.0 molar equivalents of the protected imine, i.e. N-
(benzylidene)trimethylsilylamine, in benzene at room temperature (eq.22). The analogous reaction of poly(isoprenyl)lithium in cyclohexane resulted in amine-functionalized products with 90% yield and low vinyl content (6.6%).
H H+ PLi + C N Si(CH3)3 PCH NH2 (22)
C6H5 C6H5
It is also important to note that although this protected imine was an efficient reagent for preparing primary amine functionalized polymers, its alkyl derivatives gave polymers with much lower amine functionality (30-44%). This was attributed to the
29
isomerizaton of the imine which eventually led to the termination of living chain ends
(eq.23).
R-CH2 R-CH H
CNSi(CH3)3 C N Si(CH3)3 (23)
C6H5 C6H5
Later attempts by Quirk and Summers87 to reproduce the results of Hirao et al.86 were not successful. The yield of aminated polystyrene was only 69% and the polymer was contaminated with dimeric products (19% yield) and acetophenone type functionalized polymer (12% yield).
The proposed mechanism for the formation of side products involved a
Cannizzaro-type reaction. The initially formed polymeric lithium silylamide (C) reacted with excess N-(benzylidene)trimethylsilylamine to form the corresponding polymer with imine chain-end functionality (D). The resulting polymer (D) reacted with another molecule of poly(styrly)lithium to form dimer product or hydrolyzed to the acetophenone functionality on work-up (eq.24).
30
Si(CH3)3
NLi H
PSLi + C NSi(CH3)3 PS C H C6H5
C6H5
Si(CH3)3
NLi H hydride PS C H C NSi(CH3)3 PS C NSi(CH ) + (H3C)3Si NLi transfer 3 3
C6H5 C6H5 C6H5 CH2 (C) (D) C6H5
PS Si(CH3)3 PS
PSLi H2O PS C NSi(CH ) 3 3 PS C NLi PS C NH2 acid
C6H5 C6H5 C6H5 (D)
(24) H O PS 2 C NSi(CH3)3 PS C O acid C H 6 5 C6H5
(D)
Therefore, in order to effect an efficient functionalization, the protected imine should not contain acidic alpha hydrogens (enolization-like side reactions)86, nor should it be a derivative of aldehydes (Cannizzaro-like side reactions).87 Following this rationale,
N-trimethylsilylimine derivative of benzophenone (E) was reacted with
poly(styryl)lithium in benzene at room temperature (eq.25).88 The functionality of the
resulting polymer determined by end group titration was 93%. The addition of THF (5 31
vol %) after polymerization increased the reaction rate and polymers with higher amine
functionalities (97%) were obtained. In addition, no dimer formation was observed in this
reaction as expected.
C6H5 C6H5 C6H5 Li H O+ PSLi + NSi(CH ) 3 3 3 PS C NSi(CH3)3 PS C NH2 (25) C6H5 C6H5 C6H5 (E)
Ueda et al.89 described the synthesis of primary-amine functionalized polymers by
terminating the anionic living polystyrenes and polyisoprenes with α-halo-ω-
aminoalkanes with protected amino functionality (eq.26).
H3C H3C CH3 CH3 Si Si CH3OH PLi + X (CH2)2 N P (CH2)2 N P(CH2)2NH2 (26) Si Si H3C H3C CH3 CH3
The typical procedure involved the reaction with excess reagent at -78ºC in the presence of THF, followed by standing for 1 h at 25ºC. The products were characterized
by end-group titration, TLC, TLC with flame ionization detection, vapor phase
osmometry (VPO) and SEC. Interactions of the primary amine end group with the
stationary phase led to the broadening of molecular weight distribution. In order to
overcome this problem, the primary amine end group was converted to a benzoyl
derivative which showed no retention effect on chromatographic columns. 32
The analysis of the polymers with these characterization methods revealed that
polystyrenes and polyisoprenes with high degrees of amination (>95%), predictable
molecular weights and narrow molecular distributions could be obtained by using this
functionalization chemistry. These results seem surprising because one might expect that
halogen-metal exchange and dehydrohalogenation would compete with the desired
substitution reaction. In fact, evidence for Wurtz-coupling reactions leading to the
formation of dimer was observed for the α-iodo derivative of the terminating reagent. The
coupling mechanism simply involved a halogen-metal exchange reaction between the
terminating agent and a polymer carbanion, followed by the coupling of the iodide-
terminated polymer with another polymer carbanion.
DeSimone and coworkers90 reported another synthetic methodology for the
preparation of amine-terminated polymers. In this methodology instead of alkyl halides89, a more reactive chlorosilane derivative that contained protected functional groups was utilized to terminate living polymeric anions (eq.27).
CH 3 Si(CH ) 3 3 PS-NH2 PSLi 1) Cyclohexane PDMSLi + Cl Si (CH2)4 N PDMS-NH2 (27) 2 vol%THF PILi Si(CH3)3 RT PI-NH2 CH3 2) CH3OH
The functionalities of the amine-terminated polymers, i.e. polystyrene (PS), poldimethylsiloxane (PDMS) and polyisoprene (PI), obtained from this reaction were 87 to 94%. This was determined by TLC, end-group titration, SEC, multinuclear magnetic resonance spectroscopy as well as time-of-flight secondary ion mass spectrometry (TOF-
33
SIMS). The TOF-SIMS spectra showed the presence of unfunctionalized polymer, but
the relative amounts of protected, deprotected and unfunctionalized polymers were not
determined.
It has been shown that the addition of substituted 1,1-diphenylethylenes to either
poly(styryl)lithium or poly(dienyl)lithium proceeds via a simple and quantitative addition reaction resulting in the incorporation of a single functional group at the chain end.91,92
Quirk and Lynch93 utilized this chemistry to prepare primary amine functionalized
polystyrenes. The reaction involved termination of living polystyrenes with 1-[4-[N,N-
bis(trimethylsilyl)amino]phenyl]-1-phenylethylene in benzene at room temperature
followed by acid-catalyzed hydrolysis and neutralization as shown in eq. 28.
1) CH3OH Benzene PS-H C H PSLi + CH2 C PS-H2C C Li 2 C (28) 2) 1% HCl / THF + - 3) Bu4N OH
N[Si(CH3)3]2 N[Si(CH3)3]2 NH2
The functionality of the primary amine terminated polystyrene was 97% as
determined by titration and a small amount (0.5%) of the unfunctionalized polymer was
isolated by column chromatography.
In addition to the above stated approaches where molecules containing protected
primary amines were used as either initiator or terminating agent, some alternative methods to prepare primary-amine functionalized polymers have also been reported in the literature. For example Fallais et al.5 described a multistep method for the synthesis of 34
aliphatic primary amine terminated polystyrene (eq.29). This method involved end- capping of poly(styryl)lithium by a hydroxyl group, followed by tosylation of the
hydroxyl end group and reaction with sodium azide. The azide terminal group was finally
reduced into primary amine. Polystyrenes with a high content of amine groups (98%)
were obtained as determined by end-group titration using perchloric acid.
Characterization by TOF-SIMS, FTIR and 1H NMR spectroscopy was consistent with the
presence of amine functionality.
CH OH / HCl - + O THF - + 3 PS Li + PS C2H4 O Li PS C2H4 OH -780C
O O CH2Cl2 PS C H OH Cl S CH PS C H O S CH 2 4 + 3 pyridine 2 4 3 O O (29) O NaN PS C H O S CH 3 PS C H N 2 4 3 DMF 2 4 3 O
LiAlH4 PS C2H4 N3 PS C2H4 NH2 THF
The aminated polymers were treated with trichloroisocyanate, and the SEC chromatograms of resulting urea derivatives exhibited molecular weight distributions comparable to the hydroxyl-functionalized polystyrene precursors. However, it must be pointed out that some of the samples showed a low percentage of coupling. Although the origin of this side reaction was unclear, the authors suggested that it stemmed from the reduction step of ω-azido-polystyrene into ω-amino-polystyrene since a higher percentage of coupling was observed when the THF used was not freshly distilled.
35
Macosko et al.6 described a similar procedure for the synthesis of amine-
terminated polystyrenes. The procedure consisted of termination of living polystyrene
anions with an excess (2 equiv) of 4-chlorobutyldimethylchlorosilane, substitution of the pendant alkyl chloride by azide and reduction of the azide to the primary amine (eq.30).
PSLi + Si diethyl ether Si Cl Cl PS Cl
NaN , TBAI Si 3 Si PS Cl DMF PS N3 (30)
LiAlH Si 4 Si PS N Ether / Toluene 3 PS NH2
Chloro(chloroalkyl)silanes were shown to be regioselective end-capping reagents
that allowed subsequent transformation to the primary amine groups.94 Thus, the
poly(styryl) anions reacted exclusively at the more electrophilic silicon center of the
terminating agent to give the alkyl chloride functional polymer. The amine functionalities
of 87-99% were observed as determined by 1H NMR spectroscopy and size-exclusion
chromatography techniques.
36
CHAPTER III
EXPERIMENTAL
3.1 Handling of Air Sensitive Materials
3.1.1 High Vacuum Apparatus
The removal of contaminants (e.g. O2, H2O, CO2) and volatile impurities from the
glassware, solvents, monomers and reagents was necessary to avoid unwanted
termination and chain transfer reactions. This was accomplished by using a high vacuum line.
The vacuum line consisted of primary and secondary manifolds made of Pyrex® glass tubing. These two manifolds were separated by teflon Rotaflo® stopcocks. A mechanical
vacuum pump (Edwards RV18) and a silicone oil (Dow Corning 704 diffusion pump oil)
diffusion pump (Chemglass) were connected in series to the primary manifold to achieve
a high vacuum. A liquid nitrogen trap (Chemglass) was placed between the primary
manifold and the diffusion pump to protect the pump oils from contamination. The
secondary manifold was equipped with four 24/40 ground glass joints and two grease traps. Each arm was also separated by a teflon Rotaflo® stopcock. Flasks containing
monomers and solvents were attached to the vacuum line via the ground glass joints. The
polymerization reactors were sealed directly to the line through the grease traps by glass-
blowing. One teflon Rotaflo® stopcock attached to the secondary manifold was utilized to introduce nitrogen gas (4.8 grade, Praxair) into the vacuum line. The pressure of the
37
nitrogen gas in the vacuum line was controlled using a U-tube bubbler. To check the
quality of the vacuum, a tesla coil was brought close to the glass. Lack of noise indicated
that there was high vacuum (~10-6 mmHg).95 The tesla coil was also used to detect
pinholes on the vacuum line and the glassware attached to the vacuum line. Morton and
Fetters15 described detailed procedures for the use of high vacuum line and anionic
polymerization techniques.
3.1.2 Inert Atmosphere Dry Box
Some reactions were done in an inert atmosphere dry box (Vacuum Atmospheres
Model HE-493). It was equipped with two drying columns and a circulation pump. The
drying columns were filled with R5 copper catalyst (Schweizer Hall) and a mixture of
13X and 4 Å molecular sieves. The regeneration of the columns was effected by heating
at 3000C for 4 hours, followed by purging with 5% hydrogen in nitrogen (Praxair) to
remove the resulting water and to reduce the catalyst. The glassware and chemicals were
placed in an antechamber which was pumped down using a Welch vacuum pump. After
the antechamber was evacuated, it was refilled with argon (technical grade Argon,
Praxair). This procedure was repeated two more times before transferring the glassware
and chemicals from the antechamber to the drybox. A green indicator solution that was
made using the method of Sekutovski and Stucky96 was utilized to test the oxygen level of the drybox atmosphere. A color change from green to orange indicated that the oxygen
level was > 5ppm. The moisture level was checked qualitatively by cutting a piece of
sodium metal and observing its luster.
38
3.2 Purification of Solvents, Monomers and Reagents
3.2.1 Benzene
Benzene (Reagent Grade, Fisher Scientific) was first stirred over concentrated
sulfuric acid for several days. The organic layer was separated from the acid and
neutralized with a saturated sodium bicarbonate (Fisher Scientific) solution and then
washed with distilled water. The washed benzene was then dried over anhydrous
magnesium sulfate (Aldrich) overnight, and filtered gravitationally into a flask containing freshly ground calcium hydride (Aldrich). The flask was attached to the vacuum line and stirred with intermittent freezing and degassing. The solvent was then transferred to an evacuated flask containing sodium dispersion (Aldrich, 50 wt % in paraffin) via vacuum distillation. After occasional freezing and degassing, the benzene was finally distilled into a storage flask containing oligomeric poly(styryl)lithium which was generated through the reaction of sec-butyllithium with styrene monomer. The characteristic orange color of poly(styryl)lithium served as an indicator for the purity of benzene. The desired amount of benzene was distilled directly from the storage flask to reactors or ampoules when needed.
3.2.2 Tetrahydrofuran (THF)
THF (Fisher, Certified Grade) was dried over calcium hydride (Aldrich) with
intermittent degassing on the vacuum line. The solvent was then distilled onto successive
sodium mirrors until no degradation of the mirror was observed. Tetrahydrofuran was
then distilled into a storage flask containing sodium dispersion (Aldrich, 50 wt % in
39
paraffin) and benzophenone (Fisher). The dark blue color of the resulting moisture-
sensitive ketyl radical anion was an indication of the dryness of the solvent.
3.2.3 Styrene
Styrene (99%, Aldrich) was first stirred over freshly ground calcium hydride for
several days in the vacuum line with periodic degassing. It was then vacuum-distilled into
a storage flask containing a small amount of neat dibutylmagnesium (FMC, Lithium
Division). After stirring the solution for several hours at room temperature a yellowish green color developed. It was then stored in the refrigerator until needed. The desired amount of styrene was distilled into calibrated ampoules with breakseals and sealed off from the vacuum line using a hand torch.
3.2.4 sec-Butyllithium
Solutions of sec-butyllithium in cyclohexane (12 wt %) were obtained from FMC,
Lithium Division. The concentration of carbon-bound lithium was determined using
Gilman double titration method.97 Total base was determined by placing 2 mL of sec- butyllithium into an Erlenmeyer flask containing 20 mL of dry benzene and the flask was stoppered with a rubber septum in the drybox. After removal of this solution from the drybox, 20 mL of water was added to react with the organolithium compound. The resulting solution was titrated with 1.0 N standardized aqueous hydrochloric acid (Fisher) using phenolphthalein as indicator. In the dry box, free base (non-carbon-bound lithium) was determined by placing 4 mL of sec-butyllithium into an Erlenmeyer flask with a septum containing 20 mL of hydrocarbon solvent and 4 mL (excess) of allyl bromide
40
(Aldrich). The allyl bromide reacted with carbon-bound lithium. After removal from the
drybox water (20mL) and phenolphthalein were added and the contents were titrated using 0.1 N standardized aqueous hydrochloric acid (Fisher). The difference between total base and free base concentration gave the concentration of the active initiator
(carbon-bound lithium).
3.2.5 Methanol
Methanol (Fisher, Reagent Grade) was used as received for the precipitation of
polymers. The methanol used for termination of polymerizations was placed in a round bottom flask with a Rotaflo® stopcock, degassed several times on the vacuum line and
distilled into ampoules containing breakseals. The ampoules were sealed off from the line
using a hand torch. These ampoules were stored at room temperature and attached to
polymerization reactors when needed.
3.2.6 2-Chloroethylmethyldichlorosilane
2-Chloroethylmethyldichlorosilane (Aldrich) was purified by stirring over
calcium hydride with intermittent degassing. The contents were then fractionally distilled
into a storage flask with a Rotaflo® stopcock. Only the middle fraction was collected in
the storage flask. The desired amount of the silyl halide was vacuum distilled into
calibrated ampoules, followed by flame-sealing.
41
3.3 High Purity CO2
Carbon dioxide (Praxair, Research Grade, 99.998%) was used as supplied. The
carbon dioxide cylinder was connected directly to the vacuum line via Cajon Ultra-Torr
fitting and flexible stainless steel tubing.98 In carbonation reactions, the system between
the breakseal of the reactor and the carbon dioxide cylinder was evacuated and then
refilled with carbon dioxide several times to remove the residual air. The carbon dioxide
pressure was monitored by a mercury manometer attached to the vacuum line.
3.4 Trichloroacetyl isocyanate
Trichloroacetyl isocyanate (Aldrich) was used as received. Two drops of
trichloroacetyl isocyanate was added to CDCl3 and THF solutions of middle-chain, primary amine-functionalized polystyrene for 1H NMR and SEC analysis, respectively.
The isocyanate reacted instantaneously with the amine group resulting in formation of the corresponding urea derivative.
3.5 Synthesis and Purification of 1,4-bis(1-phenylethenyl) benzene (PDDPE)
1,4-bis(1-phenylethenyl) benzene (PDDPE) was synthesized by the Friedel-Crafts
reaction of terephthaloyl chloride with benzene followed by conversion of the resulting
diketone to diolefin by Wittig reaction.
3.5.1 1,4-Dibenzoylbenzene
Under an inert atmosphere, aluminum trichloride (Aldrich, 15.0 g, 0.112 mol) was
placed in a three-necked round-bottom flask having a condenser, a drying tube and a
42
magnetic stirrer. Benzene (70 mL) was added to the reactor. Terephthaloyl chloride
(Aldrich, 10.0 g, 0.049 mol) was dissolved in a mixture of 70 mL of benzene and 70 mL
of methylene chloride. The resulting terephthaloyl chloride solution was slowly added to
the reactor which had aluminum chloride. After complete addition of terephthaloyl
chloride, the solution turned yellow within one hour. This mixture was stirred overnight
at room temperature and then quenched by pouring onto a mixture of ice-water and
concentrated HCl mixture. Using a separatory funnel the organic layer was isolated and
the aqueous phase was extracted with methylene chloride. The combined organic layers
were washed three times with sodium hydroxide solution and then water. The methylene
chloride solution was dried over anhydrous magnesium sulfate, filtered and the solvent
was removed using a rotary evaporator. The crude product was recrystallized from
benzene and the crystals had a melting point range of 159-162°C (lit.99 mp 159-160°C)
1H and 13C NMR spectra of the pure 1,4-Dibenzoylbenzene are shown in Figure 3.1 and
Figure 3.2 respectively.
Figure 3.1 1H NMR spectrum of 1,4-Dibenzoylbenzene.
43
Figure 3.2 13C NMR spectrum of 1,4-Dibenzoylbenzene.
3.5.2 Methyltriphenylphosphonium ylide
Methyltriphenylphosphonium iodide (Aldrich, 18.19 g, 0.045 mol) was placed in
a round-bottom flask with a Rotoflo® stopcock and the flask was connected to the vacuum line. Dry tetrahydrofuran (100 mL) was distilled into the flask. While purging with argon, methyllithium (Aldrich, 1.6 M in ether, 25.0 mL, 0.04 mol) was slowly transferred into the reactor using a syringe. After the addition of methyl lithium, an orange color appeared which indicated the formation of ylide.100
3.5.3. 1,4-bis(1-phenylethenyl) benzene (PDDPE)
1,4-Dibenzoylbenzene (5.0 g, 0.017 mol) was placed in a round-bottom flask
having a a Rotoflo® stopcock. The flask was attached to the vacuum line and degassed.
Dry tetrahydrofuran (60 mL) was distilled into the flask which was removed from the line after purging with argon. 1,4-Dibenzoylbenzene did not completely dissolve in THF at room temperature, so the solution was heated to 50°C. While it was hot, this solution was
transferred slowly by the cannula transfer technique into the reactor which had the ylide
solution. The reactor was kept in an ice-water bath during the addition process. The 44
mixture was stirred overnight at room temperature and then quenched by slow addition of
distilled water. The contents were extracted with diethyl ether and the aqueous layer was
washed two times with diethyl ether. The combined organic layers were dried over anhydrous magnesium sulfate. The solution was filtered and ether was removed using a rotary evaporator. The crude product was dissolved in dichloromethane and purified by column chromatography using a mixture of hexane and dichloromethane (5/1, vol/vol) as the eluent and activated silica gel (EM science, 220-400 mesh) as the support. The chromatographic process was monitored by thin layer chromatography using silica gel coated flexible plates (silica gel 60, Selecto Scientific) with fluorescent indicator. The sample having only one spot as visualized in short-wave, UV light was collected and the solvent mixture was removed under reduced pressure. Recrystallization from a toluene/ethanol (1/1, vol/vol) mixed solvent system resulted in white crystals with mp
136.4-137.1°C [lit.101 mp 137°C]. The structure was confirmed by 1H and 13C NMR spectral analyses; the characteristic resonance peaks for vinyl protons appeared at δ = 5.5 ppm in 1H NMR spectrum (Figure 3.3), and the vinyl carbons appeared at δ = 114.5 ppm
in 13C NMR spectrum (Figure 3.4).
Figure 3.3 1H NMR spectrum of 1,4-bis(1-phenylethenyl) benzene (PDDPE). 45
Figure 3.4 13C NMR spectrum of 1,4-bis(1-phenylethenyl) benzene (PDDPE).
3.6 Anionic Polymerization of Styrene
The polymerization of styrene to generate “living” poly(styrly)lithium was
conducted in Pyrex® glass sealed reactors with breakseals under high vacuum
conditions.15 The reactor consisted of a styrene ampoule, a methanol ampoule, aliquot
ampoules with methanol and a thick-walled side-arm tube. After closing the side-arm
tube with a rubber septum, the reactor was attached to the vacuum line using a hand torch
and evacuated until no noise was produced by a Tesla coil. Nitrogen gas (4.8 grade,
Praxair) was introduced into the reactor through the vacuum line. A small diameter tube
was inserted into the side arm in order to avoid the contamination of the side arm walls.
Under positive nitrogen pressure, the desired amount of sec-butyllithium was added
through this tube using a syringe which was previously flushed with nitrogen. After the
side arm was recapped by the rubber septum, the solvent of the initiator was evaporated.
The reactor was cooled in a Dry Ice/Isopropyl alcohol bath (-78ºC) and the side arm was
heat-sealed from the reactor. The appropriate amount of purified benzene was distilled
into the reactor. The amount of solvent distilled was sufficient to obtain a 10/1 (v/v) ratio
of solvent to monomer. The reactor was sealed off from the vacuum line and the frozen
solution was completely melted. The breakseal of the styrene ampoule was smashed by a glass-coated magnetic hammer and the yellowish red color of poly(styryl)lithium 46
developed immediately. The polymerization reaction was carried out overnight at room
temperature. After polymerization a small amount of living polymer was taken from the
reactor by sealing off the aliquot ampoule having degassed methanol. Then the living polymer was terminated by breaking the methanol ampoule. The polymer was precipitated into a four-fold excess of methanol, filtered and dried in a vacuum oven for characterization as the base polymer.
3.7 Preparation of middle-chain carboxyl-functionalized polystyrene
3.7.1 Preparation of PDDPE-polystyrene macromonomer
3 Poly(styryl)lithium (Mn=2.2x10 g/mol) was prepared in benzene at room
temperature using 11.9 mL of styrene monomer and sec-butyllithium (3.62 mL, 1.49M,
5.41x10-3 mole) as the initiator. This living polymer solution was split into two ampoules.
One was used to prepare the polystyrene macromonomer and the other was kept for
addition to the macromonomer after it was purified. To synthesize polystyrene
macromonomers having 1-phenylethenylbenzene groups at the chain end, the
poly(styryl)lithium (70 mL, 3.14x10-3 mol) solution was added into a solution of PDDPE
(0.974g, 3.45x10-3 mol) in benzene which contained THF (5.1 mL; 20x[PSLi]) as the
polar additive. The color of the solution immediately turned dark red after the addition of
the living polymer solution. The addition reaction was carried out at 6°C for 1 h with stirring and was monitored by UV-visible spectroscopy. After the reaction was completed the polymer solution was quenched with methanol and then precipitated into methanol.
The resulting macromonomer was dissolved in toluene and then precipitated three times
47
into methanol to remove excess PDDPE. The freeze-drying method was employed to
remove methanol, oxygen and volatile impurities from the macromonomer. The
polystyrene macromonomer was transferred into a round-bottom flask attached to the vacuum line and benzene was distilled into the flask. After stirring for one hour, the
solution was frozen using a dry ice/isopropyl alcohol bath and the frozen benzene was sublimed in the vacuum line. This procedure was repeated two times.
3.7.2 Coupling reaction of poly(styryl)lithium with the PDDPE-polystyrene macromonomer
The freeze-dried polystyrene macromonomer (4.94 g, 2.02x10-3 mol) was
dissolved in 10 mL of benzene and added to the solution of poly(styryl)lithium (50 mL,
1.94x10-3 mol). An excess of the macromonomer was used to ensure complete
consumption of living polymer chains. The reaction was monitored by UV-vis spectroscopy and the reactor was reattached to the vacuum line through a breakseal after
stirring the solution for 6 hr.
3.7.3 Carbonation of the adduct of poly(styryl)lithium with PDDPE-polystyrene
macromonomer
The carbon dioxide cylinder was connected directly to the line via Cajon Ultra-
Torr fitting and flexible stainless steel tubing.98 The system between the breakseal of the reactor and the carbon dioxide cylinder was evacuated and then refilled with carbon dioxide several times to remove the residual air. A standard U-type mercury manometer was used to monitor the carbon dioxide pressure. Before measurements were made, both
48
sides of the manometer were evacuated on the line. When monitoring the carbon dioxide
pressure, one side of the manometer was closed; thus, the difference between the top and bottom of the mercury levels gave the pressure in the system.
Prior to the carbonation reaction, 20 mL of THF was added to the living polymer
solution. Carbon dioxide was introduced into the reactor by smashing its breakseal. The
solution was not stirred during carbonation. The red “living” color of the solution turned colorless layer by layer within 4 hours. The polymeric carboxylate salt was then treated
with 10% hydrochloric acid-methanol mixture to obtain the carboxylic acid functionalized polymer. After stirring for several hours, the resulting polymer was precipitated into methanol, filtered and then dried in a vacuum oven. The excess macromonomer was separated from the carboxylated product by column chromatography with toluene as eluent and activated silica gel as support.
3.8 Preparation of middle-chain primary amine-functionalized polystyrene
3.8.1 Synthesis of middle-chain alkyl chloride functionalized polystyrene
sec-Butyllithium (3.65 mL, 1.49M, 5.45x10-3 mole) was used to initiate the
polymerization of 12.0 mL of styrene monomer in benzene. The polymerization was
allowed to proceed at room temperature overnight. After the polymerization, 2 mL of
living polymer was poured into an aliquot ampoule containing a methanol breakseal, and
the ampoule was removed from the reactor by heat-sealing while cooling its content with a Dryice/Isopropyl alcohol bath. The previously attached 2- chloroethylmethyldichlorosilane (Aldrich, 0.35 mL, 2.48x10-3 mole) ampoule was
49
opened by smashing its breakseal. The solution was stirred at room temperature until the
characteristic orange color of poly(styryl)lithium disappeared (~5 h). The resulting
polymer was precipitated in methanol, filtered and dried in a vacuum oven at room
temperature.
3.8.2 Synthesis of middle-chain azide-functionalized polystyrene
Under argon 20 mL of dimethylformamide (Aldrich) was placed in a round-
bottom flask. Sodium azide (Aldrich, 0.42g, 6.52x10-3 mol), tetrabutylammonium iodide
(Aldrich, 0.02g, 6.52x10-5 mol) and 3.0 g of chloride functionalized polystyrene
(6.52x10-4 mol), were added. The resulting slurry was stirred at 60°C for 12 h and
allowed to cool to room temperature. The polymer was precipitated twice into methanol
and dried under vacuum for 24 h.
3.8.3 Synthesis of middle-chain primary amine-functionalized polystyrene
The azide-functionalized polystyrene (2.0 g, 4.35x10-4 mol) was dissolved in 20
mL of dry THF, and 2.17 mL of lithium aluminum hydride solution (Aldrich, 1.0M in
THF, 2.17x10-3 mole) was added dropwise using a syringe while stirring the polymer solution vigorously. After 2 h of stirring at room temperature, 0.5 mL of water, 0.5 mL of
15% NaOH and 1.5 mL of water were added sequentially. The resulting slurry was filtered and the filtrate was poured into methanol to precipitate the polymer. The polymer was filtered and dried at room temperature under vacuum. The amine-functionalized polystyrene was isolated by column chromatography using toluene as the eluent.
50
3.9 Characterization
3.9.1 Size Exclusion Chromatography (SEC)
The molecular weights and molecular weight distributions of polymers were
determined using a Waters 150C-Plus system equipped with a Waters differential
refractometer, a Viscotek viscometry detector (Model 150R) and a four Phenomenex
Phenogel column set (500, 103, 104, 105Å). Samples placed in 5 mL volumetric flasks
were dissolved in THF and filtered through 0.45μm Teflon filters. The SEC
measurements were performed at 33°C at a flow rate of 0.6 mL/min using THF as the
mobile phase, after calibration with standard polystyrene samples obtained from Polymer
Laboratories, Ltd. The results were analyzed with universal calibration methods102 using
TriSEC SEC software (Viscotek Corp.)
3.9.2 UV-visible Spectroscopy
UV-vis spectra of living polymer solutions were obtained using a Hewlett
Packard 8452A diode-array spectrophotometer. A quartz UV-vis cell (1 mm thickness) was attached directly to the polymerization reactor and pure solvent was distilled into the cell for the background reference prior to obtaining data for a living polymer solution.
3.9.3 Nuclear Magnetic Resonance Spectroscopy (NMR)
1H and 13C NMR spectra were measured on a Varian Mercury-300 NMR spectrometer. Deuterated chloroform (Chemical Isotope Laboratories, 99.8% CDCl3) was
used as the solvent. The sample concentrations were 10-30 weight %. The undeuterated
51 chloroform peaks were used for internal reference (δ = 7.27 ppm for 1H NMR and δ =
77.23 ppm for 13C NMR).
3.9.4 Fourier Transform Infrared (FTIR) Spectroscopy
Fourier Transform Infrared (FTIR) spectra were recorded on a ATI Mattson
Genesis series FTIR spectrophotometer. A THF solution of the polymer was placed on a
KBr plate and the solvent was evaporated to obtain a thin film. The data were analyzed using WinFIRST® software.
3.9.5 Thin Layer Chromatography (TLC)
Thin layer chromatography was done on silica gel coated flexible plates (silica gel
60, Selecto Scientific) with fluorescent indicator using different eluents depending on the functional groups. A small amount of sample solution was placed on the plate and the plate was eluted with the appropriate solvent. The plate was then analyzed with a UV lamp (λ=254 nm)
3.9.6 Column Chromatography
Column chromatography was used for the purification of compounds. Silica gel
(EM science, 220-400 mesh) was activated by heating to 140°C for several hours. The silica gel was added to a beaker containing the appropriate solvent. The resulting slurry was then poured into the column which was half filled with the solvent. After all of the silica gel was added, the solvent was allowed to drain through the column until the solvent level was just above the surface of silica gel. The column was gently tapped
52
during this time in order to insure that there were no air bubbles in the silica gel. After the
column was uniformly packed, the crude product dissolved in a minimum amount of
solvent was carefully loaded onto the column using a pipette. The solution was allowed
to drain until the level reached the top of silica gel again. A small amount of sand was
added carefully to protect the top of the silica column. The solvent was allowed to run
through the column and the fractions were analyzed by TLC. The fractions containing the
pure product were combined and the solvent was removed using a rotary evaporator.
3.9.7 End Group Titration
3.9.7.1 Carboxylic acid group titration
0.4902 g of polymer dissolved in 30 mL of toluene was titrated with 0.0103 N
standardized KOH-methanol solution (Fisher) to the phenolphthalein end-point. The amount of KOH-methanol solution used for titration was 11.0 mL. The mole percent of carboxylic acid functionality in the polymer was calculated using the following equation
(eq. 31)
×× CVM X % = bbn (31) 10×Wa
where X % : mole percent carboxyl groups in the sample
Mn : number average molecular weight of the polymer (g/mol)
Vb : volume of KOH-methanol used in titration (mL)
Cb : concentration of KOH-methanol solution (mol/L)
Wa : weight of polymer sample (g)
53
3.9.7.2 Amine group titration
The amine functionality of the polymers was determined by titration with perchloric acid.103 Amine terminated polymer (0.4554 g) was dissolved in 40 mL of a
50/50 (vol/vol) chloroform/glacial acetic acid mixture and four drops of methyl violet indicator was added. This solution was then titrated with standardized 0.1035 N perchloric acid in glacial acetic acid (Fisher) to a blue end point. Two blank solutions were used as color references. One solution consisted of 4 drops of indicator solution in
40 mL of the chloroform/acetic acid mixture and the second solution was the same as the first one except that it contained two drops of the standardized perchloric acid solution.
The amount of standardized perchloric acid used for titration to the blue end point was
0.97 mL. The following equation was used to determine the mole percent amine groups in the polymer (eq.32):
×× CVM X % = bbn (32) 10×Wa where X % : mole percent amine groups in the sample
Mn : number average molecular weight of the polymer (g/mol)
Vb : volume of perchloric acid used in titration (mL)
Cb : concentration of perchloric acid solution solution (mol/L)
Wa : weight of polymer sample (g)
54
3.9.8 Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry
(MALDI-TOF MS)
MALDI-TOF mass spectra were acquired on a Bruker REFLEX-III time-of-flight
(TOF) mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with an LSI model
VSL-337ND pulsed nitrogen laser (337 nm, 3 nm pulse width), a two-stage gridless reflector and a single stage pulsed ion extraction source. Separate THF solutions of dithranol (20mg/mL), silver trifluoroacetate (10 mg/mL) and polymer (10 mg/mL) were mixed in the ratio of matrix:cationizing salt:polymer (10:1:2), and 0.5 µL of the mixture was placed onto the sample holder. The spectra were obtained in the reflection mode. The attenuation of the nitrogen laser was adjusted to minimize unwanted polymer fragmentation and to maximize the sensitivity. The calibration of mass scale was done externally using polystyrene standards having similar molecular weights to the sample.
55
CHAPTER IV
RESULTS AND DISCUSSION
4.1 Anionic synthesis of middle-chain, carboxyl-functionalized polystyrene
4.1.1 Preparation of PDDPE-polystyrene macromonomer
Divinylidene compounds not capable of anionic polymerization can be used as
difunctional initiators for anionic polymerization after the addition of 2 moles of sec-
butyllithium in hydrocarbon solution.104 For example, 1,3-bis(1-phenylethenyl)benzene
(MDDPE) has been used to prepare hetero, 4-arm star polymers via anionic
polymerization.105 This reaction involved the coupling of two living carbanionic polymer
chains with the living coupling agent (MDDPE) followed by a crossover reaction to a
second monomer to generate two new polymer chains thus forming a 4-arm star polymer.
Leiz and Höcker106 investigated the kinetics of the reaction of 1,3-bis(1- phenylethenyl)benzene (MDDPE) and 1,4-bis(1-phenylethenyl)benzene (PDDPE) with excess sec-butyllithium in toluene at 20°C. They reported that for MDDPE the rate constants of the first and second addition step of sec-butyllithium were 1.98x10-2 L/ mol s in toluene. For PDDPE, the rate constant for the first addition was 2.06x10-2 l/mol s and
that for the second addition was 1.51x10-3 mol/ L s. Broske and coworkers107 reported that the monoadduct of MDDPE with sec-butyllithium could be prepared in THF at -
78°C. Ma108 also obtained similar results for the corresponding reaction with
56 poly(styrly)lithium. Therefore, THF can be utilized to prepare macromonomers with a terminal 1,1-diphenylethylene functionality by the addition of one equivalent of polymeric organolithium with either MDDPE or PDDPE.
For the coupling of MDDPE with poly(styryl)lithium in benzene it was necessary
to use a mixture of THF and benzene at low temperature to get the highest functionality
of the macromonomer. However, PDDPE exhibits a high tendency to form the
monoadduct (macromonomer) in both hydrocarbon solutions and in the presence of THF
due to the one order magnitude difference in reaction rates for the first and second
addition reactions with living anionic species. This difference in reaction rates is a result
of the delocalization of negative charge in the monoadduct into the adjacent phenyl rings
and the unreacted vinyl group which causes a repulsion between the anionic charge
delocalized into the vinyl group and the incoming living polymer chain end making the
monoadduct less reactive towards the second addition.109 Similar delocalization into the
unreacted double bond is not expected for MDDPE (eq. 33)
PDDPE anion
(33)
MDDPE anion 57 Quirk and Yoo109 investigated the addition of poly(styryl)lithium with PDDPE
under a variety of experimental conditions to obtain the maximum yield of
macromonomer, and they found that the macromonomer with the highest functionality
(0.98) and a minimum amount of dimer formation (1.4%) was obtained at 5-8°C in benzene in the presence of THF ([THF]/[Li] = 20). THF was used as a polar additive to increase the rate of the coupling reaction since it favored the formation of solvent- separated ion pairs.
3 The addition of poly(styryl)lithium (Mn=2.2x10 g/mol) to an excess of PDDPE
([PDDPE]/[Li] =1.2) was carried out in benzene with small amount of THF ([THF]/[Li]
= 20) at 6°C (eq.34). The reaction was monitored by UV-visible spectroscopy. The
resulting UV-visible spectrum is shown in Figure 4.1. The absorption peak of
poly(styryl)lithium at λmax.=334 nm was replaced by the absorption peak of the
diphenylalkyl anion (λmax.=532 nm) within one hour.
PS
CH2 CH2 CH2 CH2 1) PS- Li+ C C C C C6H6 / THF H 2) CH3OH 1,4-bis(1-phenylethenyl)benzene (PDDPE) (34)
58
3 Figure 4.1 UV-visible spectra of (A) poly(styryl)lithium (Mn=2.2x10 g/mol) and (B) its addition product with PDDPE in benzene in the presence of THF ([THF]/[Li]=20).
The product was characterized by 1H and 13C NMR spectroscopy. The proton
NMR spectra of base polystyrene and that of the macromonomer were compared and the macromonomer exhibited characteristic resonance peaks for the vinyl protons and the terminal diphenyl alkyl methine proton at δ = 5.4 ppm and δ = 3.5 ppm respectively
(Figure 4.2). Similarly, the peaks at δ = 114.0 ppm (CH2=C(C6H5)2) and δ = 49.0 ppm
13 ((C6H5)2CH) in C NMR spectrum corresponded to the vinyl carbon and the diphenyl alkyl methine carbon respectively (Figure 4.3).110
59
Figure 4.2 1H NMR spectra of (A) base polystyrene and (B) PDDPE-polystyrene 3 macromonomer (Mn=2.4x10 g/mol).
60
Figure 4.3 13C NMR spectra of (A) base polystyrene and (B) PDDPE-polystyrene 3 macromonomer (Mn=2.4x10 g/mol).
61 The formation of dimeric product was not observed under these reaction
conditions as shown in the SEC chromatogram (Figure 4.4). The resulting non-
homopolymerizable macromonomer was used as the starting material for the synthesis of
middle-chain, carboxyl-functionalized polystyrene (eq.35).
3 Figure 4.4 SEC chromatogram of PDDPE-polystyrene macromonomer (Mn=2.4x10 g/mol).
4.1.2 Coupling reaction of poly(styryl)lithium with the PDDPE-polystyrene macromonomer
3 The macromonomer (Mn=2.4x10 g/mol) dissolved in benzene was added to a
3 benzene solution of poly(styryl)lithium (Mn=2.2x10 g/mol) under high vacuum
conditions. An excess of macromonomer ([PDDPE]/[Li]=1.1) was used to ensure the
complete consumption of poly(styryl)lithium, and the reaction was monitored with UV- 62 visible spectroscopy. The absorption peak of poly(styryl)lithium at λmax.=334 nm disappeared and a new peak formed at λmax.=460 nm (Figure 4.5). No residual peak for
poly(styryl)lithium was detected after 6 hours. It is known that the absorption peak for
the adduct of poly(styryl)lithium with 1,1-diphenylethylene [2-polystyryl-1,1-
111 diphenyl(ethyl)lithium] appears at 440 nm. The slight bathochromic shift in λmax to
460 nm might be due to an additional benzyl-type substituent present in the para position
to one of the aromatic rings where charge delocalization occurs. It could also be a result
of formation of a different degree of aggregation and/or ion pair structure because of the
different steric environment of the chromophore.
PS PS PS
CH2 CH2 CH2 CH2 PS- Li+ C C C C C6H6 H H Li 1) THF
2) CO2
3) 10% HCl / CH3OH PS PS
PS PS CH2 CH2
C C
H COOH COOH
(35)
63
3 Figure 4.5 UV-visible spectra of (A) poly(styryl)lithium (Mn=2.2x10 g/mol) and (B) the 3 coupling reaction product of PDDPE-polystyrene macromonomer (Mn=2.4x10 g/mol) with poly(styryl)lithium in benzene.
The coupling reaction of poly(styryl)lithium with the macromonomer (6h) was slower than the addition of poly(styryl)lithium with PDDPE (1 h) because of the higher steric hindrance at the macromonomer chain end. The higher stability of the resulting anion in the latter case due to a better charge delocalization might be another reason for the second reaction to go faster.
4.1.3 Carbonation of the adduct of poly(styryl)lithium with PDDPE-polystyrene macromonomer
The polymeric organolithium compound formed from the coupling reaction of poly(styryl)lithium with PDDPE-polystyrene macromonomer was carbonated in the
64 presence of THF using gaseous carbon dioxide (eq.35). The reaction was complete within
4 h. Since the product contained both middle-chain carboxyl-functionalized polystyrene
and unreacted PDDPE-polystyrene macromonomer, the macromonomer was removed
from the carboxylated product by silica gel column chromatography. Thin layer
chromatography (TLC) analysis of the crude product using toluene as eluent indicated
two well-separated spots (Figure 4.6). Due to the strong interaction between the carboxyl
groups and silica gel on the TLC plate, the carboxylated polymer had an Rf value of around 0.3. The excess macromonomer moved to the top of TLC plate because it did not contain any polar groups. After fractionation by column chromatography the product showed only one spot corresponding to the carboxylated polystyrene. The SEC chromatograms before and after the column chromatography provide further evidence for this successful fractionation (Figure 4.7).
excess macromonomer
carboxylated product
origin
Figure 4.6 TLC of the crude carbonation product (eq.35) using toluene as the eluent.
65
Figure 4.7 SEC chromatograms of middle-chain carboxyl-functionalized polystyrene 3 (Mn=4.2x10 g/mol) before (------) and after (------) fractionation by column chromatography.
The carboxyl functionality of the isolated product was determined by simple acid-
base titration. The number average molecular weights from stoichiometric calculation,
size exclusion chromatography and titration are given in Table 2 and all of these results
show good agreement.
Table 2. The number average molecular weights of carboxylated polymer determined by different methods. Method Number Average Molecular Weight (Mn) (g/mol) Stoichiometry 4.0x103 SEC 4.2x103 Titration 4.3x103
66 The FTIR spectrum of the carboxyl-functionalized polystyrene was compared with that of base polystyrene. The carbon-oxygen stretching frequency near 1700 cm-1 for the carbonyl absorption band of carboxylic acids112 was observed in the spectrum of
carboxyl functionalized polystyrene and no absorption band in the same region was seen
in the case of base polystyrene (Figure 4.8).
13C NMR was another method used for the qualitative characterization of the isolated carboxyl-functonalized polystyrene. 2,2-Diphenylpropionic acid was utilized as a model compound to assign the correct carbon resonance for the carboxyl carbon of the polymer. In the 13C NMR spectrum of 2,2-diphenylpropionic acid, the carboxyl carbon
appears as a weak signal at δ=181.7 ppm.113 Furthermore, Yin61 observed a hardly
detectible signal δ = 179.1 ppm in the 13C NMR spectrum of polystyrene with a terminal
13 carboxylic acid group prepared with ordinary high purity CO2 gas, and when C enriched
CO2 was used, the carbonyl carbon signal appeared clearly at δ = 179.5 ppm. Therefore,
the peak at δ = 179.0 ppm in the 13C NMR spectrum of the middle-chain carboxylated
polystyrene can be attributed to the carboxyl carbon of the polymer (Figure 4.9).
67
Figure 4.8 FTIR spectra of (A) base polystyrene and (B) middle-chain carboxyl- 3 functionalized polystyrene (Mn=4.2x10 g/mol).
68
Figure 4.9 13C NMR spectrum of middle-chain carboxyl-functionalized polystyrene 3 (Mn=4.2x10 g/mol).
It is known that in the carbonation of polymeric organolithium compounds, polar
additives (e.g.THF) and atmospheric CO2 pressures favor the formation of carboxyl
functionalized chains and hinder the formation of corresponding ketone (dimer) and
tertiary alcohol (trimer).2,58 Polar additives promote the dissociation of polymeric
organolithium compounds and thus prevent the formation of dilithio intermediate which forms dimeric ketone after hydrolysis (eq.13). Similarly, a high local concentration of
CO2, i.e. higher CO2 pressures, reduces the amount of dilithio intermediate by promoting
lithium carboxylated polystyrene formation and removing poly(styryl)lithium from the
system (eq.7). Following this rationale, the adduct of poly(styryl)lithium with PDDPE-
69 polstyrene macromonomer was carbonated in the presence of 25/75 (v/v) THF/benzene mixture under atmospheric CO2 pressure and no dimeric or trimeric side product were
observed as shown in the SEC chromatogram (Figure 4.7)
It has also been reported that the carbonation of 1,1-diphenylethylene end-capped poly(styryl)lithium in benzene resulted in carboxylated polymers with almost quantitative yields.3 Although the average degree of association of the polymeric 1,1-
diphenylalkyllithium chain ends has been reported to be two in hydrocarbon solutions60, the corresponding dissociation constants were shown to be larger than those of poly(styryl)lithium114 because of the increased steric hindrance of the chain end.
Therefore, it can be anticipated that the dimeric or trimeric side-products do not form in
the carbonation of the adduct of poly(styryl)lithium with PDDPE-polystyrene
macromonomer regardless of the CO2 pressure or the presence of Lewis base due to the
even higher steric hindrance around the chain end. Thus, the same experiment was
repeated both in the presence and absence of THF at CO2 pressures of 120 mm Hg and
760 mm Hg, and quantitative carboxylation of the chain ends was observed. SEC
chromatograms of the crude carboxylation product with the unreacted excess
macromonomer are shown in Figure 4.10 and no formation of dimeric and trimeric side-
products was detected as expected. As a result, it can be concluded that the competing
reaction to form dimeric and trimeric side-products is highly sensitive to the steric
requirements of the chain end.
70
Figure 4.10 SEC chromatograms of the carboxylated polystyrenes with unreacted macromonomers at different reaction conditions: (--∆--) in the presence of THF at 120 mm Hg, (--o--) in the absence of THF at 120 mmHg, (-----) in the presence of THF at 760 mm Hg, (-----) in the absence of THF at 760 mm Hg.
4.2 Preparation of middle-chain, primary amine-functionalized polystyrene
4.2.1 Synthesis of middle-chain, alkyl chloride-functionalized polystyrene
It has been reported by Macosko and coworkers94 that living polystyryl anions
treated with chloro(chloroalkyl)silanes, ClSiR2(CH2)Cl, reacted chemoselectively at the
silicon atom due to the greater reactivity of the Cl-Si bond over Cl-C bond even at
relatively high temperatures. Gas chromatography/mass spectrometry analysis of the
crude reaction mixtures showed no evidence for the formation of products derived from the termination of oligo(styryl)lithium species at the chloroalkyl functional group.
Therefore, it can be expected that when a dichloro(chloroalkyl)silane is used,
71 poly(styryl)lithium chains react at the more electrophilic silicon center of the silane resulting in a middle-chain, alkyl chloride-functionalized polystyrene.
2-Chloroethylmethyldichlorosilane, which was a commercially available reagent, was used to introduce the chloro functionality in the middle of the polystyrene chain by the coupling reaction of two poly(styryl)lithium chains (eq. 36). The poly(styryl) anions reacted only at the silicon center of the coupling agent. This was confirmed by SEC chromatography. As expected for reaction only at the silyl chloride groups, the molecular
3 weight of the base polystyrene (Mn=2.2x10 g/mol) doubled after the coupling reaction;
3 the resulting polymer had a molecular weight of Mn=4.5x10 g/mol (Figure 4.11). The poly(styryl)lithium was used in excess to ensure complete coupling of the living chains and to avoid complications in the subsequent steps which might result from the unreacted coupling agent. The low molecular weight shoulder on the chromatogram was due to excess unfunctionalized polystyrene.
CH3 CH3 C H - + 6 6 PS Li +SiCl Cl PS Si PS + PS + CH3OLi + LiCl (36) (excess) CH3OH
Cl Cl
72
3 Figure 4.11 SEC chromatograms of (A) base polystyrene (Mn=2.2x10 g/mol) and (B) 3 middle-chain, alkyl chloride-functionalized polystyrene (Mn=4.5x10 g/mol).
73 The 1H NMR spectrum of the resulting polymer is shown in Figure 4.12. The methylene protons alpha to the chloride (CH2Cl) appeared as a weak signal at δ = 3.3 ppm. The chemoselectivity of the reaction between poly(styryl) anions and the silicon atom was also confirmed by 1H NMR spectroscopy. The integration ratio of twelve methyl protons from the sec-butyllithium initiator at around δ = 0.7 ppm to two protons from the methylene group alpha to the chloride was 12.0 : 2.3.
Figure 4.12 1H NMR spectrum of middle-chain chloride-functionalized polystyrene 3 (Mn=4.5x10 g/mol).
74 4.2.2 Synthesis of middle-chain, azide-functionalized polystyrene
The alkyl chloride group was converted to azide by a simple substitution reaction
using sodium azide as reported by Macosko and coworkers (eq.37).6
Tetrabutylammonium iodide (TBAI) was used as a phase transfer catalyst to increase the solubility of the polymer in dimethylformamide. The completeness of the reaction was confirmed by 1H NMR spectroscopy (Figure 4.13). The methylene protons of the
chloromethylene group (CH2Cl) formed at δ = 3.3 ppm were replaced by a new peak at δ
= 2.9 ppm upon conversion to azide (CH2N3). The FTIR spectra of the chloride- and
azide- functionalized polymers were also compared (Figure 4.14). In the spectrum of
azide-functionalized polystyrene, the characteristic peak at 2100 cm-1 was assigned to the
asymmetric stretching of the azide group115 and there was no such narrow band around
that region in the chloride-functionalized polystyrene spectrum.
CH3 CH3
NaN3/TBAI PS Si PS PS Si PS + NaCl (37) DMF
Cl N3
CH3 CH3
1) LiAlH4 / THF PS Si PS PS Si PS + LiOH + Al(OH)3 (38) 2) H2O
N3 H2N
75
Figure 4.13 1H NMR spectrum of middle-chain, azide-functionalized polystyrene 3 (Mn=4.5x10 g/mol).
4.2.3 Synthesis of middle-chain, primary amine-functionalized polystyrene
The azide group in the middle of polystyrene was reduced to primary amine using
lithium aluminum hydride (eq.38). The reaction was monitored with FTIR spectroscopy.
The characteristic absorption peak of the azide group near 2100 cm-1 disappeared within
2 hours. Although the azido absorption disappeared, no characteristic band of the amine
was observed in the 3300-3500 cm-1 range as a result of the relatively low concentration
of the amine functional groups at the polymer chain ends and the low molar absorption of
the amine group.5
76
Figure 4.14 FTIR spectra of middle-chain (A) chloride and (B) azide functionalized 3 polystyrenes (Mn=4.5x10 g/mol).
77
Figure 4.15 FTIR spectrum of middle-chain, primary amine-functionalized polystyrene 3 (Mn=4.5x10 g/mol).
After the purification by column chromatography, the polymer showed only one spot on TLC plate with Rf = 0 as expected when toluene was used as the eluent. Figure
4.16 shows the TLC of chloride-, azide- and amine-functionalized polymers.
origin a bc
Figure 4.16 TLC of (a) alkyl chloride-functionalized polystyrene, (b) alkyl azide- functionalized polystyrene, (c) primary amine-functionalized polystyrene; eluent:toluene. 78 The protons of the methylene group in the alpha position to amine could not be detected in 1H NMR spectrum due to the large resonances of methylene and methine
protons of the polystyrene (Figure 4.17). Trichloroacetyl isocyanate is known to be a
useful in situ derivatizing reagent for alcohols, phenols and amines in 1H and 13C NMR spectroscopy studies.116 Therefore, the primary amine-functionalized polystyrene was
reacted with trichloroacetyl isocyanate to obtain corresponding urea derivative (eq.39).
The resonances for the alpha methylene protons shifted downfield as expected116 and they
appeared at δ = 3.15 ppm (Figure 4.18). The additional peaks were due to the hydrolysis of the excess trichloroacetyl isocyanate. The relative integration ratio of the methyl
protons from the sec-butyllithium initiator to the methylene protons alpha to the nitrogen
atom was determined to be 12 : 2.4 which was close to the expected value of 12 :2. The
13C NMR spectra of chloride-, azide- and primary amine-functionalized polystyrene as
well as corresponding urea derivative are given in “Appendix”.
CH3 O CH3
PS Si PS + OCN C CCl3 PS Si PS (39)
H2N HN
C O
NH
C O
CCl3
79
Figure 4.17 1H NMR spectrum of middle-chain, primary amine-functionalized 3 polystyrene (Mn=4.5x10 g/mol).
Figure 4.18 1H NMR spectrum of middle-chain, primary amine-functionalized 3 polystyrene after the reaction with trichloroacetyl isocyanate (Mn=4.5x10 g/mol). 80 SEC analysis of the amine-functionalized polystyrene resulted in a curve with
relatively low detector response and broad molecular weight distribution (Mw/Mn=1.22) due to the physical interaction of the primary amine group with the stationary phase
(Figure 4.19).83,89 After derivatization of the primary amine using trichloroacetyl
isocyanate, no retention effect was observed on SEC chromatogram and the polymer had a molecular weight distribution of Mw/Mn=1.06 (Figure 4.20).
Figure 4.19 SEC chromatogram of middle-chain, primary amine-functionalized 3 polystyrene (Mn=4.5x10 g/mol).
81
Figure 4.20 SEC chromatogram of middle-chain, primary amine-functionalized 3 polystyrene derivatized with trichloroacetyl isocyanate (Mn=4.5x10 g/mol).
The amine functionality of the isolated product was determined by colorimetric acid-base titration using perchloric acid and was found to be 99.5 %. The number average molecular weights from stoichiometric calculation, size exclusion chromatography and titration are given in Table 3 and all of these results show good agreement.
Table 3. The number average molecular weights of middle-chain primary amine- functionalized polystyrene determined by different methods. Method Number Average Molecular Weight (Mn) (g/mol) Stoichiometry 4.2x103 SECa 4.5x103 Titration 4.5x103 a the number average molecular weight (Mn) of urea derivative.
82 Matrix-assisted laser desorption ionization time-of-flight mass spectrometry
(MALDI-TOF MS) is known to be a powerful tool for the characterization of functional
polymers117 and was utilized for the determination of the amine functional group. Since the molecular weight of the resulting polymer was relatively high, the individual isotopic peaks could not be resolved, and thus the average mass of all isotopes for a given element was used in calculations.
The MALDI-TOF mass spectrum of the amine-functionalized polymer with silver
trifluoroacetate/dithranol as cationizing agent/matrix system is shown in Figure 4.20. The
average distance between the peaks is m/z 104.1 which corresponds to a styrene repeat
unit. An enlargement of the area between (m/z) 3700 and 4100 Da is also shown in the
same figure. The calculations using the average atomic weight values indicate that the
peaks correspond to the expected structure. For example, a peak at (m/z) 3954.22 Da corresponds to the 35-mer of polystyrene with two butyl groups and a silver complexed
2-(methylsilyl)ethyl amine group {m/z = 35 x 104.15 [(C8H8)35] + 2 x 57.11 [(C4H9)] +
+ 87.20 [SiC3H7NH2] + 107.87 (Ag ) =3954.54 Da}.
Based on these results, it can be concluded from the NMR, FTIR and mass
spectral analyses that middle-chain, primary amine-functionalized polystyrenes can be prepared by the chemical modification of middle-chain alkyl chloride-functionalized polystyrenes.
83
Figure 4.21 MALDI-TOF mass spectrum for middle-chain, primary amine-functionalized 3 polystyrene with silver trifluoroacetate as cationizing agent [Mn(calc.)=4.2x10 g/mol].
84 CHAPTER V
SUMMARY
One of the objectives of this research was to prepare pure middle-chain, carboxyl- functionalized polystyrene and to investigate the effect of chain end structure on the carboxylation reaction. PDDPE-polystyrene macromonomers with terminal 1,1- diphenylethylene functionality were prepared by the reaction of poly(styryl)lithium with
PDDPE [1,4-bis(1-phenylethenyl)benzene] ([PDDPE]/[Li]=1.2). The resulting macromonomer which was characterized by 1H and 13C NMR spectroscopy was used as a
starting material for the synthesis of middle-chain carboxyl-functionalized polystyrene.
3 After the linking reaction of poly(styryl)lithium (Mn=2.2x10 g/mol) with the
3 macromonomer (Mn=2.4x10 g/mol), the anionic living chain end in the middle of the
polystyrene chain was carbonated in the presence of THF using gaseous carbon dioxide
under atmospheric pressure. The SEC chromatogram of the crude product indicated the
absence of side products, i.e. dimeric ketone and trimeric alcohol. After isolation of
carboxylated polymer by silica gel column chromatography, the product showed only one
spot upon TLC analysis and the molecular weight of the base polystyrene doubled as
expected. The carboxyl functionality of the polymer was determined by acid-base
titration and the resulting number average molecular weight (Mn) calculated from the
titration results showed good agreement with the one obtained from SEC measurement.
The presence of carboxyl group was further confirmed by FTIR and 13C NMR
85 spectroscopy. In order to examine the effect of chain end structure on carbonation under
different reaction conditions, the same reaction was repeated both in the presence and in
the absence of THF at CO2 pressures of 120 mm Hg and 760 mm Hg. It was determined
by SEC measurements that the polymer was free of dimeric or trimeric side-products
regardless of reaction conditions and quantitative carboxylation of polystyrenes could be
accomplished due to the high steric hindrance around the living chain end.
Another goal of this research was to prepare middle chain, primary amine-
functionalized polystyrene. Living anionic polymerization techniques have been used to
prepare middle chain, alkyl chloride-functionalized polystyrene, and the alkyl chloride
was successfully transformed into a primary amine group. 2-Chloroethylmethyl
dichlorosilane was used as the linking agent with poly(styryl)lithium to introduce chloro
functionality into the middle of a polystyrene chain by the reaction of two polystyryl
anions at the more reactive chloride atoms bonded to the silicon atom. This
chemoselectivity of the reaction was confirmed by 1H NMR and SEC. The integration ratio of twelve methyl protons from the sec-butyllithium initiator to two protons from the
methylene group alpha to the chloride was comparable to the theoretical value.
Furthermore, the molecular weight of the base polystyrene doubled after the coupling
reaction as determined by SEC measurements. The conversion of the chloride
functionality to the primary amine was achieved by substitution of the chloride with azide
followed by the reduction of the azide to the amine group. Trichloroacetyl isocyanate was
used as an in situ derivatizing reagent for the resulting amine-functionalized polystyrene
to allow the identification of the characteristic methylene protons alpha to the amine
group in 1H NMR spectrum and to avoid the adsorption effects observed for the amine-
86 functionalized polymer in SEC measurements. MALDI-TOF mass spectrometric
measurements confirmed the presence of primary amine group in the polymer. The
spectrum corresponded well to the expected structure. The amine content (99.5 %) was determined by acid-base titration using perchloric acid. Based on all these characterization data, it can be concluded that this synthetic method is an efficient way of preparing middle-chain, primary amine-functionalized polystyrenes, although it is a multi-step method.
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94 APPENDIX
Figure 1. 13C NMR spectrum of middle-chain, chloride-functionalized polystyrene 3 (Mn=4.5x10 g/mol).
Figure 2. 13C NMR spectrum of middle-chain, azide-functionalized polystyrene 3 (Mn=4.5x10 g/mol).
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Figure 3. 13C NMR spectrum of middle-chain, primary amine-functionalized polystyrene 3 (Mn=4.5x10 g/mol).
Figure 4. 13C NMR spectrum of middle-chain, primary amine-functionalized polystyrene 3 after the reaction with trichloroacetyl isocyanate (Mn=4.5x10 g/mol).
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