ANIONIC POLYMERIZATION OF ETHYLENE OXIDE
AND PROPYLENE OXIDE
by
HRIRE MIRZAKHAN-GHARAPETIAN, B.Sc.,M.Sc.
A thesis submitted for the degree of
Doctor of Philosophy
in the
University of London
Physical Chemistry Laboratories Department of Chemistry Imperial College of Science and Technology London SIR 2AY June 1974 2
ACKNOWLEDGEMENTS
I am deeply indebted to my supervisor, Dr Maurice George, who patiently guided and encouraged me throughout the entire course of this work.
I would also like to express my sincere gratitude to the Calouste
Gulbenkian Foundation, Lisbon, Portugal, for the award of a Scholarship which enabled me to perform this work.
Grateful acknowledgement is made to Dr J.A. Barrie for allowing me to use some of the apparatus in the Polymer Characterization
Laboratory.
I am grateful to Dr J.M. Evans, Rubber and Plastics Research
Association, Shawbury, for performing gel permeaton chromatography analysis on polymer samples, and to Mr G.C. Goode, Atomic Weapons
Research Establishment, Aldermaston, for carrying out neutron activation analysis.
Thanks are due to the technical staff of the Chemistry Department of Imperial College.
I should like to thank Andrew J. Tinker, David C. Evans and
Ronald N. Sheppard for many helpful and interesting discussions.
Finally, I wish to thank Mrs U.O. Fowler for typing the manuscript,
HRIRE M, GHARAPETIAN 3
1. ABSTRACT
Kinetic studies of ethylene oxide and propylene oxide were performed
dilatometrically using cumyl potassium and tetrahydrofuran as initiator
and solvent, respectively.
Ethylene oxide polymerizations were carried out usually at 40°C.
Due to the gradual change in the density of growing short-length living a polyethylene oxide chains, the rate of decrease in the volume of polymeriz-
ing solutions was initially slow. The rate of decrease in volume gradually
increased until it reached a constant value.
The order of the reaction rate with respect to monomer and initiator
concentration was 0.935 + 0.15 and 1.00 + 0.27, respectively. The second
order rate constant, k , was (6.61 + 0.42) x 10 2 dm3 mol-1 s-1 at 40°C. p Rates of ethylene oxide polymerization using cumyl potassium/THF-dioxane
were measured for different THF-dioxane compositions at 40°C. No
significant variation in the reaction rates were observed for different
solvent mixtures.
Propylene oxide polymerizations were performed usually at 50°C.
The initial increase in the rate of change of volume for propylene
oxide was much less pronounced than that for ethylene oxide. This
was due to the slower change in the density of the growing polypropylene
oxide chains and the occurrence of transfer to monomer. The order of
the reaction rate with respect to the monomer and initiator concentration
was 0.91 + 0.26 and 1.01 + 0.21, respectively. The k value was -3 3 -1 -1 (1.62 + 0.10) x 10 dm mol s at 50°C. The transfer constant
to monomer, a, was (2.04 + 0.22) x 10-2. The end group unsaturation was
in the range of (5.42 - 9.33) x 10-5 equivalents g-1. -1 The overall activation energies were 3.94 + 0.37 kJ mol and -1 4.3 + 0.81 kJ mol for ethylene oxide and propylene oxide polymerizations,
respectively. 4
The homopolymers prepared by polymerization of ethylene oxide and
propylene oxide were isolated and their molecular weights were measured
by vapour pressure osmometry. The higher molecular weight polyethylene
oxides and some polypropylene oxides were analysed by gel permiation
chromatography. The molecular weights of some samples were also measured
by viscometry. *. A monodisperse sample of polystyrene was prepared using cumyl
potassium in tetrahydrofuran.
Monodisperse polystyrene samples were chloromethylated and attempts
were made to prepare graft copolymers of styrene and ethylene oxide by
anionic coupling technique. .5
CONTENTS D292. 1. Abstract 3
PART ONE LITERATURE SURVEY
2. Anionic Polymerization of Alkenes 11
2.1. The kinetics of Anionic Polymerization of Alkenes 11
2.2. Molecular Weight and Molecular Weight Distribution
in Anionic Polymerizations 13 3. Anionic Polymerization of Epoxides 15 3.A. Initiation by Alkali Metal Hydroxides, Alkyls and
Alkoxides 15
3.B. Initiation by Alkali Metal-Aromatic Hydrocarbons 15
3.C. The Kinetics of Anionic Polymerization of Epoxides 15
3.D. The nature of Propagating Species 16
3.D.1. The Effect of Solvent on the Nature of the Propagating
Species 17
3.E. The Association Phenomena 17
3.E.1. Associations in Alkali Metal Alkoxides 18
3.1. Anionic Polymerization of EpoXides Initiated with
Hydroxides, Alkoxides and Alkyls of Alkali Metals 19
3.1.A. Anionic Polymerization of Ethylene Oxide with Sodium
Methoxide
3.1.A.1. The Proton Exchange Reactions 19
3.1.B. Propylene Oxide Polymerization with Hydroxides and
Alkoxides of Alkali Metals 20
3.1.C. Polymerization of Epoxides with Potassium t-Butoxide
in Dimethyl Sulphoxide (LUSO) 21
3.1.C.1. Polymerization of Ethylene Oxide with Potassium
t-Butoxide in Dimethyl Sulphoxide (DMSO) 22 b
Page
3.1.C.2. Polymerization of Propylene Oxide with Potassium
t-Butoxide in Dimethyl Sulphoxide (DMSO) 23
3.1.C.3. Polymerization of Propylene Oxide with Potassium
t-Butoxide in DMSO and THF Mixtures 23
3.1.C.4. Polymerization of neo-Pentylethylene Oxide
Initiated with Potassium t-Butoxide 24
3.1.C.5. Polymerization of 1,2 Butylene Oxide in DMS0 and
DMSO and THF Mixtures 24
3.2. Anionic Polymerization of Epoxides with Alkali Metal-
Aromatic Hydrocarbon Initiators 25
3.2.A. Polymerization of Ethylene Oxide with Alkali Metal
Naphthalene Initiators in THF 27
3.2.B. Polymerization of Propylene Oxide with Sodium Naphthalene
in DMSO and THF Mixtures 28 3.3. Transfer Reactions in Alkyl Substituted Ethylene Oxide Polymerizations 28
3.3.A. Transfer Reactions to Monomer 28
3.3.B. Transfer to Solvent 30 3.3.C. The Unsaturation Content 30 3.3.D. The Effect of Transfer Reactions on Molecular Weights
in Alkyl Substituted Ethylene Oxide Polymerizations 31
4. Preparation of Monodisperse Polystyrene by Anionic
Polymerization 32
4.A. The Stability of Alkali Metal Initiators and Living Polymers in THF 35
5. Graft Copolymers 37 5.1. Preparation of Graft Copolymers by Anionic Methods 37 7
Page
5.1.A. Preparation of Graft Copolymers by Growing Chains
from Active Centres on the Polymer Backbone 37 5.1.B. Preparation of Graft Copolymers by Deactivation 44
PART TWO MATERIALS
6. Purification and Storage of Materials 50
6.1. Purification Techniques 50
6.1.A. Preparation of Sodium Mirror 50 6.1.B. Preparation of Sodium Potassium Alloy 50
6.2. Solvents 51 6.2.A. Tetrahydrofuran (THF) 51
6.2.B. Dioxane 51 6.3. Monomers 51 6.3.A. Ethylene Oxide 51
6.3.B. Propylene Oxide 52
6.3.c. Styrene 52 6.4. n-Butyl Bromide 52
6.5. Chloromethylation Catalysts 53
6.5.A. Stannic Chloride 53
6.5.B. Zinc Chloride 53
6.6. Chloromethyl Methyl Ether 53 6.7. Initiators 54
6.7.A. The Solution of n-Butyl Lithium in Hexane 54 6.7.B. Solutions of Cumyl Potassium in THF 51+
6.7.B.1. Determination of Cumyl Potassium in Cumyl Potassium-THF Solutions 54 8
PART THREE EXPERIMENTAL Page 7. Apparatus 56
7 .1. The High Vacuum Inert Gas System 56 7.2. The Thermostatted Bath 57 7.3. The Use of the "Sovirel" Taps and Joints 57 7.4. The Use of the Splash Head in the Purifications 58 7.5. The Containers used for Liquid Storage in Argon Gas Atmosphere 59 7.6. The Syringes and Needles 59 8. Kinetic Studies of Ethylene Oxide and Propylene Oxide Polymerizations 60 8.1. The Dilatometric Measurements 60
• 8.1.A. Filling the Dilatometers 60
8.2. Determination of the Unsaturation Content in
Propylene Oxide 61
8.3. Measurement of Molecular Weights by Vapour
Phase Osmometry 61 8.4. The Automatic Viscometer and Viscosity Measurements 62
8.5. The Calculation of Rates of Polymerization 63 8.6. Calculation of Activation Energies of Polymerizations 64 9. Preparation of Monodisperse Polystyrene Initiated by Cumyl Potassium in THE 65
10. Attempted Preparation of Graft Copolymers of Styrene
and Ethylene Oxide 67
10.1. Determination of Chlorine in Chloromethylated
Polystyrene by Neutron Activation Analysis 70 9
PART FOUR PROPERTIES OF ETHYLENE OXIDE PROPYLENE
OXIDE AND THEIR HOMOPOLYMERS Page 11. Epoxides 78
11.1. The Structure of Epoxides 78 II.I.A. The Physical Properties of Ethylene Oxide 79 11.1.B. The Physical Properties of Propylene Oxide 79 11.2.A. The Physical Properties of Polyethylene Oxide 79
11.2.B. The Physical Properties of Polypropylene Oxide 80
PART FIVE RESULTS AND DISCUSSION
12. The Kinetics of Ethylene Oxide Polymerization with.
Cumyl Potassium,in THE and THF-Dioxane Mixtures 82
12.1. The Kinetics of Ethylene Oxide Polymerization with
Cumyl Potassium in THF 82
12.1.A. Initiation 82
12.1.B. Propagation 83
12.1.C. The Dependence of the Rate of Polymerization, Rp ,
on Monomer Concentration 88
12.1.D. The Dependence of the Rate of Polymerization, R p,
on Initiator Concentration 90
12.1.E. The Effects of Counter Ion on the Polymerization
Rates of Ethylene Oxide 92
12.I.F. The activation Energies of Polymerization of
Ethylene Oxide 93
12.2. The Ethylene Oxide Polymerizations in THF-Dioxane
Mixtures 94
12.3. Molecular Weights and Molecular Weight Distributions
in Ethylene Oxide Polymerizations 96
12.4. Kinetics of Ethylene Oxide Polymerization Initiated
with n-Butyl Lithium in THF 96 10
Page
13. The Kinetics of Propylene Oxide Polymerization
with Cumyl Potassium in THE 106
13.A. Initiation 106
13.B. Propagation 106
13.C. The Dependence of the Rate of Polymerization, R ,
on Monomer Concentration 107
13.D. The Dependence of the Rate of Polymerization, R ,
on Initiator Concentration 108
13.E. The Effect of Counter Ion and Solvent on the
Polymerization Rates of Propylene Oxide 110
13.F. The Activation Energies of Polymerization of
Propylene Oxide 112
13.G. Transfer Reactions in Propylene Oxide Polymerizations
Initiated by Cumyl Potassium/THE 113
13.H. Molecular Weights and Molecular Weight Distributions
in Propylene Oxide Polymerizations 118
14. A Comparison between Ethylene Oxide and Propylene
Oxide Polymerizations Initiated with Cumyl Potassium
in THE 127
15. Preparation of Monodisperse Polystyrene using Cumyl/
Potassium/THE 130
16. Attempted Preparation of Graft Copolymers of Styrene
and Ethylene Oxide 133
17. CONCLUSIONS 139
18. ABBREVIATIONS 141
19. REFERENCES 143 11
PART ONE LITERATURE SURVEY
2. Anionic Polymerization of Alkenes
Compounds of a basic nature such as alkali metal amides, alkali
metal alkyls, alkali metal ketyls, alkali metals and alkali metal-aromatic
hydrocarbon mixtures initiate anionic polymerizations.
Normally the initiation step involves the addition of the negative
fragment of the initiator, to the monomer to form a carbanion.
A variety of organometallic compounds such as n-butyl lithium,(1)
tri-phenyl methyl potassium( 2) and cumyl potassium(3) act as initiating
systems.
When the alkali metals or their complexes with aromatic hydrocarbons
are used as initiators, the initiation takes place by an electron transfer
from initiator to monomer to form a radical ion,
e f CE6 = CHY -4 .CH2 -.6HY (2.1)
Sodium naphthalene and sodium biphenyl complexes have been used in styrene polymerizations.(-6) Usually the radical anions undergo dimerization via their free radical ends to form dianions.
The propagation takes place by successive addition of monomer to carbanion or radical anion. Many anionic polymerizations take place without transfer or termination reactions. They are very useful for the preparation of monodisperse polymers.
2.1. The Kinetics of Anionic Polymerization of Alkenes
In anionic polymerizations where no transfer or termination reactions take place, the polymerization rate, Rp, is expressed as:
Rp = kp [14][M] (2.2) where [M] is the initial monomer concentration, [1,1 ] the living chain concentration and k is the second order rate constant for propagation. 12
R and k pare dependent on the counter ion and the solvating
power of the solvent. The polymerizations are faster in more -polar
solvents.(5'7)
Both free ions and ion pairs are present in equilibrium with one
another
- + + P Me P + Me (2.3)
where P Me represents an ion pair from a living polymer anion, P
and counter ion Me+.
[P ]1M}+] K - if [P-] - [Me+] K °P32 (2.4)
CPMe+J [PMe+]
where K is the equilibrium constant.
The total reaction rate is,
kp-me [P-Me+] Rp = kp- [4]D P 7 + (2.5)
where kp_ and kp_me+ are the second order rate constants for the
propagations with free ions and ion pairs, respectively.
From the combination of equations (2.4) and (2.5)
1
kp- K2 kp_me+ (2.6) [1,1 ][P-me+ ] [p-me] , 2
As [P-] is frequently very small DP-Me+J may be considered as the
total concentration of the propagating species. The right side of the
equation (2.6) becomes equal to k .
P 1
kp_ K2
k = + (2.7) p r [p-me412
According to equation (2.7) k should have a linear dependence on
(1/[P-Me+]1). This is in agreement with experimental data(5'7) that
show greater reactivity of free ions compared to ion pairs.
In tetrahydrofuran as solvent the reactivity of ion pairs and also
+ + + + + K, decrease along the series Li > Na > K - Rb > Cs , which is due to
the greater solvation of smaller counter ions. 13
The reactivity of ion pairs is much less in dioxane with a much
lower solvating power than tetrahydrofuran, also their reactivity is + + the reverse of that in tetrahydrofuran along the series Li < Na < K
Rb < Cs .
In dioxane of lower dielectric constant the Coulombic forces between
counter ions and carbanions are of greater importance. Coulombic forces
decrease with increasing atomic readius of the counter ion and thus
facilitate the addition of monomer and the formation of free ions.
In hydrocarbon solvents where the homogeneous polymerizations
are restricted to the organolithium initiators the kinetics of the
polymerizations are more complex due to the associations of the initiator molecules and the propagating carbonium ions.
2.2. Molecular Weight and Molecular Weight Distribution in Anionic
Polymerizations
In anionic polymerizations where all the mono functional initiator units initiate polymerization, the active centres have an equal chance of adding to monomer. If no transfer or termination reactions take place, the number average degree of polymerization, DP, is very near to the weight average degree of polymerization, DP and is given by w [M]o = (2.8) [-1.1 ] where [M] is the initial monomer concentration. o As the living chain concentration is equal to the initial initiator concentration, [In] o DP = [M]o ]. (2.9)
A very narrow molecular weight distribution is expected in anionic polymerizations. The size distribution expressed by the Poisson () Distribution for anionic polymerizations
DP DP w - 1 + (2.10) DP (1 + DP)2
The equation (2.10) shows how at higher molecular weights DPw/BF becomes smaller. 15
3. Anionic Polymerization of Epoxides
The anionic polymerization of epoxides can be initiated by alkali metal hydroxides, alkali metal alkoxides„ alkali metal alkyls and alkali metal-aromatic hydrocarbon systems.
3.A.Initiation by Alkali Metal Hydroxides Alkyls and Alkoxides
The initiation takes place by means of a nucleophilic addition of monomer to the carbanion, A, 4- 0. + AMe M -4 AM Me (3.13 and then the propagation takes place by further additions of monomer to the alkoxide ion.
3.B.Initiation by Alkali Metal-Aromatic Hydrocarbons
The initiation by alkali metal-aromatic hydrocarbons takes place by nucleophilic addition of monomer to the initiating radical anion to form an aromatic hydrocarbon alkoxide radical anion. Then an electron transfer from an initial radical anion to the alkoxide radical anion occurs and a bifunctional carbanion is formed and the propagation occurs from two growing ends(9-11,4) The formation of bifunctional carbanion is concentration dependent and at radical anion concentrations of less than 10-ilmol dm-3I only monofunctional growth is observed.(13)
3.C.The Kinetics of Anionic Polymerization of Epoxides
The anionic polymerization of epoxides is usually a step polymeriza- tion, as the chain lengths increase slowly with conversion,, The epoxide polymerizations are similar to the anionic polymerization of alkenes.
The polymerization rate is given by
R = - .14110 k [M] [In] p dt where [M] and [In] are the monomer and initiator concentrations, respectively.
16
However the anionic polymerization of propylene oxide initiated (14) by potassium hydroxide is a chain reaction.
In the anionic step polymerizations of the epoxides where chain transfer reactions do not occur, the number average degree of polymerization at time t during the propagation is given by the ratio of the concentration of the monomer which has reacted and the initial initiator concentration, [In]o,(15)
f[M]o-[M]/ D'- (303) [In]o
[/%1] is the initial monomer concentration. o In the alkali metal-aromatic hydrocarbon initiated polymerizations, the average degree of polymerization for low initiator concentrations, -4 (13) e.g. 1U mol dm73 for Na naphthalene, equation (3.3) can be used, while for high initiator concentrations, where carbanions with two growing ends exist 2{[m]o [M]} DP ( 3. 4) [In]o
Devietions from simple kinetic scheme occur in some reaction media and with certain propagating species.
3.D. The Nature of Propagating Species
The propagating species exist in the forms of ion pairs and free ions depending on the nature of the reaction media and the counter ion.
The order of the reactivity of propagating species increase in the + (11,13,16) series Li < Na+ < le - Rb+ < Cs which is due to the increase id the Coulombic forces between the propagating end and the counter ion, by increasing the counter ion size. Among alkali metal hydroxides only
K, Rb and Cs hydroxides are capable of initiating anionic polymerization (17,18) of propylene oxide. The order of the reactivity of alkoxides with respect to counter ion size is the reverse of that observed in the 17
anionic polymerization of alkenes in solvents of high dielectric
constant.
3.D.1. The Effect of Solvent on the Nature of the Pro.a.atin. S. ecies
The solvating power of the solvent plays an important part in the
dissociation of the alkali metal alkoxide propagating species. In a
strong dissociating solvent such as dimethyl sulphoxide (ENSO) there is (20) a high possibility of ion pair dissociation.(19) Blanchard et al.
have observed an order of about 1.7 with respect to the initiator
concentration in propylene oxide polymerization initiated by potassium
tertiary butoxide in dimethyl sulphoxide, which has been attributed to
the presence of both free ions and ion pairs in the reaction medium.
Even in a highly dissociating solvent such as dimethyl sulphoxide, the
counter ion size plays an important part in the dissociation of the
ion pairs. Lithium and potassium alkoxides have lower dissociation (19) constants than potassium and caesium alkoxides. In solvents of
lower dielectric constant such as tetrahydrofuran (THF) only ion pairs
are present.
The ionic dissociation constants of living polyethylene oxide in THF o -10 -11 3 + + at 40 C are 2 x 10 and 8 x 10 mol dm for Cs and K as counter (13) ions, respectively.
A reaction order of one with respect to the initiator concentration
has been observed even when propylene oxide has been polymerized by (20) t-butoxide in mixtures of DMSO and THF, which indicates_
the existence of only ion pairs in these reaction media.
3.E. The Association Phenomena
Association of various organolithium species take place in anionic
polymerizations initiated by organolithium compounds in non polar media such (1.21) as benzene and toluene. ' 3.8
Initiation and propagation rates are both affected, at n-butyl -4 lithium concentrations of 10 - 10-2mol dm-3. The initiation and the propagation rates are respectively proportional to the n-butyl lithium 1 concentration to the 1/6 and /2 power, respectively.
3.E.1. Associations in Alkali Metal Alkoxides
The alkali metal alkoxide ion pairs form associiations due to the large Coulombic forces present between the propagating ends of the
alkoxide anions and the counter ion, in solvents of relatively low
dielectric constant such as THF.
Formation of such associations of ion pairs has been studied by reacting an alkoxide forming compound with the living polymer chains in alkene polymerizations. A large increase in the viscosity of the polystyryl alkali metal living polymer solution has been observed when (22,23) (9,24) it has reacted even with small amounts of THF, or ethylene oxide.
This phenomenon has been attributed to the formation of association of the alkoxide ion pairs that have been formed by the reactions of ethylene oxide and THF with the living polystyryl alkali metal chains.
R CH2 -FH(C6H5)Li++[(CH2)40j ... CH2 -CH(C6H5) -(CH2 )40 Li+ (3.5)
R CH2 -EH(C6H5 )Na+ n[(CH2 )2°] ... CH2 -CH(C6H5) -[(CH2 )20]n_i -(CH2 )20 Na+
(3.6)
The ethylene oxide polymerization initiated by sodium, potassium (13) and caesium naphthalene initiators in THF are of the order of one with respect to the initiator concentration at low initiator concentrations, while the orders are less than one at high initiator concentrations, about 0.33 for caesium and potassium and about 0.25 for sodium counter ions.
This has been attributed to the formation of associations at high initiator concentrations.
Association factors of 3 for caesium and potassium and If for sodium counter ions have been obtained. (13) 19
The stability of associates is dependent on counter ion size. The
larger the Coulombic force the less is the ability of solvent molecules
to enter between the counter ion and the propagating chain end. In a
solvent of higher solvating power such as DMSO the association is less (19,20,25) pronounced.
3.1. Anionic Polymerization of Epoxides Initiated with Hydroxides,
Alkoxides and Alkyls of Alkali Metals
3.I.A. Anionic Polymerization of Ethylene Oxide with Sodium Methoxide
In the anionic polymerizations of epoxides initiated by alkali
metal alkoxides, some alcohol is added for solubilizing the initiator.
Anionic polymerizations of ethylene oxide have been carried out by sodium methoxide initiator both in bulk and in dioxane, in the presence of some methanol.(15)
The propagation reactions are first order with respect to the monomer concentration, while orders of less than one are observed with respect to the initiator concentration, which has been attributed to the nature of the growing ends. The reaction medium, especially when no dioxane is present, is not homogeneous throughout the course of the reaction. 5 3 -1 -1 The second order rate constant is about 2 x 10 dm mol s at 30C and is relatively low, partly due to the sodium counter ion but mainly due to the non-dissociating reaction medium. Higher second order rate constants are obtained when relatively large amounts of alcohol are present. (15)
The alcohol destroys the associations of growing ends by solubilizing them and monomer addition to the growing ends is facilitated. The -1 activation energy of polymerization is 4.25 kJ mol and A is about 8 3 -1 -1 (15) 2 x 10 dm moi s .
3.1.A.1. The Proton Exchange Reactions
The proton exchange reactions take place between the growing chains and the added alcohol, also between the growing chains and chains terminated 20
due to proton transfer
RO(CH2C1120)n...1-CH2CH20Me++ROH z-*RO(CH2 CH2 OH + ROMe+ CH20)n-l-CH2 (3.7)
RiO(CH2CH2 0Me++ RO(CH2CH20)n_1-CH2CH2 OH 11 10(CH2CH20) n...1 °)11""CCH2 C"2
-CH2 CH2 OH RO(CH2CH20)n_i-CH2 CH20Me+ (3.8)
Each molecule is a potential growing point for a polymer chain, like
an alkoxide ion. Therefore the degree of polymerization is given by N o- [M]t (3.9) [In]o-1-+ where [q] and [i] are the monomer concentrations at time t equal to 0 o t [In] and [ROB] represent the initial initiator and alcohol and t. o concentrations.
The exchange reactions are dependent on the acid dissociation constants of the alcohols and the polymers terminated due to proton transfer. If the added alcohol is much more acidic than the terminated polymer the equilibrium (3.7) will be displaced to the right. All the free alcohol will react before any polymerization takes place. Due to the absence of the reaction (3.8) narrow molecular weight distribution will be produced.
If the added alcohol is less acidic than the polymeric alcohol, no initial reactivity of the alcohol would be expected. The exchange reactions will take place in the later stages of the polymerization with broadening molecular weight distributions.
In ethylene oxide polymerization by sodium methoxide in the presence (15) of methanol, the evidence from molecular weight studies and the observation that all the monomer reacts but no initiator is lost show that no transfer or termination reactions take place.
3.I.B. Propylene Oxide Polymerization with Hydroxides and Alkoxides of
Alkali Metals
Sodium and lithium hydroxides are inactive, but potassium, rubidium 21
and caesium hydroxides are capable of initiating propylene oxide
polymerization.(17,18)
The generally accepted mechanism for base initiated polymerization
of propylene oxide is - ROH + HO .72- RO + H20 (3.10)
OH-or RO + (CH3 CH - CH2 -4 HO or RO - CH2 - CH(CH3 )-0 (3.11)
where R represents any possible moities including the initiator.(26)
In sodium methoxide initiated polymerizations of propylene oxide
in bulk or dioxane, the rapid initiation is followed by a slower head
to tail addition of monomer to the propagating chain.. (26) The initial
rapid reaction is due to the presence of the primary alkoxides and
alcohols, which react faster than the secondary alcohols and alkoxides
S present during the propagation. The propagation step is first order with respect to monomer
concentration and initiator concentration. The value of k is -4 -1 -1 -4 3 -1 -1 1.3 x 10 dm3me1 s for the bulk polymerisations and 2.5 - 3 x 10 dm moi s
for the polymerizations in dioxane, at 70°C. The activation energy is -1 about 4.1 kJ mol and A is 2.3 x 107 dm3 mol-1s -1 , which is a power of
ten less than for ethylene oxide, due to the greater restriction in the
propagation reaction.
3.1.C. Polymerization of Epoxides with Potassium t-Butoxide in Dimethyl
Sulphoxide ) The following mechanisms have been suggested for the initiation and
propagation stages of the polymerizations initiated by potassium t-butoxide
in DMSO,
K = 1.5x10-7dm3mol'4 + + t-BuO K + CH3 - S - CH3 < > CH3 - S - CH2 K + t-BuOH i II L
(D) (3.12)
- 22
where K. is the equilibrium constant. CH 0 3 \ I D R CH - CH2 -+ D CH2 - CH- 0 (3.13)
R 0 I - / \ D CH2 - CH- 0 R CH - CH2 -0 D - CH2 - CH - 0 - CH2 - CH - 0- (3.14) R is either H or alkyl. (29) The methyl sulphinyl carbanion (CH3SOCH2 ) known as the dimsyl ion,
plays an important part in the majority of base catalysed reactions in
DMSO, although the equilibrium constant of this reaction is only of the - 7 -1 order of 10 dm3mol (20) Blanchard et al. have established the presence of dimsyl ions
in the polymerization of propylene oxide with potassium tertiary butoxide
in DMSO but they have not been able to detect sulphoxide groups in the
polypropylene oxide.
3.1.C. 1. Polymerization of EthyleneOxide with Potassium t-Butoxide in
Dimethyl Sulphoxide (DMSO)
During the initial stages of ethylene oxide polymerization with
potassium t-butoxide in DMS0 the reaction is first order both in monomer
concentration and in initiator concentration.(25) if the polymerization
is carried out at 25°C, micelles are gradually formed, the polymer precipitates
and a decrease in the values of (R /[M] ) is observed. Raising the t temperature to 50°C or adding 20 vol. % of 192-dimethoxyethane which is
a good solvent for polyethylene oxide eliminates this effect. DMSO
undergoes a phase transition at higher temperatures and becomes a better
solvent for living polyethylene oxide. The second order rate constant 3 -1 -1 is of the order of 0.10 dm mo1 s at 250C.(25) Addition of small
amounts of t-butanol has a pronounced effect on the propagation rate 23
due to the strong solvation of alkoxide ions. The second order rate -1-1 constant 0.016 dm mn1 has been obtained in the presence of a very (25) small amount of t-butanol.
The large value of k in ethylene oxide polymerization with potassium t-butoxide, compared with the corresponding value when the (15) initiator is sodium methoxide indicates the presence of free ions in DMSO solutions.
Ethylene oxide polymerization by potassium t-butoxide in DMSO a.(30) has also been carried out by Price et but no kinetic details have been reported.
3.1.C.2. Polymerization of Propylene Oxide with Potassium t-Butoxide
inDinieth cide(DMS0) (30) Price et al. have found the propylene oxide polymerization with potassium t-butoxide pseudo first order. They have obtained the second -4 3 -1 -1 order rate constant 2.5 x 10 dm mol s at 30°C. (20) Blanchard et al. have found the order of thepolymerization one in monomer concentration, but about 1.7 in initiator concentration.
They have observed that the second order rate constant varies with the inverse square root of initiator concentration. The value of the 3 3 -1 -1 second order rate constant is 3.31 x 10 dm mo1 s for the initiator -2 concentration of 1.71 x 10 mol dm-3 at 50°C.
The large value of the second order rate constant and its variation with initiator concentration suggest that polymerization takes place by free ions and ion pairs.
3.1.C.3. Polymerization of Propylene Oxide with Potassium t-Butoxide in
DMSO and THF Mixtures
In propylene oxide polymerizations with potassium t-butoxide in
DMSO and THF mixtures the reaction order is one in monomer concentration 24
and initiator concentration. A second order rate constant of
5.6 x 10-4dm3mol-1s-1 at 50°C in a THF:DMSO 1:1 (V/V) mixture has been (20) obtained. Unlike the polymerization in DMSO the second order rate constant does not vary with initiator concentration.
The above evidence suggests that in the less dissociating reaction medium of THF and DMSO, the polymerization takes place only by ion pairs.
3.1.C.4. Polymerization of neo-Pentylethylene Oxide Initiated with
Potassium t-Butoxide 0 /\ In the bulk polymerization of neo-pentylethylene oxide [CH3(CH2 )3 -CH-CH2 ] initiated with potassium t-butoxide, unlike the propylene oxide polymerization with sodium methoxide (2b) the initiation is slower than the propagation step. This has been attributed to the incomplete solubility of the (31) initiator during the first 5 to 10% of the reaction. But the propa-
gation is similar to the propylene oxide polymerization with sodium (26) methoxide in dioxane. -4 3 -1 -1 o Second order rate constants of 1.3 x 10 dm mo1 s at b0 C were -1 measured.(31) An activation energy of 4.20 kJ mol and A equal to 3 -1 -1 2.3"x 10 dm mol s were also obtained for the neo-pentylethylene oxide
polymerization.(31)
Similar to propylene oxide polymerization, transfer to monomer occurs in neo-pentylethylene oxide polymerization which will be discussed in
Section (3.3).
3.1.C.5. Polymerizations of 1,2 Butylene Oxide Initiated with K t-BuO
in DMSO and DMSO and THF Mixtures
The kinetic studies of 1,2 butylene oxide were performed using
K t-BuO and K t-BuO/DMSO-THF systems in the temperature range of 30 - 60°c.(127) 25
The order of the reaction rate with respect to monomer concentration
was unity for polymerizations performed in both DMSO and DMSO-THF. The
order of the reaction rate with respect to the initiator concentration
was ^,l.8 and unity for the polymerizations performed in DMSO and DMS0-
THF, respectively. The second order rate constant, k , varied with the
inverse square root of the initiator concentration. It was concluded
that the polymerizations performed in DMSO involved both ion pairs and
free ions, while there were only ion pairs present in DMSO-THF mixtures. -4 3 -1 The k value of 7.5 x 10 dm mols was obtained for the poly-
merizations performed in DMSO-THF at 60°C. -1 An activation energy of 6.16 kJ mol was calculated for the
polymerizations performed in DMSO-THF mixtures.
9 3.2. Anionic Polymerization of Epoxides with Alkali Meta -Aromatic
agrocarbon Initiators
The initiation of vinyl monomer, e.g. styrene, by alkali metal-
atomatic hydrocarbon initiators as discussed in Section (2) takes place
by an electron transfer to the monomer. 26
In epoxides the initiation takes place by direct nucleophilic addition of the monomer to the radical anion. (9-11,4)
The structural studies of the polyethylene oxides prepared by alkali metal naphthalene initiators have shown the existence of the naphthalene group in the polymer.(9) It has been shown that the addition of the radical anion to ethylene oxide is the rate determining step.(12)
O \ Me 4. CH2 - CH2 -+ ( 3.15)
Due to the radical activity, a further conversion of the radical anions takes place by reaction with another initial radical anion.
(3.16)
Reaction (3.16) is dependent on the concentration of the reactants.
It has been observed that at radical anion concentrations less than (13) 10 4ino1 dm-3, reaction (3.16) does not take place.
It has been suggested that the following reaction takes place at (13) low initiator concentrations.
(3.17)
where RH is the solvent.
There is not enough evidence to justify reaction (3.17). It is not clear why reaction (3.16) does not occur at low initiator concentrations.
27
3.2.A. Polymerization of Ethylene Oxide with Alkali Metal Naphthalene
Initiators in THF
The initiation has been discussed in Section (3.B.) The propagation
takes place by addition of ethylene oxide to the carbanions. The
conversion vs time plots show an initial acceleration after which the (13) process is first order in monomer concentration. • (13) Kazanskii et al. have suggested that the reason for the initial
acceleration is the low solubility of the low molecular weight living
oligomers of ethylene oxide in THF. They suggest that the living
polyethylene oxide chains become soluble in THF after their molecular
weights reach 660 - 880.
Faster propagation rates are observed when larger counter ion
• naphthalene initiators are used. This is due to the smaller Coulombic
forces present when the counter ion is large.
The propagation is first order with respect to the initiator
concentrations, e.g., [In] > 0.1 mol dm3 for potassium counter ion at 40°C.
This has been attributed to the association phenomena of alkoxide
ions due to the high charge density on terminal oxygen atoms. The
association phenomena is more pronounced at higher growing end concen- (13) trations.
The growing ends exist in the form of ion pairs, as the dissociation 11 -3 constant of living polyethylene oxide ion pairs in THF is 8 x 10 mol dm + -1 + for K and 2 x 10 mol dm-3 for Cs at 40°C.(13)
A cyclic structure, consisting of three growing ion pairs, has been
assumed for the associates.
The second order rate constants for propagation are: + -1 -1 + for K , k equal to 0.94 dm3mol s and for Cs , k equal to (13) 3.5 dm 3mols, at 700C. 28
The activation energies of propagation are: -1 -1 for e, E equal to 18.9kCal mol = 4.51 kJ mol , and + -1 .(13) for Cs E equal to 11.9 kCal mol = 2.84 kJ mot-1
3.2.B. Polymerization of Propylene Oxide with Sodium Naphthalene in DMS0
and THE Mixtures
Propylene oxide polymerizations have been carried out by sodium naphthalene initiator in THE and DMS0 mixtures, but no detailed kinetic (2U) results have been reported.
Transfer reactions occur which give rise to unsaturation and lowering of molecular weights. 3.3. Transfer Reactions in Alkyl Substituted Ethylene Oxide
3.3.A. Transfer Reactions to Monomer
In base catalysed polymerizations of propylene oxide, the products have considerably lower molecular weights than the calculated molecular weights. (18) St Piere et al. using the infrared spectroscopy have detected unsaturatiens in the polypropylene oxides prepared in the presence of potassium hydroxide.
The generally accepted mechanism for the source of unsaturation and the decrease in molecular weight involves a proton transfer from the methyl group of a propylene oxide molecule to the propagating chain, causing its termination and formation of a new active centre.
R CH2CH(CH3 )0- + H - CH2 -C-- tH2 (3.18) R CH2CH(CH3)0HCH2 = CH - CH20-
Simons and Verbane(32) have observed that the relatively acidic hydrogens of the methyl group in propylene oxide react with metallic sodium to produce hydrogen gas and polymeric materials containing hydroxide and other groups, together with allyl alcohol. Styrene oxide, however, 29
does not liberate hydrogen gas on reacting with metallic sodium. When propylene oxide has been polymerized with trans-2,3-epoxy butane, in the o I(14) presence of potassium hydroxide at 30 C an increase in the unsaturation and a decrease in the molecular weights of the polymerization products is observed by increasing the 2,3-epoxy buLane/propylene oxide ratio.
2,3-epoxy butane enters into the polymerization only after it has been converted to an unsaturated secondary alkoxide by a proton transfer involving the hydrogens of its methyl groups and the propagating alkoxide (14) anion chains, 0
RO- + H CH2 - CH CH - CH, ROH + CH2 = CH - CH(CH3)0- (3.19)
Tri and tetra methylene oxides produce only unsaturated alcohols in the presence of k t-BuO in DMSO, involving reactions between methyl groups of the epoxides and the alkoxide anions. () Examination of the 0 polymerization of deuterated propylene oxide [(CD)3CH - CH2 ] in the (14) presence of anhydrous potassium hydroxide or potassium t-butoxide in
DMS0( 30)has shown that the polymeric products contain less unsaturation than the propylene oxide polymerized under similar conditions.
Anionically polymerized propylene oxides contain allyl and isopropenyl unsaturations. It has been suggested that the isopropenyl groups are obtained as a result of intramolecular arrangement of the allyl groups.
In propylene oxide polymerization with anhydrous potassium hydroxide, the rate of allyl to isopropenyl conversion is a maximum when the degree of polymerization is three and it drops to a minimum when the degree of polymerization is seven. Only 27 mol % isopropenyl unsaturation is present at high conversions.(14)
In neo-pentylethylene oxide bulk polymerization by potassium t-butoxide, transfer reaction similar to those taking place during the 30
propylene oxide polymerizations occur: 0 / R CH2 - CH - CH2 + R'O R CH = CH - CH20- + R'OH (3.20)
3.3.B. Transfer to Solvent
The possibility of transfer reactions to solvent, when ethylene
oxide or propylene oxide are polymerized with potassium t-butoxide in (25) (21) DMSO, have been suggested by Bawn et al. and Blanchard et al. (25) Bawn et al. after considering the molecular weight of polyethylene
oxides prepared in solution do not find solvent transfer to be very
important.
RO + CH3 SOCH3 ROH + CH3 SOCH2 (3.21)
3.3.C. The Unsaturation Content
The unsaturation content is affected by factors such as initiator concentration, the type of initiator and solvent.
Increasing the initiator concentration causes an increase in the 29,)0) unsaturation content,( Table (l). (20) Blanchard et al. have observed that the unsaturation content is larger when propylene oxide is polymerized with sodium naphthalene, rather than with potassium tertiary butoxide.
Also the unsaturation content for propylene oxide polymerized with sodium naphthalene in THE is larger than when the solvent is a mixture of THE and DMSO.
Unsaturation content values have not been reported for neo-pentylethylene (19) oxide polymerizations, but the transfer constant for this polymerization is near to the transfer constant for propylene oxide bulk polymerization (26) with sodium methoxide. This indicates the similarity of the transfer reactions occurring in the polymerizations of these two monomers. 31
3.3.D. The Effect of Transfer Reactions on Molecular Weights in Alkyl
Substituted Ethylene Oxide Polymerizations
The polymers produced from alkyl ethylene oxide monomers have
molecular weights much lower than the calculated values. This is due
to the transfer reactions to monomer.
In propylene oxide polymerization by sodium methoxide at 70°C and
93°C, a linear relationship is observed between the reciprocals of the calculated degrees of polymerization and the measured degrees of poly- (26) merization.
1 = 1 4. a (3.22) DP DPo 1+a where DP and DP are the calculated and measured degrees of polymerization respectively. a is the transfer constant to monomer which has a value of 0.013 at 70°C and of 0.026 at 9
In propylene oxide polymerization by potassium t-butoxide in HMSO, a greater decrease in the molecular weight is observed,(20'30) indicating that attack on the methyl hydrogens is favoured in DMSO which has a high dielectric constant. (20) Blanchard et al. have observed that by increasing the monomer to initiator ratio, the polypropylene oxide molecular weights reach a limiting value. Thus a plot of 1/717 vs 15F0 which is at first linear finally reaches a plateau.
In neo-pentylethylene oxide polymerization with potassium t-butoxide (19) a linear relationship between 1/171F and 1/TOW0 is also observed. The transfer constant a is 0.0116 and 0.007 at 60°C and 39.9°C, respectively. 32
4. Preparation of Monodisperse Polystyrene by Anionic Polymerization Anionic polymerization is a very useful method for the preparation
of monodisperse polystyrene.
There are no termination or transfer reactions during the polymeriza-
tion and the growing chains can compete equally for the available monomer.
Under these conditions, polystyrenes of very narrow molecular weight
distribution (i.e. a Poisson distribution (8)) can be prepared.
The ratio of initiator rate over propagation rate is important.
The faster the initiation rate the faster is the rate of the formation
of the propagating centres that can have equal opportunities of addition
to monomer. However Gold(33) has calculated the effect of the propagation/
initiation rate ratio on the molecular weight distribution. He has shown that even when kp /k. 1 is equal to 10b, (where k and k. are the second order rate constants for the propagation and initiation steps respectively), the Mw/ii n ratio does not exceed 1.3 - 1.4. The rates of initiation and propagation are dependent on the initiator, the solvent and the temperature of the reaction.
In polar solvents such as dioxane and triethyl amine, the initiation step is rapid, whereas in hydrocarbon media such as benzene, heptane, hexane and cyclohexane it may be very slow. This may complicate the overall kinetics of polymerization due to the formation of associations of the initiating species in hydrocarbon solvents.
The formation of free ions, which is dependent on the counter ion and the solvent, also influences the polymerization rates to a great extent.
Section (2.1).
Waak and Doran 34) have studied the reactions between various organo- lithium compounds and styrene in THF, where partial dissociation of the 33
organolithium aggregates takes place. They have observed that the
reactivity of the organolithium compounds with styrene is greatly
dependent on their tendency for forming associations. In turn, this
tendency is dependent on the structure of the organolithium compound.
Vinyl lithium and phenyl lithium initiate styrene polymerization in
benzene very slowly, due to strong associations in benzene.(34)
Kuntz(35) has observed that the initiation of styrene in benzene with
secondary alkyl lithium is much faster than with a primary lithium
compound, though a primary carbanion is more reactive than a secondary
one. This has been attributed to the larger dissociation of secondary
alkyl lithium associations than those of primary alkyl lithium in benzene.
Welsh(36) has observed that the rate of propagation in hydrocarbon solvents becomes independent of initiator concentration when the initiator concentration exceeds some critical value. Worsfold and
Bywater 7) have also observed that the concentration of growing polymers increases with time in benzene and toluene. Thus the concentration of the active centres during the initiation is much lower than it would have been in the absence of association of the initiating species.
The experimental conditions are very important in the preparation of monodisperse polystyrene. The reactants are very sensitive to oxygen, carbon dioxide and the other compounds present in air, therefore the polymerizations should be carried out in the absence of air, either under vacuum or in an inert gas atmosphere. The reaction apparatus should be purged with either initiator or living polymer solution, prior to the polymerization, in order to destroy the impurities that may inter- fere with the polymerization. 34
The ratio of initiation and propagation must be taken into consideration in devising experimental procedures. Where initiation and propagation are both very rapid, it is better to add the monomer solution gradually to the initiator solution. It is also preferable to carry out the reaction at low temperatures. When initiation is rapid, but propagation is slow, monomer and initiator can be thoroughly mixed and allowed to polymerize. If the initiation and propagation reactions are competitive, then initiation can be carried out at one temperature, while propagation can proceed at a lower temperature
"seeding technique".
Stirring is also very important in all anionic polymerizations.
The polymerizing solutions should be stirred continuously throughout the course of the reaction.
Brower and McCormick(38) and Wenger(39) have prepared monodisperse polystyrene by polymerizing styrene with sodium naphthalene in THF. It was found necessary to initiate the polymerisation by adding a small amount of monomer to the initiator solution at room temperature. Then the propagation was carried out by gradually adding the rest of the monomer to the solution at about -78°C. It was observed that relatively polydisperse polystyrene was produced if the initiation and propagation reactions were carried out at low temperatures. This can be attributed to the competitivity of the initiation and propagation reactions. (40) However Waak et al. have prepared monodisperse polystyrenes using sodium naphthalene in THF at temperatures ranging from -78°C to 0°C.
They have reported that the dispersity of the polymer is not dependent on temperature or the ratios of the concentrations of monomer and initiator. Morton et al. have polymerized styrene by sodium naphthalene 35
in THF(41) and by n-butyl lithium and ethyl lithium in benzeneZ
THF.(42) It was observed that in polymerizations in benzene, the initiation and propagation reactions were competitive. They initiated the polymerization with a very small amount of styrene at room temperature and then the solution was cooled to 0°C. The remaining styrene was finally added gradually. The propagations were performed in THF at o (42) about -7o C because of the relatively rapid reactions of the lithium (22,23) compounds with THF above -35C. The propagating polystyryl lithium species do not react markedly with THF during the actual polymerization, but the reaction becomes important when all the monomer has been consumed. (43) Altarez et al. have prepared monodisperse polystyrene in benzene in the presence of a very small amount of THF. The n-bUtyl lithium solution was added to a half thawed mixture of benzene and styrene.
Then a small amount of THF was added. The solution was stirred continuously ) without further cooling. Pannell(44 has carried out styrene polymerizations 0 under similar conditions, in the presence of argon gas at -78 C.
Monodisperse polystyrenes have also been prepared by other organo- metallic initiators in THF, where the polymerizations are very rapid.
The initiation and propagation reactions have been performed at Dry Ice (45) (46) temperatures. Wenger has used sodium biphenyl initiator and Yen has, used oi(phenyl ethyl potassium to obtain polystyrene with values of
equal to 1.05 and M equal to 2 x 104. Pannell(3) has also prepared MMn n monodisperse polystyrene, but used cumyl potassium (a-phenyl isopropyl potassium) as initiator.
4.A. The Stability of Alkali Metal Initiators and Living Polymers in THF
The stability of initiators and living polymers in THF is dependent on the counter ion. The lithium initiators and the living polymers are (22,23) relatively unstable in THF. Section (5.E.1). (47,48) Spach et al. and Asami et al. have studied the stability
of cumyl potassium, benzyl sodium, polystyryl sodium and polystyryl
potassium in THF. They have observed that the cumyl potassium in
THF are stable when stored for a week at room temperature. The measured molecular weight of polystyrene prepared using cumyl potassium was close to the calculated molecular weight.(48)
The loss in the activity of benzyl sodium in THF starts a few hours after preparation of solution.(48) Polystyryl sodium is terminated gradually when stored in THF for a week.(47)
Sodium hydride, NaH, and [-CH = CH(C6H5)] polymer end groups are formed by the self termination of polystyryl sodium in THF. This termina- tion is accelerated in the presence of metallic sodium.
The measured number average molecular weight of the polystyrene, prepared by benzyl sodium, was close to the calculated molecular weight only when a freshly prepared benzyl sodium solution in THF was used.
This indicated a gradual decrease in the activity of benzyl sodium with time.
37
5. Graft Copolymers
Graft copolymers are polymers with long sequences of two monomers.
The sequences of one monomer, B, are attached to a backbone of the
second type of monomer, A, which may be represented:
....AAA....AAA....AAA... (5.1) I I I B B B B B B B B B . . • • . • . • • • •
5.1. Preparation of Graft Copolymers by Anionic Methods
Graft copolymers are prepared in two principal ways using anionic
methods,
(a)active centres are created on the polymer backbone, from which
side chains can grow by anionic polymerization. Section(5.1.A.1),
(b)the side chains are prepared separately by anionic polymerization
and then they are reacted with the deactivating groups present on
the polymer backbone chain. Section(5.1.B).
5.1.A. Preparation of Graft Copolymers by Growin Chains from Active
Centres on the Polymer Backbone
In this type of copolymerization, organometallic sites are created
on the main chain backbone. Then the side chains grow by the addition
of the second type of monomer. The main disadvantage of this method
if the difficulty in controlling the chain length of the grafts and their
molecular weight distribution.
The active organometallic sites can be created in different ways.
(i) Polymers containing vinyl aromatic monomer units, which have acidic
hydrogen atoms on the aromatic rings, can be metalated directly by the
addition of alkali metals. 38
Aromatic radical ions are then formed. Therefore, only monomers
that can be polymerized by nucleophilic addition to aromatic radical (11,49-53) ions may be used for the preparation of the branches. Two
growing ends for each aromatic hydrocarbon are formed by a mechanism
discussed in Section (2). The radical ions are relatively unstable.
Electron migration from the aromatic rings to the alpha carbon atoms of
the aliphatic chain cause bond dissociation and formation of anionic
centres on the polymer chain. The stability of polymer anions is
related to the resonance stabilization of the aromatic groups. Poly-4-
vinyl-biphenyl (PVB) and 1 poly-2-vinyl naphthalene (PVN) radical ions
have relatively high resonance stabilization. They start to degrade
at a measurable rate at 75°C, while polyacenaphthalene undergoes
instantaneous chain scission at that temperature. Graft copolymers of (11,49- ) ethylene oxide have been prepared with PVB and PVN poly radical ions. 54
Pyridine forms a radical ion by direct metalation with sodium. The radical ions form dimer dianions. These sodium pyridine complexes can (55) initiate anionic polymerizations.
Pyridine can be utilized for initiation of anionic polymerizations when incorporated in a polymer chain. Poly-4-vinyl pyridine radical ions are capable of initiating polymerization of methyl methacrylate in THE but not styrene.(55)
9-Fluorenyl lithium is basic enough to initiate methyl methacrylate polymerization but it cannot polymerize styrene. Fluorenyl lithium is prepared either by direct reaction of fluorene with lithium or with n--butyl lithium.(56) Lithiated poly vinyl fluorene is capable of initiating methyl (56) methacrylate polymerization in THF.
Block copolymers of styrene with methyl methacrylate, acrylonitrile and alkinylstyrenes have also peen prepared. Then the monomer units of the last three of the above mentioned monomers have been metalated directly (57 with metallic sodium. '58) 39
- CH -CH2 -CH I -CH2 - CH - -CH2 - CH- H2 ° C = N- 1 nM H-C=N-(M)nH 4- *C = N -. - .0 = N-(M)n_1 I C = N I --CH2 -CH- -CH2 - CH H-C=0
/1214(M)nH
(5.2)
-CH2 - CH- -CH2 -CH- -CH2 - CH- Na Na
■•■ (5.3)
CH2 -CH = CH2 CH2 joH-C71.2
The metal at ion can be carried out with alkali metal-aromatic hydrocarbon compounds, polydiphenY1-313-propane has been metalated with sodium-naph- thalene.
-CH2 -CH-CH2 - -CH2 -CH-CH2 - I Na naphthalene I -C- (5.4) - H Na Then graft copolymers have been prepared by the addition of styrene, methyl methacrylate and vinyl-4-pyridine.(51) Starch has been metalated with sodium naphthalene in DMSO. The graft copolymer has been prepared by adding ethylene oxide.(59) 4o
Alkali metal ketyls, the products of the reaction between alkali metals and ketones with no alpha hydrogen atoms, are capable of initiating the polymerization of monomers such as acrylonitrile and methyl methacrylate in THF.(60-62)
0
(5.5)
0
(b) (a)
The polymerization of a mixture of styrene and acrylonitrile in the presence of sodium benzophenone monoketyl has produced only homopolymers (62) of acry1onitrile. The monoketyl is not basic enough to be able to initiate styrene. But if the strong initiator dianion (b) is formed due (60,61) to the presence of excess sodium, styrene can also be polymerized.
When the ketyl groups are attached to a polymer backbonek they are capable of ihitiating acrylonitrile and methyl methacrylate polymerizations.
In the presence of excess sodium, the dianion (b) is formed which makes possible the preparations of polystyrene and polybutadiene side chains.(63-65)
(ii) The metalation can take place by exchange reactions between metals and halogens that are attached to the aromatic rings of the polymer.
The lower the electronegativity of the halogen the easier is the exchange reactions.
Poly-p-sodium styrene has been prepared by metalating poly-p-chloro- (66,67) styrene with sodium naphthalene in the THF.
Some undesirable crosslinking of poly-p-chlorostyrene also takes (67) place. 2 - CH - CH2 - - CH - CH2 - I 2 Na naphthalene (5.6)
Cl
- CH - CH6 -
Graft copolymers have been prepared by the addition of acrylonitrile, (66,6 vinyl pyridine and methyl methacrylate to poly-p-sodium styrene polyanions.
Partially iodinated polystyrene has been prepared by reacting poly-
styrene with iodine and iodic acid in nitrobenzene for 30 hours at 90°C.
• The poly-p-iodostyrene has been treated with n-butyl lithium. Lithiation (6-71) at para positions occurred.
The polymer metalated by metal halogen exchange reactions are capable (6971) of initiating easily polymerizable monomers.
Metalated polystyrene has been prepared by exchange reaction of (74) poly-p-bromostyrene with n-butyl lithium,(72'73) lithium-naphthalene
and sodium-naphthalene.(75) Then graft copolymers have been prepared
by addition of acrylonitrile, methyl methacrylate and styrene.(74175)
The metalation of halogenated polyolephines has been performed by
metal-halogen exchange reactions. Polyvinyl chloride detala ion has (76) been carried out in THF. Bromopolyethylene and chloropolyethylene
have been metalated by n-butyl lithium(77) and phenyl lithium,(78) in THF.
Due to the similar electronegativities of the aliphatic functional
groups, the exchange reactions are not as efficient as in halogenated
polystyrene where quantitative metalation occurs.
Three simultaneous reactions take place between n-butyl lithium and
the halogenated polyolephine; metal-halogen exchange reactions,
42
Wurtz-Fittig type reactions and dehydrohalogenation.
Metal-halogen CH2 - CH- + R'X Exchange Li Wurtz - CH2 - CM- + R'Li - CH2 - CH + LiX (5.7) Fittig X R' Dehydrohalogenation - CH = CH - CH - CH 2 -
X
In the case of bromopolyethylene the metalation is inefficient as
the dehydrohalogenation is the prominant reaction.
When phenyl lithium is used in exchange reactions, all the three
above-mentioned reactions take place simultaneously.
(iii) n-Butyl lithium and tetramethylenethylenediamine (TMEDA) complexes
are used for direct metalation of polymers containing reactive hydrogen
atoms, e.g., polymers containing aryl groups, diene polymers and polyphenylene
The metalation takes place by a nucleophilic attack of the complex
anion on the sites that have reactive hydrogen atoms according to the
scheme:
CH3 CH3 CH CH3 / , N A / CH2 CH2 Li - C + C4 H10 (5.8) I 2 n-C4 H3 o-Lio+ + H8+- C a -I, I CH \ A CH2‘ N N ,/ N, ,- ■ CH3 CH3 CU3 CH3
The metalation of polyphenylene ethers has been carried out in
benzene, toluene and THF. It has been observed that the metalation of
43
poly-2,6-dimethy1-114-phenylene ether with n-butyl lithium - TMEDA (79,80) occurs on the nucleus as well as on the methyl groups. The
methyl group substitution has been attributed to the slow isomerization
of the ring metalated polymer to form polymer metalated in benzylic
positions. Grafts of polystyrene havo been prepared by adding styrene
to metalated polyphenylene ether. The metalation occurs only on the
methyl groups when organosodium or organopotassium compounds are used (79) as metalation agents.
The metalation of diene polymers in n-heptane has been carried out o at 80 C. 27% and 9% metalation per monomer unit has been obtained for (81,82) polybutadiene and polyisoprene, respectively.
The exchange reactions have taken place with the allylic hydrogens
of the polymer, but no addition to double bonds has taken place. CH3(Li)
- CH2 C = CH - CH2 - - CH2 - CH = CH = CH2 - (5.9) (Li) (Li) (Li)
Then graft copolymers have been prepared by adding styrene, methyl
methacrylate and acrylonitrile.
The metalation of polymers containing aryl groups, mainly polystyrene,
has been carried out in n-heptane, hexane and cyclohexane, at temperatures (78,83-86) ranging from 50°C - 60°C.
The metalation of toluene with n-butyl lithium-TMEDA complexes has
resulted in ring and methyl lithiation.(87)
When metalated polystyrene has reacted with D20, trimethylchlorosilane
or dimethylsulphate, ring-substituted products were obtained.(87) It has
been suggested that a benzylic anion is formed first, then, the substituted
derivatives are formed. 44
It has been shown recently that the metalation of polystyrene by n-butyl lithium-TMEDA in cyclohexane at 55°C occurs both at meta and (83,84) para positions on the aromatic rings.
The Iithiated polymers have been used to initiate polymerizations (78) (83,84) of styrene and diene monomers.
The rate of propagation of styrene polymerization in n-heptane initiated with n-butyl lithium is slightly faster than the rate of propagation of styrene initiated by polystyryl lithium. However, in the presence of TMEDA, the polymerization of styrene proceeds faster with the polystyryl lithium than with n-butyl lithium. This effect is explained by the association of active centres linked in the same polymeric chain in the absence of TMODA.(78)
(iv) Ester groups are capable of initiating C-caprolactam polymerization.(88)
Hence ester groups, attached to a polymer chain backbone, can be utilized for grafting on poly-C-caprolactam.
Block copolymers of styrene-ethyl acrylate, styrene-methyl methacrylate can also be used as starting materials. Then the ester groups present (89) in the copolymer can be used to initiate e-caprolactam polymerization.
5.1.B. Preparation of Graft Copolymers by Deactivation
(i) In this method, suitable living homopolymers are reacted with electrophilic deactivating groups present on a different dead polymer backbone.
Electrophilic groups such as esters, nitriles, halogen atoms and anhydrides attached to the polymer backbone can act as deactivators.
Polystyrene grafts on polymethyl methacrylate (PMMA) and polyvinyl (81,90,91) chloride (PVC) have been prepared by this method. The living 45
monodisperse polystyrene, prepared with cumyl potassium in THF, has been
added to PMMA and PVC in THF solutions at -78°C. Then the temperature
has been allowed to rise while efficient stirring has been maintained
during the grafting process. The percentage of grafting, i.e., the (90) percentage of the original deactivating groups, has been less than 10%.
This method has also been used for the preparation of branched homo- • polymers of styrene (comb-like).
The disadvantages of this method are: (a) the grafts are attached to
the main chain by means of a carbonyl group which is sensitive to photo-
chemical oxidation; (b) the number of the grafts is proportional to the
length of the main chain. Thus the molecular weight distribution of the (74,92) copolymer is as broad as that of the backbone itself.
With pyridine as the deactivating group in polyvinyl pyridine, the
deactivation with polystyryl potassium forms graft copolymers which (92) decompose at high temperatures. This method of grafting is restricted
to a small number of cases but it is a useful method for the preparation
of polymers for morphological studies. The length of the backbone, the
number and average length of grafts are experimentally accessible. Also
the random distribution of the grafts and the low level of fluctuations
in composition within a sample have been established.
(ii) Anionically prepared monodisperse polystyrene is chloromethylated
on the benzene ring. The electrophilic chloromethyl groups can act as
deactivators on reacting with living polymer chains and form grafts on
the backbone.
The chloromethylation of polystyrene is carried out with chloromethyl (93,44) (4394) methylether in the presence of Zn and Sn chlorides in (93) chloromethyl methylether or carbon tetrachloride. (43) 46
ZnC12 CH2 - SnC12 CH3°H (5.10) SnC14 CH2 C1
• • The substitution occurs predominantly but not exclusively in the (43) para position, the order of the substitution being p >> V >m positions.
* This method has been used extensively in the preparation of branched (43,44,95) (96 homopolymers of styrene and also styrene-isoprene graft copolymers.
The coupling reaction has been carried out in THF and THF-benzene
mixtures.
The counter ion of the living homopolymer plays an important part in (43,44 ,95,96) the deactivation reaction. Side reactions during the coupling
reaction between the chloromethyl group and the counter ion results in the (43144) cross-linking of polystyrene. These reactions are metal-halogen
exchange processes which compete with the deactivation reactions. The
metal-halogen exchange results in the formation of metalated sites,
followed by the reaction of metal containing coupler with another coupler
molecule. If the degree of chloromethylation of polystyrene is low and
its solution in either THF or benzene-THF is added gradually to the living
homopolymer solution, then the concentration of the chloromethyl groups
present in the homopolymer solution at any given time is low. Thus the (43,44 ) possibility of the halogen-metal exchange reactions will be redUed.
The exchange reactions occur mainly when lithium is the counter ion .4- of the living homopOlymer. Thus with Li as counter ion the coupling
reactions are not complete due to the covalent bond character of the (46) carbon-lithium bond. (97) Hoeg et al. have suggested that the following reactions take
place betweeen benzyl chloride and n-butyl lithium in the presence of THF. 4-7
C6H5CH2C1 + n -BuLi -*C6H5CHC1Li + n -Butane
C6H5CHC1Li + C6H5CH2 C1 -■ C6H5CHC1CH2 C6H5 + Lid (5.11)
{ C6H5CHC1CH2 C6 H5 -■ C6 H5CH = CHC6 H5 + HC1 (22,23,98) n -Butyl lithium reacts with THF. During the coupling reaction of polystyryl lithium with chloromethylated polystyrene, the side reactions may be due to the products derived from n-BuLi and THF () reacting with the chloromethylated sites.
It has been observed that during the preparation of styrene-isoprene graft copolymers the reaction of polyisopropenyl lithium with chloro- methylated polystyrene in benzene results in cross-linking, while the addition of TMEDA has prevented this problem. The high solvating power of TMEDA may be responsible for ensuring coupling by lithium chloride elimination.( 96)
The side reactions that occur during the coupling are eliminated to a large extent if potassium is the counter ion of the living homopolymer.
The preparation of (star shaped) branched polystyrene using tetra-chloro- methyl-benzene and polystyryl potassium in benzene, suggest that cross- (46) linking does not occur. (95) Pannell has used polystyryl potassium in THF for the preparation of (comb-like) branched homopolymers of styrene with long chain branching.
This was a satisfactory method for avoiding side reactions during (99) coupling. But Fujimoto et al. have obtained branched (comb-like) polystyrenes of somewhat higher molecular weight than expected when partially chloromethylated polystyrene was reacted with polystyryl potassium in THF-benzene mixtures.
Various compounds containing polyfunctional electrophilic deactivator
(43,46) (1 ) groups, such as tri- or tetra-chloromethyl-benzene and SiC14, 91° 48
react with living polymers and produce branched (star shaped) polystyrenes. (43) Altarez et al. have observed that the functionality of the branched
(star shaped) polystyrenes is greater than the expected four when tetra- chloromethyl-benzene has been used as the deactivator of polystyryl lithium in THF-benzene (1:1) vol/vol mixtures. This higher functionality has been attributed to metal-halogen exchange reactions resulting in cross-linking.
They have suggested that reactions similar to those between benzyl chloride (97) and n-butyl lithium in THF may also take place which result in some decrease in functionality of the branched (star shaped) polystyrene.
The possibility of the control of the graft lengths and their molecular weight distribution is an advantage in the preparation of graft copolymers by deactivation of chloromethylated sites.
But.always.there are some homopolymers mixed with the copolymers.
They are formed from the excess living homopolymers that have reacted with the chloromethylated sites during coupling. The separation of these homopolymers may be difficult. Again, the reproducibility of the chloro- methylation is not very satisfactory. It is highly dependent on the concentrations of the reactants and catalyst, the reaction temperature and the reaction time. Table (1) : Summary of Kinetic Data for Propylene Oxide Polymerizations
Refs. [M] [In]x102 M.wt Unsat- Transfer M.wt (Meas.) k Monomer Initiator Solvent T E A (Calc.) uration p a constant
26 4.62 1.3 prop. NaOCH3 (bulk) 70 4.15 2.3x107 0.013 oxide 26 5.1 1.92 15400 2.7 " n dioxane 93 0.026 26 5500 1980 It It (bulk) 7(70) 4.15 26 1500 1056 11 to it It It 11 II 26 5000 1300 93 tt 11 11 tt 26 360 300 93 30 3.3 6.7 2850 1160(96% yield) 4.25 2.5 " K t.BuO DMS0 30 If 11 30 3.3 2 9570 1135 3.85 2.5 30 n n 30 3.3 6.7 2850 2500(91% yield) 2.71 1.2 (a) 30 n n 30 3.3 2 9570 2085(78% yield) 2.57 1.2 (a) 30 20 6.48 17.1 2200 1270(GPC) 0.21 13.6 prop. n n 50 oxide 11 II It 20 5.6 1.71 19000 3000( " ) <).05 33 50 II t1 II 20 4.2 0.17 142000 2700/7000 (GPC) <0.05 26.3 50 3.96 11 II 20 5.13 0.74 40000 4400/15000( " ) <0.05 5.6 DMSO:THP 50 1:1 11 II 20 6.0 7.4 4700 3900 (GPO <0.05 1.3 " 40 20 5.6 1.71 19000 5300 ( " ) <0.05 1.07 " n n 40 20 2.69 0.74 21000 3600/11000(GPC) <0.05 1.54 " It 11 40 20 3.9 1.71 13500 3500 (GPc) <0.05 0.458 " t, THE 40 it 20 0.015 1200/5300 (GPO) 0.80 Na naph- THE 100 thalene 20 0.015 2250/8000 ( " ) 0.50 it It DMSO:THF 1:1 100 31 4570 3220 (b) K t.BuO (bulk) 39.9 4.2 0.007 It 31 1230 990 60 0.0116 It 31 0.6 60
(a) 313,3-Trideuteriopropylene oxide Units: [M] and [In] MO1 dM 3 1 (b) neo-pentylethylene oxide Unsaturation meg. g- dm -1 -1 k and A 7 mol s P T degrees centigrade B kJ mob
50
PART TWO MATERIALS
6. Purification and Storage of Materials
6.1. Purification Techniques
The following drying agents were ozed for the purification of
solvents and the reactants: calcium hydride, sodium mirrors and • sodium-potassium alloy.
6.1.A. EmalraLLan aLs9dium Mirror
About 0.2 - 0.4 grams of freshly cut sodium metal was immersed
consecutively in two beakers containing petroleum ether of 80 - 100°C
and 50 - 60°C boiling points, respectively, to remove the protective
layer of liquid paraffin covering the metal. Then it was dropped
• into a round bottom Quickfit flask containing a glass encased magnetic
follower. The flask was quickly attached to the vacuum line, and it
was evacuated to 10 4 torr. The bottom of the flask was gently heated
by a gas flame, while the flask was still being evacuated. The magnetic
follower was kept in the neck of the flask by means of a magnet. The
sodium melted, started to evaporate and formed a mirror on the wall of
the flask.
The flame was removed, and the follower was lowered to the bottom
of the flask after it had cooled down. (101) 6.1.B. Preparation of Sodium-Potassium Alloy
Sodium and potassium form an alloy molten at room temperature, which
is useful for drying liquids at room temperature.
The sodium-potassium alloy was prepared in a similar way to the
sodium mirror. The size of the lump of potassium was about twice the
size of the lump of sodium.
On being heated, the potassium which had a lower melting point,
melted first, dissolved the sodium and formed the liquid alloy. 51
6.2. Solvents
6.2.A. Tetrahydrofuran (THF)
Tetrahydrofuran (Koch-Light Laboratories Ltd puriss grade) was stirred over calcium hydride for two days. It was distilled away from calcium hydride and was dried on the vacuum line over sodium- potassium alloy at room temperature. It was distilled into the evacuated storage vessels, Figure (8) [THF for direct use as solvent in polymeriza- tions] and Figure (1) [THF for initiator solution preparations].
6.2.B. Dioxane
Dioxane (BDH R grade) was used. It was purified in a similar way to THF. Section (6.2.A). b.3. Monomers
6.3.A. Ethylene Oxide
The ethylene oxide gas (Cambrian Chemicals) was 99 weight % pure and was supplied in 100 cm3 bottles. For further purification and prevention of contamination during storage on the vacuum line, the ethylene oxide was passed through a tube filled with B.D.H. molecular sieves size 4A (F). Figure (4). The ethylene oxide was condensed on to a sodium mirror in a flask, immersed in liquid nitrogen, attached to the vacuum line. The liquid nitrogen was removed and the flask was heated up to 00C. The flask was then kept immersed in an ice bath to keep the ethylene oxide in liquid form. The ethylene oxide was stirred by means of a magnetic follower for about half an hour.
Quantities of ethylene oxide were measured by distillation into a constant temperature calibrated tube. Figure (2). Then the ethylene oxide was distilled into the polymerization vessel. 52
6.3.B. Propylene Oxide
The propylene oxide was 99 weight ,O pure (Ralph N. Emanuel Ltd).
It was stirred over some calcium hydride while attached to the vacuum line. Then it was distilled into a flask containing sodium-potassium alloy and was stirred over it for about half an hour at room temperature.
The propylene oxide was partially polymerized initially by the sodium-potassium alloy, which ensured the absence of materials which would interfere later with anionic polymerizations.
Quantities of propylene oxide were measured in a manner similar to that previously described for ethylene oxide.
6.3.C. Styrene
The styrene (BEM) was stabilized by 0.001 to 0.002 weight % t-butyl catechol. It was washed three times with 10% w/v aqueous sodium hydroxide and six times with distilled water. It was then stirred over anhydrous calcium sulphate followed by calcium hydride.
It was next distilled under reduced pressure. The middle fraction was finally dried over a sodium mirror. The temperature of the flask was kept at about 0°C to prevent any sudden violent polymerization of styrene initiated by metallic sodium. Measured quantities of styrene were distilled into the storage tubes, (Figure (3)) and were sealed off in vacuo. The measurements of known quantities of styrene were carried out by means of a calibrated tube attached to the vacuum line.
6.4. n-Butyl Bromide
The n-butyl bromide (Hopkin and Williams) was dried over calcium hydride on the vacuum line. Then it was distilled into the containers fitted with break seals, spring loaded taps and suba seals. Figure (8). 53
6.5. Chloromethylation Catalysts
6.5.A. Stannic Chloride
The stannic chloride (Koch-Light Laboratories Ltd) was about
95 weight % pure and was supplied in 100 cm3 bottles. The stannic chloride was a moisture sensitive colourless liquid. It was transferred, under partial vacuum through fine steel tubing, into a container.
Figure (7). Then the container was filled with argon gas until the pressure inside the container was approximately one atmosphere.
Known amounts of stannic chloride were transferred into the reaction vessels by means of Luer-Lock syringes and needles. Section (7.6).
The piston of the syringe was immediately removed from the barrel to avoid jamming because of formation of a solid layer inside the syringe due to the fast reaction of stannic chloride with moisture present in air.
6.5.B. Zinc Chloride
The zinc chloride (Koch-Light Laboratories Ltd) was about
95 weight % pure and was supplied as a white powder in 500 g bottles.
The commercial sample was stored in a glove box, filled with argon gas, to prevent the rapid reaction of zinc chloride with moisture present in air.
6.6. Chloromethyl Methyl Ether
The chloromethyl methyl ether (CME) (Ralph N. Emanual Ltd) was
97 weight % pure and was supplied in 100 cm3 bottles.
The CME was a colourless moisture-sensitive liquid. It reacted with moisture present in air to produce hydrogen chloride fumes. As
CME was very carcinogenic, it was kept in the original commercially supplied bottle and was stored in a deep freezer at -250C. The CM was used directly in the chloromethylation reactions, without any further purification. It was warmed to room temperature and then it was transferred into the reaction-vessels, through steel tubing, in a sufficiently ventilated fume-cupboard.
6.7. Initiators
6.7.A. The Solution of n-Butyl Lithium in Hexane
The n-butyl lithium initiator (Koch-Light Laboratories Ltd) was supplied as a 15% (w/v) solution in hexane.
The n-butyl lithium was very air and moisture sensitive. It was usually handled in an argon atmosphere. The commercial sample of the initiator solution was stored in a container. Figure (7).
6.7.B. Solutions of Cumyl Potassium in THF
The cumyl potassium initiator (Orgmet, Inc. Hampstead, N.H., U.S.A.) was supplied as a suspension in n-heptane, in the presence of an excess of metallic potassium to prevent any degradation.
As cumyl potassium is very air and moisture sensitive it was handled in a glove boxi in an argon atmosphere.
The cumyl potassium in n-heptane suspension was filtered through a porosity 3 glass sinter to remove the n-heptane. The solid cumyl potassium and potassium was then stirred with purified THF. The solution was filtered through a porosity 3 glass sinter to leave behind the metallic potassium. The cumyl potassium solution was stored in a container.
Figure (7). The container was kept in a deep freezer to prevent the slow reaction between the cumyl potassium and the THF. The solution had a cherry red colour.
6.7.B.1. Determination of Cumyl Potassium in Cumyl Potassium-THF Solutions
Cumyl potassium reacts with n-butyl bromide and forms potassium bromide which can be determined by titration with silver nitrate in the presence of fluorescein acid indicator. 55
A known quantity of cumyl potassium in THF solution (about 1 - 2 cm3); was transferred to a suba-seal stoppered tube filled with argon gas, previously purged with cumyl potassium in THF solution. About U.5 cm3 of purified n-butyl bromide was introduced into the tube by means of a syringe. The n-butyl bromide immediately reacted with the cumyl potassium and the cherry red colour of the cumyl potassium disappeared. Some distilled water was added to the solution to dissolve the potassium bromide. The solution was slightly acidified by nitric acid and was titrated with a standard silver nitrate solution in the presence of fluorescein acid indicator.
The silver nitrate solution was standardized against a standard sodium chloride solution. 5.
56
PART THREE EXPERIMENTAL
7. Apparatus
7.1. The High Vacuum-Inert Gas System
Anionic polymerizations are sensitive to gases such as oxygen,
carbon dioxide, and water vapour, which cause termination of the
reactions. Therefore these polymerizations should be carried out
in the absence of air.
The purification of reactants, solvents, and the preparation of
the polymerizing solutions were carried out under vacuum in the presence
of inert gas (Argon B.O.C. High Purity). Figure (4).
(i)The Pumping Part
The pumping part consisted of a high vacuum rotary oil pump
(Edwards Speedivac 2SC30) (A), and a mercury diffusion pump (B), with
two liquid nitrogen traps, one between the two pumps, and one between
the diffusion pump and the vacuum lines (C)(C1). This prevented
mercury diffusing into the rotary pump and the vacuum lines, and solidified
the condensable vapour. The pressure was measured by a McLeod gauge (D).
(ii)The Ethylene Oxide Line
A 100 cm3 ethylene oxide cylinder (E) was attached to a one metre
long, 2 cm o.d. glass tube, filled with size 4A molecular sieves (F) by
means of a short length of pressure tubing. The tube (F) was connected
to a one dm3 flask (G), a mercury manometer (H), and seven outlets of
(0.6 cm i.d.) straight quill high vacuum (Springham) taps, and B-24
extended cones (J)(I).
(iii)The Argon Gas Line
The argon gas line consisted of two gas drying bottles containing
sulphuric acid, and sodium hydroxide (K)(L), with a five dm3 flask (M), 57
and a mercury manometer (N). This line could be connected to the ethylene oxide line by the tap (0). The two argon gas outlets (P) and
(Q) were used to create an inert atmosphere whenever air-sensitive liquids or solutions were being transferred or handled.
The ethylene oxide and argon lines could be connected to the rotary vacuum pump via the tap (R) and a short length of pressure tubing. This pump was used to create a rough vacuum in the ethylene oxide line, whenever it was being purged by argon gas.
N-Apiezon grease was used for the pumping part, and MS-Silicone
High Vacuum Grease for the rest of the system.
The vacuum line (S) was used for the purification and distillation of all the reactants and solvents apart from ethylene oxide and propylene oxide.
7.2. The Thermostatted Bath
The bath was of cylindrical glass and was 35 cm high and 30 cm in diameter. It was filled with water and was kept covered with poly- propylene balls of 2 cm diameter to reduce evaporation. The water was constantly stirred to obtain a uniform temperature.
The temperature of the bath was controlled by a mercury thermostat
(Gallenkamp 10510/1A8.Sc.Tetcol) connected to a heater via a mercury electronic control relay (Tetcol). A standard thermometer was used as a reference thermometer. 7.3. The Use of the "Sovirel" Taps and Joints Originally, PTFE-glass "Sovirel" tans and joints (Figure (5)) were used in the vacuum line and in the polymerization apparatus, instead of the more conventional high vacuum glass taps and break-seal stoppered containers. 58
It was hoped that the use of "Sovirel" taps in the vacuum system would prevent contact between any distilling solvents and the grease of the high vacuum glass taps. The high vacuum system could then be altered by connecting different sections to the main pumping part of the vacuum line using these "Sovirel" joints. Again use of "Sovirel" taps in polymerization vessels would have eliminated the use of the break-seals and solutions could be transferred very conveniently through the tapsie The use of the "Sovirel" joints could also facilitate the connection of the different sections of polymerization vessels and thus eliminate glass blowing.
The vacuum system involving "Sovirel" taps and joints was evacuated continuously for four weeks, but it was not possible to reduce the pressure inside the system to below 10-4 torr. This was due to air leakage through the "Sovirel" joints and taps. Thus these joints and taps were unsuitable for performing anionic polymerization reactions under high vacuum.
However, the "Sovirel" joints and taps were successfully used in the experiments that were not performed under vacuum. The "Sovirel" taps were particularly superior to glass taps where solutions had to be transferred through them, due to the better control of flow rates.
Also the blocking and jamming of taps, which may occur with glass taps were avoided.
7.4. The Use of the Splash Head in the Purifications
When the liquids that had been driad over calcium hydride on the vacuum line were being distilled from the solid some fine solid particles of calcium hydride and the products of its reactions with impurities were also carried over. Vertical splash heads were used to prevent this undesirable phenomenon during the drying of all the liquid reagents and 59
solvents over calcium hydride on the vacuum line. Figure (6). One splash head connected the flask containing the liquid drying over calcium hydride, to the vacuum line. Another splash head connected the flask into which the distilled liquid was collected to the vacuum line. As may be observed from the construction of the splash head, the fine solid particles travelling in the vacuum line during the
distillations were collected in the bulb of the splash head.
7.5. The Containers used for Li uid Storage in Argon Gas Atmosphere
Two types of containers were used for the storage of the solutions under argon gas:
(a) One container that consisted of a round-bottomed flask, of about
100 cm3 capacity, joined to a 0.3 cm spring-loaded tap and a size 21 or 25 suba-seal. The container also had a 0.3 cm i.d. capillary tubing side arm joined to a B-24 Quickfit socket. The purified liquid was distilled into the container on the vacuum line through the side arm and the container was then sealed off. The break-seal was broken and argon gas was introduced into the container. Figure (8).
7.6. The Syringes and Needles
Luer-lock syringes (Chance supplied by A.R. Horwell Ltd) were of
1, 2, 5 and 20 cm3 capacity. Stainless steel tubing of about 35 cm length and about 0.1 cm internal diameter were used for transferring liquids. 6o
8. Kinetic Studies of Ethylene Oxide and Propylene Oxide Polymerizations
8.1. The Dilatometric Measurements
The kinetic studies of cumyl potassium initiated homopolymerizations
of ethylene oxide and propylene oxide in THF and ethylene oxide polymeri na-
tions in THF-dioxane mixtures were performed by conventional diIatometry. Volume contraction of the solutions due to the polymerizations were
followed at temperatures ranging from 30 - 65°C, using the thermostatted
bath.
8.1.A. Filling the Dilatometers
(i) Polymerizations in THF
The dilatometers (Figure (9)) were connected to the vacuum line by
means of the outlets (J), Figure (4). They were evacuated and filled
with argon gas for a few consecutive times, to create an inert atmosphere
in the dilatometers. The dilatometers were purged with cumyl potassium
in THF solution, which was introduced into the dilatometers by means
of syringing through the suba-seals and the spring loaded taps, while
the dilatometers were filled with argon. The pressure of the argon
gas was kept about 2 cm Hg below the atmospheric pressure.
The measured amounts of cumyl potassium in THF solutions were
introduced into the dilatometers by means of syringing. The dilatometers
were frozen by immersing them in liquid nitrogen, and they were then
evacuated.
The required volumes of the monomers, ethylene oxide or propylene
oxide were measured by the calibrating tube, Figure (2), previously
purged by cumyl potassium in THF, attached to the vacuum line by means
of the outlets (J), Figure (4). The temperature of the calibrating
tube was kept at 00C, by immersing it in an ice bath. 61
The measured amounts of the monomers were then distilled into the
dilatometers. The liquid nitrogen was removed and the dilatometers
were heated up to room temperature. Argon gas was re-introduced into the dilatometers. The dilatometers were filled up to the required
volume by syringing into them required amounts of purified THF. Finally the dilatometers were frozen in liquid nitrogen, and after evacuation they were sealed off.
The spring loaded taps were kept closed when the dilatometers were being evacuated, to prevent air being introduced into the dilatometers.
Figure (9).
(ii) Polymerization in THF-dioxane mixtures
The dilatometers were connected to the vacuum line. They were purged with cumyl potassium solution in a manner described in Section (i).
Then measured amounts of cumyl potassium in THF solutions were introduced into the dialtometers. The THF was distilled away from the dilatometers.
Measured amounts of ethylene oxide were then distilled into the dilatometers.
The liquid nitrogen was removed and the dilatometers were heated up to
0°C. The dilatometers were filled with the required amounts of THF and dioxane. Finally the dilatometers were frozen in liquid nitrogen and after evacuation they were sealed off.
The spring loaded taps were kept closed when the dilatometers were being evacuated, to prevent air being introduced into the dilatometers.
Figure (9). (18) 8.2. Determination of the Unsaturation Content in Polypro ylene Oxide
A weighed amount of polymer was dissolved in 10 cm3 AnalaR chloroform and 10 cm3 glacial acetic acid. After cooling in the ice bath, 10 cm3 62
of a very dilute solution of bromine in glacial acetic acid was pipetted
in.
After standing for exactly two minutes, 25 cm3 of 10)0 (w/v) potassium iodide solution was added, which reacted with the excess of bromine which had not reacted previously with the double bonds and liberated iodines.
The solution was immediately titrated with standard sodium thiosulphate using starch indicator. A blank titration was performed at the same time. The unsaturation was calculated from the difference in the results.
The sodium thiosulphate solution was standardized by titrating the toluene liberated from a weighed amount of potassium dichromate with excess potassium iodide in concentrated hydrochloric acid solution.
8.3. Measurement of Molecular Wei hts b Vapour Phase Osmometry
The Mechrolab model 3O2 Vapour Pressure Osmometer (WO) (Hewlett-Packard) was used for measuring the molecular weights of the polyethylene oxides and polypropylene oxides prepared in the present work.
Benzyl was used for calibrating the instrument and toluene was used as solvent. The instrument was maintained at 37°C.
8.4. The Automatic Viscometer and Viscosit1 Measurements
A "FICA model 52000 Viscomat" automatic capillary viscometer was used for the viscosity measurements of the low molecular weight propylene oxide in toluene solution. A "Haake mod,A. FS Constant Temperature
Circulator" was used for controlling the viscometer temperature.
The viscosity measurements were carried out in a conventional way at 25°C and full details of the methods of analysing the results are (102) described elsewhere.
The following K and a values were used for calculating the molecular weights of the propylene oxides from the viscometric data: (103) K equal to 12.9 x 105 and a equal to 0.75 at 250C.
63
8.5. The calculation of Rates of Polymerization
The rates of polymerization were calculated using the data obtained by dilatometric measurements in the following way: 0 ..1, Percentage Concentration 1 . 100 for a time t (8.1) in volume V where V is the initial total volume of the polymerizing solutions, v is the volume of unit length of the capillary, (10 - 11 ) is the change in the height of the solution in the capillary tubing occurring in a time t.
One hundred per cent = Vm - Vp 100 per cent volume conversion (8.2) V contraction where V is the initial volume of the monomer in the system and V m is the polymer volume in the system after one hundred per cent conversion.
The percentage polymerization corresponding to time t (minutes) is given by: - ) v . V 100 at a fixed Percentage polymerization = Vm dm V temperature Vm - d - p (8.3)
[(1 0 - 11 ).v.100 Percentage polymerization = dm (8.4) V (1 d ) -)1 where d and d are the monomer and polymer densities respectively. m (10 - 11 ).v.100 Percentage polymerization 1 ■••■■••■1111.1. (8.5) per second dm 60t Vm - 71-1 p Hence: 10dm (10 - 11 ).v.100 - -1 Rate of Polymerization - d_ mol dm 3s (8.6) m.6o.t. v (1 - m dp where M is the molecular weight of the monomer. 64
8.6. Calculation of Activation Energies of Polymerizations
The Arrhenius equation was used for the calculation of activation energies of polymerizations of ethylene oxide and propylene oxide by plotting log k vs 1/T, where T is the absolute temperature. 65
9. PreparationofMonodiserse.I AiatedbyCumlPotassium
in THF
The reaction apparatus (Figure (10)) was connected to the vacuum line - argon gas system (Figure 4)) by the B-24 socket (D). The reaction apparatus was then evacuated. About 400 cm3 THF, previously purified and dried over sodium potassium alloy on the vacuum line, was distilled into the one dm3 flat-bottomed reaction flask (C)- Argon gas was introduced into the reaction apparatus through the B-24 socket (D), to produce an inert gas atmosphere. The argon gas pressure was kept about
2 cm Hg below the atmospheric pressure. The hollow key tap (F) was closed and the apparatus was then disconnected from the vacuum line.
The apparatus was purged by introducing enough of a solution of cumyl potassium in THF to react with all the impurities that might interfere with the polymerization. This purging solution was syringed into the reaction vessel (C) via the suba seal (G) and the apparatus was repeatedly tilted to ensure that all the impurities were destroyed.
A known amount of cumyl potassium in THF was introduced to the reaction vessel (C). The apparatus was connected to the vacuum line by the tap (F), was partially evacuated and was sealed off at (H).
The reaction vessel (C) was immersed in an alcohol bath (temperature about - 76°C). The cumyl potassium solution in THF was rapidly stirred by means of the magnetic follower (J), and the stirrer (K).
The break seal of the tube (A) containing a measured amount of purified styrene, was broken by means of the glass encased breaker (L).
A slow flow of styrene was introduced into the reaction vessel (C) via the tube (N), which let the styrene drop into the cumyl potassium solution near the wall of the reaction vessel (C). This ensured even mixing of the styrene with the initiator solution. The styrene flow was controlled 66
by the slow movements of the (M), which had been placed in the upper
part of the tube (N) and acted as a valve. The breaker (M) movements were controlled by a magnet from outside. All the styrene was added in about thirty minutes.
The living polystyrene had a red colour distinctly different from the cherry red colour of the cumyl potassium.
The polymerization was terminated by addition of methanol to the reaction mixture. The dry ice bath was removed, the break seal of the tube (B), containing 0.5 cm3 methanol, was broken and the methanol was introduced into the reaction mixture through the tube (N).
The polystyrene was precipitated twice into methanol, once from a solution in THE followed by once from a solution in toluene.
The dried polymer was sent for GPC analysis to R.A.P.R.A. 67
10. Attempted Preparation of Graft Co olymers of Styrene and Ethylene Oxide
It was attempted to prepare a graft copolymer of styrene and ethylene oxide using the deactivation technique.
A sample of monodisperse polystyrene was chloromethylated, then it was reacted with pre polymerized living polyethylene oxide.
(i) The Chloromethylation
The chloromethylation of polystyrene was performed with chloromethyl methyl ether in the presence of stannic chloride as catalyst in carbon tetrachloride.
The reproducibility of the chioromethylations were not very satis- factory. Thus it was necessary to perform a series of exploratory reactions, by varying the reaction time, the catalyst and the concentrations of the reactants and catalyst, to find the most suitable experimental procedure.
The conditions eventually chosen were as follows:
8 g Polystyrene, Mn equal to 110000 and Till/Fin less than 1.06,
(Pressure Chemical Company) was dissolved in 300 cm3 of carbon tetra- chloride fractionally distilled into the one dm3 flat-bottomed reaction flask (C). Figure (11). The vessel was partially evacuated by connecting it to the rough vacuum line by means of steel tubing. The steel tubing was connected to the reaction vessel by the suba seal (A).
About 50 cm3 of chloromethyl methyl ether was transferred into the reaction vessel through steel tubing by the suba seal (A). Argon gas was introduced into the vessel until the pressure inside the vessel was about 2 cm Hg below atmospheric pressure. 0.5 cm3 of stannic chloride was syringed into the reaction vessel. The "Sovirel" tap (B) was closed, to prevent any possible reaction of the reactants with the suba seal (A). The flask was then placed in an ice bath. The solution 68
was stirred by means of the glass encased magnetic follower (D) and the stirrer (E) for five hours. The reaction was terminated by adding wet dioxane, followed by distilled water. The solution was shaken three times with 10% w/v sodium hydroxide solution and five times with distilled water.
The carbon tetrachloride layer was separated by the use of a separation funnel. Then the carbon tetrachloride was distilled away from the polymer at room temperature by means of a thin film evaporator.
The polymer was dissolved in 400 cm3 toluene and was precipitated in about 4 dm3 methanol. The polymer was filtered and dried, redissolved in 400 cm3 THE and was precipitated in 4 dm3 methanol and water mixture
(1:1 by volume), to ensure the complete separation of the inorganic salts from the polymer. Finally the polymer was filtered and dried under vacuum at room temperature to prevent cross-linking due to the presence of chlorine in the polymer at temperatures above 40°C.
(ii) A Grafting Experiment
A solution of 8 g chloromethylated polystyrene in 50 cm3 benzene was introduced into the 100 cm3 round-bottomed flask (B) (Figure 12)) by syringing through the capillary tube (C). The reaction apparatus was connected to the vacuum line (Figure (4)) by the B-24 socket (D).with the spring loaded tap (F) and the hollow key tap (E) were closed. The polystyrene was freeze dried by continuous evacuating of the apparatus for one day.
The reaction apparatus was sealed off at (C) and then it was reconnected to the vacuum line by the B-24 socket (G). The tap (E) was opened and argon gas was introduced into the reaction apparatus until the pressure inside the apparatus was about 2 cm Hg lower than atmospheric pressure. Then the apparatus was purged by gradually introducing a 69
solution of cumyl potassium in THF. This purging solution was syringed
into the one dm3 flat-bottomed flask (A) through the suba seal (H) and spring loaded tap (F). The tap (E) was then closed. The apparatus was
disconnected from the vacuum line and was repeatedly tilted so that all the impurities had been destroyed, i.e. when a faint cherry red colour
persisted inside the flask (A).
A further known amount of cumyl potassium in THF was introduced into the flask (A). The apparatus was reconnected to the vacuum line by the socket (G) and the apparatus was evacuated. Then 410 cm3 of previously dried THF and a known amount of ethylene oxide, (previously dried over cumyl potassium) were distilled into the flask (A).
The apparatus were finally sealed off at the capillary tube (J).
The ethylene oxide and cumyl potassium solution was stirred by means of the glass encased follower (K) and the stirrer (L) for three days at room temperature to ensure that all the ethylene oxide had been polymerized.
About 50 cm3 of THF was distilled away from the flask (A) and was collected in the flask (B). The chloromethylated polystyrene in flask (B) was then dissolved in this THF. The apparatus was tilted so that the polystyrene solution was gradually introduced into flask (A) containing the living polyethylene oxide solution.
The solution was subsequently stirred for four days at room tempera- ture to ensure that the coupling reaction between the chloromethylated sites on the polystyrene and the living polyethylene oxide had taken place.
A fine precipitate of inorganic potassium compounds was formed during the first 24 hours of the coupling reaction. The product of the reaction, 70
which was expected to be a graft copolymer of styrene and ethylene oxide together with some polyethylene oxide homopolymers were precipitated by adding a solution in 400 cm3 of THE to 4 dm3 of n-heptane in a beaker.
The polymer did not form a fine precipitate but a large gelled lump of polymer was obtained and some fine particles of polymer were also deposited on the walls of the beaker.
The clear solution was removed by tilting tho beaker. The remaining solid polymers were dissolved in about 50 cm3 of benzene and were freeze dried.
The dry white-coloured solid products of the grafting reaction were sent to R.A.P.R.A. for GPC analysis.
10.1. Determination of Chlorine in Chloromethylated Polystyrene by
Neutron Activation Analysis
Determination of chlorine in chloromethylated polystyrenes could not be performed by conventional analytical techniques, due to the small amounts of chlorine present in the polymer, i.e. less than one per cent. 104,105) The Neutron Activation Analysis (NAA) technique( which was very useful for the microdeterminations of trace amounts of elements was utilized for analysing the chloromethylated polystyrenes.
The basic principle of NAA is that a stable isotope when irradiated by neutrons can undergo nuclear transformations to produce a radioactive nuclide.
Most radio nuclides exhibit characteristic beta- and gamma-ray energies and half-lives. The radiation from an irradiated sample can be characterized using appropriate detection apparatus and relate these to sample composition. Thus the induced activity due to a particular element in a sample is proportional to the amount of that element in the sample. It is possible to determine several elements in the same sample.
However, NAA does not give information about the chemical form in which a particular element is present. Figure (1) Vessel for THE storage Figure (2) Apparatus for volumetric Figure (3) Ampoule for storage measurements of monomer of purified monomers (a)B-24 socket B-24 socket 1 B-24 socket (b)Capillary tubing (a) (a) (b)suba-seal (b)capillary tubing (c)100 cm3 round-bottomed flask (c)spring-loaded tap (c)break-seal
Figure (4) The high vacuum-Inert Gas System Vacuum tap T Spring-loaded tap A B-24 extended cone S 1 t t
D N
K L
A •argon 2
Figure (5) "Sovirel" joint and tap Figure (9) Dilatometer with vacuum line attachment •
a
Figure (8) Container for liquid Figure .(7) Container for liquid Figure (6) Splash-head storage under vacuum storage under inert and inert gas gas (a)B-24 socket (a) suba-seal (a) suba-sea3. (b)B-24 cone • (b) break seal (b) springloaded tap (c)springloaded tap (c) 100 cm3. round-bottomed flask (d)B-24 socket (e)100 cm3 round-bottomed flask 75
Figure (10) Apparatus for preparation of monodisperse polystyrene 76
Figure (11) Apparatus for dhloromethylation
of polystyrene 77
Figure (12) Apparatus for preparation .
of graft copolymers of
styrene and ethylene oxide 78
PART FOUR PHYSICAL PROPERTIES OF ETHYLENE OXIDE,
PROPYLENE OXIDE AND THEIR HOMOPOLYMERS
11. The Epoxides
Due to the relatively strong basic ether linkage, cyclic ethers
are generally polymerized by cationic catalysts. Cyclic ethers of
less than five or more than six members are relatively easily polymerized,
e.g., tetra hydropyrane and dioxane are relatively unreactive.
11.1. The Structure of Epoxides
The methylene groups in ethylene oxide lie in places perpendicular to the ring plane. Figure (13). The electron density of the oxygen
atom is abnormally small compared with the oxygens of acyclic and large ring cyclic ethers. The strain energy, defined as the difference
between the calculated and experimental heats of formation, has been -1,(106) shown to be54.3 kJ mol which may be regarded as the driving force for the ring opening polymerizations.
7,0 \ P H • H ‘1.1
Figure (13) (107) Zimatov has proposed a resonance hybrid of the structures
(1), (2) and (3). Figure (14).
0 0 0 0+ /\ + _ CH2 - CH2 CH2 - CH2 CH2 - CH2 CH2 CH2 CH2 CH2
( 1) (2) (3) (4) (5)
Figure (11+) 79
11.1.A. The Physical Properties of Ethylene Oxide (109) Ethylene oxide is a colourless liquid boiling at 10.4C and o (109) the freezing point of this liquid is -112.51 C. The density of (11o) ethylene oxide liquid is 0.882 g cm-3 at /0C and 0.89713 g cm ' o (111) at 0 C. The following linear relationship between temperature and density was derived and was used for calculating the approximate density values for ethylene oxide at all temperatures relating to the present work:
d = -0.001513 t + 0.89713 g cm3 (11.1) where d is the density and t the temperature in °C.
11.1.B. The Physical Properties of Propylene Oxide
Propylene oxide is a colourless liquid boiling at 36.5 - 38°C.
The density of propylene oxide is 0.8598 g cm-3 at 0°C(112) and 0.8287 g cm-3 o (113) at 20 C. It exists in (d) and (1) forms and the density of the
(d) form is 0.8412 g cm-3 at 20°C.(113) The following linear relationship was derived between temperatures and density which was used for calculating the approximate density values for propylene oxide at other temperatures:
d = -0.001555 t + 0.8598 g cm-3 (11.2) where d is the density and t the temperature in °C.
11.2.A. The Physical Properties of Polyethylene Oxide
Polyethylene oxide exists in liquid, waxy and solid forms depending on molecular weight and crystallinity. At molecular weights below 440 (114,115) it exists in a liquid form. Up to molecular weights of about
4,400 it is a relatively soft wax. The hardness of the wax increases with increasing molecular weight and it eventually becomes a tough solid.
Fordyce et al.(115) prepared 6, 18 and 42 membered poiyoxyethylene glycols. The polymer containing six repeat units was in a liquid form while both the higher molecular weight polyethylene glycols were 80
in a solid form.
Solubility tests on polyethylene oxide showed that it dissolved rapidly in water and methanol whereas it dissolved slowly in benzene (114) and THF. Polyethylene oxide is soluble in: (a) chlorinated hydrocarbons: e.g. methylene chloride, carbon tetrachloride.
(b) Aromatic hydrocarbons: e.g. toluene, benzene. (c) Ketones: e.g. acetone and methyl ethyl ketons. (d) Alcohols: e.g. methanol and isopropanol. The solubility of polyethylene oxide in water is due to the ability of the polyether to form hydrogen bonds with water.
There is an upper solubility limit in water for polyethylene oxide which varies with temperature.
The crystalline density of polyethylene oxide is 1.23 g cm-3 .(116)
The density of the polyethylene oxide in solution in THE was calculated from the data obtained by dilatometric polymerizations of ethylene oxide performed in these laboratories and was 1.18 g cm-3 at 40°C.
11.2.B. Physical Properties of Polypropylene Oxide
Polypropylene oxide exists in liquid or solid (crystalline and amorphous) forms. The physical form of the polymer is dependent on the monomer from which it has been prepared and the catalyst of the polymerization. (d), (1) or (dl) propylene oxides form liquid or solid polymers depending on the catalyst or the initiator of the polymerization.
1-Propylene oxide polymerization using powdered potassium hydroxide at 0 (117) 25°C produced a solid crystalline polymer, m.p. 55.5°C - 56.5 O, (18'118) while Price and St Piere have prepared liquid polymers by polymerizing racemic propylene oxide under similar conditions. Solid propylene oxide (3% by weight crystalline) has been prepared by polymerizing racemic propylene oxide in presence of a ferric-chloride catalyst.(119)
The polymerization of racemic propylene oxide in the presence of sodium 83.
o catalysts at 140 C and high pressure has produced a liquid polymer.(18)
Thus the physical properties of the polymer are influenced by the optical form of the monomer.
The polymerization catalyst exerts a great influence on the configuration (119) of the assymetric centres during the polymerization. This gives rise to the formation of liquid or solid polypropylene oxides. -3 The crystalline density of polypropylene oxide is 1.102 g cm .(120)
The density of polypropylene oxide in solution in THF was calculated from the data obtained from the dilatometric polymerizations of propylene oxide performed in these laboratories and was 1.16 g cm-3 at 50°C. 82
PART FIVE RESULTS AND DISCUSSION
12. The Kinetics of Ethylene Oxide Polymerization with Cumyl Potassium
in THF and THF-Dioxane Mixtures
12.1. The Kinetics of Ethylene Oxide Polymerization with Cumyl Potassium
in THF
The anionic polymerisation of ethylene oxide was studied by conventional dilatometry.
12.1.A. Initiation
Cumyl potassium (0J-phenyl isopropyl potassium) which was a very efficient initiator was used in the present studies. Unlike certain lithium or sodium initiators cumyl potassium/THF solutions are quite stable at room temperature. (Section (1.A)). The stability of cumyl potassium in THF is due in part to the presence of the potassium counter ion.
The cherry red colour of cumyl potassium in THF disappeared immediately, even at very low temperatures, when ethylene oxide was introduced. This indicated that the initiation reaction was rapid.
The polymerizing solution was homogeneous both during the initiation and propagation reactions.
In some cases a small amount of a white precipitate was present throughout the course of the polymerizations, the amount of this precipi- tate did not change during the polymerizations. It was believed that the precipitate consisted of insoluble products of the reaction betwecit cumyl potassium and impurities. These insoluble products could be formed during the transfer of the cumyl potassium solution to the dila- tometers and also during the purging of the dilatometers prior to the filling. A similar white precipitate was formed when some cumyl potassium/THF solution was terminated by exposing to air. 83
12.1.B. Propagation
The whole course of the propagation reactions could be conveniently
followed dilatometrically at 40°C.
As shown in Figure (15), the decrease in the volume of the polymerizing solutions was initially very slow. Indeed at lower temperatures of o 30 - 35 C, no measurable change in the volume of the solutions were observed during the first 30 - 40 minutes.
The initial slow decrease in volume was followed by a more rapid decrease in volume. Eventually the decrease in volume became linear with time. This initial slow decrease in the volume of a polymerizing solution has also been observed by Kazanskii et al.(13) who studied the kinetics of polymerization of ethylene oxide, in THF. They attributed this observation to the insolubility of low molecular weight living polyethylene oxides which became gradually soluble when their molecular (13) weights exceeded 660 - 880.
Kazanskii et al. have also reported that in some cases they actually observed the formation of insoluble living polymers visually during the initial stages of the propagation which became soluble when their molecular weights increased. At this latter stage a rapid decrease in the volume of the solution was noted.
However in the present experiments as- was previously described in
Section (12.1.A), the polymerizing solution remained homogeneous throughout the course of the polymerization.
The propagation reaction was expected to be a simple step involving addition of ethylene oxide monomer to the propagating species by a ring opening reaction. It was thought that the initial slow changes in the volume of the polymerizing solutions were related to the physical properties of the living polymers (low molecular weight polyethylene oxides are 84
usually liquid but they become soft waxes and then solids with increasing molecular weight).
A study of the densities of low molecular weight polyethylene oxides showed that the density of the oligomers change with the molecular weight.
Table (2) and Figure (16). It has also been observed that the density of low molecular weight polyglycollides is dependent on the nature of the end groups. The density of the polyglycollides decreases in the series of end groups OH >CH3 ).CH3CH- >CH3(CH2 )2 - >CH3(CH2 )3-.
Table (2) and Figure (16).
The density of ethylene oxide is 0.882 g cm-3 at 100C(110) and the density of high molecular weight polyethylene oxide is 1.18 g cm-3 at 0 (114) 40 c. It can be concluded that the density of polyethylene oxides increases gradually with the molecular weight until it reaches a constant value at high molecular weights. It is therefore suggested that during the initial stages of ethylene oxide polymerization, the rate of change in density due to addition of monomer to very short length polymer chains must be small. Hence the initial rate of change in volume of a polymerizing solution must also be small. As the chains grow longer, their densities increase accordingly, so the change in volume of a polymerizing solution becomes larger. When the density of the growing chains reaches a maximum value, a linear decrease in the volume of the solution with time is expected. The relatively large cumyl end group is expected to make the density of low molecular weight polymers relatively 0 (110) small. The density of cumene itself is 0.8617 g cm-3 at 20 C.
It was decided to prepare low molecular weight polyethylene oxides
(oligomers) and measure their densities to check whether the expected gradual increase in density with increasing molecular weights could be observed. 85
Table (2). The Change in the Density of Low Molecular Weight Polyglycollides with Molecular Weight. (110)
o Formula Temperature C Density g cm-3
HO(CH2 CH2 0)2 H 20 1.1164 HO(CH2 CH2 0)311 20 3..1235 110(C112 CH2 0)4 H 15 1.2850
CH3 (CH2 )3 0(CH2 CH2 0)H 20 0.9007 CH3 (CH2 )3 0(CH2 CH2 0)2 H 20 0.9553 CH3 (CH2 )30 (CH2 CH2 0)3 H 20 0.9890
C113 0(CH2 CH2 0 )CH3 20 0.8665 CH3 (CH2 )3 0(CH2 CH2 0)H 20 0.9007 CH3 CH2 0(CH2 CH2 0)H 20 0.9294
HO(C1-12 CH2 0)2 H 20 1.1640 CH30( CH2 CH2 0)2 H 20 1.0210 CH3CH2 0(CH2 CH2 0)2 H 20 009885
HO(CH2 CH2 0)4 H 15 1.2850 CH3 0(CH2 CH2 0)4 CH3 20 1.0132
CH3 0(CH2 CH2 0)H 0.9646 CH3 CH2 0(C112 CH2 0)H 0.9294 CH3 CH2 0(CH2 CH2 0 )C1-13 20 0.8529 CH3 0(CH2 CH2 0 )CH3 20 0.8665 CH3 CH2 0(C112 CH2 0)CH2 CH3 20 0.8484 86
The preparations of the polymers and measurements of their
densities were performed by initiating ethylene oxide polymerization
in THF at 20°C.
Relatively large quantities of cumyl potassium were used together
with a dilatometer having a 30 cm3 bulb. The polymerizations were
performed only for a short time so that only oligomers of the required
molecular weights were prepared. The chains were terminated by adding
small amounts of methanol. The decrease in the volume of the polymerizing
solutions started from the moment the initiator solutions were added to
the monomer solutions. In standard size dilatometers, however, with
7 - 8 cm3 bulbs, the initial small changes in volume were not measurable.
The changes in the volumes of the solutions were due to the formation
of oligomers of polyethylene oxide. The liquid low molecular weight
polymers were isolated by filtering the polymer solutions through
sintered glass crucibles (porosity 4) followed by evaporation of THF
under vacuum using a thin film evaporator.
The possibility must be considered that during the isolation
procedure, molecules of low molecular weight such as C6H5C(CH3 )2 (CH2 CH2 0),IR,
with n equal to 1,2 and above, were lost together with the ethylene oxide 0 Clc - CH2 remaining.
However, the boiling points of ethylene oxide and tetrahydrofuran (109) (110) are relatively low being 10.4°C/760 mm Hg and 64-5 /C,
respectively. The boiling points of C6 H5 -CH2 OH, C6H5CH(CH3 )CH2 OH,
C6 H5 (CH2 )3 0H, C6 H5 (CH2 )4 OH, C6H5CH(CH3 )CH2 CH2 OH, C61:5 -CH(CH2 CH3 )CH2CH2 OR
are 205.35°C/760 mm Hg,(110) 114°C/14 mm Hg,(121) 235°C/760 mm Hg,(121) mm Hg, (121) ii7oc/8 mm Hg,(121) C/2 mm Hg,(121) 140°C/14 8l° respectively.
The densities of low molecular weight polyethylene oxides were calculated using the equation: 87
1 1 111(— =AV (12.1) dm - dp where m is the weight of the polymerized monomer, d and d are the m monomer and oligomer densities, respectively and AV is the change in the volume of the polymerizing solutions. The results are given in
Table (3).
m was calculated using the equation:
= [1,0 _ 11.1000 (12.2) [mn]o M.V. [In]o where [M] is the concentration of the monomer(mol dm-) that has been converted to polymer and [In]o the initial concentration of the initiator
(mol dm-3). M is the molecular weight of the monomer and V the initial volume of the polymerizing solution (dm3). DP was calculated using the equation
n- (M uth MH) 31" - 120 c yl (12.3) M 1.4 where M u and MH are the molecular weight of the cumyl end-group and c myl the atomic weight of the other end-group, hydrogen, respectively.
Table (3). The Effect of )P on the dp of Low Molecular Weight Polyethylene Oxides. Temperature 20°C.
DP Molecular Weight [In]ox 102 [M] x 10 LIT x 10 d (Measured'by VPO) mol dm-3 mol dm-3 cm3 g cm 3
3.63 280 4.21 1.53 1.68 0.930 4.32 310 5.56 2.40 4.56 0.983 12.72 68o 5.32 6.77 30.04 1.195 0
88
The densities of the low molecular weight polymers are plotted
against the degrees of polymerization calculated from the measured
molecular weights of the polymers as shown in Figure (17). The
densities of the polymers increase gradually until they reach a constant
value at high molecular weights.
As described previously in Section (8.1.A), stringent precautions
were taken to purify all reagents and all dilatometers were purged
prior to use with freshly prepared cumyl potassium solutions. Therefore
there seems little likelihood that the above dilatometric results can be
explained by assuming that trace impurities were present during the
experiments. Furthermore, the final rates of polymerization measured
at different initiator concentrations (Section (12.1.D.)) were related
in a simple way, which would not be expected if impurities were present
either in the monomer or the THF. Therefore it was concluded that the
initial slow rate of change in the volume of the polymerizing solution
was due to change in the density of the growing chains. Thus as the
active chains grow the rate of change in volume of the systems and the
polymerization densities reach constant limiting values.
The ethylene oxide polymerization reactions were followed up to
ten per cent conversion of monomer to polymer. After this stage, the
rate of change in volume tended to decrease due to the decrease in monomer
concentration.
In all subsequent sections, when analysis of polymerization rate
data are made, the final constant rates of polymerization are used.
12.1.C. The Dependence of the Rate of Polymerization, R , on Monomer
Concentration
in Figure (18) shows that the The plot of log R against log [M]o rate of ethylene oxide polymerization initiated with cumyl potassium
in THF is of the order 0.935 + 0.15 with respect to the monomer concentration.
The results are given in Table (4). 89
The Effect of Dqj on R in Ethylene Oxide Polymerization. Table (4) : o Solvent THF. Temperature 40°C.
R x 103 [In]cy x 103 Molecular Molecular p [M]o 7IA -3 weight mol dm mol dm-3 mol dm-3 weight (Measured (measured -1 by GPC) s (calculated) by VPO)
1.01 1.52 11 . CO 6079 6500 1.23 1.51 1.89 11.80 7047 7500 1.37 1.61 2.06 11.20 8092 8100 1.30
1.90 2.34 11.40 9031 8900 1.45 2.17 2.88 12.00 10056 10200 1.34
As it has been discussed in Section (3.1.A.) the rate of ethylene oxide polymerization with respect to the monomer concentration was also first order when polymerizations were initiated with sodium methoxide (15) (25) in bulk or dioxane, potassium t-butoxide in DMSO and sodium naphthalene in THF.
In the polymerization of ethylene oxide initiated with K t-BuO in
DMSO, where the reaction medium gradually becomes inhomogeneous due to the gradual precipitation of living polymer, the IRp/[M]t} values (25) decrease gradually. In the present study, the polymerizing solutions remained homogeneous throughout the reaction and the second order rate constants were equal, within experimental error, at different monomer concentrations. Figure (19).
Thus in the anionic polymerization of ethylene oxide, monomer units add gradually to the growing ends of the living polymers and the molecular weights increase slowly throughout the course of the reaction. 12.1.D. The De endence of the Rate of Pol rnerization R , on Initiator
Concentration
The dependence of the rate of polymerization on the concentration of the propagating species or the concentration of the monofunctional initiators is a means of examining the nature of the propagating ends.
The nature of a growing end is dependent on the counter ion and the solvent used in the polymerization of ethylene oxide where associations of growing ends are present, the order of the polymerization rate with (15) respect to the initiator concentration was less than one. In the polymerization of ethylene oxide with K t-BuO in DMSO, the polymerization rate is first order with respect to the initiator concentration during the initial stages of the reaction until an inhomogeneous system is formed. The inhomogeneity is due to the insolubility of the living (25) polymer in DMSO. However the order of the polymerization rate is expected to be larger than one as both free ions and ion pairs are (25) present in the highly dissociating BMS0 medium.
In the polymerization of ethylene oxide initiated with alkali metal naphthalene in THF, the order of the reaction with respect to the initiator concentration is dependent on the initiator concentration. At low initiator concentrations the order of the reaction is one while at high initiator concentrations orders of less than one have been observed. (13)
As was discussed in Section (3.2.A.), the formation of associations of alkoxide-alkali metal counter ion growing ends in THF are responsible for this observation - + + nP Me V: (PMe ) (12.4) n + + PMe + M PMe (12.5) where P and Me+ are the carbonium ion and the counter ion, respectively, and M is the monomer. As shown in Equation (12.5) chain propagation takes place by addition to the unassociated growing ends, while both 91
associated and unassociated forms are present in the reaction medium.
An increase in the initiator concentration facilitates association of ion pairs. Thus at high initiator concentrations, where high concen- tration of associated growing ends and a low concentration of active growing ends are present, the order of the reaction rate with respect to the initiator concentration equal to the growing end concentration becomes less than unity.
An order of less than unity has also been observed for the bulk polymerization of ethylene oxide with sodium methoxide due to the (15) association of growing ends.
The growing ends exist in the form of ion pairs in THF due to the very small dissociation of living alkoxide alkali metal species in this medium.(13) Section (3.B.2.).
In the present work, most polymerization reactions were performed in THF. The dilatometric results show that the rate of reaction is of the order 1.00 + 0.27 with respect to the initiator concentration, at least for the initiator concentrations involved. A plot of log R vs log [In] is shown in Figure (20). The results are given t in Table (5). [In] is the mean initiator concentration when the t rate of polymerization had reached a constant value. The highest initial initiator concentration used in the present work was
22.40 x 10-3 mol dm-3. Table (5).
When potassium naphthalene is used to initiate ethylene oxide polymerizations in THF at 30 - 40°C, the order of the reaction with respect to the growing end concentration is unity at concentrations -1 of less than about 10 mol dm-3.
A comparison has been made of the concentrations of the mono- functional initiators used in the present experiments with the concen- trations of the growing ends produced using potassium naphthalene 92
Table (5): The Effect of [In]t on R in Ethylene Oxide Polymerization.
Solvent THF. Temperature 40°C.
3 [1]0 [In]ox 103 [In] x 103 Molecular Molecular ViTil R x 10 t -3 weight weight -3 moi mol dm mol dm mol dm (calculated) (measured (Measured -1 s dm -3 by VPO) by GPC)
1.17 2.10 7.63 7.77 12110 11600 1.18 1.65 2.10 13.50 13.73 6844 7100 1.25
1.85 2.18 13.60 13.79 7052 7500 1.21
2.99 2.20 19.20 19.63 501 5500 1.20 3.18 2.15 22.40 22.45 4223 4300 1.15
initiator.(13) The initiator concentrations used in the present study correspond to the lower region of growing end concentrations formed from potassium naphthalene.(13)
In this concentration region the order of the rate of reaction with respect to the potassium naphthalene concentration was unity. These observations suggest that the polymerization of ethylene oxide initiated with cumyl potassium in THE involves ion pairs since the dissociation constant for the potassium alkoxide ion pairs is very small.(13) The ion pairs form associations which are dependent on their concentration. Propagation occurs by the gradual addition of monomer units to the growing ends.
12.1.E. The Effects of Counter Ion and Solvent on the Polymerization
Rates of Ethylene Oxide
The effect of solvent on the nature of the propagating species has been discussed in Section (3.D.1.). Polymerization rates are relatively fast in highly dissociating solvents where there is a preferred
93
formation of free ions rather than ion pairs and their associations.
The second order rate constant, k , for ethylene oxide polymerization
initiated with sodium methoxide in bulk or in dioxane is of the order of
2 x 10 5 dm3 mol-1 s-1 at 30°C.(15) In ethylene oxide polymerizations -1 -1 -1 initiated with K t-BuO in DMSO, k is of the order of 10 dm s for p monomer concentrations of about 4 mol dm-3 at 25°C.(25) In ethylene
oxide polymerizations initiated with alkali metal naphthalene initiators + in THF, the second order rate constants are: for K , k is equal to 3 -1 -1 0.94 dm3 mot-1 s-1, and for Cs+, k is equal. to 3.5 dm mol s at 3) 70 oC. (1 The second order rate constants, k , in the present work 3 -1 -1 have values of about 6.6x10-2 dm mol s at 40°C and 13.6 x 10-2 dm3 -1 -1 mol s at 48.9°C.
A comparison of the above-mentioned second order rate constants
shows that reaction rates increase in the series dioxane < THF < DMSO.
The presence of associations of ion pairs in dioxane and THF
clearly affects the polymerization rates. In DMSO where free ions
are present, the reaction proceeds more rapidly than in dioxane and
THF where the propagating ends are present only in the form of ion pairs.
The effect of counter ion on the polymerization rates is shown by the
above second order rate constants obtained with potassium naphthalene
and caesium naphthalene initiators in THF.(13)
12.1.F. The Activation Energies of Polymerization of Ethylene Oxide
The activation energy values obtained for ethylene oxide polymeriza-
tions using various initiators are similar and are of the order of -1 4 - 4.5 kJ mol in dioxane and THF, when the polymerizations involve -1 ion pairs.(13115) An activation energy value of 3.94 + 0.37 kJ mol 10 3 -1 and A equal to 2.4 x 10 dm mol-1 s was obtained from a log k vs 1/T
plot (T is the absolute temperature) (Figure (21)) for the ethylene
94
oxide polymerizations performed in these laboratories. The k values
for ethylene oxide polymerizations at different temperatures were
obtained by dilatometric measurements. The results are given in
Table (6).
Table (6): Ethylene Oxide Polymerizations at Different Temperatures.
S Solvent THF.
t°C [M]o [In]xo 103 k x 102 Molecular Molecular Viin weights weights mol mol dm-3 dmP3 mol -1 (measured (calculated) (measured -1 by GPC) dm-3 s by 17p0)
30.1 2.32 1.38 2.94 7397 8000 1.55 35 2.40 1.42 3.75 7436 8300 1.50 4o 2.26 1.35 7.10 7365 7500 1.40 48.9 2.72 1.40 13.60 7605 7700 1.48
Bawn et ai.(25) have reported a lower activation energy of the -1 order of 1.5 kJ mol for polymerizations performed in DMS° which is
a highly dissociating solvent.
12.2. Ethylene Oxide Polymerizations in THF-Dioxane Mixtures
Cumyl potassium was almost insoluble in pure dioxane. Only
a faint red colour developed when some dioxane was distilled on to
cumyl potassium initiator previously dried on the vacuum line. Nearly
all the cumyl potassium remained insoluble. The addition of a very
small amount of THF to the dioxane caused the rapid development of a
red colour in the solution and the cumyl potassium rapidly dissolved.
An attempt was made to study the kinetics of ethylene oxide
polymerizations in dioxane-THF mixtures. Polymerization rates were
followed by conventional dilatometry. Section (8.).
95
The monomer and initiator concentrations were similar to those
used for the kinetic studies in pure THF. It was expected that due
to the better solvating power of THF rates of polymerization should
increase as the THF content in dioxane-THF mixtures increased. However
results obtained for this series of experiments showed that the reaction
rates did not vary greatly from a dioxane/THF(vol/vol) ratio of 0.209 to
4.71. The second order rate constants for the different solvent mixtures
were calculated and are shown in Figure (22) and Table (7). The second
order rate constants measured in various dioxane/THF mixtures do not
vary significantly with the dioxane/THF(Vol/voi) ratios. The k values
are also relatively close to those obtained for ethylene oxide polymerisa-
tions in THF.
Table (7): Ethylene Oxide Polymerization in Dioxane/THF Mixtures.
Temperature 40°C.
[In]x 3 k x 102 Dioxane/ Molecular Molecular [41]o 10 THF weight weight mol dm dm-3 mol dm-3 (measured moimol (measured vol/vol (calculated) by GM) s-1 by GPC. and VPO 2.33 9.33 7.01 0.209 , 10988 11100 1.57 2.16 6.05 6.40 0.505 15709 15800 1.35 2.44 7.98 6.81 2.082 13453 136ov 1.33 2.46 7.28 7.63 3.516 14868 14600 1.29 1.17 8.73 6.51 4.710 5896 5900 (vPo)
It may be concluded that even small additions o3 THF to dioxane
greatly accelerate the propagation rates due to the increased solvation
of the growing species. A similar effect has been previously observed 96
for the anionic polymerization of styrene in benzene, using n-BuLi, on adding small amounts of THF. (43,122)
12.3. Molecular Weights and Molecular Weight Distributions in Ethylene
Oxide Polymerizations
The polyethylene oxide homopolymers formed during ethylene oxide polymerizations followed dilatometrically were analyzed by GPC. The number average molecular weights, M , were measured also by vapour n pressure osmometry.
The MWD data obtained by GPC (Tables (4), (5), (6) and (7)) show that the MWD of the polyethylene oxides prepared in THF and in THF-dioxane mixtures is 1.15 - 1.55 and 1.29 - 1.57, respectively.
An example of the GPC analysis chromatograms and the differential
MWD curves obtained from 4W/d (log M) vs log M plot is included as shown in Figures (23) and (24).
12.4. Kinetics of Ethylene Oxide Polymerization Initiated with n-But 1
Lithium in THF
An attempt was made to study the kinetics of ethylene oxide polymerization initiated by n-butyl lithium in THF by dilatometry.
The initiator and monomer concentrations were similar to those used in the polymerizations performed with cumyl potassium/THF.
No change in the volume of the polymerizing solution was observed for about 8 hours at 40°C. After this period a very slow decrease in volume occurred which was considered to be difficult to follow accurately by dilatometry. The very slow rate of polymerization of ethylene oxide with n-BuLi/THF was mainly due to the small lithium counter ion. This has been discussed previously in Section (3.D.).
Ethylene oxide was also polymerized using n-BuLi in the presence of
N,N,N',N'-tetramethyl ethylene diamine, (TMEDA), with THF as solvent.
TMEDA is known to cause accelerated polymerization rates when used with 97
certain vinyl monomers and N-BuLi. This latter effect is thought to be associated with the complexing of the Li+ counter ion by TMEDA.
Increased polymerization rates were observed to occur when examining the ethylene oxide/n-BuLi/TMEDA system in TIFF, at room temperature o (about 25 C). An extensive investigation of the system was not considered to be worthwhile, however, in view of the known ability of amines including tertiary amines, to initiate epoxide polymerizations, by themselves.
It was noticed that an orange coloured precipitate, presumably polymeric in nature, was formed when polymerizing ethylene oxide with TMEDA alone. Figure (15) Volume change as a function of time for ethylene oxide polymerization with cumyl potassium/THF at different temperatures (a) 30.1°C (b) 35°C (c) 40°C (d) 48.9°C.
E U Cv a 0
0 z
U LU 5
0 > 0 0 20 40 60 8 0 100 120 140 TIME (Minutes)
99
Figure (16) Density vs degree of polymerization for low molecular weight polyglycollides at 20°C. 1. C113 0(CH2 CH2 0)H 2. CH3C}12 0(CH2 CH2 0)H 3. Cit3 (cH2 )30(cH2 cH2 0)H 4. cH3 o(cH2 cH2 0)cH3 5. Hc(cH2 cH2 0)2 H 6. cH3 o(cH2 cH2 0)2 H 7. cH3cH2 0(cH2 CH2 H 8. cH3 (CH2 )3 o(cH.2 CH2 o)2 H 9. Ho(cH2 cH2 0)3 H. 10. CH3 (CH2 )3 0(CH2 C112 0)3 H 11. CII30(CH2 CII2 0)4 CII3
>- 0 90 1— 1 .1 5 z ho 1.O , 7-0 jo-o 1 o' 0.9 20'" 3 0' 40
O 2 3 4 DP
• Figure (17) Density vs degree of polymerization, DP, for the low molecular weight polyethylene oxides prepared by polymerizing ethylene oxide with cumyl potassium/THF at 20°C. The points (o) represent the values of density for the oligomers. The point OM represents the density of ethylene oxide and the point (0) represents the density of high molecular weight polyethylene oxide at 20°C.
DP 101
Figure (18) log R vs log No for the polymerization of ethylene oxide with cumyl potassium/THF at 40°C.
0.5
0.I
0.1 0.2 0.3 0.4 0.5 log [M]o 102
Figure (20) log R vs log [In] for the polymerization t of ethylene oxide with cumyl potassiumAHF o at 40 C.
o .5
O .4 cn 0.3 0
O .2
O .1
0.9 1.0 Id 1-2 1.3
3+log t 103
Figure (21) log k vs 1/T) for the polymerization of ethylene oxide with cumyl potassium/THF
0.9
0.6
0.5
3.10 3.20 3.30 103/7 (k. ) • 104
Figure (19) lc as a function of [In] for the polymerization o of ethylene oxide using cumyl potassium/THF at 40°C.
0 ME
17, NO
I 0 18 3 22 [In] x 1 0 (mol dmi
22) k as a function of dioxane/THF mixtures for the poly.r, of ethylene oxide. using cumyl potassiumANF-dioxane mixtulgeP,i1 40°c.
2.5 5 DIOXANE/THF 105
Figure (23) GPC chromatogram for a sample of polyethylene oxide
HO 130 150 170 ELUTION (crn 3 VOLUME
Figure (24) Differential molecular weight distribution curve for a sample of polyethylene oxide the GPC chromatogram of which is observed in Figure (23). The sample has been treated -1 as polystyrene and K and a values of 1.2 x 10-4 (100 mlg ) and 0.71, respectively, have teen used for the GPC analysis.
4
log M lob
13. The Kinetics of Propylene Oxide Polymerization with Cumyl
Potassium in THE
The anionic polymerization of propylene oxide was studied by conventional dilatometry.
13.A.Initiation
The polymerizations were initiated with cumyl potassium in TIIF.
As with ethylene oxide polymerizations the cherry-red colour of the initiator solution disappeared very rapidly even when propylene oxide was introduced at low temperatures. This indicated that the initiation reaction was fast. The polymerizing solution was homogeneous both during the initiation and propagation reactions. In scme cases a small but constant amount of a white precipitate was present during the polymerizations. This precipitate was thought to consist of various insoluble products formed by reaction between cumyl potassium and various impurities. These insoluble products could be formed during the transfer of the cumyl/potassium solution to the dilatometers and also during the purging of the dilatometers prior to the filling.
13.B.Propagation
The whole course of the propagation reactions could be conveniently followed dilatometrically at 50°C. As it is shown in Figure (25), the rate of decrease in the volume of the polymerizing solution was much slower than that for ethylene oxide polymerizations. An initial small acceleration in the rate of decrease of volume of the polymerizing solution with time was observed until the volume eventually decreased linearly with time.
The initial acceleration was less pronounced when the polymerizations were performed below 55°C. Figure (25). The initial period of acceleration was much larger for propylene oxide polymerizations than for ethylene oxide polymerizations (Section (12)) conducted under 107
similar conditions. These observations may be due to transfer to monomer reactions which cause the molecular weights of the living polypropylene oxide chains to be much lower than the expected molecular weights throughout the course of the polymerization. Section (13.H.).
Thus the gradual increase in the polymer density with increasing molecular weight, which is a characteristic of anionic polymerizations of ethylene, pxide,msy not be so marked in propylene oxide polymerizations.
The propylene oxide polymerization reactions were followed up to about ten per cent conversion of monomer to polymer. After this stage, the rate of polymerization decreased due to the decrease in monomer concentration.
13.C. The Dependence of the Rate of Polymerization, R104 on Monomer
Concentration
The plot of log R against log [M]o in Figure (26) shows that the rate of propylene oxide polymerization initiated with cumyl potassium in THE is of the order 0.91 0.26 with respect to the monomer concen- tration.
As has been discussed in Section (3) the rate of propylene oxide polymerization with respect to the monomer concentration was approximately first order for initiation involving sodium methoxide in bulk or in (20) dioxane,(26) K t-BuO in DMSO(20,30) and 11450-THF mixtures. Thus, in the anionic polymerizations of propylene oxide, most monomer units add gradually to the growing ends of the living polymers, by a "head to tail" addition. Some living polymers form new growing centres by transfer to monomer reactions (section (3.3)) which do not affect the polymerization rates.
The collected results of such studies for propylene oxide polymeriza- tions are given in Table (8). 0 io8
Table (8) : The Effect of [M] on R in Propylene Oxide o Polymerization. Solvent TIIF. Temperature 50°C.
R x 10 [In] 3 Molecular Weight (Unsaturation N J 0x10 Molecular ° Weight (Measured (Measured Content) x -3 mol dm s-1 mol dm-3 mol dm-3 (calcu- by WO) by Visco- 105 lated) metry) eq 9-1
7.81 4.04 10.40 22,530 2800 4400 5.92 8.2b 4.00 12.00 19,333 2700 4000 5.39 6.89 3.70 12.00 17,864 2600 3500 4.31
6.24 3.18 11.50 16,038 2500 5500 6.90 4.94 2.46 12.20 11,695 2300 2700 15.20
13.D. Thelendericeoftheitstec a,tion,p. , on Initiator
Concentration
The nature of propagating species can be examined by studying the
effect of the concentration of the arowing species or the concentration
of the mono-functional initiators on the rate of polymerization.
The nature of a living polypropylene oxide-alkali metal counter
ion end group is dependent on the counter ion and the solvent used in
the polymerization.
In the polymerization of propylene oxide initiated with sodium (26) methoxide in bulk or in dioxane, the polymerization rate is first
order with respect to the initiator concentration but the order is 1.7
for propylene oxide polymerizations initiated with K t-BuO in DHSO.(20)
However, the order of the polymerization rate is about one if the same (20) polymerizations are performed in DMSO-THF, 1:1 (vol/vol) mixtures.
This difference between the behaviour of the polymerizing system in DMSO
and DMSO-THF mixtures has been attributed to the presence of both free
109
ions and ion pairs of growing ends in DMSO but only ion pairs in the
less dissociating DMSO-THF mixture.
The plot of log R vs log [In] in Figure (27) shows that the rate p t of propylene oxide polymerization initiated with cumyl potassium in THF
is of the order 1.01 + 0.21 with respect to the initiator concentration.
[In] is the mean initiator concentration in the region where the rate of t polymerization is constant. The collected results are given in Table (9).
Table (9) : The Effect of [In] on R in Propylene Oxide Polymerization. t Solvent THF. Temperature 50°C.
5 [In] x 103 [In] x103 Molecular Weight Mw Mn (Unsatura- Rp x 10 o t (calcu- (meas- tion con- (measured -3 tent)x 105 mol dm-3s-1 mol dm mol dm-3 mol dm fated)lated) uredby vpo) by GPC) eq g-1
7.06 3.33 13.40 13.54 14,413 2200 1.24 9.33
6.92 3.34 12.20 12.55 15,622 2200 1.32 7.00
6.05 3.50 12.10 12.22 16,846 2400 1.28 5.55
5.93 3.46 11.30 11.45: 17,759 2600 1.29 5.42 , 4.19 3.26 7.90 8.o5 239,341 4300 1.30 6.2o
These results are quite similar to those for polymerizations (20) involving the potassium counter ion and conducted in fl1SO-THF mixtures.
This suggests that the propagation of propylene oxide involves only
monomer addition to ion pairs. However, this suggestion that ion pairs
and not free ions are involved in the propagation reaction is in accord
with the fact that the dissociation constants of various living alkoxide
alkali metal species are very small in this medium.(13) 110
The highest initiator concentration used for the polymerizations (20) performed in DNSO-THF, 1:1 (vol/vol) mixtures was 7.45 x 10-2 mol dm-3.
In such mixture the order of the reaction rate with respect to the initiator concentration did not change with increasing initiator concen- tration. This showed the absence of associations of ion pairs for the initiator concentration range investigated.
In the present experiments, the highest initiator concentration used m was 13.4 x 10-3 mol d -3. As can be observed from the log R vs log [In]t plot, (Figure (27)) the order of the reaction rate with respect to the initiator concentration is also unity up to this initiator concentration.
This suggests that the nature of the propagating species is unaffected by the formation of ion pair associations in this concentration range of propagating ends.
The overall second order rate constant, k $ varies inversely as the square root of the initiator concentration for polymerizations per- (20) formed in DMSO. However, k does not vary with initiator concentration (20) for polymerizations performed in DMSO-TN: mixtures. These observations have also been attributed to the presence of ion pairs in DMSO-THF mixtures and the presence of both free ions and ion pairs in DMSO medium.
In the present study the second order rate constant did not vary within experimental error, at different initiator concentrations.
Figure (28).
13.E. The Effect of Counter Ion and Solvent on the Polymerization Rates
of Propylene Oxide
The effect of solvent on the nature of the propagating species in the polymerizations of epoxides has been discussed in Section (3.).
The second order rate constant, k , for the polymerizations initiated with sodium methoxide in bulk or in dioxane is of the order 111
-4 -1 of 1.3 x 10 dm3mol-1 s for the bulk polymerizations and -4 3 -1 -1 0 (26) 2.5 - 3 x 10 dm moi s for the polymerizations in dioxane at 70 C. (2o) Blanchard et al. have obtained the second order rate constant, - -1 k , equal to 3.31 x 10 3dm3mol-1 s at an initiator concentration of 2 1.71 x 10 mol dm-3, for the propylene oxide polymerization initiated (20) with K t-BuO in DMSO at 50°C. They have also obtained a second '; -1 -I 4 3 order rate constant of 5.6 x 10 dm-mol s at 50C and 7.16 x 10 dm -1 -1 - (30) moi s at 60 C$ in DMSO-THF, 1:1 (vol/Vol) mixtures. Price et al. 4 3 -1 -1 have obtained a second order rate constant 2.5 x 10 dm moi s for
K t-BuO/DMSO system at 30°C. The second order rate constant obtained (30) by Price et al. is. relatively low for the polymerization performed in a dissociating solvent like DMSO. This could be due to the presence of impurities present in the polymerizing solution. The second order rate constant, k , in the present work has a value of about 16 x 10 4 dm3mol-i s-1 at 50°C.
A comparison of the above-mentioned second order rate constants shows that the polymerization rates are felatively fast in highly dissociating solvents where polymerizations involve both free ions and ion pairs. The order of the growing polymer end reactivity increases in the series dioxane The second order rate constant obtained in the present studies (20) is lower than that obtained by Blanchard et al. for the polymerizations in DRSO at 50°C but it is unexpectedly greater than the k value obtained (20) in DMSO-THF mixtures at 500C. The unusually large value of k obtained in the present study (20) compared with the value reported by Blanchard et al. may be due to the different experimental technique employed. It is generally accepted that trace impurities in anionic polymerizations tend to lower the poly- merization rates and hence the values of k . Thus the experimental 112 methods employed in this work may have been superior, in some way, (20) to those of Blanchard et al. 13.F. The Activation Energies of Polymerization of Pro lene Oxide The activation energy values obtained for propylene oxide polymeriza- -1 tions using various initiators are of the order of 3.9 - 4.2 kJ mol (26) in dioxane, THF-MS0(26) and DMSO. (26) An activation energy value of 4.3 + 0.81 kJ mol-1 was obtained in this study from a log k vs 1/T plot, where T is the absolute temperature. Figure (29). The k values for propylene oxide polymerizations at different temperatures were obtained by dilatometric measurements and the results are given in Table (10). A frequency factor, A, of 2.3 x 107 dm3mo1-1s-1 has been reported for propylene oxide polymerizations initiated with sodium (26) 3 -1 methoxide in dioxane and a value of 2 x 109 an mol-1 s was obtained in these laboratories. Table 10 : Propylene Oxide Polymerizations at Different Temperatures. Solvent THF. o t C x 10 k x 4 D4lo [In] 3 10 3 P -1 -1 mol dm-3 mol dm-3 dm mol s 45 3.10 14.30 7.64 50 3.02 12.90 17.80 55 3.30 12.81 19.50 65 3.01 12.12 27.20 3.13 13.G. Transfer Reactions in Propylene Oxide Polymerizations Initiated by Cumyl Potassium/THF The transfer reaction to monomer in propylene oxide polymerizations has been discussed in Section (3.3.). It involves a proton transfer from the methyl group of a propylene oxide molecule to the propagating chain causing its termination and formation of a new active centre. 0 / R-CH2 CH(CH3)0 H - CIi2 - CH - CH2 -4 R - CH2CH(CH3 )OH + CH2 = CH -• CH2 0 (13.1.) As it is observed from the equation (13.1.) a double bond is present in the newly formed active centre. Thus the polymer chain initiated by this new active centre will contain end group unsaturation. Anionically prepared propylene oxides contain allyl and isopropenyl unsaturations. It has been suggested that the isopropenyl groups are (32) formed as a result of an intramolecular rearrangement of the allyl groups. The transfer reaction and thus the end group unsaturation content is affected by factors such as initiator concentration, the type of initiator and solvent. An increase in the initiator concentration (29,30) causes an increase in the unsaturation content. The results of unsaturation content at different initiator/monomer concentrati',n ratios obtained for the polypropylene oxides prepared in the present experiments are given in Table (11). The plot of ([In] /[M ]0) or vs unsaturation content (Figure (30)) shows a scattering of points. This may be attributed to the relatively large experimental errors involved in accurate determinations of very small unsaturation contents. However, an increase in the unsaturation content is observed with an increase in initiator monomer concentration ratio. 114 Table (11) : Effect of [In]/[M] on End Group Unsaturation o o Content in Propylene Oxide Polymerizations. Solvent THF. Temperature 50°C. [In]0/[M]ol x 103 - 2.57 3.00 3.24 3.60 4.96 (Unsaturated) 5 15.20 (Content )x 10 - 5.92 5.39 4.31 6.90 -1 eq. g 411•11...• 4.02 3.71 3.44 3.26 2.44 9.33 7.00 5.55 5.42 6.22 The unsaturation content values obtained for the polypropylene oxides prepared in these laboratories were near to the values obtained o (20) for polymers prepared with K t-BuO/DMSO and K t-BuO/DMS0 -THE', at 50 C. Table (1). The unsaturation content values for polypropylene oxides prepared with sodium naphthalene/THF and sodium naphthalene/THF-DMSO at 100°C, were higher than the values reported by the same workers for (20) the K t-BuO/DMSO and k t-BuO/DMSO-THF systems. Price and Carmelite(30) have reported higher unsaturation content values for the polymers prepared using K t-BuO/DHSO at 30°C. They also found that the unsaturation content in the homopolymers prepared by deuterated propylene oxide is lower than in polymers prepared from propylene oxide under similar conditions. This is evidence for the involvement of the methyl group in the transfer reactions. In propylene oxide polymerizations with sodium methoxide/dioxane or /bulk, a linear relationship was observed between the reciprocals (26) of the calculated and measured degrees of polymerization. 115 As was discussed in Section (3.3 D ) 1 1 a (13.2) DP DP a41 o where IX' and 53 are the measured and calculated degrees of polymerization o respectively. a is the transfer constant to monomer which has a value o c.(26) of 0.013 at 70°C and 0.026 at 93 Equation. (13.2) has also been used for calculating the transfer to monomer constant in neopentylethylene oxide polymerizations and a values of 0.007 at 39.9°C and 0.0116 at 60°C have been reported for this monomer. In the mathematical treatment by (26) Gee et al. the rate of monomer consumption is given by the sum of the rates of propagation and transfer reactions -d[M] - (k k ) [q] [In]o dt p fm (13.3) where k and k are the second order rate constants of propagation and fm transfer to monomer, respectively. The increase in the concentration of polymer chains, [P], is given by the rate of the transfer reaction: d[P] kfm [4] [In],a dt (13.4) Dividing equation (13.4) by equation (13.3), and combining the result with equation (13.5) k. fm k (13.5) p [11] = DP Jo f[m]0 - [M]} (13.6) a where [P] is the concentration of polymer chains in the absence of chain o transfer to monomer and is equal to [In] . o The degrees of polymerization in the absence and presence of chain 11—3 transfer, o and IT, respectively, are given by 116 [g] o [14] (13.7) [P]0 CH) - [H] (13.8) [P] Combination of equations (13.6). (13.7) and (13.8) yields equation (13.2). (20) Blanchard et al. have observed that by increasing the monoer/ initiator concentration ratio, in propylene oxide polymerizations, the • polymer molecular weights reach a limiting value. (123) Litt and Szwarc have assumed that m moles of monomer react with one mole of mono functional initiator, the initiation being instantaneous. Each growing centre has a probability p to add a further monomeric unit and a probability (1-p), to transfer to a monomer molecule. Based upon these assumptions they have calculated the number average degree of polymerization, DP, for mono functional initiators: DP = m/11 + (m-1)(1-p)1 (13.9) which is valid for high degrees of polymerization. m, as previously defined, is equal to the calculated degree of polymerization, DP0, assuming no transfer (i.e. when p = 1), [14]0 m = DP = — ° [In]. 1 DP' = p + (1-p) TiCI DP0 At low DP values, when p > (1-p)DP a plot of I1 vs DP would o o o be almost linear since under these conditions equation (13.11) reduces to: EP = ( ) Dpo (13.12) where p = fk / (kfm + kr,)1 = (14-a)/ (13.13) 117 • PINIIONIO awrow■I values of unity and zero, the corresponding values of DP For DPo should also be unity and zero, respectively. Again it would be feasible to choose a "best value" of p in equation (13.11) to provide a theoretical curve to fit the points involved and hence evaluate C. in a plot of DP against DPo At very high values of DP0, a limiting value of DP is reached, which may be designated Wilma From equation (13.11), when p <<(1-p)DP0 DP1im = 1 (13.14) 1-p (123) The equation of Gee et al.(26) and Litt and Szwarc become equivalent for high degrees of polymerization. Thus from equation (13.11), with DP0 >> 1 (DP0 - 1)(1-p) 1 = 1 - + a (13.15) DP DP DP 1+a o o EPo which is equivalent to equation (13.2), since if p is equal to 11,14/(14.0/ then (1-p) = (--1") (13.16) a+1 A line of slope unity, (A), was drawn through the points shown in the plot of 1/TOF vs 1/51170. In this plot the point corresponding to the value of (1005F) equal to 0.024 has not been taken into consideration. Figure (31). a 2 From equation (13.2) a value of (I a) equal to (2.00 0.21) x 10 was obtained, equal to the value of the intercept when (1005F0) was equal to zero. Figure (31). Thus a transfer constant to monomer, a, equal to (2.04 + 0.22) x 10 2 at 50°C was calculated. A plot of DP is shown in Figure (32). Using this value of a a theoretical vs DPo plot of ITC, vs 53 can be constructed and this is also shown in Figure (32). O The experimental values of DP, measured in this study, are located near to the plateau of the curve. Thus the degrees of polymerization obtained in the present experiments are quite close to the limiting degree of polymerization for polypropylene oxides. Again the DP value corres- ponding to DP0 equal to 4126.6 was considered to be too high. It was considered that a calculation of a from the limiting degree DPli ' obtained from the plot of W vs DP , would not of polymerization, o 7 ) plot. be as accurate as the calculation from the (1/N) vs (1/5 o Figure (31). However, from the intercept of this latter plot a/(1+0 was found to be (2.00 + 0.21)"x 10 2. Hence for this system rp = 1 = a+1 - 50.61 4. 5.56 • lim 1-p a This derived value of DPlim is shown in Figure (32). 13.H. Molecular Weights and Molecular Weight Distributions in Propylene Oxide Polymerizations The molecular weights of the homo polymers formed from propylene oxide polymerizations were measured by vapour phase osmometry. Tables (8) and (9). The corresponding rates of polymerization for these experiments were followed dilatometrically prior to isolation of homopolymers. Five homopolymer samples were analysed by GPC. (Table (9)), Figures (33) and (34) and the molecular weights of another five samples — '— I/M were measured also by automatic viscometry. Table (8). The M n values of the polypropylene oxide samples measured by GPC were 1.24 - 1.32. An example of the GPC chromatogram and a differential molecular weight distribution curve, dW/a(log M) vs log M is plotted. Figures (33) and (34). The measured molecular weights are much lower than the calculated molecular weights due to the transfer reaction to monomer, which restricts the growth of the living polypropylene oxide chains. • 119 - Figure (25) Volume change as a function of time for propylene oxide polymerization with cumyl potassium/THF at different temperatures. (a) 45°c (b) 50°C (c) 55°C (d) 65°C 6 w 0 I w 2 2 0 2 4 6 8 10 TIME huors) .120 Figure (26) log R vs log [q] for the polymerization o of propylene oxide with cumyl potassium/THF at 50°C. 0.6 0-3 0.4 0-5 O.6 log [M]o 121 Figure (27) log R vs log [In] for the polymerization p t of propylene oxide with cumyl potassium/THF at 500C. _0.9 0.6 0.5 0.8 0.9 1-0 1.1 1.2 3 -Flog [InI t 122 Figure (29). log k vs (1/T) for the propylene oxide polymerization with cumyl potasSium/THF 0 0.9 •••■•••111•••••••••• 3.00 3.10 3.20 3 1\ 10 123 . Figure (28) k as a function of [In]o for the polymerization of propylene oxide using cumyl potassium/THF at 50°0. O 0 12 Figure (30) 1/To vs unsaturation content for propylene oxide polymerization using cumyl potassium/THF at 50°C. 0 0 2 5 10 15 UNSATURATION 5 CONTENT xl0 (eq Figure C31). (1/117)Irs (1/57 ) for the polymerization of propylene oxide with cumyl potassium at 50°C. The line (A) has been drawn with a slope of unity in accordance with theoretical prediction. The points (C) represent experimental values. 0.1 0.2 0.3 0.4 0.5 2 - 10 DP Figure (32). (Jr) vs (LP) for the polymerizations of propylene oxide with cumyl , poiassium/THF at 50°C. The points (0) represent the experimental values of W. The curve (---) represents a theoretical plot assuming a transfer to monomer constant, a, equal to 2 x lo. . The line (---) represents the limiting degree of polymerization (DP1im) calculated using the above value of a. 80 0 60 40 00 20 200 400 4126.6 DP 0 126 Figure (33) GPC chromatogram for a sample of polypropylene oxide. 52 0 600 3, ELUTION (cm ) VOLUME Figure (34) Differential molecular weight distribution curve for a sample of polypropylene oxide the GPC chromatogram of which is observed in Figure (33). The sample has been treated_l as polystyrene and K and a values of 1.2 x 10-4 (10U ml g ) and 0.71, respectively, have been used for the GPC analysis. 5 3 log M 427 14. A Comparison between Ethylene Oxide and Propylene Oxide Polymerizations Initiated with Cumyl Potassium in THE The initiation step takes place by addition of a monomer unit to the initiator molecule and is similar for both ethylene oxide and propylene oxide polymerizations. The propagation step occurs by gradual addition of monomer to the propagating chains. The propylene oxide adds by a "head to tail" mechanism and the growing end has the structure CH2CH(CH3 )0- Ring opening polymerizations of ethylene oxide are slower than the polymerizations of vinyl monomers. However, as was discussed in previous sections, the polymerization rates vary to a considerable extent with the type of solvent employed. The effect of counter ion on the polymerization rate is also very important. Anionic polymerizations of ethylene oxide are faster than those for propylene oxide when performed under similar conditions. A comparison of the kinetic data obtained for ethylene oxide and propylene oxide polymerizations performed in these laboratories shows this expected difference in k values. Table (12). Table (12). Kinetic Data Obtained for Ethylene Oxide and Propylene Oxide Polymerizations. Solvent THF. k x 10 A E Monomer a dm3mo1-18-1 dm3Mol-i s-1 kJ mol -1 10 ethylene oxide 13.6 (at 48.9°C) 2.4 x 10 3.94 + 0.37 propylene oxide .4.6 (at 50°C) 2.0 x 109' 4.3 + 0.81 The k and A values are larger for the ethylene oxide polymerizations, while the value of E for propylene oxide polymerizations is larger than a that for ethylene oxide. 128 The reason for this difference has been attributed to the steric effect of the methyl group present in the propylene oxide molecule, which causes hindrance during the addition of propylene oxide to the propagating chains. The decrease in the volume of the polymerizing solution of ethylene oxide, initiated with cumyl potassium in THE is initially very slow. This initial slow decrease in volume is followed by a more rapid decrease and eventually the decrease in volume becomes linear with time. Section (13.8.). This effect is also important in propylene oxide• - polymerizations with cumyl potassium/THF. It is suggested that the transfer to monomer reactions occurring in the propylene oxide polymeriza- tions maintain the molecular weight of the living polypropylene oxide chains lower than the expected molecular weight, throughout the course of the polymerization. Thus the gradual change ir► the density with molecular weight which affects the rate of change in the volume of the polymerizing solution of ethylene oxide is lower-for propylene oxide polymerizations where the molecular weights increase more slowly. The transfer to monomer reaction occurring in propylene oxide polymerizations give rise to the formation of polymer containing end group unsaturation. The measured molecular weights of the polypropylene oxides are always lower than the calculated molecular weights and they reach a limiting value on increasing the monomer/initiator concentration ratio. This behaviour was not observed for ethylene oxide polymerizations. The high molecular weight polyethylene oxides were white solids, but all of the polypropylene oxides isolated in these laboratories were faint yellow liquids. In summary then, both the ethylene oxide and propylene oxide polymerizations occur by opening of the epoxide rings. In propylene 129 oxide polymerizations the rates are slower due to the steric restrictions caused by the methyl group. Also transfer reactions to the methyl group occur which cause end group unsaturation and limit the molecular weights of polypropylene oxides. 130 15. - PREPARATION OF MONODISPERSE POLYSTYRENE USING CUMYL POTASSIUM/THF The preparation of polystyrenes having chains of equal lengths is important for structural studies of this polymer and especially for the preparation of branched homopolymers and graft copolymers having main chains of equal length and grafts of equal, but different, lengths. Anionic polymerization is a very useful method of preparing monodisperse polystyrene. Section (4.). It is not experimentally possible to prepare a polymer having chains all of the same length and a Poisson distribution(8) of molecular weights is always produced, which is reflected by the value of 1q461 . However polystyrenes with n M n values very near to unity have been prepared using the anionic polymerization method. In the present work an attempt was made to prepare a polystyrene sample of narrow molecular weight distribution involving a relatively straightforward technique using cumyl pctassium/THF. The experimental details have been described in Section (9). Cumyl potassium is an efficient initiator and unlike some other anionic initiators, such as n-butyl lithium and benzyl sodium, it is quite stable in THF at room temperature. Section (4.A.). The rate of addition of styrene to living polystyryl potassium in THF is slower than the rate of addition to living polystyryl lithium in the same solvent. Thus there is more time for the growing ends to react with the monomer which is gradually introduced into the reaction medium. A homogeneous distribution of monomers is ensured by stirring and thus, due to the relatively long reaction time each growing end has an equal opportunity of adding monomer. However, in the present experiments the 131 polymerization reaction was performed at Dry Ice/Methanol temperature and the reactivity of the polystyryl potassium was reduced to a great extent. Although the styrene was gradually added to the polymerizing solution, it was not possible to maintain continuous rapid stirring throughout the course of the reaction. The glass encased magnetic follower could not follow the rapid rotations of the stirrer. The polymer was analysed by GPC. Sample P-I, Figure (35). The number average molecular weights M' measured by GPC was equal to 61,900 which was very near to the calculated molecular weight of 60,000. This was evidence for the high efficiency of this initiator. 448) Asami et al. also observed that the measured molecular weight of the polystyrene, prepared by cumyl potassium/THF, was near to the calcu- lated molecular weight. The Mw/M was equal to 1.19. A value of n equal to 1.16 was reported for a sample of monodisperse polystyrene n (Pressure Chemical Co.] with M equal to 110,000. Thus the value of n mw/Rn for the polystyrene sample prepared in these laboratories was satisfactory, bearing in mind the problem of stirring encountered during the polymerization. 132 Figure (35) Differential molecular weight distribution of sample P-1, a monodisperse polystyrene. I 5 - 0 0 4 log M 133 16. ATTEMPTED PREPARATION OF GRAFT COPOLYMERS OF STYRENE AND ETHYLENE OXIDE Graft copolymers having ethylene oxide grafts have been prepared using both the deactivation technique and also by addition of monomer to metalated sites on another type of polymer backbone chain. Thus graft copolymers of vinyl biphenyl (VB) and vinyl naphthalene (NN) with ethylene oxide have been prepared. Homopolymers of (VN) and (VB) have been metalated with alkali metals or alkali metal naphthalene and then ethylene oxide has been added to the metalated sites on the (4,54) polymers. Graft copolymers of nylon 6 with ethylene oxide have also been prepared by metalation of nylon 6 with metallic sodium/ammonia followed by addition of ethylene oxide to nylon 6 metalated at NH positions in THF.(124) Polyethylene oxide has been grafted on to polymethyl methacrylate (PMMA) by trans-esterification involving the reaction of living polyethylene oxide chains having potassium counter ions with the (59) -COOCH3 groups of PMMA. Branched polystyrenes have been prepared using the deactivation technique. Section (5.1.B.). Monodisperse polystyrenes were chloro- methylated on the benzene ring and then the chloromethylated sites were (43,44 ) (95) coupled with living polystyrene chains having lithium or potassium counter_ions. Complications involving halogen-metal exchange reactions_ resulting in cross-linking were observed when lithium was the counter ion. Section (5.1.B.). The halogen-metal exchange reactions were (95) not important when potassium was the counter ion. Graft copolymers (96) of styrene and butadiene have also been prepared by the same technique. In the present work an attempt was made to graft living polyethylene oxide chains, polymerized with cumyl potassium in THF, on to chloro- methylated polystyrene. 134 The experimental details have been given in Section (10). The reproducibility was a major problem during the preparation of chloromethylated polystyrenes containing small numbers of chloromethylated sites. The reaction has already been found to be dependent on factors such as the type, purity and concentration of catalyst, solvent and polystyrene. Section (10). One of the problems involved was the reaction of methanol, a product of the chloromethylation reaction, with stannic chloride to form, initially, hydrogen chloride and methoxy tin trichloridp. Both the latter substances could act as catalysts for further chloromethylation, when stannic chloride was the original catalyst. (93) Originally zinc chloride was used as the catalyst for the chloro- methylation reactions but later it was observed that relatively reproducible results had been obtained using stannic chloride catalyst/carbon tetra- (43) chloride systems. The handling of stannic chloride was easier than that of zinc chloride. In particular, liquid stannic chloride could be transferred by syringing whereas the zirc chloride powder had to be transferred in a glove-box. After a few preliminary attempts the optimum conditions for chloro- methylating polystyrene to the low required extent were established. The chlorine content analyses were performed using the Neutron Activation Analysis (NAA) technique. Section (10.1.). Figure (36). An initial small scale attempt was made to perform a coupling reaction between living polyethylene oxide chains, polymerized with cumyl potassium/ THE and a chloromethylated sample of polystyrene having a molecular weight of about 60,000. The chlorine content of the polystyrene had not been previously determined, but it was expected to be high (about 5 weight %). The coupling reaction in THE was performed with stirring at room temperature. A gradual increase in the viscosity of the solution 135 was observed until it gelled completely and became insoluble. The final product was then found to be insoluble in THF, benzene, toluene and their mixtures. As the coupling reaction had been performed at room temperature, cross-linking was not expected. Howe7er it was thought that a coupling reaction together with cross-linking of polystyrene due to the high chlorine content had occurred. Thus it was decided to perform a coupling reaction using a polystyrene sample chloromethylated to a much lesser extent. equal to 110,000 A monodisperse polystyrene (MITI < 1.06) and Mn (Pressure Chemical Company) was chloromethylated up to 0.33 weight per cent (Measured by NAA) and was coupled with living polyethylene oxide having potassium counter ions. This time the polymerizing solution in THF did not gel. The product of the coupling was a white polymer and was analysed by GPC. Figures (37) and (38). A cdomparison of the GPC results obtained for the initial polystyrene and the coupling products shows a slight increase of 3000 in the molecular weight of the polystyrene from the grafting reaction. However this change, compared with the molecular weight of the polystyrene, (M equal to 101,000, measured by GPC) was so small that it was considered n to be within the limits of experimental error. The separate peaks in polystyrene and polyethylene oxide chromatograms (Figure (37)) show that almost no coupling reaction took place. The peak for polyethylene oxide is much smaller than that for polystyrene. The reason for this is the difference between the refractive index increments of polyethylene oxide and polystyrene solutions. The specific refractive index increment, (125) dn/dc, is 0.198 cm 3g -1 for polystyrene and 0.068 cm3g-1 for poly- ethyleneglycol in THF at 25°C. 136 It was known from classic chemistry (Willamson Reactions) that alkoxide ions couple with halogen containing compounds. Thus it was expected that the coupling reaction should take place. The concen- trations of the polyethylene oxide and polystyrene in the coupling 4 solution were each about 2 x 10 mol dm-3 and as described previously, the coupling solution was stirred for four days at room temperature. It is expected that the coupling should take place by altering the conditions of the reaction, such as temperature, reaction time and reactant concentrations. 137 Figure (36) Spectrum of Chloromethylated polystyrene after irradiation for one minute in the pneumatic tube facility of the HERALD reactor Uta.,1 &P31 Is SAMPLE (N aas NO . 5000 4500 ,.000 3500 0 2500 '0 U N 2000 T 1500 U 1000 LI 4.,'" • "-• 600 200 400 Boo boo iboo 1200 1400 1800 1800 2000 EWAN( KtV/ COUNTED AT 3442.34 HOURS 1974 FOR 50.0 MINS CRYSTAL, 80 CAIN 1.6 080m ,3 138 Figure (37) GPC chromatogram of the product of the attempted preparation of a graft copolymer of styrene and ethylene oxide. 4 0 c I 440 540 640 3 ELUTION (cm ) VOLUME Figure (38) GPC chromatogram of the polystyrene sample used in the attempted preparation of a graft copolymer of styrene and ethylene oxide. L. 60 C o 40 380 460 540 3 ELUTION (cm VOLUME 139 •17. CONCLUSIONS Anionic polymerizations of ethylene oxide and propylene oxide at 30 - 60°C using cumyl potassium initiator and tetrahydrofuran as solvent occur by the gradual addition of monomer to the growing alkoxide end groups. The polymerizations in tetrahydrofuran involve ion pairs only since the dissociation constants of alkoxide and alkali metal ion pairs in this solvent are very small. The anionic polymerization rates of these monomers are much slower than that of most vinyl monomers under similar conditions. The kinetics of the polymerizations were studied dilatometrically, although the polymerizations of propylene oxide were relatively slow. Due to the gradual change in density of growing low molecular weight living polyethylene oxide chains, the rate of decrease in the volume of polymerizing solutions was initially slow. The rate of decrease in the volume gradually increased until it reached a constant value. The initial increase in the rate of change of volume for propylene oxide polymerizations was slower than that for ethylene oxide polymerizations. This was attributed to the transfer to monomer involved in the anionic polymerization of propylene oxide which greatly lowered the molecular weights of the living polypropylene oxide chains. The effect of initiator concentration on end group unsaturation was also studied in propylene oxide polymeiization. It was found that the unsaturation content tended to increase with increasing initiator/ monomer concentration ratio. The anionic polymerization of propylene oxide using different initiators and reaction media have been studied widely by other workers. Nevertheless it appears that the detailed mechanisms of these polymeriza- tions are quite complicated. Further extensive studies are required 1.40 for a better understanding of the polymerization of propylene oxide. The reproducibility of chloromethylations of polystyrene samples was not very satisfactory due to its dependence on the purity and concentrations of reactants and solvent. However, chloromethylotion was considered to be a relatively useful method for the preparation of polystyrenes of low chlorine content. Preliminary attempts to prepare graft copolymers of ethylene oxide and styrene were not satisfactory. However, it is thought that the coupling reaction between living polyethylene oxide chains and chloro- methyl groups on the polystyrene rings should be possible by varying the reaction conditions. 141 18. ABBREVIATIONS P Me ion pair free ion + Me counter ion. A carbonium ion M monomer In initiator initial monomer concentration [M]o [In]0 initial initiator concentration DI-3 living chain concentration R rate of polymerization P k second order rate constant for propagation P k. second order rate constant for initiation 1 kfm second order rate constant for transfer to monomer second order rate constant for propagation with free ions kP- second order rate constant for propagation with ion pairs kP Me+ K equilibrium constant for ion pair dissociation K. equilibrium constant for dimsyl ion formation 1 [ROB] alcohol concentration [P] concentration of polymer chains in the presence of transfer to monomer concentration of polymer chains in the absence of transfer °Plo to monomer DP number average degree of polymerization DP weight average degree of polymerization w DP calculated degree of polymerization DP1im limiting degree of polymerization M number average molecular weight n M weight average molecular weight w 3.42 molecular weight W weight of polymer molecular weight of cumyl group • cumyl M atomic weight of hydrogen H absolute temperature t and t°C temperature degrees centigrade t time t ✓ initial volume of polymerizing solution (dilatometry) ✓ volume of unit length of capillary tubing (dilatometry) 1 height of solution in capillary tubing (dilatometry) V volume of initial solution m and d monomer density d m dp polymer density QV change in volume of polymerizing solution moles of monomer p probability to add a monomer unit. 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