Polymerization of Alpha.-Methylstyrene at High Pressures

POLYMERIZATION OF ALPHA.-METHYLSTYRENE AT HIGH PRESSURES

Thesis Presented for the Degree of

DOCTOR OF PHILOSOPHY

in the Faculty of Engineering in the University of London

by ISY ELROY, B.Sc. (Eng.)

Department of Chemical Engineering, Imperial College of Science and Technology, LONDON, S.W.7

October, 1961 ABSTRACT

The free radical polymerization of alpha-methylstyrene has been studied at pressures up to 8000 atm. The initiator used in most of the experiments was azo-bis-isobutyronitrile, but tert-butyl-perbenzoate was employed at temperatures above 100°C. The existence of two limiting factors, the ceiling temperature for polymerization and the freezing of the monomer at high pressures, has been investigated. At temperatures above the ceiling temperature the main product is dimer. The rate of dimerization is increased by pressure and temperature and straight lines are obtained for the log (rate) vs. pressure and vs. liT plots respectively. Below the ceiling temperature solid polymer is formed together with a small amount of low molecular weight liquid polymer, which is believed to be formed by a transfer to monomer reaction involving an allylio transfer mechanism. The freezing of the monomer has been investigated and a linear relationship between the freezing temperature and the pressure has been established. The dependence of the rate of polymerization on the initiator concentration is in agreement with existing results for styrene and alpha-methylstyrene, the order with respect to initiator being 0.44. Pressure increases the overall rate of reaction between 1500 atm. and 4700 atm. and there is a linear relation between log (rate) and the pressure in the range 3000 — 4500 atm. at 60°C. The rate falls off sharply above the freezing point at 4860 atm. due to partial solidification of the reagents.

The degree of polymerization is only slightly affected by the duration of the reaction, and the molecular weight of the polymer increases with increasing pressure. In agreement with the usual kinetic equations, the degree of polymerization is proportional to the initiator concentration raised to the power —0.485. The dependence of the polymer molecular weight on the temperature agrees with preaiction. At 3000 atm. there is a decrease in molecular weight between 40°C and 60°C, followed by a region in which it is constant and then by a further sharp decrease as the ceiling temperature

(109°C) is approached. ACKNOWLEDGEMENTS

The author wishes to express his sincere thanks to Dr. K.E. Weale for his advice and encouragement throughout the course of this research, and to Mr. A.M. Alger and the Departmental Workshop Staff for their assistance in maintaining the high pressure equipment.

The author also thanks the Scholarship Fund Committee, B'nai B'rith Leo Baeck Lodge (London) for the award of a Postgraduate Scholarship held during the course of this work.

Thanks are due to Miss C.V. Rawlings for her help in the prepara— tion of this thesis. 5 CONTENTS

Page PART ONE: INTRODUCTION

I.1 General Review of Vinyl Polymerization 8

(i)Initiation 10

(ii)Chain Propagation 13

(iii)Chain Termination 14

(iv)Chain Transfer 15

1.2 Kinetics of Free Radical Chain Polymerizations 18

(i)Kinetic equations 18 (ii)Polymer molecular weight 23

(iii)Ceiling temperature and thermodynamics of polymerization 25

1.3 The Effect of Pressure on Polymerization 28 1.4 The Polymerization of Alpha-methylstyrene 35

(i)Atmospheric pressure polymerizations 35

(ii)High pressure polymerizations 37 1.5 Scope of the Work 39

PART TWO: APPARATUS, MATERIALS, AND EXPERIMENTAL PROCEDURE

11.1 Apparatus 41

11.2 Preparation of Reagents 48 6 Page

11.3 Experimental Procedure 50

(i)Preparation of reaction mixture 50

(ii)Polymerization reaction 50

(iii.) Separation of high polymer 51

(iv)Determination of yield of liquid polymer 51

(v)Molecular weight determination 53

PART THREE: EXPERIMENTAL RESULTS

111.1 Rates of Polymerization 57

(i)Dependence of rate on initiator concentration 57

(ii)Dependence of polymerization rate on pressure 59 (iii)Dependence of the yield of polymer on the

temperature 63

111.2 Molecular Weights 66

111.3 Freezing Pressures 74

TII.4 Formation of Liquid Polymer 79

Tables of Results 87

PART FOUR: DISCUSSION

IV.1 Freezing and Ceiling Temperatures as Limiting Factors

in the Polymerization of Alpha—methylstyrene 99

(i)Freezing phenomonon under pressure 101

(ii)Ceiling temperature at high pressures 103 7 Page IV.2 The Effect of Pressure on Polymerization Rate 107

(i)Order of reaction with respeot to initiator

concentration 107

(ii)Overall rate of polymerization 109 (iii)Dependence of overall rate on temperature 114 IV.3 The Effect of Pressure on the Molecular Weight 116

IV.4 The Effect of Pressure on the Formation of Low

Molecular Weight Polymer 125

CONCLUSIONS 134

BIBLIOGRAPHY 136 8

PART ONE: INTRODUCTION

I.1 General Review of Vinyl Polymerization Vinyl polymerization, a term frequently used to describe the addition polymerization of substituted ethylenes, is a chain reaction,

in the course of which the unsaturated molecules of olefins are transformed into large saturated molecules of very high molecular weight. Studies, initiated by Herman Staudinger (1) over forty years ago, led to a series of investigations in the field of the

chemical structure of addition polymers. He suggested that a

typical addition polymer was essentially a saturated linear molecule with a head—to—tail linkage of the monomers in the polymer chain. A chain reaction takes place in three distinct processes& r 1.Initiation 2.Propagation

3.Termination It is the initiation process which determines the nature of the

polymerization reaction. In most of the cases a free radical or an ionic mechanism are in control of the course of the reaction. As in this work interest lies only in the free radical reactions, polymerizations with an ionic mechanism will not be considered

further. The quantity known as the degree of polymerization of a polymer which is proportional to its molecular weight, is of great importance

as it is chiefly the very large number of monomer units incorporated 9 in the polymer molecule which give it its distinctive properties.

Polymer molecular weights may be determined by methods of degradation and end-group analysis but as the molecular weights are so high, this way can be used to a very limited extent. There are other, more sensitive physical methods, the most important of which are measurements of intrinsic viscosity, osmotic pressure, light scattering, sedimentation and diffusion using an ultracentri- fuge. The intrinsic viscosity method is empirical and requires calibration against one of the other techniques.

Chain reactions are known for their sensitivities to impurities.

Only the use of highly purified reagents can give reliable experimental data for the investigation of free radical polymerization kinetics. Evidence of the free radical mechanism in polymerization reactions was obtained by approaching the problem from different directions: a)Initiating polymerization by introducing free radicals to vinyl monomers. b)Substances known to decompose and give free radicals when mixed with certain monomers at the proper conditions gave products of high molecular weight. Incorporated fragments of the initiator were detected by means of isotopical or chemical labeling.

c)Using inhibitors, such as quinones, which on reacting with the free radicals of a system produce stable radicals, or substances known as free radical 'scavengers' such as diphenylpicrylhydrazyl

(DPPH). 10

As stated before,the basic steps by which any homogeneous free radical polymerization procedes are radical formation, successive radical additions to a double bond and radical destruction. These processes are described in detail by Walling (2), Bamford et al. (3), Boundy and Bwer (4), Burnett (5) and several other authors. It is essential that a short summary on these reactions and their kinetics should be included in this introduction. (i) Initiation a) Thermal initiations There are certain monomers known to undergo a spontaneous thermal polymerization by a radical process even in the absence of any added initiator. The best known example is styrene. Taking precautions to exclude any traces of air and by rigorous purification of the monomer, reproducible polymerization rates have been obtained, e.g. by Walling and Briggs (6). In contrast, certain olefins including vinyl ketone, acrylonitrile and tetrafluoroethylene give only dimeric Diels —Alder products at temperatures about 20000. Fiory (7) proposed as the mechanism a bimolecular initiation process leading to the formation of a diradical 20H2 =

Although energetically feasible, this reaction suffers from the consequence that the resulting diradical is subject to cyclization thus decreasing enormously the probability of growing long chains.

Mayo's study of the thermal polymerization of styrene in bromobenzene (8) proposed that the reaction is more nearly 5/2 order than second 11 order. He suggested a third order initiation reaction process producing monoradicals:

3CH = CHBz----“H 2 3-tHBz + CH-CHBz = CH-tHBz An alternative bimolecular reaction producing monoradicals was proposed by Walling(2). The process 2CH = CHBz--- 2 )PCH3 6HBz + CH2 = tBz is also thermodynamically feasible and gives an alpha -phenylvinyl radical with a resonance energy of >- 18.5 kcal/mole. The feasibility of these processes depends on the resonance stabilization of the radicals, hence only monomers which may produce such highly stabilized radicals as methylmethacrylate can be polymerized thermaly. However, what actually occurs during the thermal initiation process remains obscure and most uncertain. The effect on the rate of a relatively low concentration of an initiator is far more significant than thermal initiation. b) Chemical iniation: The initiation of chain growth is caused by the reaction of a primary radical R*, produced by the decomposition of an initiator molecule In, with the double bond of an olefin M. In the case of the decomposition of azo-bis- isobutyronitrile (AIBN) into free radicals via the process

CH3 CH I 3 F* NC-C-N = Y-C-CN----.1.,2 CH -C + N t t 3 1 2 CH CH CH 3 3 3 kr]. * In ._-_-30. 2 R* (1) followed by 12 CH3 I* k. 3 1* NC - C + CH2 . CHX-->NC - C - CH, -- C . k . CH CH X 3 3 R (2)

The fraction of initiator fragments formed by the decomposition of the initiator, which were used in initiating the polymerization process is defined as f - initiator efficiency or elset

= Rate of initiation of chain radicals f 2 x Rate of decomposition of initiator In many systems f varies between 0.5 and 1.0. Initiators are usually induced to decompose by means of heat or photochemical treatment. Many substances can initiate polymerization reactions, the most commonly used being benzoyl peroxide and azo-bis-isobutyronitrile and its homologs. As mentioned before, initiator fragements are included in the polymer. Chemically-bound halogen which cannot be removed by precipitation is found in polystyrene and polymethyl methacrylate produced by using halogenated benzoyl peroxide. Initiators labelled with radioactive isotopes provide a very sensitive method of detecting initiators-inclusion in polymer molecules. Bevington and co-workers (74) (80) used azo-bis- isobutyronitrile containing C14. Radioactive determinations indicated one or two initiator fragments per polymer molecule. Initiators for which the rate of decomposition can be measured with accuracy are used to determine the efficiency of initiations. Benzoyl peroxide (by titration) and AIBN (rate of nitrogen evolution) 13

have been employed. One of the methods involves comparing the rates of initiator decomposition and polymer production. If each decomposed initiator molecule produces one polymer molecule (provided the termination is by combination), f = 1. Any lower number will indicatealowareffeciancy. The rate of initiation and the efficiency are generally accepted to be independent of monomer concentration above Di.] . 1 mole/liter. Below this value Bevington (9) found a rapid decrease in f with (X]. He reported an efficiency of 0.6

for azo-bis-isobutyronitrile with styrene at 60°C. The same author reports a value of f = 1.0 for benzoyl peroxide under the same condition.

The rate of disappearance of inhibitors and retarders are

usually used to estimate the rate of production of free radicals. Bamford et al (10) used ferric chloride, a very convenient and

accurate method. Much less accurate is the method using diphenylpicrylhydrazyl since the mechanism of reaction is too uncertain to yield an exact estimation. (ii) Chain Propagation

This reaction consists of addition of a monomer to a polymer radical to produce a larger but structurally similar radical, capable of further growth.

CH CH H H 3 I* + 3 I NC-C - CH -C CH H 2 2 >N0-0 - CH2-0-CH2- CH X 3 3 14 +M2kpl * MI• kp2 - * followed by M2• + M ---47111 et cetera u3 + k n n + 1 (3) It is usual to make the simplifying assumption that the rate of propagation is independent of the length of the growing polymer chain. (iii) Chain Termination Two processes leading to the self-termination of the chain radicals may take place: a)Combination of two chains to give one inactive polymer molecule P CH H H CH 1 1* *1 1 3 NC-C- (CH2 -CHX)1_, - CH -C + C-CH2- (CHX-CH2)0-1 - - aa 1 . 2 i 1 1 C CH3 X X CH3 CH H E CH 1 1 3 --kt - (CH2-CliX)h-1 -CH2 -C-C-CH--(CHX-CH11 e 2)1714.- C - CN CH3 X X CH3

* * kt (4a) Mn +Mra"-Pm+1 b)Disproportionation, involving the transfer of a hydrogen atom or a small radical between two growing polymer chains with the production of two inactive polymer molecules Pm and Pn. CH. H H CH. 3 lie *1 1 3 kta NC-c - (cH2-cHx)ri_i - CH, -C + C -CH2 - (cHx-cyni_i - C - CN CH X X CH 3 3 15 kta >CNC(CH3)2(CH2-CHXL3. - CH. CHX

▪ CH2 X-CH2 -(CHX-CH_c)m-1 - 3) 2 cag * *td Mn + Mm >PM Pn (4b) There is a question as to the relative importance of the two reactions, and considerable evidence showing thattermination by combination is tae predominant one was given by Flory (11) and Bamford et al. (3) for the polymerization of styrene at atmospheric pressure. The formulation of the disproportionation reaction was based entirely upon analogy to non-polymer systems. However, for kinetic) purposes, the two are considered as equivalent and usually combined as k:t + M 4. P + P Mn* m y.* Pmn m n (5) here kt kto ktd Two other termination processes, which may take place, involve primary radicals: * * M (6) * * R + R (7) These reactions can affect the polymerization kinetics only when the initiator concentration is very high and monomer concentration very low. (iv) Chain Transfer In many polymerization systems the degree of polymerization (or the polymer molecular weight) is found to be notably lower than that, predicted by theoretical calculations due to the 16 intervention of radical displacement reactions in the course of

the polymerization. Flory (7) introduced the term 'dhain

transfer' for reactions of this kind. They involve the transfer of a labile atom or group between a molecule in the system and a growing chain radical, thus terminating the chain, while transferring its activity to the molecule. A second step, eventually, takes place, in the course of which the newly produced radical adds to

a monomer molecule, starting a new polymer chain. When the rate of this reaction is comparable to the ordinary chain propagation there

will be no appreciable effect on the overall rate of polymerization, yet the resulting product will necessarily be of a lower molecular weight. In some extreme cases the resulting product will be a collection of small molecules in which, fragments of the transfer agent are added across the double bond of the olefin. Initiator, solvent, monomer and the polymer molecules can take part in transfer reaction. The different transfer reactions are outlined below: a) Monomer transfer, k • " mt + r u* (8) b) Polymer transfer, k pt u* 4N111 Am (9) 0) Initiator transfer, kit M•h + R (10) d) Solvent transfer, * M•h + o kst + S (ii) 17

The re—initiation reactions are of the type: following (8) * * M + and following (11) * S + (12). There is very slight probability that the transfer agent will take part in termination reactions such as: * * S 11,n—h•pP (13) * * or S + S --> Inactive product In principle, any molecule in a polymerizing system may undergo chain transfer reaction with an active polymer chain. i8 1.2 Kinetics of Free Radical Chain Polymerizations

In order to obtain tractable kinetic equations it is necessary

to introduce certain simplifying assumptions: a)That by allowing the polymerization reaction to proceed only to a low degree of conversion it is usual to assume that

concentrations of the reagents are kept virtually constant. b)That since the polymer chain produced is of very high molecular weight, monomer is consumed mainly in the propagation

reaction.

0 A general problem in treating any radical chain process is to eliminate radical concentrations from the kinetic equations.

For this purpose it is convenient to assume that a steady state is obtained in the system. In other words, the rate of change of free radical concentration is very small compared to its rates of

formation and disappearance and may be assumed to be zero. d) That the various rates of reactions are independent of

the length of the growing polymer radicals. This assumption is acceptable if the growing end of the polymer chain has considerable freedom of movement and its reaction properties can be influenced

only by the monomers in its vicinity. This was confirmed by

Matheson et al. (12) for styrene.

(i) Kinetic equations Without considering the different chain transfer reactions,

as in the simplest case they do not affect the overall rate of reaction,

the main reactions as described before are:

* In 2R kd L In_ 1 (1) * R + M > J (2)

Mn + M r n+11 *] (3) * * Mn + Mm Fm+n kt [mn*I (4)

As assumed above, rate constants are independent of polymer chain length and for convenience the subscripts m an n's will be

omitted from the subsequent equations. From eqs. 2 and 3 the rate of monomer disappearance is given as a Da. 3 [R*I LM] + kp I1 M*1 CM] (15) dt since for the high molecular weight production process

k. [R* kp Lei EM] for first term can be neglected:

d DM] kp Dit* rld (16) dt From eqs. 2 and 4 the rate of radical production is

d LM* (17) = ki I— R _ _ — 2kt CM*-11 2 dt (The factor 2 is being used according to the convention of Flory, Matheson and others of writing the equation to correspond to the

number of radicals produced or consumed.) Applying the steady state assumption, eq. 17 may be equated

to zero thus giving

ki 11111.* CM1 = 2k [M* 12 (18)

Similarly, from 'eqs. 1 and 2, with the introduction of the initiator efficiency f

[41 [In] Di] • 0 (19) dt By combining eqs. 15 - 19 the rate of propagation Rp is:

k f [in] d- [ d Rp k M (20) dt [ kt

SincetherateofinitiationR.• is equal to

2k LM 2 Ri k.1 [R*1[14]- t 2fkd [In] the equation for the rate of propagation can be written R i R k Li] (21) 2kt P P Another way to express R is by defining k and replacing it in eq. (20) k 0 (- (22) (21kd Fill CIn *V - C In many cases polymerization reactions employing initiators which decompose spontaneously give excellent agreement with the predicted relationship in eq. 22. Mayo et al. (13), Schulz and Blaschke (14) reported that the polymerization rate of styrene and methyl methacrylate initiated with benzoyl peroxide, respectively, was proportional to the square root of the initiator concentra— tion. Some slight deviations were observed below 10-3 molar initiator 21 concentration due to thermal initiation of the monomer.

Arnett (15) showed that using different azonitrile initiators at the same concentration and temperature conditions a 0.5 exponent for the relationship between the rates of polymerization and initiator was obtained. A marked rise in the exponent was observed when a phase separation occurs during the polymerization reaction resulting from the production of monomer-insoluble polymers, which precipitate in the medium. These systems are, apparently, intermediate between homogeneous polymerization and polymerization in emulsion. Because of these precipitation complications it is very difficult to obtain absolute rate constants (16), (18), (19) and the picture of polymerization with phase separation is still largely qualitative. By use of suitable solvent the separation can be avoided thus reducing the exponents very near to the predicted value of 0.5 (17). There have been reports of exponents smaller than 0.5 when high initiator concentrations were used in polymerizations without solvents, or at reduced monomer concentrations with moderate initiator concentrations (20). Chapiro et al. (21), (22) observed similar trends in radiation-induced polymerizations.

While many indicate that under such conditions primary radicals from the initiator participate in chain termination, the dependence of polymerization rate on monomer concentration, however, is less clear-cut than its relation to initiator concentration.

In the systems mentioned above (13), (14), (15) consisting styrene

22 and methyl methacrylate with benzoyl peroxide and azo-bis- isobutyronitrile as initiators, the rate of reaction is close to first order in respect to monomer concentration. Values ranging between 1.0 and 1.5 have been reported (23), (24), (25), (26). An empirical relation was developed Amp

B 51) d k 51] CIO (23) dt 1 B where B is a constant depending on the particular system at constant temperature. Here the order of reaction can vary between values of 1.0 and 1.5 according to the magnitude of B[11]. When toluene was used as a solvent in the polymerization of styrene, initiated with benzoyl peroxide at 80°C, the order in respect to monomer conoentration was found to be between 1.18 at PC= 1.8 moles/liter and 1.36 at [113 = 0.4 moles/liter. Horikt and. Herman (23) explaining their findings pointed out the importance of using an experimental technique employing a stirred flow reactor which proved very useful in the study of radical chain processes, in contrast to the conventional methods. In this case the constant B was 1.19 litres/mole. However, it appears that several factors such as: the variation in kd, and f- the initiator effeciency with the medium, the presence of adventitious retarders in the solvent, and finally, reactions generally of small extent, between polymer radicals and solvent which produce radicals of low reactivity, may all be involved in the increasing of order of the polymerization reaction with respect to monomer.

(ii) Polymer molecular weight

The average number of monomer molecules consumed by every radical employed in the initiation of a new polymerization chain is given by Rp the ratio known as the kinetic chain length CL R -1 R = or (2fkjk Ri 2fka [In]

CL = (2fka.)4 [In] 4 [Id (24) From eq. 18 it is evident that for steady—state conditions

R. = Rt therefore k rIL*1 kp [113 P — CL = R = (25) t 2kt [ie".1 2 2kt Er 1

In the simplest case it is possible to consider that the formation of polymer molecules depends upon the relative importance of chain termination by disproportionation and combination. The first reaction produces one polymer molecule for every polymerization chain, whereas the second combines two chains into one inactive polymer molecule. Thus the relation between the kinetic chain length CL and the average degree of polymerization DP (otherwise known as polymer molecular weight) is: DP = CL for disproportionation, and re = 2CL for combination. If all the transfer reactions (eqs. 8 - 11) are also taken in

24 account, bearing in mind that they can produce polymer molecules in the same proportions as termination by combination, the general equation for average degree of polymerization is:

E (ktc/2 ktd) kmt ban [M] pt [M1 kp Lel

kst — + k. LM -n k [IA j LM* k transfer Introducing a new parameter, the transfer constant C = kp the equation above is rewritten as

(kt0/2 + ktd) [111 FPI Es ] [In] (26) 1 = -+C + C =----+C -+C. DP m PP [M 1 P [M] s PI] I M-1 This equation is known as 'the general transfer equation.' For low conversions transfer to polymer may be neglected and using the equivalent for the radical concentration LM j from R eq. 16 as the general transfer equation is:

Ck /2 + k ) ER 5] [in] 1 td td + C + C + C . (27) DP 2 j2 m s [m] pt or replacing Rp from eq. 22

1 (kto/2 ktd) (2fkdIcf [I/J1 (28) DP - + um + u +C. (kto km) [ 25

1 If - is defined as the degree of polymerization observed DPo in the absence of transfer then

1 1 [In + C + Cs + C (29) M DP DP Li] DI o

(iii) Ceiling temperature and thermodynamics of polymerization Polymerization reactions are known to be exothermic processes. While the heat of polymerization Ls:El for the conversion of ethylene to polyethylene is approximately -22 kcal/mole of monomer, lower values ranging from -9 kcal/mole for alpha-methylstyrene to -21.3 kcal/mole for vinyl acetate have been reported. The reasons for the lower heats of polymerization are two-fold sources. In the case of conjugated olefins there is the loss in resonance energy due to the disappearance of the double bonds which amounts to as much as 44 kcal/ mole for styrene and butadiene. In addition, strong steric repul- sions between substituents in successive monomer units in the polymer chain and especially in 1,1 -disubstituted compounds apparently account for a large reduction inAlip (up to 10 kcal/mole for isobutylene and vinylidene chloride). It is obvious that in the case of alpha- methylstyrene where both effects are combined,4NH has the lowest absolute value of all common monomers. Since in the polymerization reaction there is a large number of propagation steps for a single initiation or termination, All can be taken as the overall heat of polymerization,aH for the production of a polymer molecule thus 26 leading to the conclusion that 4H is independent of catalyst concentration and the presence of transfer agents. When tali is low, a relatively low activation energy and a high rate of reaction are expected and observed. However, for these monomers at higher temperatures the reverse of the polymerization reaction may become appreoiable. The thermal cracking of hydrocarbons to low molecular weight olefins is a kind of depolymerization, a reversal of propagation reaction. In contrast to the heat of polymerization which favours the reaction, the entropy decreases when monomer units are linked into long polymer chain so that the entropy factor favours depolymerization. Toboldbor (27) suggested that a polymer molecule represents an equilibrium between a forward and a backward reaction i.e. polymerization and depolymerization. The conditions under which such an equilibrium k * p Mn MMAr7k -1 (30) dp becomes important were discussed in detail by Dainton and Ivin (28).

They introduced the term 'ceiling temperature' T0, for polymerization reactions. The free energy of polymerization ea passes from a negative to a positive value as the temperature is raised above the value o. When equilibrium is reached by the system represented in eq. 30

RT1nK = T p = 0 (31) k, where K = The thermodynamic quantities refer to the monomer, kdp 27 and polymer in their standard states i.e. pure liquid and dilute solution in monomer, respectively.

From equ. 31 To - p (32) o p The values of aS were found by Dainton and Ivin to range between -26 to -36 E.U./mole for various polymers. Small (29) determined the monomer-polymer equilibrium for methyl methacrylate at 100-160°C. Prom his dltatbsceiling temperature for this monomer should be 190°C. Alpha-methylstyrene does not polymerize above

61°C at atmospheric pressure. The ceiling temperature of monomes such as styrene is so high that side reactions or cracking occur before the ceiling temperature can be reached, while in other cases the ceiling temperature is below room temperature. 28

1.3 The Effect of Pressure on Polymerization Free radical reactions with reactants in the gaseous phase are

influenced by pressure due to changes in the thermodynamic properties of the system. The high compressibilities of gases cause large changes in reagent concentrations and therefore marked changes in rate are observed even for reactions carried out at moderate pressures.

Liquids, on the other hand are far less compressible and only pressures of the order of 103 atm. can produce easily measurable effects. However there is a practical limit to the pressure which

can be imposed on a particular reaction system since freezing of one of the components may occur, thus altering the composition. In many

polymerization reactions very low molecular weight polymers are produced because of steric effects. Pressure is known to overcome to certain extent steric hindrances allowing the production of long polymer chains (Holmes - Walker and Weale, 30). Pressure also has

a pronounced effect on liquid viscosities. Liquids in which

hydrogen-bonding does not occur follow the relation

op coast. (32)

This is of particular interest for diffusion controlled

reactions. If the rate of initiation is controlled by diffusivity of the free radicals produced by the decomposition of the initiator depends on the viscosity of the medium. the rate constant ki According to the Stokes - Einstein relation for spheric molecules,

29 the diffusivity D is inversely proportional to the viscosity therefore

and from eq. 32 ki a which means that the rate of initiation will be reduced by pressure. Termination reactions are also reduced as they become diffusion controlled at high pressures.

The energy required to bring about a chemical reaction is normally supplied by the kinetic energy of the molecules in the system. From the kinetic theory of gases the Arrhenius equation for the relation between the rate constant and the energy is -EAT k = Pzo (33) where E is the activation energy, z collision number, and P - the probability that a collision of molecules with sufficient energy will lead to chemical reaction. A second form of expressing the same relation is derived from the transition-state theory developed by Glasstone, Laidler and Eyring (31)

(34) --17- e Here tsE is the heat of activation equivalent to E in eq. 33,

A.S is entropy of activation, negative values of which correspond; to small values of P, and finally, the quantity kT (about 6 x 1012 h at room temperature) being roughly equal to the collision number Z. 30

Unimolecular radical reactions involving thermal dissociation have 10 12 'normal' Pz factor of 10 - 10 while others show small Pz factors .44:',40-0 of 106 - 109. The later values correspond to values of A S of -20 to -34 entropy units. Perrin (32) noted that bimolecular processes with small Pz factors were strongly accelerated by pressure, whereas those with 'normal' values were slightly affected, to the

extent that some unimolecular processes were slowed. Evans and

Polanyi(33) were the first to apply the transition state theory to

the effect of pressure on rate constants deriving the expression

) V* (35) (77--)T = RT * in which AV is the change in volume in going from reactants to

transition state and k is the rate constant for the reaction. The * overall effect of the pressure depends on how AV will change, overall taking in account the different steps, e.g. initiation, propagation and termination. From eq. 21 R. x1/2 R = k p 2kt ) the overall rate of reaction is proportional tos -1/2 1 2 k kp kt ki / , hence employing eq. 35 * (36) vV*overall = tat.Vp 4-1- * - Vt In radical processes the propagation step is bimolecular reaction which involves the close approach of the reactants, therefore 4NAT

should have large negative values, on the other hand, by producing

the rigid link between the molecules, some degree of freedom is lost, 31 * hence aS is negative as well. However, 4.1/ has the largest effect on the overall rate and accounts for most of the usual increase in polymerization rate at high pressures.

The effect of high pressures on polymerization reactions was first reported by Bridgman and Conant (34). They noted that substances such as isoprene, vinyl acetate, 2-3 dimethylbutadiene and styrene formed high polymers at pressures in the range 9000-12000 atm. and room temperature under conditions where negligible reaction was noticed at atmospheric pressure. Starkweather (35) and Tamman and Pape (36) also reported an increase of the rate of reaction with pressure, the latter giving results for the polymerization of styrene with a very 6 high acceleration factor (up to 2 x 10 at 140°C and 1500 atm.). Compared to these results, the work of Gillham (37) shows much less pressure acceleration of the polymerization. He suggested that the very large increase in rate previously reported was due to inadequate experimental technique, e.g. inclusion of dissolved oxygen and non- isothermal conditions. By taking the necessary precautions to avoid impurities and temperature fluctuations he found a ten-fold increase in the rate of polymerization of styrene at 3000 atm. and 100°C with an increase in the polymer molecular weight of about 1.55 fold. A deficiency in Gillham's work is that he allowed the reaction to proceed to high conversions (up to 100%) thus permitting for the 'gel effect' or 'autoacceleration' to influence the course of the reaction.

A very detailed study on the polymerization of styrene initiated 32 by benzoyl peroxide at 6000 under pressures up to 3000 atm. was carried out by Merrett and Norrish (38). They attempted to relate the pressure effect to the specific reaction processes in the course of polymerization. Their main findings are: a)In the pressure range of 3000 to 5000 atmospheres the logarithm of the overall rate of reaction is nearly directly proportional to the pressure at constant initiator concentrations. Below 3000 atm. the relationship is more complicated. Over the same range there is up to 15 fold increase in the rate of reaction (with 0.043 mole% initiator). b)The rate of polymerization is proportional to the square root of the initiator concentration at one atmosphere with the index of power decreasing to 0.40 at 3000 atm. and then increasing to 0.45 at

5000 atm.

c)There was a two to three fold increase in the molecular weights of the polymer produced under pressures up to 3000 atm. and thereafter approximately constant values were observed. Discussing these results Merrett and Norrish gave the following explanations: 1)The rate of initiation is slightly reduced by pressure. 2)Pressure accelerates greatly the propagation reaction with slight increase in the rate of transfer processes.

3)There is a decrease in the rate of termination by combination due to increased viscosity of the polymerizing medium at high pressures. 33

These postulates were later confirmed by Nicholson and Norrish

(39) (40). In their work they mostly concentrated on investigating the pressure effect on the propagation and termination steps at pressure up to 3000 atm.

Similar findings were reported for the polymerization of methyl methacrylate by Melekhina and Kuvshinsky (41). The pressure accelerates the rate of polymerization, molecular weights are increased to a constant value at the higher pressures, and the rate of reaction is proportional to the square root of the initiator concentration.

Summary of effect of pressure on the various steps during polymerization

a)Initiation process - the rate of initiation is slightly reduced by pressure. At high pressures there is evidence of the increasing importance of thermal initiation. Initiators such as benzoyl peroxide and azo-bis-isobutyronitrile show a retardation of decomposition at elevated pressures. An increase in free radical wastage is also observed.

b)Propagation - the propagation reaction is accelerated exponentially with pressure. For styrene there is an approximate 5.5 fold increase in the propagation rate constant at 3000 atm. At this region cOT for the propagation reaction is about -11.5 cc/mole (57). Pressure does not affect the mechanism in emulsion polymerization. 34

c)Termination - termination by combination which involves reactions between two large polymer chains becomes diffusion- controlled and retarded due to the increase in viscosity at high pressure. This also applies to termination by disproportionation. It ray be possible to decrease the viscosity effect on the diffusion-controlled process by use of diluents which do not have high transfer constants. The rate of termination constant for styrene kt was found to decrease rapidly up to 1000 atm. with a more moderate decrease up to 3000 atm., forming a nearly linear relation between log(kt)pilog(kt), and the pressure at 30°C and pressures from 1000-3000 atm. d)Transfer - transfer processes generally involve reaction between a large radical and a small molecule, therefore they are accelerated by pressure, yet the effect is smaller than in propagation. Evidence for this, is the trend to a constant value of the molecular weights at high pressures which is due to the increased effect of the various transfer reactions leading to the termination of the growing chain radical, 35

1.4 The Polymerization of Alpha—methylstyrene

(i) Atmospheric pressure polymerization

Investigationson the polymerization of alpha—methylstyrene

have usually been carried out at atmospheric pressure. This olefin

is known to have the relatively low ceiling temperature of 61°C at one atmosphere. As mentioned before, alpha—methylstyrene has the

lowest heat of polymerization among the well known monomers. Jessup and Roberts (42) found that the heat of polymerization had a marked

increase in cases where the polymer chain had less than few units.

The conclusion from this was that the ceiling temperature should necessarily increase for polymers of the order of pentamer and lower

degrees of polymerization. Reactions at room temperatures produced chiefly dimers and Dainton and Ivin (28) (43) suggested that a de—

propagation reaction of appreciable rate was the reason for these

results. An alternative explanation was proposed by Worsfold and

Bywater (44), who used boron trifluoride etherate as catalyst with

excess water and diethyl ether in ethylene chloride solution to

investigate the termination process. Thqrconcluded that the propa—

gation rate is very low in comparison to the termination rate and

that deproragation was not important. Dainton and Tomlinson (45)

polymerized alpha—methylstyrene using stannic chloride in ethyl chloride solution obtaining low molecular weights which they explained

as a result of an unusually low rate of propagation.

Other authors (46) have considered that the steric hindrance is the major reason for the low molecular weight. By using low 36

temperature conditions (-130°C) Hersberger, Reid and Heiligmann (47) obtained the comparatively high molecular weight of 84000. The

catalyst was aluminium chloride in ethyl chloride. They proposed

that at low temperatures the effect of transfer reactions leading to termination is decreased. A slightly different explanation put forward by Jordan and Mathieson (48) was that at these conditions there is an increase in the termination by combination in comparison

to transfer reactions leading to the high molecular weight. Worsfold

and Bywater (49) investigated the anionic polymerization of alpha-

methylstyrene with sodium naphtalene in tetrahydrofuran, a system in which monomer-polymer equilibrium is established, thus eliminating the possibility that low polymer is produced because of a low rate of propagation in comparison to that of transfer. By the instantaneous

change in thecolour from green to red of the reagents it was shown

that the initiation was completed very quickly. At -80°C a 100% ma-

version was noted whereas at temperatures between 0°C and -40°C an

equilibrium was achieved, thus leaving some of the monomer unreacted. A linear relationship between the reciprocal value of the ceiling temperature 1 and ln(m), the monomer concentration was found by Tc McCormick (50) who studied the same system. He used several initial

monomer concentrations and obtained the same results for each temperature, i.e. aenfirmed the ceiling-temperature phenomenon. The

same author determined the ceiling temperature to be 61°C at atmos-

pheric pressure and Brown and Mathieson (70) estimated e.t. at 60°0

from their experiments with trichloroacetic acid as catalyst in 37 ethylene dichloride.

(ii) High pressure polymerizations

The use of high pressures in the polymerization of alpha- methylstyrene causes marked acceleration in the overall rate of reaction and production of polymer of increased molecular weight was Observed. Sapiro, Linstead and Newitt (51) carried out an extensive investigation on the polymerization of alpha -methylstyrene at temperatures up to 150°C and pressures up to 10000 atm. They found that the pol3merization could be carried out without initiator at pressures above 2000 atm. and temperatures in the range of 100°C. No differences in the molecular weights obtained or in the rate of reaction were observed when hydrogen chloride or benzoyl peroxide were added as initiators. They claimed that the main effeot of the chemical initiation was to produce more low molecular weight polymer at the expense of the monomer, The effect of anhydrous zinc chloride was to prevent any formation of high polymer. These authors concluded that increase in pressure at constant temperature affected only the conversion but the molecular weight remained unchanged. At pressures of about 4000 atm. the molecular weight decreases from 6000 to about 1000 with the increase in temperature 100-125°C. However, the results of these workers are of limited reliability, since no precautions against the inclusion of oxygen were taken. Their main value is that they showed the possibility of obtaining high polymers of alpha - methylstyrene at temperatures abofe the ordinary ceiling temperature 38

(61°C), by using high pressures.

Recent work by Kilroe and Weale (52), using ionic as well as free radical initiators in the polymerization of alpha -methylstyrene o at pressures up to 12000 atm. and temperatures up to 170 0, showed a linear relationship between the ceiling temperature and the pressure which, extrapolated to one atmosphere, agreed with the ordinary ceiling temperature of 61°C. The dependence of the rate of initiation on the initiator concentration is similar to the existing results for styrene. The logarithm of the rate of polymerization at constant temperature and initiator concentration increases with pressure, reaches a maximum value and falls off sharply. These workers suggested a partial solidification of reactants at higher pressures.

This thesis describes a more detailed investigation of these phenomena at high pressures. 39

1.5 Scope of the Work

The polymerization of alpha—methylstyrene has been studied

by many workers but the information available on several important

aspects of the reaction is by no means complete. The object of this

work was to carry out a systematic investigation of the effects of

pressure on the free radical polymerization of this monomer. In

view of recent findings of the existence of limiting factors for

the polymerization including a pressure—dependent 'ceiling temperature'

and a possible phase separation due to freezing at high pressures,

a detailed study of these phenomena was undertaken.

The main aims in the present study wereg

1)To confirm the effect of pressure on the rate of reaction

and the order of reaction with respect to initiator concentration.

2)To determine the effect of pressure on the molecular weight

of the polymer, and the dependence of the degree of polymerization on

the temperature, the initiator concentration and the degree of

conversion.

3)To establish the freezing temperature dependence on the

pressure and to investigate the effect of increased viscosities and

of freezing at high pressures on the rate of reaotion and the

molecular weight.

4)To determine the effect of pressure and temperature on the formation of liquid polymer above the ceiling temperature limit. 40

5) To investigate the possible existence of competing reactions leading to the formation of either solid or liquid polymer at conditions between the limiting ceiling temperature and freezing pressure. 41

PART TWO: APPARATUS, MATERIALS AND EXPERIMENTAL PROCEDURE

11.1 Apparatus The equipment used for generating high pressures including reaction vessel and ancilliary units is shown diagramatically in

Fig.l. A hydraulic hand pump P, made by Blackhawk Manufacturing Company for pressures up to 2700 atm. is fitted with an oil reservoir and a by-pass valve B. The pressure produced by the pump is trans-

mitted through valve D to the intensifier I by stainless steel tubing

o.d. and i.d. and measured by gauge G. From the intensifier, the fluid, liquid paraffin B.P. under increased pressure is transmitted to the high pressure reaction vessel V through i° o.d. and 1/16" i.d.

Vitra° steel tubing. Both the intensifier and the reaction vessel are shown in Fig.2. In order to achieve constant temperature during

the reaction the high pressure vessel V is totally immersed in a thermostatically controlled oil ',ath 0 heated by a 1400 watt immersion heater H. Synchronions motor S is used to drive a propeller at 200 r.p.m. to ensure effective stirring and equally dissipated heat. The mercury-toluene thermoregulator T and a Sunvic Hot-wire relay

are sufficient to give temperature control of t0.1°C in the oil of

the bath and ±0.02°C within the vessel in the range of temperatures employed during the runs. The intensifier, as shown in section

in Fig.2 consists of two separate parts connected by a screwed sleeve.

Pressure from the pump is applied to a piston 1.50" in diameter 42

FIGURE I 43 FIGURE 2

INTENSIFIER REACTION VESSEL 44 moving in the bottom low pressure cylinder I.'. and obturated by a

Bridgman unsupported-area packing. The force is transmitted through a thrust block and a thin copper washer to an 0.651" piston, sealed with a hard rubber Poulter packing, which travels up to the high pressure chamber B. The thrust block and the copper washer are a necessary arrangement to take up any small differences in alignment between the two pistons. The high pressure chamber is built up of two Vibrac steel cylinders shrunk together. An approximately five- fold pressure intensification is given by the two pistons for which geometric ratio of the cross sectional area is 5.131. The difference arises from losses due to friction of the packings as well as some very small changes in the relative areas of the pistons due to elastic compression. The high pressure created in cylinder

B is transmitted to the reaction vessel shown also in Fig.2. It is made of two high tensile molybdenum steel cylinders shrunk together and has an overall length of 15.5" outside diameter of 4" and a 0.7" bore, A screw plug obturated by a Poulter packing is used to close the upper end. The pressure guage, a standard Bourdon tube connected to

valve D, measures the low pressure in the bottom part of the intensifier. Calibrations carried out with a primary free piston gauge connected to the high pressure side of the intensifier showed

a constant and linear ratio of the pressures in the two gauges up

to 1000 atm. The relationship between the true pressure in the 45 reaction vessel and that shown by the Bourdon gauge is given by the equation p = v 5.12 P - 140

Pv - vessel pressure; P - gauge pressure It was assumed that this relationship remained constant up to 5000 atm. as this had been checked to 4500 atm. in previous work (53). The recorded pressures taken from the gauge readings are accurate within 1% and in view of the magnitude of the effect measured this is considered adequate. In order to achieve better pressure control the intensifier was enclosed and provided with an air thermostat consisting of two 150 watt electric bulbs, a motor driven fan to distribute the heat and a mercury-toluene thermoregulator which main- tained constant temperature in this enclosure of about 5°C above the ambient temperature in the laboratory. The purpose of this arrangement was to minimize the effect of air-temperature fluctuations on the intensifier. The reacting solution was placed in a container of two types. These reaction tubes are shown in Fig.3. The first is a Pyrex tube of about 4 cc capacity fitted with a ground glass stopper and a small hole at the bottom to allow equal pressure inside and out, In order to avoid direct contact and contamination of the reagents with the liquid paraffin the tube contained a small amount (about VI length) of mercury and floated on the same material contained in the steel bucket, thus transmitting the pressure to the reactants through a 46 FIGURE 3 47 mercury seal. The second type was a length of PTFE tubing of

o.d. and wall thickness of 1/32". The tube was fitted with crass sleeves at each end and closed by plugs made of the same material,

This reaction tube also avoids direct contact with the oil in the vessel, the pressure being transmitted through the flexible walls. The steel bucket has dimentions which allow sufficient clearance between it and the walls of the vessel, as well as the reaction tube.

This ensures the rapid transmission of pressure changes at the extreme conditions, when the liquid paraffin has its highest viscosity.

Viscometers For viscometric molecular weight determinations, microviscometers of size M1 and M2 were used according to British Standard Specification,

BS 188 s 1957 (56). A water-filled glass thermostat bath at a constant temperature of 30°C ±0.1, controlled by a mercury-toluene thermoregulator operating a vacuum relay switch, gave the proper conditions for the viscosity measurements. 48

11.2 Pre aration of Reagents

Alpha-methylstyrene -supplied 95-99% pure by Light & Co. Ltd., contained hydroquinone as inhibitor. To remove the

inhibitor the reagent was washed four times with its volume of 10% sodium hydroxide solution, followed by several washings with distilled water until there were no traces of alkali. After drying over calcium chloride for 24 hours it was distilled under

vacuum in an atmosphere of oxygen-free nitrogen, the middle 60% of the distilate being collected. The boiling point at 14 mm Hg

was 54.5-55.00C. The monomer was stored in the dark in the same oxygen-free conditions for not more than 48 hours, although reagent

which was kept for more than one week did not give any precipita-

tion in methanol.

Azo-bis-isobutyronitrile,AIBN (Eastman Kodak) - used as an

initiator was purified by dissolving in chloroform, filtration

and recrYstallisation over an ice bath. The crystalline product

was dried in vacuum and kept in the dark in a vacuum desiccator over calcium chloride. Melting point 1060C,

Toluene - 'Analart (Hopkin and Williams) was dried over calcium chloride and kept in the dark.

Bensene 'Analar' quality (Hopkin and Williams) was used without further purification after drying over anhydrous calcium

chloride. 49

Methanol - 'Analar' was used without further purification.

1, 2 Epoxypropane - (B.D.H.) was not further purified.

Tert-butyl-perbenzoate - (B.D.H.). An attempt to purify this initiator by distillation under nitrogen at 70°C, 0.05 on ag vacuum (81) resulted in a serious explosion and it was afterwards used as received. 50

11.3 Experimental Procedure

(0 Preparation of reaction mixture The initiator and the monomer were weighed in the desired

proportion into a standard flask of 10 ml. capacity. After thorough mixing of the reagents a sample was pipetted into the reaction tube, which was previously filled with a small amount of mercury and subsequently flushed with nitrogen. The ground glass

stopper was then inserted carefully, to avoid inclusion of air

bubbles. The tube, containing the reaction mixture and about im length of mercury, was inverted into the bucket containing

the mercury seal. When PTFE tube was used, the same procedure was followed without using any mercury.

(ii) Polymerization reaction

The steel bucket containing the reaction tube was inserted into the pressure vessel which was at the required temperature.

With valves B and D opened (Fig.l), a hard rubber Poulter packing

was pressed into the bore of the vessel by means of a special

tool. The screwed plug, which together with the loading tool was kept in liquid paraffin at the temperature of the vessel, was turned down until hand—tight. Valve B was closed and

pressure was applied by means of the hydraulic hand pump at a controlled rate, until the desired pressure was obtained. In

pressurising the system care was taken that the gauge pressure 51

did not exceed the required value. Finally, valve D was closed

and valve B opened in order to release the pump from any pressure

during the run. The time for the beginning of the run was

recorded when half the desired pressure had been obtained.

At the conclusion of the run valve D was opened slowly,

thus allowing a gradual release of the pressure. The time for

the end of the run was recorded when the pressure was half of the

maximum value. The plug was unscrewed and by applying pressure

with a few strokes of the pump (valve B closed) the packing was

ejected. The thermometer was replaced after taking out the

bucket, and the temperature was recorded again after 15. minutes.

(iii)Separation of high polymer

On removal from the reaction vessel the product was weighed and dissolved in cold 1,2-epoxypropane. The solution was made up to 25 ml. after successive washing with the same solvent into a dropping funnel. The high polymer was precipitated by dropwise addition with vigorous stirring into 150 ml. of cold methanol.

The precipitate was coagulated by heating the solution on a bath and filtered in a weighed sintered glass crucible of porosity 3. After several washings with cold methanol the polymer was dried to constant weight at 300C for about 24 hours in vacuo.

(iv)Determination of the yield of liquid polymer

Polymerization products consisting of dimers and other products below pentamer cannot be separated by the precipitation 52 method as these low polymers are known to be soluble in methanol.

A drying technique at reduced pressure (4 mm Hg. at 25°C) was therefore applied. The polymerization products were weighed in a special vacuum flask made of Pyrex glass and kept under 4 mm Hg pressure for about 24 hours, evaporating all the monomer until constant weight was attained. At these conditions no substantial amount of low polymer could evaporate according to

Newitt et al. (51), who reported that the first fraction of low polymers at 0.1 mm vacuum was taken at 110°C. The residue was weighed, the solid polymer precipitated by the method described above, weighed, and the difference between the residue and the solid polymer gave the yield of the low polymer. When there were no traces of high polymer in the product, the run was repeated and the whole amount of residue after the vacuum drying was considered as low polymer, the molecular weight of which was determined eryoscopically. A callibrated apparatus for determination of the depression of the freezing point of

'molecular weight' benzene solvent, caused by a known amount of dissolved polymer, was used.

The molecular weights were calculated from the formula: 5.81x1000xW M.W. S x.c.T W — weight of polymer sample added

S — weight of solvent used 53

- depression of the freezing point of the solvent

5.81- constant for the apparatus (54). (v) Molecular weight determination

Most of the polymers produced had molecular weights in the 4 5 range 2 x 10 to 2 x 10 and the viscometry method for determination of molecular weight is the simplest and most convenient way

for this range. Toluene was used as a solvent and viscosities

of polymer solution were determined at 30 C using M viscometers 2 reproducibly positioned on a frame with a three point suspension.

The correlation used for the calculation of the molecular weight

was given by Bywater et al. (55). This gives the following relationship between the molecular weight of the poly-alpha-

methylstyrene and the intrinsic viscosity in toluene at 30°C.

[TO . 1.08 x 10-4 m0.71

where PO - intrinsic viscosity and M - molecular weight.

The definition of intrinsic viscosity (or limiting viscosity number) is

[17) = lim ,Pic

tsp - specific viscosity 0 - concentration of the solution in gm. polymer /

100 ml. solution.

-7.sP 54

1- viscosity of the solution - viscosity coefficient of the solvent o The value of[ li]may be determined directly by extra- /C vs. C to infinite dilution polation of the plot of sp according to its definition but this method required several measurements at different concentrations. By using the Schulz-

Sing correlation (82) sic

1 + k" I/ sp after determination of k" a single viscosity measurement gives the intrinsic viscosity. The constant k" was determined by finding the values of specific viscosity Isp over a range of concentration C, from 0.4 to 0.9 gm./100 ml. and plotting ysp/C

sp/C =[7] + k" [7] against 7sp. From the equation above 7 The ratio between the slope of the straight line and the intercept at 1/sp = 0, which in this case is equal tortl, gives the value of the constant k". The assumption was made that k" was not affected by the pressure at which the polymerization took place. In this case sp/ 0 k" was found to be k" = 0.546, hence[ - and 1 + 0.546 sp the molecular weight M 1.4084 fi x 104 L 1.08 For most of the molecular weight determinations an 55

approximately constant concentration C'A.'-50..60 gm./100 ml. of the

polymer in toluene was,used. A sample of precipitated polymer of

about 0.03 gr. was weighed into a 5 ml. calibrated flask. The solvent was added to within two mm. of the graduation and the

flask was immersed in the thermostatic bath and left there until the solution reached the temperature of 3000. Subsequently, the

solution was made up to the 5 ml. volume by adding a few more drops

to the flask. The viscometer in use was carefully washed with

toluene and dried, and then filled with pure solvent. After 20

minutes in the thermostatic bath, the excess amount of toluene was removed, thus bringing the liquid level to the graduated mark

on the viscometer. The fall of the miniscus between the two

graduations on the capillary tube was measured with a stop—watch

several times, until three successive measurements gave a smaller

difference in flow time than 0.2 sec. and the average was taken as the result. After washing and dying the viscometer, the solution

was vigorously shaken to make sure of thorough homogenisation and

then filtered into the viscometer. The same procedure as for the

pure solvent was followed. A measurement of the viscosity of

the pure solvent was made before every new solution was introduced for molecular weight determination, thus making sure that dust

particles had not entered the viscometer as these could have seriously affected the flow times. When not in use, the viscometers

were filled with toluene, sealed and kept in the thermostatic bath. 56

At the right conditions the flow time for the toluene was about 186.2 sec. in the M2 viscometer, therefore the viscosity may be taken directly proportional to the flow time. The error allowed by neglecting kinetic energy corrections is not more than 0.1% (56), hence it was possible to use flow—times ratios for solution and solvent as viscosity ratios. 57

PART THREE: EXPERIMENTAL RESULTS

III.1 Rates of Polymerization

(i) Dependence of rate on initiator concentration

The rate of polymerization as function of the initiator

concentration is given in table No.ld and plotted in Fig.4. The initiator used throughout the greatest part of this work was azo—

bis—isobutyronitrile (AIBN). The reason for choosing it was its stability within the temperature range employed in most of the

runs. In addition, there were no reports of AIBN forming white precipitates on the mercury surface in the reaction tubes as in the case when benzoyl peroxide is used above certain concentrations

(57). The experiments were carried out at a pressure of 3000 atm.

and constant temperature of 6000. The choice of these conditions ensured that the polymer formed will be mainly solid (>.--pentamer)

and that the yield was of reasonable amount in order to allow

measurements of molecular weights. For each initiator concentra—

tion runs of different duration were made. The results, expressed as wt.% of conversion into solid polymer, were plotted against

the time, and the slope was taken as the rate of the reaction at

that particular initiator concentration. It is obvious that it is more correct to use the term 'average' or 'mean' rate, since

it is impossible to follow the formation of solid polymer at 58

FIGURE 4

9 8 7

1 0.06 0.1 0.2 0.3 0.4 0.5 46 0.7 Initiator Conce ntration Cvit°10) 59 different times during one run without disturbing and changing the basic conditions. From Fig.4 the slope of the straight line is calculated to be 0.44 i.e. that the rate is roughly proportional to the square root of the initiator concentration which is in accordance with the accepted kinetic equation given in part one. This result is also in good agreement with the values of the exponent given by previous workers. Nicholson and Norrish (39) reported

0.50 ± 0.02 at 1 atmosphere and 0.45 at 3000 atmospheres for styrene at 30°C. Merrett and Norrish (38) found that the value of the exponent decreases from 0.5 at 1 atm. to 0.40 at 3000 atm. for styrene initiated by benzoyl peroxide at 60°C. The results of Kilroe and Weale (52) show that the rate is proportional to x r-Initiato;] where x is 0.44 and 0.45 at 8100 and 9000 atm., respectively, in the alpha-methylstyrene benzoyl peroxide system at 94.5°C. It is obvious that there is similar dependence of polymerization rate on the initiator concentration for styrene and alpha-methylstyrene.

(ii) Dependence of polymerization rate on pressure The conditions chosen for this series of experiments were a temperature of 600C and an initiator concentration about 0.555 weight % i.e. about 0.4 molar %. The pressure range employed was between 1500 atm. and 5000 atm. As described in 60 section III.1(1), runs were carried out at different periodsof time, the results plotted as % conversion vs. time and the slope of the line taken as the overall rate of reaction at this particular pressure. The final results are given in table No.lc and plotted in Fig.5. It is obvious that the rate of polymerization increases with pressure and there is a linear relationship of log (rate) vs. pressure in the region between 3000 and 4500 atm.

There is a ten-fold increase in the rate of polymerization between 1500 atm. and 4500 atm., whereas in the pressure range distinguished by the linear relationship (3000 - 4500 atm.) the increase is about three-fold. Similar behaviour for styrene was reported by

Merrett and Norrish (38). They found that the logarithm of the rate of polymerization was directly proportional to the pressure at the pressure range of 3000 - 5000 atm.

Thc,.Te was a sharp fall in the rate of polymerization at 5000 atm. This peculiar phenomenon in the polymerization reaction of alpha-methylstyrene was noticed by Kilroe and Weale (52). They reported a similar decrease in the rate at pressures higher than

6500 atm. and at 94.5°C. It was therefore decided to carry out a further and more detailed investigation in the region between

4500 and 5000 atm. The runs were mainly of 22 hours' duration and the results, together with those of the same duration at different pressures, are given in Fig.6. The wt. % conversion vs. pressure plot gives a very clear picture of the large increase 61

FIGURE 5

8 7

firl 4b 4 0 a 3

1 I 0 00 2 000 3000 4000 5000 Pressure (at m.) 62 FIGURE 6

L. d

E >ft. 20

1-0. a

%No 0

10 O 60°C

CI 45°C

• 1 000 200 0 300 0 4000 5000 6000 7000 800 0 Pressure (atm.) 63 and abrupt fall of the polymerization rate between 4500 atm. and 5000 atm. A run carried out at 8000 atm. shows a very low yield very similar to that at 5000 atm. Similar behaviour was observed at 45°C and pressures in the region of 4000 atm. The yield of polymer falls off from 4.23% at 3920 atm. to 0.28% at

4070 atm. as given in table No.6. Since it was established by Kilroe and Weale that with different initiators including the ionic catalyst trichloro—acetic acid the relations took the same shape, the conclusion was that the fall off in the rate was not due to the effect of pressure on the free radical mechanism or a very rapid decomposition of the initiator. It was considered that a sudden increase in viscosity followed by freezing of the liquid reactant might be the reason for this behaviour. The influence of pressure on the freezing point of the monomer was investigated and described in section 111.3.

(iii) Euendence of the yield of polymer on the temperature

A series of runs has been carried out at constant pressure of 3000 atm. using AIBN at initiator concentration of 0.555 wt. %. 0 o The temperature range employed was between 40 C and 100 C. The experimental results are given in table No.3 and plotted in Fig.7.

At the conditions prevailing during these experiments the yield of solid polymer increases about ten—fold between 40°C and 80°C. The highest point is probably at about 87°C, then the production of solid polymer falls off. When extrapolated to zero 64

FIGURE 7

e. 14 •••%,„0 0, NO V 12

20 40 60 80 10 0 120 Temperature 00 65 conversion, the curve gives a ceiling temperature of 109°C at 3000 atm. which is in excellent agreement with the results obtained by Kilroe and Weale (52). 66

111.2 Molecular Weights This section deals with molecular weights of solid polymers

produced under conditions deliberately selected to decrease the

influence of competitive side reactions leading to the formation of substantial amounts of low molecular weight liquid polymers.

The results for Figures 8 — 10 are give in tables No.la and No.lb, whereas these from table No.3 are plotted in Fig.11.

The change in the molecular weights with the duration of the polymerization reaction at two initiator concentrations, e.g.

0.140 wt.% and 0.555 st.% is represented in Fig.8. At 3000 atm.

and 60°C the molecular weights obtained at 0.140 wt.% initiator concentration are of magnitudes between 4.6 x 104 and 5.7 x 104,

about 24% increase in a period of 22 hours, whereas for the 4 concentration of 0.555 wt.% the increase is from 2.4 x 10 to 2.8 x 104 — about 17%. Fig. 9 gives the dependence of the molecular weight on the initiator concentration. The runs were carried out at 3000 atm.

and 60°C in two different times of durations 14 hours and 22

hours. It is evident that a linear relationship between the logarithms of the molecular weights and the initiator concentra—

tion exists. For the reaction time of 22 hours the molecular weight at 0.070 wt.% initiator is 7.63 x 104 and falls to

2.42 x 104 at 0.691 wt.% initiator conc., the decrease for 14 hr. time is from 7.03 x 104 to 2.33 x l04, respectively. The slope 67

FIGURE 8

4 8 12 16 20 24 Time (hr,) 68

FIGURE 9

9 8 '7 6

lar 2 lecu Mo

006 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Initiator Concentration (vit!lo) 69 of both in -0.485. Accoreing to the general transfer equation-(28), when the various transfer reactions are negligible the average degree of polymerization is proportional to the initiator concentration raised to the power -0.50. The dependence of the polymer molecular weight on the pressure is presented in Fig.10. The experiments were performed at constant temperature of 60°C and initiator concentration of 0.555 wt.% of 22 hr. and 18 hr. duration. The plot of log. M.W. against pressure gives a straight line for pressures between 1500 atm. and 4500 atm., the corresponding values of the molecular weight increase from 0.99 x 104 to

9.41 x 104. Between 3000 and 4500 atm. a nearly three-fold increase in the molecular weight was observed. A much larger increase of the molecular weight (18.48 x 104) was observed at

5000 atm., which is beyond the freezing pressure at this temperature. Unusually high molecular weights of up to 59.4 x 104 were obtained in a. series of experiments given in table No.2. At constant temperature of 60°C, three runs of duration between 14 hr. and 22 hr. were performed at a nominal pressure of 5000 atm., although during the reaction the pressure fen to about 4700 atm. for a period of two hours and was then raised to the original value. This may have resulted in a temporary 70

FIGURE 10

10 9 8

It 0 7 X 6

2 5 .5.12 V 4 X L. 3 a 3 U V O 2 2 Is

1000 2000 3000 4000 5000 Pressure Cam.) 71

de—freezing of tho ror.ction miT:ture and the Tyroauction of a

certain amount of high polymer which, when brought back to the

frozen state with the rest of the reactants at the higher

pressure eventually gave the unusually long chains. Another

experiment at the same temperature and 0.555 initiator concentra— tion, gave 86.0% yield of polymer. The pressure was released

three times to one atmosphere for a few hours, the overall duration

of reaction being 58 hr. The average pressure was 4600 atm.

The reaction tube had to be broken in order to remove the polymer which was completely solid. The determined molecular weight was

39.6 x 104. The apparent increase in molecular weight at pressures near the freezing point was investigated also at a temperature of 45°C. Table No.6 gives the results obtained with 0.555 wt.% initiator at a constant time of 22 hr. At a pressure of 3920 atm.

M.W.. 81.3 x 104; values much higher than those obtained at even higher pressures at 60°C.

The effect of the temperature on the degree of polymerization was investigated and the results obtained are given in table No.3 and plotted in Fig.11. The temperature range was between 40°C and 100°O at constant pressure of 3000 atm. AIBN was used as initiator at a concentration of 0.555 wt.% Time of reaction was

22 hr. At 40°C the molecular weight of the polymer was 3.95 x 104. With the increase in temperature there was a decrease

FIGURE II

•

40 50 60 70 80 90 100 Temperature CO 73 of about 30% at 60°C (11.117. 2.79 x 104) after which the molecular weight kept an almost constant value to 95°C. A further fall off in the degree of polymerization was observed at 100°C. At this point the molecular weight was 1.58 x 104, or a decrease of 43% in a range of 5°C. It is of interest to point out that the temperature of the last experiment at 100°C is only 9°C lower than the ceiling temperature at the corresponding pressure, whereas 40°C is about 12°C higher than the freezing point at

3000 atm. Discussions and probable explanation of these phenomena are given in a later chapter. 74

111.3 Freezing Pressures

For the determination of the freezing pressures, alpha—

methylstyrene inhibited with hydioquinone, as supplied by L. Light & Co. Ltd., was used. The experiments were carried o o out at three different temperatures, e.g. 30 CI 45 C and 60 C, in order to establish the relationship between the freezing

pressures and the temperature of the liquid, respectively. The results are given in table No.7 and plotted in Fig.12. They

represent the position of the piston in the intensifier at a

particular pressure. The movement of the piston was followed by a telescope mounted on a stand with a device for measuring the

position with an accuracy of 0.02 mm. The pressure readings are given in the original gauge divisions, and the observed freezing points transferred to actual pressures in atmospheres.

The pressure was raised gradually in order to give time for the discipation of the temperature and reaching some sort of equilib—

rium in the system. When the change in phase took place, a sudden fall in pressure was observed and in order tonaintain the

pressure steady there was need of additional pumping, which

caused a further movement of the piston. It is worth mentioning that the time needed tocbtain steady pressure was about three

times longer at the freezing point. From Fig.12 it can be seen that the freezing is an isobaric process. A certain degree of

superpressing is probable since if the pressure is being raised 75

F IGURE 12

E 80 •. 4•P

700 C ce 600 tt)

S 00 0

QS 10 LS 20 Piston Height eh (cm)

76 rapidly the freezing takes place at lower pressures. This behaviour was noticed throughout the runs taken in the region near the freezing point. It is also evident that the compressibility decreases after this particular point. This fact points to a change in phase having taken place. The different freezing points are plotted in Fig.13 and they fall on a straight line which. when extrapolated to pressure of one atmosphere gives the normal freezing point of alpha- methylstyrene at atmospheric pressure to be -23.2°C as reported by Boundy and Boyer (4).

Calculation of all and4NV for the freezing phenomenon

Alpha -methylstyrene M.W. 118 Sample 4.4 gr. . = 3.74 x 10-2 gr. mole 118 • Temperature = 60°C = 333°K Piston displacement 0.124 cm. (table No.7)

Piston diameter 0.651" = 1.65 cm. dT 43 -2 / 2 Slope of Fig.l3 1.72 x 10 0Agfem dP 2500

17.-- .6.1/ = x 0.822 x 1.24 0.060 cm3/gr./ 4.4 From the Clapeyron equations

dT T alf T AH = AH = dT dP 77

FIGURE 13

0 30

20 ture era

10 Temp

-10

-20

1000 2000 3000 4000 5000 Pressure (atm.)

333 x 0.06 nH -2 - -1160 kg.cm./gr. 1.72 x 10

1160 x 118 . -1370 kg.m/gr.mole 100

= 7.08 cm3/gr.mole At 60°C and 4860 atm.

AIR = -1370 kg.m/gr.mole, COT = -0.060 am3/gr. 79

111.4 Formation of Livid Polymer

The formation of low molecular weight polymer was investigated at conditions near the ceiling temperature line. At

temperatures between 80°C and 95°C and pressures in the range of 1000 - 2500 atm. with AIBN as initiator, the polymerization reactions produced both high molecular weight solid polymer, as

well as low molecular weight liquid products, consisting chiefly

of dimers. The experimental results are given in table No.4a.

The initiator concentration throughout the runs was 0.555 wt.%. At 80°C, between 1500 atm. and 2500 atm. the yields of both the

solid and the liquid polymers increase with the pressure. The

dependence of the rates of polymerization on the pressure is given in table No7.4b and the plot in Fig.14 shows that the

logarithm of the rate of reaction for solid polymer is almost in linear relationship with the pressure, with an eleven-fold

increase for this pressure range. For liquid polymer there is only a 39% increase in rate and the line is more curved. At

1500 atm. the rate of the reaction leading to the low molecular

weight product is about five times higher than the corresponding

rate of reaction for the solid polymer. At 2000 atm. the

formation of both products is almost equal, and at 2500 atm. the polymerization to high polymer is about twice the rate of

dimerization.

At 1000 atm. and 95°C the only product is liquid polymer,

which consists chiefly of dimer as the molecular weight is between 80

FIGURE 14

IS 00 2000 2500 Pressure (atm.) 81

230 and 248 (The molecular weight of unsaturated dimer is 236).

At the more elevated pressures of 1500 atm. and 2000 atm. at 95°C a very low yield of solid polymer (M.W. of 2280 at 2000 atm.) was obtained. The molecular weights of the solid polymers obtained at 80°C form a straight line in a log M.W. vs. pressure plot (Fig.15) and when extrapolated to 3000 atm., a molecular weight in agree— ment with the data shown in Fig.11 is obtained. (Under these conditions the M.W. determined by experiment was 2.81 x 104, whereas the extrapolation gives a value of 2.92 x 104). This indicates that under these conditions the formation of solid polymer takes the same course of reaction as observed before, and is accompanied independently by an increase in the competitive reaction leading to the formation of the low molecular weight polymer. However, the yields of solid polymer at 95°C are lower than those obtained at 80°C which is due to the temperature being in the region of the ceiling temperature. The yield of liquid polymer is higher at 95°C. In order to investigate the rate of the polymerization reaction for the low molecular weight polymers beyond the ceiling temperature limit, a different initiator was chosen. The less reactivetert—butyl—perbenzoate at 1.00 wt.% concentration w used througkeutft series of runs, the results of which are given

in table No.5. Under the conditions prevailing during these 82

FIGURE 15

30 g

X 20 4•P

j.. 10 0 9 5 8

(V 7

1500 2000 2500 3000 Pressure (atm.)

runs no high polymer was formed, as expected.

Fig.16 gives the dependence of the rate of formation of

liquid polymers on the pressure at the constant temperature of

135°C. There is a linear relationship between the logarithm of the rate and the pressure and a 3.5—fold increase in rate for the pressures between 1750 atm. and 3900 atm. was found.

The effect of temperature on the rates of formation of 1 liquid dimer is shown in Fig. 17. The log.(rate)vs. y, plot at a constant pressure of 2500 atm. is a good straight line with

50% increase in rate between 123°C and 146.2°C. The determina— tion of the molecular weight at the highest temperature employed

shows that the main product is dimer (M.W.229).

Calculation of the activation energy E From table No.5a and Fig.17 o 1 2 T C T° K — x 10 3 k x 10 T°k 1)123 396 2.53 2.02 2) 146.2 419.2 2.38 3.12 using the Arrhenius equation for the rate constant

k = Ae —E/RT

log k = —413ek log A RT

1 1 ) k1 log -- = 2.303R T T ) k2 2 1 84

FIGURE 16

1000 2000 300 0 4000 Pressure (atm.) 85

FIGURE 17

4.0

3.5

3.0

2.5 • X

•a00 cr 2.0 •

2.38 2.40 2.4 5 2.50 2.53

.1 x 103 T

ki. ( 1 ) E = log — x 2.303R ( 1 1c2 1 .. T2 — T1

3.12 ( 103 E = log— x 2.303 x 1.987 x 2.02 2.53 - 2.38 ) E = 5800 cal/. mole = 5.8 kcal/' mole

TABLE NO.1

a) Initiator: AIBN Pressure: 3000 atm. Temperature: 60°C Time Initiator conc. Yield of polymer Molecular weight hr. wt.% wt.% M.W. x 10 4

24 0.136 5.82 MID 18 0.136 4.77 5.10 22 0.137 5.69 5.46 20 0.140 5.32 5.27 16 0.145 4.53 — 26 0.140 5.66 5.68 10 0.142 2.66 — 14 0.137 3.81 5.16

4 0.140 1.18 4,59 6 0.140 4.59 — 8 0.139 2.15 4.91 12 0.139 3.23 4.93 22 0.555 10.10 2.79 18 0.555 8.06 2.50 14 0.552 6.76 2.51 4 0.552 2.04 2.43 9 0.550 4.29 2.73

Table No.la conted.

Time Initiator conc. Yield of polymer Molecular weight hr. wt.% wt.% M.W. x 104

22 0.277 7.49 4.08 22 0.691 10.72 2.42 22 0.070 3.92 7.63 14 0.694 7.07 2.33 14 0.277 4.97 3.59 14 0.070 2.84 7.03

b) Initiator: AIBN Initiator conc.: 0.555 wt.% Temperature: 600C

Time Pressure Yield of polymer Molecular weight hr. atm. wt. M.W. x 10

•••••.••••••••••••• 18 1500 1.74 1.03 22 1500 2.36 0.99 14 1500 0.70 22 4000 18.45 6.25 18 4000 15.91 6.19 14 4000 12.08 10 4000 9.26

14 4500 16.68 ••••• 18 4500 21.50 9.23 89

Table No.lb conttd.

Time Pressure Yield of polymer Molecular weight hr. atm. wt.% M.W. x 10

22 4500 24.98 9.41 22 4200 22.44 — 22 4600 26.78 - 22 4700 45.74 - 22 4850 1.56 — 22 8000 1.38 — 10 5000 0.84 14 5000 1.02 — 18 5000 1.32 14.60 22 5000 1.75 18.48 c) Dependence of rate of polymerization on pressure Initiator: AIBN Initiator conc.: 0.555 wt.% Temperature: 60°C Pressure Rate of Polymerization x 103 atm. wt.% polymer/Min.

1500 1.75 3000 7.78 4000 14.40 4500 19.60 5000 1.34 90 d) Dependence of rate of polymerization on initiator concentration Initiator: AIBN Pressure: 3000 atm. Temperature: 6000 Initiator conc. Rate of Polymerization x 103 wt.% wt.% polymer/Min.

0.070 3.20 0.140 4.27 0.277 5.76 0.555 7.78 0.692 8.25 91

TABLE NO.2

Initiator: AIBN Temperature: 6000

Time Pressure Initiator conc.Yield of polymer Molecular weight hr. atm. wt.% wt.% M.W. x 10-9"

22 5000 0.276 3.38 59.4 18 5000 0.276 4.21 55.8 14 5000 0.276 3.38 56.o 58 4600 0.555 86.0 39.6

TABLE NO.3

Initiator: AIBN Pressure: 3000 atm. Times 22 hr. Temperature Initiator conc. Yield of polymer Molecular weight 00 wt.% wt.% M.W. x 104

40.0 0.554 1.60 3.95 50.0 0.554 6.25 3.76 60.0 0.555 10.10 2.79 80.0 0.554 15.26 2.81 95.0 0.553 13.43 2.77 100.3 0.553 9.02 1.58 92 TABLE NO.4 a) Initiator: AIBN Initiator conc.: 0.555 wt.%

Time Pressure Temperature00 Solid polymer Liquid polymer Molecular hr. atm. wt.% yield wt.% yield weight

5 moo 95 NIL 2.75 248 14 1000 95 NIL 3.18 242 22 1000 95 NIL 3.67 230

22 1500 95 0.51 4.95 11•01. 14 2000 95 1.66 6.42 22 2000 95 2.51 6.71 2280 *

14 1500 8o 0.86 3.95 ONO 22 1500 8o 0.96 4.79 5440 * 14 2000 8o 4.34 5.43 - 22 2000 80 5.42 5.56 9090 * 14 250o 8o 9.61 5.22 22 2500 80 10.72 5.95 16700 *

*Mai. of solid polymer 93 b) Dependence of rate of polymerization on pressure

Initiator: AIBN Initiator conc.: 0.555 wt.% Temperature: 80°C

3 Pressure Rate x 10 (wt.% polymer/min.) atm. liquid solid

2500 5.77 9.81 2000 5.34 4.64 1500 4.16 0.87 94

TABLE NO.5

Initiator: tert—butyl perbenzoate Initiator cone.: 1.000 wt.% Time: 6 hr. -2 Pressure Temperature Rate of polymerization x 10 Molecular atm. oc wt.% polymer/Min. weight

3900 135.0 7.24 OWN 3200 135.0 3.03

2500 135.0 2.46 ••• 1750 135.0 2.01

2500 123.0 2.02 4•11

2500 130.0 2.32 ONO 2500 146.2 3.12 229 a)Dependence of rate of liquid polymer formation on temperature o 2 t C T°K (140k) x 103 k x 10

123 396 2.53 2.02 130 403 2.48 2.32 135 408 2.45 2.46 146.2 419.2 2.38 3.12 95

TABLE NO.6

Initiator: AIBN Initiator conc.: 0.555 wt.% Temperatures 45°C Time: 22 hr.

Pressure Yield of polymer Molecular weight atm. wt.% M.W. x 10

4070 0.28 101,

3950 0.41 3920 4.23 8.89

3750 3.97 8.13 96

TABLE NO.7 a)Freezing point determination at 60°C

Gauge reading Height of piston c:Nh = (h ho) atm. h — cm. cm.

800 ho = 11.604 0 850 11.764 0.160 875 11.848 0.244 900 11.924 0.320 925 11.988 0.384 950 12.056 0.452

975 12.180 0.576 1000 12.236 0.632 1025 12.290 0.686 From Fig.12 the freezing point is at 975 atm. gauge indication; the actual pressure is 4860 atm.

b) Freezing point determination at 45°C

Gauge reading Height of piston tall= (h — ho) atm. h — cm. cm.

600 ho = 9.960 0 650 10.166 0.206

700 10.364 0.404 750 10.560 0.600

775 10.652 0.692 97

Table No.7b cont'd. Gauge reading Height of piston ho) atm. h — cm. cm.

800 10.748 0.788 825 10.940 0.980 850 10.998 1.038 875 11.058 1.098 From Fig.12 the freezing point is at 825 atm. gauge pressure; actual pressure — 4080 atm.

c) Freezing point determination at 30°C Gauge reading Hetght of piston oh = (h ho) atm. h cm. cm.

.11 400 ho = 10.265 450 10.514 0.250 500 10.776 0.512 550 11.034 0.770 600 11.282 1.018 625 11.408 14 650 11.622 1.358 675 11.694 1.430 700 11.782 1.518 From Fig.12 the freezing point is 650 atm. guage indication; the actual pressure is 3180 atm. 98

d) Dependence of freezing temperature on the pressure Temperature P — freezing pressure oc f atm.

—23.2 * 1 30.0 3180 45.0 4080 60.0 * The freezing point at 1 atm. is given ::6:oundy and Boyer (4). 99

PART FOUR: DISCUSSION

IV.l Freezing and Ceiling Temperatures as Limiting Factors in the

Polymerization of Alpha-methylstyrene In the course of this work experiments, carried out at

different pressures and temperatures, confirmed the existence

of three distinct areas in the pressure-temperature diagram for

alpha-methylstyrene. Fig.18 gives a general review of the P T conditions at which the runs were performed. The dotted isobaric

and isothermal lines represent series of experiments carried out

at specific pressure and temperature. The end points, e.g. a, b, c... are runs at the extreme conditions along the particular

isobaric or isothermal line. The P T diagram is divided into three areas by the freezing curve and the ceiling temperature

curve. The experimental procedure in obtaining the freezing

curve was given in section 111.3 and results plotted in Figs. 12 and 13. The freezing curve in Fig.18 is identical to that shown

in Fig.13. The ceiling temperature line represents the linear

relation between polymerization ceiling temperatures and the

corresponding pressures as reported by Kilroe and Weale (52) for alpha-methylstyrene initiated by di-t-butylperoxide and confirmed

in the present work. Polymerization ceiling temperatures at high pressures:

Pressure (atm.) 1 2200 4210 4860 6480 Ceiling temp. (°C) 61 97 131 143 171 8000" d

7000.

6000 -n

5000 C

0 rn I II II I 4000' bt Fo" L. U) lin 3000- 0 0- 411, CL k — ---e 1

2000

1000

-20 0 20 40 60 80 100 I'20 140 160 Temperature C3C) 101

(i) Freezing phenomenon under pressure

With nearly all liquids the freezing temperature increases

with the pressure. The dependence of the freezing temperature T

on the pressure P is usually determined by the method of volume discontinuity as used by Mack, Tamman and Bridgman (59). The liquid is compressed isothermally until the solid phase appears, followed by a large contraction of the substance at constant pressure until all the liquid is solidified. Sometimes super—

pressing is observed as in the case of methanol which is expected

to freeze at 30000 atm. at 25°C, but does not change phase even

at 50000 atm. This metastable condition probably results from the very high viscosity of the liquid at that high pressure which

prevents freezing. Measurements using the volume—discontinuity

method require a perfectly leak—proof apparatus in order to

give accurate results. By making a series of readings at

different temperatures and plotting the results as shown in Fig.12, it is possible to determine all the parameters specifying

the thermodynamics of freezing.

Clapeyron's equation gives the temperature T as a function

of the pressure P.

d T T e-N.V dP b.H

ia and .icZ are the volume and enthalpy changes occuring during the phase change at the freezing temperature T. The difference in 102

slope of the curve of piston displacement above and below the freezing point given by the volume discontinuity is evidence for the difference of compressibility of solid and liquid phases.

As expected, Fig.12 shows the lower compressibility of the solid state as well as the decrease in the piston displacement which corresponds to AV, with the increase in pressure. For comparison, the AV vs. pressure curve for nitrobenzene, given by Bridgeman (59), has a slope which decreases gradually with increasing P and levels off at very high pressures.. The same trend is observed in Fig.12. The difference of Allh (s.›.3- ba) between 650 atm. and 825 atm. is considerably smaller than between 825 atm. and 975 atm. gauge pressure (corresponding to 3180 atm., 4080 atm. and 4860 atm. respectively). No correction was made for distortion of the pressure cylinder which is very small in the pressure range employed and the change of volume of the transmitting liquid on passing from the cylinder to the pressure vessel, due to temperature difference of the two vessels, which could not be more than 20°C.

Without applying any correction for the eventual errors in reading the pressure and the piston position, a rough estimation of &V and cbli, based on the Clapeyron equation, was made. Taking the slope dT of from Fig. 13, the calculations given in section 111.3 shows

fa = —0.060 cm3/gr or — 7.08 cm3/gr. mole tal —1370 kg.m./gr.mole or -3.3 kcal/gr.mole at 60°C and 4860 atm. 103

These values are of the expected magnitude. For example: for benzene at 4000 atm., 96.6°C, aV = -0.0675 cm3/gr. and for

nitrogenzene at 4000 atm., 87.6°C, pV = -0.0555 cm3/gr. - -1350 kg.m./gr.mole

Bridgman gives also data for some other substances which show 4.7 and 611 values of the same magnitude. This comparison tends to confirm the accuracy of the freezing

point measurements and the authenticity of the freezing curve, particularly when it is recalled that the extrapolation of the

curve in Fig.13 to one atmosphere gives the normal freezing temperature of alpha-methylstyrene (-23.2°C), as reported by

Boundy and Boyer (4).

(ii) Ceiling temperatures at hi h pressures The general concept of the existence of a depolymerization

reaction occuring above certain temperature (the 'ceiling temperature' - has already been described in the introduction -

Sec.I.2(iii).Beoatve alpha-methylstyrene has the lowest heat of

polymerization among the common monomers, it is to be expected that this substance should tend to depolymerize at comparatively lower

temperatures than monomers such as styrene or methyl methacrylate.

The propagation involves bond formation and high negative value of

4NIT therefore this reaction is accelerated by pressure, whereas the depropagation is a bond-breaking process which should show

an opposite pressure effect, as a small positive value of 411/;44 dp 104 is expected.

The retardation of the depolymerization together with the acceleration of the forward propagation reaction will result in the shifting of the equilibrium between these two processes towards higher temperatures, i.e. increase of the ceiling temperature with the increase in pressure. The increasing influence of the depropagation reaction close to the equilibrium should cause a reduction of the overall rate of polymerization and of the molecular weight of the product. The manner in which the overall rate and degree of polymerization are expected to change with temperature in the equilibrium region near the ceiling temperature has been calculated by Dainton (60). Curves based on these calculations show an increase of R with the temperature, reaching a maximum point after which there is a very sharp decrease ending with R = 0 at the particular ceiling temperature. The degree of polymerization' if termination by transfer is not considered, decreases slowly followed by a levelled region, after which a very substantial falling off leads to zero degree of polymerization at the ceiling temperature.

Dainton and Ivin (28) compared the existence of the ceiling temperature as a limiting factor in the polyermization reaction of a monomer with the freezing temperature at which the liquid state of a substance is in equilibrium with the solid state. Above the freezing point the solid cannot exist as a stable phase. Here, 105

the polymerization was taken as a chemical aggregation process,

with the monomer, polymer and ceiling temperature being physical analogies to liquid, solid and freezing temperature.

Applying this theory to the ceiling temperature curve

for alpha-methylstyrene, Kilroe (54) used the Clapeyron's equation to calculate 4W, the difference in molar volume of reactants and products, taking the slope of the ceiling dT temperature curve as the 7-p- and the value of6H .-- -8.4 kcal/Mole.

The calculated value of 6V (-14.7 cc/mole) was fouhd to be in fair agreement with estimated values ofaV * for alpha-

methylstyreno (-14.3 cc/mole). This estimation was made assuming that the ceiling temperature line represents a true equilibrium between propagation and depropagation reactions. A striking fact is that the freezing line and the ceiling temperature line, as plotted in Fig.l8, have almost equal slopes. The results obtained during the freezing pressure experiments at 60°C show a ratio of

41 -3'3 . 0.468 bal ca V 77613 cm which compares quite well with the calculated result based on the ceiling temperature line with the Clapeyron's equation, which is

431 2111 s 0.57 kcal/cm3 -14.7 a difference of only 20%. 106

This fact provides some justification to the comparison between the 'chemical' and physical aggregation processes as suggested by Dainton and Ivin. Usually, monomers in solid state cannot participate in polymerization reactions (Polymeriz— ation in solid state is known for a few monomers with favourable crystal structure by using radiation to initiate) due to the mobility of the molecules. On the other hand temperatures above the 'ceiling' values at the corresponding pressures also prevent polymerization. It is clear that these two lines as plotted in Fig.18 divide the P T diagram for alpha— methylstyrene into three dinstinct regions: Region I — even if presence of some polymer prevents complete freezing, the very high viscosities and the presence of monomer mainly in crystal form reduce the polymerization rate to zero.

Region II — polymerization favoured by high pressure producing long—chain solid polymer, together with small amounts of low molecular weight liquid polymer, especially near the ceiling temperature line. Region III — reactions leading to the formation of very low molecular weight polymer, chiefly consisting of dimer,are possible but no high polymer is formed.

Apparently, in regions near the separating lines, i.e. where conditions are near equilibrium, complex effects may occur. Some unusual phenomena which have been observed are discussed in chapter IV.2 107

IV.2 The Effect of Pressure on the Polymerization Rate (i) Order of reaction with respect to initiator concentration

The observed effect of high pressure on the initiation step is the slight decrease in'the order of reaction with respect to the initiator concentration. Theoretically, the polymerization rate should be dependent on the initiator concentration raised to the power of 0.5. The results obtained in this work show an exponent value of 0.44 at pressure of 3000 atm. and temperature

of 60°C. Kilroe and Weale (52) reported a similar reduction in the reaction order, e.g. 0.44 and 0.45 at 8100 atm. and 9000 atm., respectively, using benzoyl peroxide for runs carried

out at 94.5°C. They compared their results with those of Merrett and Norrish (38) who showed that for the polymerization of styrene

with benzoyl peroxide as initiator at 60°C, the rate of initiation is very slightly affected by pressure. They reported a variation

of the power index from 0.50 at one atmosphere, decreasing to 0.40 at 3000 atm. and again increasing to the value of 0.45 at 5000 atm.

The authors' suggestion to explain this behaviour was an unlikely termination reaction between chain radicals and an initiator molecule. Under the experimental conditions the spontaneous decomposition is retarded by the pressure as it consists ofsplitting

the initiator molecule to produce two radicals. For this first

order decomposition various workers reported a nearly constant "IV

for different solvents, the value being about 4.8 cc/mole. 108

Nicholson and Norrish (40) (41) extended the work on the same processes and slowed that because of the presence of 'radical traps' in the polymerizing system such as styrene and methyl methacrylate„ the influence of an induced decomposition of the initiator is very small, hence the overall effect of the pressure should be the slight reduction of the initiation rate. For the polymerization of styrene at 30°C with benzoyl peroxide as initiator, the rate of initiation was found to be relatively insensitive to pressure, the order being decreased from 0.5 at one atmosphere to

0.45 at 3000 atm. For azo-bis-isobutyronitrilelEwald (61) found that the decomposition of this initiator at 62.5 C in toluene was retarded at pressures up to 10000 atm. He also confirmed that the rate of radical wastage is increased at high pressure due to possible recombination of primary radicals. The azonitriles, in general, are very convenient radical sources, since they are known to decompose by strictly first order kinetics at the same rate in various solvents without being affected by induced chain reaction. They are one of the very few classes of initiators with which chain transfer is not observed. At high pressure, however, the recombination of primary radicals is favoured because it is more difficult to escape the so-called 'solvent cage'. Hence, the effect of pressure on the order of initiation can be compared to the decrease,noted with large initiator concentrations,where the increased 109 importance of recombination and terminationl and termination by primary radicals from the initiator reduced the order to values, below the square root dependence7and tending to zero at infinite initiator concentrations.

(ii) Overall rate of polymerization The increase in the rate of polymerization with pressure is already a well established fact. However, in the course of this work it was found that a ten-fold increase in the overall rate between 1500 atm. and 4500 atm. is followed by an abrupt fall off at the pressure of 5000 atm. (Fig.5). There is a linear relationship between the logarithm of the polymerization rate and the pressure in the range of 3000 - 4500 atm.lthe increase being about three-fold. Kilroe and Neale (52) studied the polymerization of alpha-methylstyrene initiated by various initiators at pressures in the range of 3000 - 12000 atm. and reported that log[ratOat first is proportional to the presssure, reaches a maximum value and decreases rapidly, the point of the maximum rate being shifted to higher pressures on dilution with solvents. They suggested that the decrease in rate is probably due to a large increase in viscosity or a phase separation, both of which could be affected by dilution. Merrett and Norrish (38) found that the logarithm of the rate of polymerization of styrene at constant initiator concentration (benzoyl peroxide) and 60°C is nearly directly proportional to the pressure ranging between 3000 and 5000 atm., with 110 a less clear relation at lower pressures. The overall rate increase at 5000 atm. was about fifteen-fold. Their explanation of these results was that the rate of initiation is slightly retarded by pressure, the rates of propagation and transfer are considerably accelerated, and the rate of mutual termination reduced because of increased viscosity at high pressures. Nocholson and Norrish (39) using the rotating sector technique for the styrene -benzoyl peroxide system at 30°C found that k increased exponentially with pressure giving a 5.5—fold increase at 3000 atm., kt decreased rapidly up to 1000 atm., followed by a much slower reduction with a linear relationship between log (kt)p/(kt)1 between 1000 and 3000 atm. They suggested that the increase in viscosity with pressure led to a diffusion controlled termination reaction. Melekhina and Kuvshinski (41) reported similar effects of high pressures on the polymerization of methyl methacrylate.

The propagation is a bimolecular process, in the course of which there is a close approach between a growing chain radical and a monomer molecule, associated with large negative values ofi.V and losses in degrees of freedom which is expressed by large decrease in entropy.

Such reactions should be accelerated by pressure (83), (84). On the other hand, transfer reactions involve at first a process of lengthen- ing and breaking a C - H bond before a further bond formation occurs. It is clear that there is a preliminary increase in volume due to the stretching of the breaking C H linkage before the contraction,which 111 follows with the formation of the new carbon-hydrogen bond. The overall effect is a smaller decrease in volume in the transition state (E.Vtr ) in comparison to &.T . Thus, the effect of pressure on the transfer reactions should be smaller than on propagation.

According to Hamann (62), (63), bimolecular reactions become diffusion-controlled and retarded if the viscosity of the medium is increased by a factor of nearly 1010. He investigated the alkaline etherification of ethyl bromide at 25°C and pressures in the range 10000 - 40000 atm. In an isopropyl alcohol -eugenol mixture used as a solvent, the reaction was hicnly reduced at high pressures, corresponding to a ten-fold viscosity increase. These postulates were based on the Einstein-Stoluchowski theory of diffusion. As for propagation reaction in vinyl polymerization, the activation energy is about 5 kcal/mole in comparison to c...f20 kcal/Mole for ordinary bimolecular reactions, therefore a smaller increase in viscosity (in the order of 105 - 106) might cause the propagation becoming diffusion controlled.

A good example can be the polymerization of methyl methacrylate at 1 atm. as reported by Hayden and Melville (64). The rate of polymerization is independent of conversions up to 10%, followed by a rapid rise due to increased viscosity, but at very high conversions the rate is retarded as the viscosity is of the magu. nitude as to cause the reduction of the propagation, and at 80% conversion the polymerization reaction is almost completely ceased, 112

In this case as well as at high pressures very high viscosities can be the reason for the self-inhibition of the polymerizing system.

As noted earlier, the abrupt fall off in the rate of polymerization at the pressure of 5000 atm. might be due to a sudden increase

in the viscosity of the polymerizing medium to such an extent as to reduce the propagation rate almost to zero. The work described

in chapter 111.3 established that the monomer at 4860 atm. and 60°C undergoes a phase change from the liquid to the solid state. Freezing

points at different temperatures are given in table No.7d and plotted

in Fig.l3 forming a linear relationship between temperature and pressure. Dodd and Hu (65) carried out measurements of the viscosities of supercooled phenyl ether. It was found that in both the ordinary and the supercooled state the liquid obeys a relation of the type

= but the value of b in the supercooled state was significantly larger than the corresponding value for liquid above the melting point. An abrupt change of 13% in the slope of the plot log

(Vvs. 1/T was observed as the liquid passed from the ordinary to the supercooled region. Other liquids gave similar increases in viscosity.

In analogy to these findings it is possible to assume that when the pressure is raised, the viscosity is increased, and in regions near the freezing point if superpressing occurs there should be large

sudden increases in the viscosity of the medium. When the actual

phase change of the monomer is achieved, there could be some sort of

phase separation of monomer-polymer aggregates surrounded by rigid 113 blocks of immobile solid monomer under high pressure which might raise the viscosity of polymer occluding monomer molecules to very high values, thus decreasing the propagation reaction to very low rates, according to the prediction of Hamann. In order to give a more clear view of the polymerization reaction at pressures between 4500 - 5000 atm., a series of runs were performed and Fig.6 shows the nearly 70% increase in wt.% conversion from

4600 atm. to 4700 atm., followed by the sudden 97% drop in the yield to 1.56 art .% at 4850 atm. which is the freezing pressure at 60°C, the experimental temperature conditions. The runs performed at 5000 atm. and. 8000 atm. show very similar conversion to high polymer. Another series of runs performed near the freezing pressure at 45°C showed exactly the same shape of the wt.% yield vs. pressure curve.

The large increase at4700 atm. is probably due to a very abrupt and substantial increase in the viscosity as this point is very near the freezing point and a superpressed state might have been achieved because a pressure difference of 100 atm. is well within the estimated experimental. error. According to what was suggested above, the high viscosity in the superpressed region, before the freezing took part, resulted in reducing suddenly the termination reactions to a very large extent with a much smaller effect on the transfer thus increasing the yield. With the freezing, the propagation rate was reduced almost to zero and caused the aprupt fall off in the overall rate of polymerization. 114

(iii) Dependence of overall rate on temperature

The conversion to high polymer at a pressure of 3000 atm. rises with increased temperature, reaches a maximum value between

85°C and 90°C followed by a very rapid fall between 90°C and 100°C. The extrapolation of the wt.% yield vs. temperature plot to zero conversion is in excellent agreement with the expected magnitude of the equilibrium temperature between the propagation and depropagation reactions according to the ceiling temperature vs. pressure line reported by Kilroe and Weale and shown in Fig.18.

In the region between 40°C and 85°C the constant increase in the yield of polymer is ascribed to the positive effect of the temperature on the rate of radical polymerization, a reaction with low activation energy, with the propagation step being the predominant factor. However, with the further increase in temperature the reaction becomes appreciably reversible with the increasing importance of the depropagation, favoured by the effect of high temperatures on the exothermic polymerization reaction. It is clear that these two reactions with opposite directions should reach an equilibrium state at certain temperature, above which no polymerization to long chains would be possible. Worefold and Bywater (49) demonstrated the possible existence

of a propagation - depropagation equilibrium by investigating the anionic polymerization of alpha -methylstyrene with sodium naphthalene

in tetrahydrofuran at temperatures between -40°C and 0°C, where 115 termination was negligible. Following the course of reaction by means of colour change they obtained equilibrium monomer concentra— tions by approaching it from both directions. McCormick (50) found that the ceiling temperature at one atmosphere was 61°C. Sapiro, Linstead and Newitt (51) could not obtain high polymer of alpha— methylstyrene above 150°C and 4000 atm. Polymethyl methacrylate depolymerizes at temperatures above 200°C (66), (67), or even lower, when put under vacuum (68). The ceiling temperature of styrene is very high (about 327°C at one atmosphere), therefore in order to study the depolymerization Madorsky et al. (69) employed a heating under vacuum technique. The yield of monomer was approximately 42%. During this series of runs no attempt was made to follow the production of low molecular weight polymer, a reaction which proved to be of a substantial magnitude at conditions near the ceiling temperature line and at higher temperatures. In the section dealing with the formation of dimer it is shown that in conditions quite similar to these mentioned here, there is competition between reactions which lead to the production of dimer or long chain molecules, the rates of which depend on the pressure —temperature conditions. 116

IV.3 The Effect of Pressure on the Molecular Weight

The degree of polymerization and the molecular weight of a high polymer are average quantities. Since the possibility of chain

termination or chain transfer exists at every step of the reaction leading to the formation of the long chains, it is obvious that polymers of different degrees of polymerization are being formed so that the product has a wide molecular weight distribution.

The simplest distribution results when the dominating termination process is disproportionation, and chain transfer is significant.

If the chains end by mutual coupling, the distribution is considerably narrower. In principle, determination of the molecular weight distribution by fractionation techniques might be used to establish the nature of the termination process. Evans et al. (73) made an attempt to apply this method to methyl methacrylate. Their results

are only in very rough agreement with theory, as there are many difficulties in carrying out a precise fractionation of polymer molecules with a wide range of molecular weights.

As the degree of polymerization is dependent on the influence of various reactions, such as termination by combination or dis-

proportionation, and chain transfer to solvent, monomer, initiator and polymer, it is necessary to discuss their specific) effects on the polymerization of alpha-methylstyrene with AIBN as initiator. For

bulk polymerizations transfer to solvent is eliminated, and for

controlled reactions, leading to low conversions, the transfer to 117 polymer may be neglected. Generally, polymerization reactions with initiators at low concentrations do not appear to be affected to a significant extent by transfer to initiator. Of special value is the azonitrile group of initiators, the best known and most studied of which is azo-bis-isobutyrnitrile (AIBN). They are known to decompose by strictly first-order knietics at a nearly constant rate in different solvents, in the presence of free radicals, with no evidence of induced chain decomposition (85),(a common complication with peroxide-type initiators), or of chain transfer to initiator. For this reason transfer to initiator in the system alpha-methylstyrene - AIBN is extremely unlikely and even if it does exist to a very small extent, this process can be omitted from further consideration. Hence, the reactions which appear to influence the molecular weight are propagation, termination by combination and disproportionation, and transfer to monomer. Except at high pressures, where high viscosity may cause termination to be diffusion-controlled, pressure should have a positive effect on the rates of these reactions, the effect of pressure decreasing in the order propagation > transfer > termination.

A significant property of radical-induced polymerizations is that although the amount of polymer increases with time, over a large range of conversion the molecular weight remains practically constant. This is due to the very short time required to produce a long chain polymer molecule. Chain propagation is a rapid process with rate constants of 10 - 104 for various monomers, and termination is even 118

7 faster'k tbeing of the order 10 . Chain transfer is much slower and requires a higher activation energy than propagation and termination. In general, in polymerizing systems radical concentrations are low and chain lives short. The longer the time of reaction, more chains are induced and terminated thus giving an increase in the conversion to polymer at the expense of the monomer. The expected approximate constancy of the molecular weight with reaction

time was observed in the polymerization of alpha -methylstyrene with AIBN at 3000 atm. and 60°C as shown in Fig.8.

The dependence of the molecular weight on the initiator concentration was investigated at constant pressure and temperature. The linear relationship between log (M.W.) and log (In), with a slope of -0.485 (calculated from Fig.9), is in good agreement with the conventional kinetic scheme. According to this wheme„ if transfer to initiator can be neglected, the mean-degree of polymerization

DP7i'Dhl 4." The conclusion to be drawn from this result is that mutual chain termination reactions do occur in the polymerization in the prevailing conditions. Had they been negligible, the kinetio equations would have resulted in the relation DP'>°[In] -1, which is not confirmed by the experimental work. The slight difference in

the coefficient is attributed to an effect of pressure on the initiator decomposition. The question that remains is whether the termination is by combination or disproportionation, or if both occur. As for kinetic purposes they are equivalent, it is difficult to determine 119 exactly the relative importance of these two processes. At present, the reported results indicate that most polymerization processes terminate by radical coupling. Bevington, Melville and Taylor (74) reported two initiator fragments per molecule for the polymerization of styrene initiates L by AIBN. On the other hand, the same authors using radioactive azo-bis-isobutyronitrile made an extensive study of the polymerization of methyl methacrylate and found approximately 1.2 initiator fragments per molecule of polymer. This means that the predominant termination reaction is disproportionation. Reported rate constants for styrene and methyl methacrylate at atmospheric pressure are as follows (75)s Styrene Methyl methacrylate 0 ktd 4 x 107

ktc 107 3 x 107 An inspection of molecular models suggests that mutual combination of polyalpha-methylstyrene radicals may be prevented by steric hindrance. In this respect alpha-methylstly.uu resembles methyl methacrylate due to steric interactions existing between substituents in successive monomer units particularly important in

1,1-disubstituted compounds. This assumption suggests that in the case of al?ha-methylstyrene termination by disproportionation is the more significant among the two alternatives. The relatively low values of molecular weight before freezing takes place (M.W.4( 105) might 120 support this postulate, although the knowledge of the magnitudes of kp and kt is essential to confirm it. Disproportionation together with a small amount of transfer to monomer will result in a wide distribution of molecular weight, and as chain propagation is considerably accelerated at high pressures the molecular weight should show an increase with pressure, corresponding to the increase in tat, overall rate of reaction. In agreement with this expectation Fig.10 gives a linear relationship between log (LW.) and the pressure over the whole range. When the freezing point is approached, the viscosity is increasing very appreciably, thus resulting in a diffusion controlled process and the termination should then be very much redueed. Bearing in mind that it was assumed that the transfer to monomer is much slower reaction than the propagation and acquires higher energy of activation, whereas the experiments were carried out at only 60°C, the conclusion is that at very high pressures, just before freezing takes place, the decisive reaction in determining the molecular weight is the propagation. This explains the increase of molecular weight together with a large increase in the rate. At pressures above the freezing point blocks of frozen monomer will surround aggregates of polymer and occluded monomer, which will retain a certain degree of mobility, but as the viscosity is very high, termination is greatly reduced relative to propagation, and with a relatively low rate of transfer to monomer this will result in a rapid increase in the molecular weight. As mentioned before, transfer should be less 121 affected by high pressures than propagation, since its absolute value oftNV is smaller. However, the nearly three-fold increase in molecular weight between 3000 - 4500 atm. is similar to results reported on the polymerization of styrene (38). The limiting values of DP given by Merrett and Norrish at high pressures are approximately equal to values obtained by estimating the molecular weight on the basis that the chain transfer to initiator (benzoyl peroxide) is the major chain ending step, an effect greatly reduced by the use of AIBN.

In this series of experiments, with no major effects caused by transfer reactions, the observed levelling-off in the molecular weight of styrene do not appear. The exceptional difference with alpha- methyl styrene which arises due to the freezing effect, does not occur in the experimental conditions employed by the various workers who have used other monomers.

The unusually high molecular weight (M.W.?"4 x 105) cannot be interpreted quantitatively as the pressure during these runs was not completely constant and the reactants may have changed several times from liquid to solid phase. During the phase changes there are increases in the relative importance of kp and kt as high viscosities would reduce kt while kp will be much less affected unless the viscosity is increased by a factor of 105 — 106. This might occur above the freezing point, but at pressures 200 - 300 atm. below, the which would account for large increase marked reduction will be on kt in molecul-.r weight, a phenomenon observed with auto acceleration (76), 122

(77), (78)• Thus, the increase in viscosity suggested above is comparable to the auto acceleration observed at high conversions with other monomers. Rotating sector experiments carried out at high conversions by Matheson et al. (79) indicate that for methyl methacrylate at 35% conversion kt decreased to less than 1% of its initial value whereas k remained practically constant.

Monomer Temp.°0 % conversion at k(35%)/k(0%) DP transition point k kt initial

Styrene 50 30 0,62 0.075 4000 Methyl methacrylate 30 15 1.12 0.0066 5000

The transition point indicates % conversion at which auto acceleration becomes detectable. These results were obtained at atmospheric pressure. It is obvious that the viscosities at high pressures, and especially near and above the preezing points, may be greatly increased even at low conversions. The same authors also pointed out that the auto acceleration increases with the initial values of DP. On the bssis of these findings it is possible to suggest a qualitative explanation for the results obtained near the phase boundary with the unusually high molecular weight. While the reactants were being brought to high pressure conditions above the freezing point, a small amount of polymerization probably occured, 123 resulting in a low conversion and a molecular weight of the order of 105 as observed in runs at similar conditions. When the pressure was reduced to about two hundred atmospheres below the freezing point, the reactants became liquid and though the viscosity might have been still very high, it permitted a substantial degree of reaction which was based on high viscosity and high initial DP, thus resulting ev-intually with the very high molecular weight. As the drop in pressure was for short periods of time, the conversion was not more than 5%, but in the run carried out at an average pressure of 4600 atm. (table No.2) and with 58 hr. duration the yield was 86% and the molecular weight 3.96 x 105. In this case freezing has not occured so that the run is quite similar to those carried out at pressures below the freezing point, the exception being the very high conversion obtained, which would have caused a large increase in vis— cosity. It is noteworthy that at the end of this experiment the reaction tube had to be broken as the polymer formed a solid block. The dependence of the degree of polymerization on the temperature has been examined theoretically. by Dainton (60). His calculation, based on thermodynamical and kinetic considerations, suggests that in polymerization reactions with no marked transfer processes the mole— cular weight should at first decrease slowly with temperature in the lower temperature range, then attain a fairly constant value, and finally decrease rapidly with the approach to the ceiling temperature.

In the polymerization of alph—methylstyrene at high pressures there 124 is another factor to be taken into account - the freezing of the monomer at temperatures much higher than the normal freezing point. Molecular weight of polymer formed at 3000 atm. and various temperatures were determined in a series of runs indicated by the line ab in Fig.18. The points a and b are very near the freezing and ceiling temperatures, respectively, at 3000 atm. These experimental results, represented by the M.W. vs. temp. curve in Fig.l1 show the empr-35ed behaviour. The change in molecular weight at low temperatures, i.e. in the vicinity of the freezing point, is more emphatic than suggested by Dainton and is due to the effsct of the increase in viscosity on the termination reaction with the approach of the freezizt, conditions. Between 6000 and 9000 there is a constant value of the molecular weight attributed to the increasing effects of transfer to monomer and termination by disproportionation, which counterbalance the increase in the rate of propagation. in this region, the result of these processes is a wide molecular weight distribution and an almost constant molecular weight. With the increasing effect of depolymerization near the ceiling temperature, the degree of polymerization falls off rapidlyland as described in a previous chapter, beyond the limit set by the ceiling temperature line the major product is dieter. 125

IV.4 The Effect of Pressure on the Formation of Low Molecular Weight

Polymer

The possibility that the polymerization of alpha— methylstyrene can follow two paths leading either to the

formation of low molecular weight liquid polymer or to long chain solid polymer has already been mentioned in some of the

previous chapters. According to Fig.18 reaction temperatures and pressure,, within Region II should favour high molecular

weight polymer with increasing production of dimer in areas near

the ceiling temperature line. Similarly, in areas of Region III in close vicinity to the boundary it is possible that the products

would consist chiefly of dimer with perhaps a certain amount of

very low molecular weight..solid polymer (M.W. 3000). Under pressure—temperature conditions situated in Region III, far from the ceiling temperature line, only the formation of dimers

should be observed. In fact, the runs performed at 80°C and 95° 0 give• clear evidence of behaviour according to these expecta— tions. Fig. 14 shows an almost linear relationship between the log. rate and the pressure for the reaction giving high molecular

weight polymer. The straight line obtained for the relation

between the logarithm of the molecular weight and the pressure (Fig.15), and the extrapolation of this line to 3000 atm., which

gives almost the same value of M.W. as wasrUtained experimentally

at 80°0 (2.92 x 104) is sufficient evidence that the polymerization

126 to solid polymer takes the same course as in other parts of

Region II. The curve representing the formation of liquid polymer is less well-defined but as is to be expected, the rate increases to a certain extent with increase in pressure at constant temperature. The exact nature and the influence of the various possible reactions taking part in the production of low molecular weight poly 9r have not yet been established. One explanation is that &ain transfer to monomer, which generally has a higher energy of activation than chain propagation, becomes more important with the increase in temperature, especially after the depropagation begins to become important when the temperature has risen close to the equilibrium region, near the ceiling temperature line. The chain transfer reaction probably involves the transfer of allylic hydrogen as in the case of allyl acetate (57), as this leads to the formation of resonance-stabilized radical. The corresponding mechanisms with alpha-methylstyrene are:

a) Propagation

Bz (1) Bz- 0I .= CH2 + M* C* - CHM cs3 CH3 b) Transfer with allylic hydrogen (stabilized by resonance)

Hz C CH2 + M* > & -C . cH2 MH CH3 CH2 (2) Bz -C -tH2 • CH2

127

c) Transfer with hydrogen located on the double bond (this reaction is much less probable as the resulting radical cannot be stabilized by electron resonance and it is most unlikely that the approaching radical M would not preferably attack the double bond).

Bz - C 1= CH z - C CH + MH 2 17 (3) CH 3 CH3 The formation of the dimer is obtained by:

d) Mutual termination between the radicals from reaction (2)

Bz - C CH2 CH2 - C - Be - C CH2- CH2- Bz CH2 62 CH2 CH2 (4) The resulting dimer includes two double bonds. e) Addition of a monomer molecule to a radical obtained from reaction (2) followed by another transfer reaction to give a stable molecule

Bz = CH +CH C 2 2 - 'GI}, 2 CH3 M Bz - CH2 - CH2 - Bz 3 M * Bz - CH - CH2 - CH2 - 2 Bz (5) CH3 CH

128

Bz C = CH- tH - C - BZ ---;e 2 H CH3 CH2 CH2 M 1 1z -C - CH2 - C - Bz > CH CH 3 2 TI3 M * (6) Bz - - CH2 - 0 - /b. CH CH 3 2 The resulting dauers include only one double bond. There is also

a possibility that the intermediate products of reactions (5) and

(6) will undergo further propagation instead of termination, thus producing long chain molecules, or especially in the case of the reaction (5) the radical could remain active and free throughout the

polymerization, without being involved in any further reaction. As mentioned already,the volume of activation for transfer reactions has a negative value, therefore the pressure should have

an increasing effect on the rate of transfer processes. Fig.14 shows a 39% increase in log. rate with the increase in pressure from 1500 atm. to 2500 atm. At these conditions, apparently, the dominating reaction is the propagation with a lower energy of activation and the overall effect of pressure on the rate of liquid polymer formation is less pronounced. At 95°C though, at pressure conditions which give points situated in Region III of Fig.18 near the ceiling temperature line, at 1000 atm., the only product is liquid polymer and at 1500 atm. and 2000 atm. there is a certain 129 amount of very low molecular weight solid polymer, the molecular weight of which was determined to be 2280 at the higher pressure.

The amounts of liquid is well as solid polymer, as expected, increase with the pressure. As mentioned before (Section 111.4), the yields of liquid polymer at 95°C are higher than these at o 80 C at the same pressure. The reason is probably due to the higher,oHtr for dimerization so that the dimer does not tend to break down, together with the higher energy of activation which would result in higher rates with the increase in temperature (71),

(72), Kilroe and Weale reported an increase in the yield of liquid polymer with the increase in temperature at constant pressure.

Newitt et al. (51) obtained large amounts of low molecular weight polymer at 125°C and 4000 atm. chiefly consisting of dimers. They suggested that two distinct types of polymerization, taking place at temperatures very narrowly separated, one of which leading to the formation of low polymer, were possible, with a stepwise type of reaction. However, these workers did not try to explain these postulates and it is most unlikely that there is a change in mechanism between narrowly separated temperatures rather than gradually increasing influence of reactions which compete with the propagation and eventually take its place after the ceiling temperature. The experiments performed at conditions permitting the production of liquid polymer only (Table No.5, Figs.16, 17) confirm 130

that the logarithm of the rate is in linear relationship with the

pressure. At these conditions no propagation to high polymer is possible therefore the predominant reaction is transfer to monomer

and the effect of pressure can be better observed. The positive effect of temperature on the rate also holds order these conditions. An extrapo1Etion- of the log rate vs. 1 line to 80°C gives a T calculated rate of about 8.0 x 10 , whereas the experimental result shows a value of 5.77 x 10-3 for the rate of formation of liquid polymer at 80°C and 2500 atm. The difference of about 27%

is partly due to higher initiator concentration (1.00 wt.% tert— butyl perbenzoate for this series at the higher temperatures in comparison to 0.555 wt.% AlBN) and to the error of extrapolation, but is probably to some extent due to an increase in rate of dimerization above the ceiling temperature, where there is no competition by the propagation reaction. A calculation of the energy of activation for

the overall reaction of dimerization at thewe conditions was made (using the results in Fig.17) and gave E X5.8 kcal. Estimates

of the activation energies for propagation Ep, termination Et, measurements of di — heat of reaction have been made transfer Etr' p for styrene and methyl methacrylate by different workers. Some

of the reported values are compared below with these for alpha—

methylstyrene. 131

Monomer Styrene Methyl Methacrylate Alpha— methyletyrena E (kcal) 7.3 4.7 ,cl3.5 (o) Et (kcal) 1.9 1.0 (c) —11 (kcal) 16.1 ± 0.2 13.0 ± 0.2 9.0 ± 0.2 Ceiling terp. 327 190 61 (b) at 1 atm. (°C) NOTES a)These values are extracted from Walling (2) b)Given by McCormick (50) c)Rough estimation made by the author On thermodynamical bases alpha-methylstyrene resembles methyl methacrylate more than styrene which might be due to sterie interactions existing between substituents in successive monomer units in the polymers of substituted olefins, which is particularly important in 1, 1 - disubstituted compounds. If this suggestion is accepted, then by following the thermodynamic values given above it appears that the values of tH and ceilitlig temperature decrease steadily from styrene to alpha-methylstyrene in similar proportions. The same trend is observed with the values of the activation energies

E and Et for styrene and methyl methacrylate. Although there need not necessarily be a close connection between LIZ and E

(which is nearly equivalent to €H , the heat of activation from the transition-state theory), one may expect the changes in E to have a similar trend to those of4Hp. Hence, a very rough 132

estimation of E ';',4 3.5 kcal and Elt.i1.0 kcal for alpha-methylstyrale

can be made. This would indicate that the observed value of E = 5.8 kcal is too high to be associated with a propagation or termination reaction, but it is possible that it might arise from a transfer reaction. Mayo et al. (13) reported a value of Cm = 0.6x104 for the chain transfer to monomer for styrene at 60°C, atmospheric pressure. The transfer oonstant of styrene with ethylbenzene, a solvent with

a very similar structure to the monomer, giving products through inclusion in the polymer chain of the same structure as the reactant,

was reported by Gregg and Mayo (72) to be Cs = o.67 x 10-4 at one atmosphere and 60°C and they calculated the value of of Etr-Ep to

be 5.5 kcal. For the transfer between styrene and isopropylbenzene (a solvent similar in structure to alpha -methylstyrene) at the same 0.82 x 104 5.5 kcal. Since the conditions Cs and Etr -E p = transfer constant to monomer (Cm) and to ethylbenzene (Cs) have almost the same values (0.6 x 104 and 0.67 x 104 respectively) it

is possible to assume that the energy of activation for transfer to monomer should be of the same magnitude. Using the value of E as given in the table above, the activation energy for transfer to monomer should be about 12.8 kcal which is nearly 75% higher than the corresponding activation energy for propagation. For alpha -methylstyrene, using a similar ratio, the value

of E should be about 6.0 kcal which is very near to the observed tr 133 value of E = 5.8 kcal. These considerations give some support to the view that the predominant reaction in the formation of liquid polymer is transfer to monomer. Of course, other reactions such as limited propagation and termination cannot be completely ruled out but the conclusion is that their participation in the formation of dimer below and, especially, above the ceiling temperature is not very significant. 134

CONCLUSIONS

The main facts which have been obtained in the course of this work ares 1)The overall rate of the free radical polymerization of alpha-methylstyrene is accelerated by increase in pressure.

2)The order of reaction with respect to the initiator concentration is 0.44 at 3000 atm. and 60°C, in agreement with previous reports.

3)The molecular weight of the polymer is increased by an increase in pressure. 4)The degree of polymerization is proportional to the initiator concentration raised to the power -0.485.

5)The degree of conversion does not much affect the magnitude of the molecular weight. 6)The pressure-dependence of the ceiling temperature found by Kilroe and Weale was confirmed. At temperatures below the ceiling temperature both solid and liquid polymers are formed.

7)Above the ceiling temperature the main product is liquid dieter. The rate of the dimerization reaction is increased with increases in temperature and pressure. 8)The freezing temperature of alpha-methylstyrene is raised from -23.2°C at 1 atm. to 60°C at 4860 atm., and there is a linear relationship between freezing temperature and pressure. 135

9)There is a sharp decrease in the rate of reaction at the freezing pressure of the monomer. 10)The polymerization of alpha—methylstyrene to high polymer can be carried out only between the limiting conditions, set by

the ceiling temperature and freezing lines, which divide the P T diagram for this monomer into three distinct regions.

These results have been interpreted in terms of the accepted kinetic and thermodynamic theories of polymerization. 136

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