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Polymerization of Alpha.-Methylstyrene at High Pressures

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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 tempera- ture 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 polymeriza- tion 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].
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