Ionic polymerization styrene

Seeded dispersion polymerization was extensively investigated for radical systems [17]. Much less is known about seeded dispersion polymerizations with propagation on ionic and/or pseudoionic active centers. Awan et al. reported seeded ionic polymerization of styrene, which at certain conditions produced particles with narrow diameter size dispersity [18,19]. We presented the first data on the seeded ring-opening polymerization with constant number of microspheres. 

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

Starting from 1956, living ionic polymerizations became the major interest for the synthesis of well-defined polymers. Szwarc reported that in the anionic polymerization of styrene (St) the polymer chains grew until all the monomer was consumed the chains continued to grow upon addition of more monomer [16],... 

Ionic polymerizations are remarkable in the variety of polymer steric structures that are produced by variation of the solvent or the counter ion. The long lived nature of the active chain ends in the anionic polymerization of diene and styrene type monomers lends itself to studies of their structure and properties which might have relevance to the structure of the polymer produced when these chain ends add further monomer. One of the tools that, may be used in the characterization of these ion pairs is the NMR spectrometer. However, it should always be appreciated that, the conditions in the NMR tube are frequently far removed from those in the actual polymerization. Furthermore NMR observes the equilibrium form on a long time scale, and this is not necessarily that form present at the moment of polymerization. 

Ise, N., H. Hirohara, T. Makino, K. Takaya, and M. Nakayama Ionic polymerization under an electric field. XIII. living anionic polymerization of styrene in the Unary mixtures of benzene and dimethoxyethane by the three-state mechanism. Presented at the 17th Discussion Meeting of High Polymers, October, 1968, Matsuyama, Preprint p. 261. 

The most important feature of ionizing radiations is, as the term implies, ionization to give ionic intermediates in irradiated systems. Though radiation-induced radical polymerization had long been studied, it is only a decade since radiation-induced ionic polymerization was first found. In 1957, Davison et al. obtained polymer from isobutene, which is known not to be polymerized by radical catalysts, by irradiating at low temperature with y-rays (7). Before long, the radiation-induced polymerization of styrene was proved to proceed as an ionic mechanism in suitable solvents (2,3,4). Since these pioneering researches, the study of the chemical kinetics of radiation-induced ionic polymerization has been extended to several vinyl, diene and cyclic monomers. 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

Overberger, C. G., G. F. Endres and A. Monaci Ionic polymerization. VII. Relative reactivities of mono- and />-dialkylbenzenes as molecular terminating agents in the cationic polymerization of styrene. J. Amer. chem. Soc. 78, 1969 (1956). 

Styrene-Divinylbenzene Networks. Using ionic polymerization methods, Rietsch et al. (1976) prepared polystyrene (PS) networks with a well-controlled length of elastically active chains and crosslinks of variable functionality. In a given series, the glass transition temperature obeys the classical free volume theory ... 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

About thirty years ago, all cases of polymerization kinetics used to be solved as statinary reactions. Hayes and Pepper [27] were the first to call attention to the non-stationary character of ionic polymerizations. They noticed the premature decay of styrene polymerization initiated by H2S04 (see Fig. 8). This was a simple case of non-stationarity caused by the slow decay of rapidly generated active centres [27, 28]. They assumed that the polymerization proceeds according to a rather conventional scheme represented in simplified form (without transfer) by the reactions... 

Figure 5 Effect of temperature on rate constants of propagation, depropagation, transfer to monomer, transfer to triflate anion, and indan formation in the carbocat-ionic polymerization of styrene (From Ref. 292).

Materials and Polymerization. Styrene and methyl methacrylate were obtained from commercial sources and were distilled to remove inhibitor. After distillation, the monomers were stored, under nitrogen, in a refrigerator. For the mixed emulsifier system, Emulphogene BC840(GAF), tridecyloxy-polyethylene-oxyethanol, was used as the nonionic constituent, and sodium lauryl sulfate (K and K Labs) was used as the ionic constituent. The sodium lauryl sulfate was at a concentration below its cms whereas the BD-840 was at a concentration above its cmc. This emulsifier system has been shown to yield mixed micelles (2)/ having a low ionic change (2)/ which produce latlces with rather narrow particle size distributions (2 ) ... 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Conjugated olefins, like styrene, butadiene, and isoprene, can be caused to polymerize by cationic and anionic as well as by free-radical processes because the active site is delocalized in all cases. The most practical ionic polymerizations for these species are anionic, because such reactions involve fewer side reactions and better control of the diene polymer microstructure than in cationic systems. Free-radical polymerization of styrene is preferred over ionie proeesses, however, for cost reasons. 

The activation energy of anionic propagation in the homopolymerization of styrene was determined to be about 1 kcal. per mole. This value refers to the reaction proceeding in tetrahydrofuran solution. The activation energy for the same reaction in dioxane was reported (1, 2) to be 9 3 kcal. per mole. This is one of many examples which stresses the importance of a solvent in ionic polymerization. 

Most Grignard reagents are inert toward styrene (up to the temperature of spontaneous thermal polymerization). This is a significant difference from lithium alkyls, which are readily able to initiate styrenic monomers [123]. The only reported exception is p-vinylbenzyl magnesium chloride, which polymerized styrene in THF at O C, but not at — 78X [50,51]. Substitution at the puru-position of a phenyl ring may stabilize the benzyl anion, owing to the delocatlization of electrons, and favor ionic dissociation of... 

Chain Homopolymerization Mechanism and Kinetics Free radical and ionic polymerizations proceed through this type of mechanism, such as styrene polymerization. Here one monomer molecule is added to the chain in each step. The general reaction steps and corresponding rates can be written as follows ... 

It is believed that the free positive and negative species annihilate each other immediately on contact. This is borne out by the strict square root relationship which is found between the rates of polymerization and the dose rate of the radiation. The growing chain ends are therefore free in nature, i.e., with no ion-pair component. This makes radiation initiated ionic polymerization an excellent method for studying free ion polymerization examples of the power of this method have been presented for p-methoxy styrene (1 9) and the vinyl ethers (16,20,21). 

Styrene monomer, as normally purified by conventional distillation, has sufficient water present (ca 10 -10 M) to prevent ionic polymerization and consequently only radical polymerization occurs. In an extremely dried sample of styrene (as obtained by distillation from sodium-potassium alloy) the polymerization, however, proceeds entirely by cationic propagation. Ionic polymerizations are generally much faster than free-radical polymerizations. 

In the field of ionic polymerization, we should mention the investigators87,114) who calculated the relative chain transfer constants for the polymerization of styrene in benzene solutions and its mixture with 1,2-dichloroethane on Friedel-Crafts catalysts. 

When the effect of impurities on radiation-induced ionic polymerization is considered, two points should be taken into account. One of them is related to the concentration limit of impurity, which affects the reaction order in dose rate, depends on the nature of the impurity, and is low. For instance, the presence of 10 2 mol/1 of water in the reaction system can suppress the cationic polymerization of styrene. 

---