Propagation steps styrene polymerization

FIGURE 11.10 Chain propagation step in polymerization of styrene. The growing polymer chain has a free-radical site at the benzylic carbon. It adds to a molecule of styrene to extend the chain by one styrene unit. The new polymer chain is also a benzylic radical it attacks another molecule of styrene, and the process repeats over and over again. 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

Many of the synthetic elastomers now made are still polymerized by a free radical mechanism. Polychloroprene, polybutadiene, polyisoprene, and styrene-butadiene copolymer are made this way. Initiation by peroxides is common. Many propagation steps create high molecular weight products. Review the mechanism of free radical polymerization of dienes given in Chapter 14, Section 2.2. 

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. 

The presence of a comonomer has, in certain cases, 9 marked influence on polymerization rate. For example, the mastication of natural rubber in the presence of maleic anhydride, even with small concentrations of the latter, about 5%, leads to accelerated polymerization of styrene monomer (11) either because of its high reactivity in the propagation step of heterochain copolymerization and/or because of a hardening effect. This reaction is discussed later. 

As was stated above, the interpretation that the field affects the dis-sodation state of the growing chain ends was not uniquely substantiated by the experimental data, except those on copolymerizations. Thus it is interesting to investigate the field influence on much simpler systems than cationic homopolymerizations. For this purpose we have chosen living anionic systems in which only propagation steps are involved. The system first studied was a living anionic polymerization of styrene with n-butyllithium in the binary mixtures of benzene and tetrahydrofuran (17,24) and in the binary mixtures of benzene and dimethoxyethane (15). 

The recent study by Overberger and Jarovitzky (37) shows clearly the cationic nature of the propagation step in the polymerization of alpha-D-styrene by Ziegler catalysis. 

Szwarc and coworkers (232) concluded from kinetic studies of sodium catalyzed polymerizations of vinyl pyridine and styrene that propagation involves two consecutive steps. In the first step, monomer complexes with the catalyst. In the rate-determining second step, the complex rearranges to yield product. For styrene polymerization, the steps were formulated as follows ... 

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. 

Typical monomers that may be polymerized by cationic methods include styrene, isobutylene, and vinyl ethers. Unlike radical polymerizations, solvent polarity can influence the rate of polymerization. This is due to the presence of the counterion (see Fig. 15.13). For example, more polar solvents can increase the degree of separation between the growing end and the counterion during the propagation step, increasing the rate of propagation.17... 

Even very small amounts of anhydrous perchloric and triflic acids (<10-3 M) polymerize styrene rapidly and quantitatively [123,126-130], Carbenium ions were initially not detected in these systems, and they were therefore proposed to proceed by a pseudocationic mechanism in which covalent esters react directly with styrene in a concerted muticenter rearrangement [128], However, short-lived carbenium ions have since been detected directly by stopped-flow UV [17-19,131]. The mechanism of the propagation step in these systems is discussed in more detail in Section lV.D.2.a. 

Polymerization reactions proceed via initiation, propagation, and termination steps as illustrated in Section 4.1. A simplified network to describe the styrene polymerization is ... 

Fig. 14. The propagation step in the polymerization of styrene (counter-ion, sodium). Variation of fegpp mole" sec" ) with inverse square root of concentration of active centres. Solvents ( ) tetrahydrofuran 25°C [16] (O) tetrahydropyran 30°C [106] (A) oxepane 30°C [105] ( ) dioxane 25°C [104].

Polymerization is the repetition of an addition propagation step, like the polymerization of styrene to give polystyrene above. The initiator radical adds to a multiple bond to produce another radical that adds again and again until termination occurs. Polymerization is favored by a high concentration of the multiple bond reactant. 

When the reaction occurs in nonpolar solvents, the propagation step is not hampered as much by a tendency of ion pairs to cluster into aggregates, as is encountered in initiation. For instance, in butyllithium-initiated polymerizations of styrene in benzene, the propagation step is much faster than the initiation. This is probably due to an absence of aggregates. Some association between the growing polymeric chains, however, does occur. It may be shown as follows ... 

However, the kinetics of polymerization of butadiene, isoprene, and styrene with alkyllithium initiators has been studied extensively by Hsieh (39c), and the polymerization rates broken down into initiation and propagation steps. The effects of BuLi concentration, the structure of the butyl group, and the solvent type were studied. The rates of initiation (i i) for the olefins mentioned were determined. For dienes, the order was ec-Bu > iso-Pr > iso-Bu > n-Bu > tert-Bu. With n-BuLi the order was styrene > butadiene >... 

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