Styrene polymerization chain transfer

Mayo, F. R. Chain transfer in the polymerization of styrene. VIII. Chain transfer with bromobenzenc and mechanism of thermal initiation. J. Am. Chem. Soc. 75, 6133 (1953). 

The concept of PO macroinitiators centers on the introduction of an initiation moiety into an olefinic polymer chain for polymerization. The most effective route for preparing PO macroinitiators is by employing functional polyolefins containing hydroxyl groups or other reactive groups. These functional POs are prepared by copolymerization of olefins with functional monomers and post-polymerization reaction, as mentioned above. In the case where an initiation moiety was at the chain-end of the polyolefins, a block type copolymer is produced. It has been reported that thiol-terminated PP was used as polymeric chain transfer agent in styrene and styrene/acrylonitrile polymerization to form polypropylene-b/odc-polystyrene (PP-b-PS) and polypropylene-btock-poly(styrene-co-acrylonitrile) (PP-b-SAN) block copolymer [19]. On the other hand, polymer hybrids with block and graft structures can be produced if initiation moieties are in the polymer chain. 

Ignatz-Hoover, F., Petrukhin, R., Karelson, M. and Katritzky, A.R. (2001) QSRR correlation of free-radical polymerization chain-transfer constants for styrene. J. Chem. Inf. Comput. Sci., 41, 295-299. 

For example, methyl methacrylate block copolymers are much less studied than those of styrene. Anion chain transfer occurs at the pendent ester group, drastically reducing the yield of block copolymers. Poly(methyl methacrylate-b-isoprene) has been prepared, however, by using an ingenious chain cap of l,l -diphenylethyl-ene(27,28). i l diphenylethylene will not anionically homopolymerize, therefore it adds only one mer to the macroanion. This anion is more stable in the presence of methyl methacrylate, but will initiate further polymerization. Other workers have reported the preparation of isoprene-methyl methacrylate block copolymers by sequential addition to "living" polyisoprene anions(29,30),... 

Table 9 summarizes two sets of propylene polymerizations using the rac-Mc2Si [2-Me-4-Ph(Ind)]2ZrCl2/MAO catalyst in the presence of p-MS/hydrogen and styrene/hydrogen chain transfer agents to form p-MS-terminated PP (PP-t-p-MS) and styrene-terminated PP (PP-t-St) polymers [56]. The in-situ chain transfer... 

Table 9 Two sets of the experimental results in rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO-catalyzed polymerization of propylene with p-MS/hydrogen and styrene/hydrogen chain transfer agents...

The polymerization of styrene in liquid ammonia, initiated by potassium amide, was one of the first anionic polymerizations to be studied in detail. In this polymerization chain transfer to ammonia terminates the growth of polymer chains and living polystyrene is not formed. The reaction is of minor importance nowadays but will be briefly considered since it provides a further example of kinetics analysis. 

In 1956 Szwarc and his associates discovered an anionic polymerization of styrene without chain transfer and termination , and this reaction was called living polymerization. Subsequent studies revealed a number of examples of living anionic systems . In the field of cationic polymerization, two research groups reported in 1965 that the propagating species in ring-opening polymerization of tetrahydro-furan have an unusually extended lifetime . ... 

This suggests that polymerizations should be conducted at different ratios of [SX]/[M] and the molecular weight measured for each. Equation (6.89) shows that a plot of l/E j. versus [SX]/[M] should be a straight line of slope sx Figure 6.8 shows this type of plot for the polymerization of styrene at 100°C in the presence of four different solvents. The fact that all show a common intercept as required by Eq. (6.89) shows that the rate of initiation is unaffected by the nature of the solvent. The following example examines chain transfer constants evaluated in this situation. 

Table 4. Chain-Transfer Constants in Free-Radical Styrene Polymerization...

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Through a study of the influence of thiophene and other aromatic compounds on the retardation and chain transfer on the polymerization of styrene by stannic chloride, the relative rates of attack of a carbonium-ion pair could be obtained. It was found that thiophene in this reaction was about 100 times more reactive than p-xylene and somewhat less reactive than anisole. ... 

The first unequivocal proof of feasibility of homogeneous polymerization free of termination and chain-transfer was reported in 1956, and the concept of living polymers and its ramifications were fully developed in those publications 2). Although the initial work dealt with anionic polymerization of styrene and the dienes, the underlying ideas were soon applied by other workers to a great variety of polymeric systems, as shown by the brief review of some of those developed recently. 

Peaking and Non-isothermal Polymerizations. Biesenberger a (3) have studied the theory of "thermal ignition" applied to chain addition polymerization and worked out computational and experimental cases for batch styrene polymerization with various catalysts. They define thermal ignition as the condition where the reaction temperature increases rapidly with time and the rate of increase in temperature also increases with time (concave upward curve). Their theory, computations, and experiments were for well stirred batch reactors with constant heat transfer coefficients. Their work is of interest for understanding the boundaries of stability for abnormal situations like catalyst mischarge or control malfunctions. In practice, however, the criterion for stability in low conversion... 

---