Chain Branching in Free-Radical Polymerization

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

Classification of Polymers Properties 1223 Addition Polymers A Review and a Preview 1225 Chain Branching in Free-Radical Polymerization 1227 Anionic Polymerization Living Polymers 1230 1 Cationic Polymerization 1232... 

Wolf, C., Burchard, W. Branching in free radical polymerization due to chain transfer, application to poly(vinyl acetate). Makromol. Chem. 177, 2519-2538 (1976)... 

If the reactions involved in free-radical polymerization proceeded precisely in the maimer outlined above (initiation, propagation, termination), in all instances, one would obtain long and linear polymer chains. However, in free radical polymerization, linear chains are, in fact, not formed. Instead, extensive branching is observed particularly with monomers such as ethylene. In this context a slight clarification of branching is required. Thus, for a polymer obtained from a substituted vinyl monomer regular side groups are present (Fig. 2.3). 

The main topic of interest is the properties of molecules of finite size, having no large rings, and in general having trifunctional branch-points. These are typically produced by chain-transfer with polymer in free-radical polymerizations, though they can of course be made in other ways. Molecules with branch-points of higher functionality are also of interest, especially star-shaped molecules with several arms, as these are both easy to synthesize and relatively easy to discuss theoretically. 

Finally, in the last Chap. E the more complex reactions are treated which are observed in free-radical polymerization and in vulcanization of chains. In the course of branching the experimentalist is often confronted with inhomogeneities in branching and chain flexibility and with chemical heterogeneity and steric hindrance due to an overcrowding of segments in space. Some of these problems of great practical importance have been solved in the past and are briefly reported. 

Chain branching occurs in cationic polymerization much as it does in free-radical polymerization. Propose a mechanism to show how branching occurs in the cationic polymerization of styrene. Suggest why isobutylene might be a better monomer for cationic polymerization than styrene. 

As in so many things in this field, if you want to work through the arguments yourself, you cannot do better than go to Flory— see Principles of Polymer Chemistry, Chapter EX. Stockmayer s equation illustrates the point we wish to make with dazzling simplicity as f the number of branches, increases, the polydispersity decreases. Thus for values of/equal to 4, 5 and 10, the polydispersity values are 1.25, 1.20 and 1.10, respectively. Note also that for / = 2, where two independent chains are combined to form one linear molecule (Figure 5-28), the polydispersity is predicted to be 1.5. Incidentally, an analogous situation occurs in free radical polymerization when chain termination is exclusively by combination. 

Branching is a special case of a process called chain transfer that is operational in free-radical polymerization. Chain transfer simply means the transfer of the radical from the growing polymer chain to another speeies. Effectively chain transfer eurtails polymer growth. For example, ehlorin-ated solvents are efiBcient ehain transfer agents (see Eq. 2.12). 

Stockmayer, W.H.J., 1945. Distribution of chain lengths and compositions in copolymers. Chem. Phys. 13,199-207. Tobita, H., 1993. Molecular weight distribution in free radical polymerization with long-chain branching. J. Polym. Sci. B Polym. Phys. 31, 1363-1371. 

No bimolecular termination reactions - termination by combination or disproportionation - as observed in free-radical polymerization take place with coordination catalysts. Some catalysts, under certain polymerization conditions, may polymerize dead polymer chains containing terminal vinyl unsaturations, leading to the formation of chains with long-chain branches. We will discuss the mechanism of long-chain branch formation with coordination catalysts in Section 8.3.4. 

The analysis of expressions (3.16) shows that side branches formed in anionic polymerization are much shorter than the backbone. Thus, for typical values (P = 10 and Cp = 10 ), the ratio ib/is is about 5 at x = 0.9 and increases up to about 20 by full conversion. On the other hand, in free radical polymerization with short-living active centers, the average lifetimes of active centers in the side and main chains are equal, that is, Ib Is. 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

One method is to measure chain-transfer coefficients with low-MW analogues of the polymer. Thus Gilchrist (140) measured the rate at which 14C labelled decane was incorporated into polyethylene in the free-radical polymerization, and hence obtained an estimate of the transfer coefficient with methylene groups this was in fair agreement with another estimate obtained from the effect of the addition of fractions of linear polyethylene on the Mn of the branched polyethylene, which could be separated from linear polymer plus grafted branched polymer by column extraction. Low MW polymer may be used as a transfer agent Schulz and co-workers (189) obtained chain-transfer coefficients in styrene polymerization from the effect of added low MW polymer on Mn. 

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