Branching in Free-Radical Polymerization

The principal mechanism involved is transfer by attack of the growing radical, R, with polymer  

The group X transferred may be H or halogen. Attack on the CH2 groups in the polymer, or on pendent groups, such as — COO CH3 in poly (vinyl acetate), would produce similar effects in the case of poly (vinyl acetate) branches attached via the acetate group can be removed by hydrolysis, unlike those formed by attack on hydrogen atoms linked to main chain carbons. 

Another mechanism, conceivable with most monomers, and believed to occur in vinyl acetate polymerization (see Section 10), is transfer with the monomer forming a polymer with an unsaturated end-group capable of copolymerizing, e.g.  

There is little evidence for branching by transfer to monomer in other polymerizations. 

The nature of the product is considerably dependent on the type of reactor and the mixing conditions within it. Thus, to take the two simplest cases, in a perfectly-stirred batch reactor, when branching is due solely to transfer with the polymer the mean number of branches per monomer unit in the polymer, q, is given by  

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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. 

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

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... 

Figure 6.9 Mechanism of ethyl and butyl branching in free-radical polymerization of ethylene. (After Ghosh, 1990.)...

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. 

The great number of possible elementary reactions in free radical polymerizations explains why only a fraction of the many thermodynamically polymerizable groups can be converted free radically to un-cross-linked high-molar-mass polymers. Vinyl, vinylidene, and acrylic compounds, as well as some strained saturated rings, belong to this fraction. Allyl compounds only polymerize to branched oligomers, but diallyl and triallyl compounds produce high-molar-mass cross-linked networks. 

In free radical polymerization a material rich in syndiotactic sequences, which can partially crystallize, is produced. The tendency toward crystallization is increased by polymerization at low temperature, since products produced at 50°C possess one branch point per 30 monomeric units, but polymers produced at — 60°C are practically unbranched. To obtain a polymer with a softening point 10-15°C above that of the conventional PVC, advantage is taken industrially of the low-temperature polymerization. Here, a prepolymerization at 60°C is carried out to a yield of up to 10 % and the resulting mixture is then further polymerized to a yield of up to 65% at — 15°C after addition of 10% methanol and the initiator system of H202/Fe(II)/ascorbic acid. 

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). 

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. 

Herein is reported a summary of recent studies of diffusion-controlled termination and propagation reactions in free-radical polymerization which are pertinent to emulsion polymerization kinetics. Also included are discussions of the effects of diffusion-controlled termination and propagation on molecular weight and branching development with particular reference to the synthesis of poly (vinyl chloride) at high conversions and to the significant reduction of thermal stability of PVC which occurs. 

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. 

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