Vinyl acetate polymerization, branching

One of the most dramatic examples of a solvent effect on propagation taken from the early literature is for vinyl acetate polymerization.78,79 Kamachi el al.n reported a ca. 80-fold reduction in kp (30aC) on shifting from ethyl acetate to benzonilrile solvent (Table 8.1). Effects on polymer structure were also reported. Hatada ef a m conducted a H NMR study on the structure of the PVAc formed in various solvents. They found that PVAc (M n 20000) produced in ethyl acetate solvent has 0.7 branches/chain while that formed in aromatic solvents is essentially unbranched. 

SOLUTION OF KINETIC EQUATIONS FOR LONG CHAIN BRANCHING IN BULK VINYL ACETATE POLYMERIZATION ... 

Graessley and his co-workers have made calculations of the effects of branching in batch polymerizations, with particular reference to vinyl acetate polymerization, and have considered the influence of reactor type on the breadth of the MWD (89, 91, 95, 96). Use was made of the Bamford and Tompa (93) method of moments to obtain the ratio MJMn, and in some cases the MWD by the Laguerre function procedure. It was found (89,91) that narrower distributions are produced in batch (or the equivalent plug-flow) systems than in continuous systems with mixing, a result referrable to the wide distribution of residence times in the latter. 

Investigation has shown that chain transfer to polymer occurs predominantly on the acetate methyl group in preference to the chain backbone one estimate of the magnitude of the predominance is 40-fold (92,93). The number of branches per molecule of poly(vinyl acetate) polymerized at 60°C is ca 3, at 80% conversion. It rises rapidly thereafter and is ca 15 at 95% conversion and 1—2 x 104 number-average degrees of polymerization. 

The development takes into account transfer to monomer, transfer to polymer, and terminal double bond polymerization. For the vinyl acetate system where transfer to monomer is high, the generation of radicals by transfer to monomer is much greater than the generation of radicals by initiation, so that essentially all radicals present have terminal double bonds hence, effectively all dead polymer molecules also have a terminal double bond. Thus, for vinyl acetate polymerization, the terminal double bond polymerization can be significant, and has been built into the development. The equations for the moments of the molecular weight distribution and the average number of branches per polymer molecule are as follows ... 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of dead polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the fiee-radical polymerization of other types of monomers. 

Fig. 1.1. Branching diagram for the vinyl acetate polymerization in a CSTR. [B = 5.458371e3 s, c, / = 16.3, all other constants are as in Teymour, Ray (1991).)...

Tobita H. A simulation-model for long-chain branching in vinyl-acetate polymerization. 2. Continuous polymerization in a stirred-tank reactor. J Polym Sci Part B Polym Phys 1994 32 911-919. 

Backbiting is an intramolecular chain transfer reaction. If transfer reactions occur between different chains, long-chain branched polymers are formed. Well-known examples include ethylene and vinyl acetate. Vinyl acetate polymerization could lead to gel formation under certain conditions. It should be pointed out that chain transfer to polymer reaction alone generates only T-type branch structures that do not result in gel formation. Theoretically, some mechanism such as radical termination by combination that brings two chains together to form H-type branch structures is an essential condition for gelation. 

This equation indicates that branching is negligible at low monomer conversion. For vinyl acetate polymerization, the following equation has also been proposed [73] ... 

Example 7.4 Polymerizing systems such as vinyl chloride and vinyl acetate give branched polymers in emulsion. Find the extent of branching in the second stage. Also, find the relations governing and in the third stage of emulsion... 

Issues to be considered in selecting the best stabilizing system are polymeric chain branching which increases with high temperature and the presence of some stabilizers, polydispersity of the particles produced, and grafting copolymerization, which may occur because of the reaction of vinyl acetate with emulsifiers such as poly(vinyl alcohol) (43,44). 

By designing the repeat unit into the parent diene (containing either an alkyl branch or functionality), only a single type of repeat unit is formed upon polymerization, giving pure polymer microstructures. To date, perfectly controlled ADMET ethylene copolymers have included ethylene-CO,34 ethylene-vinyl alcohol,35 ethylene-vinyl acetate,36 and ethylene-propylene.20 Figure 8.12... 

For example, the parameters g = 0.77, h = 0.94, p = 1.4, and C = 0.158 measured for a polymer sample and compared with the plots in Figures 7.11 through 7.13 were most consistent with athree-arm star monodisperse polymer a poly disperse three-arm star would have g= 1.12,/ = 1.05,p= 1.6, and C close to 0.2. °° The second example was poly(vinyl acetate) (PVAc) prepared by emulsion polymerization. Since no data for linear equivalent were available, g and h were not calculated. At lower conversion/MW p= 1.84 was found, only slightly higher than the theoretically expected p = 1.73 for a randomly branched architecture, p slightly decreased with increasing M, indicating... 

Under conditions of low radical initiation, Graessely (21) has shown that the following set of equations describes the molecular weight and branching development in the bulk polymerization of vinyl acetate ... 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

As a result, the formation of long branches in the polymerization of vinyl acetate by the free-radical mechanism is better understood than in the polymerization of any other monomer, though there is even so still some disagreement about the transfer coefficient with the polymer, and also in estimates of the proportion of the total branching that takes place through acetate groups in either monomer or polymer. However, the state of knowledge is still less satisfactory for other monomers such as ethylene or vinyl chloride. 

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