Vinyl polymers, polymerization kinetics

This monomer polymerizes faster ia 50% water than it does ia bulk (35), an abnormaHty iaconsistent with general polymerization kinetics. This may be due to a complex with water that activates the monomer it may also be related to the impurities ia the monomer (eg, acetaldehyde, 1-methyl pyrroHdone, and 2-pyrroHdone) that are difficult to remove and that would be diluted and partitioned ia a 50% aqueous media (see Vinyl polymers, A/-VINYLAMIDE POLYPffiRS). 

For less polar monomers, the most extensively studied homopolymerizations are vinyl esters (e.g. VAc), acrylate and methacrylate esters and S. Most of these studies have focused wholly on the polymerization kinetics and only a few have examined the mierostructures of the polymers formed. Most of the early rate data in this area should be treated with caution because of the difficulties associated in separating effects of solvent on p, k and initiation rate and efficiency. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

The polymerization kinetics of alkali salts of living vinyl polymers In ethereal solvents, such as tetrahydrofuran CD, tetrahydropyran (2), dlmethoxyethane Q), oxepane (4) and dloxane... 

In another variant of the kinetic method, the shapes of curves of Mn, Mw, or [ /], versus conversion in batch polymerization may be used to obtain transfer coefficients, both with monomer and with polymer this procedure has been used by Wheeler (142), Graessley (143), and others, to obtain transfer coefficients for vinyl acetate polymerization (Section 11). 

The information available up to 1965 on the polymerization kinetics of vinyl acetate has been reviewed by Lindemann 184% who collected data on the transfer coefficients with monomer and polymer, which may be denoted as Clm and Clp respectively. These are the ratios of rate constants for attack on monomer or polymer (per monomer unit) to the propagation rate constant they are the... 

Conix, A., and G. Smets Benzoyl peroxide initiated polymerization kinetics of vinyl monomers in various solvents. J. Polymer. Sci. 10, 525 (1953). 

One way to distinguish between the two possibilities is to study the isotope effect on the kinetics of vinyl acetate polymerization and on the polymer molecular weight. The deuterium isotope eftect has been ascribed to the difference in the zero point energies of the stretching vibrations of the C-H and C-D bond (11). The rate of a reaction in which deuterium is transferred is slower than that of the corresponding reaction for hydrogen, since the C-D bond has a lower zero point energy. 

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. 

This chapter describes the coordination polymerization of acyclic and cyclic vinylic monomers, conjugated dienes, and polar vinylic monomers with the most important catalytic systems known in this area. A chronological classitication for the development of the main coordination catalyst types is outlined, as well as polymerization kinetics and mechanisms and applications of polymers obtained through different metallic complexes. 

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. 

Most commonly, in the emulsion polymerization of vinyl acetate, anionic surfactants are used either alone or in combination with a protective colloid. Typical examples of surfactants which have found application are Aerosol OT (sodium dioctylsulfosuccinate), alkyl aryl sulfonate salts (e.g., Santomerse-3), sodium lauiyl sulfate, etc. A study of the kinetics of the vinyl acetate polymerization in the presence of sodium lauryl sulfate indicated that the rate of polymerization was proportional to the square root of the initiator concentration and the 0.25th power of the number of particles. The number of particles were proportional to the 0.5th 0.05 power of the surfactant concentration but independent of the level of potassium persulfate. The intrinsic viscosity of the final polymer was said to be independent of the initiator concentration and of the munber of polymer particles. These observations were said to suggest that the mechanism of the vinyl acetate polymerization in emulsion resembles that of vinyl chloride [153]. 

Cationic surfactants, in contrast to anionic surfactants, usually reduce both the number of particles involved in the polymerization and the rate of polymerization. The nature of the stabilizing emulsifier has a marked effect on the polymerization kinetics. For example, addition of a non-ionic stabilizer [e.g., poly(vinyl alcohol), a block copolymer of carbowax 6000 and vinyl acetate, or ethylene oxide-alkyl phenol condensates] to a seed polymer stabilized by an anionic surfactant decreased the rate of polymerization to 25% of the original rate. The effect was as if the nonionic stabilizer (or protective colloid) acted as a barrier around the seed particles to alter the over-all kinetics. It may be that the viscosity of the medium in the neighborhood of the nonionic surfactant coating of the polymer particle is sufficiently different from that of an anionic layer to interfere with the diffusion of monomer or free radicals. There may also be a change in the chain-transfer characteristics of the system [156]. 

In the case of vinyl chloride, the kinetics of miniemulsion polymerization is complicated by the fact that almost from the beginning of the reaction, two reaction zones inside the droplets need to be considered (1) a practically pure monomer phase of decreasing volume, and (2) a precipitated, monomer-swollen polymer phase which increases in volume with conversioa The general expression for the rate of polymerization (moles dm H2O) is... 

Prior to Harwood s work, the existence of a Bootstrap effect in copolymerization was considered but rejected after the failure of efforts to correlate polymer-solvent interaction parameters with observed solvent effects. Kamachi, for instance, estimated the interaction between polymer and solvent by calculating the difference between their solubility parameters. He found that while there was some correlation between polymer-solvent interaction parameters and observed solvent effects for methyl methacrylate, for vinyl acetate there was none. However, it should be noted that evidence for radical-solvent complexes in vinyl acetate systems is fairly strong (see Section 3), so a rejection of a generalized Bootstrap model on the basis of evidence from vinyl acetate polymerization is perhaps unwise. Kratochvil et al." investigated the possible influence of preferential solvation in copolymerizations and concluded that, for systems with weak non-specific interactions, such as STY-MMA, the effect of preferential solvation on kinetics was probably comparable to the experimental error in determining the rate of polymerization ( 5%). Later, Maxwell et al." also concluded that the origin of the Bootstrap effect was not likely to be bulk monomer-polymer thermodynamics since, for a variety of monomers, Flory-Huggins theory predicts that the monomer ratios in the monomer-polymer phase would be equal to that in the bulk phase. 

Most vinyl and acrylic compounds more or less obey ideal polymerization kinetics during free radical polymerization. Drastic deviations, however, occur for allyl polymerizations since a chain termination by monomer dominates in this case [see Equation (20-41)]. For this case, the formation of polymer free radicals is given analogously to Equation (20-51) as... 

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