Vinyl transfer rate constant

We originally suggested - that trauisfer to the polyvinyl alcohol could explain the effect of Elvanol 32-22 in reducing the rate of polymerisation of vinyl acetate solutions. The importance of this transfer reaction has recently been confirmed by Dimonie et al.(6) who use ferrous ion/hydrogen peroxide redox Initiation. However Okamura, Yamashlta, and Motoyama determined the transfer rate constant of vinyl acetate to polyvinyl acetate and polyvinyl alcohol in homogeneous solution they found for... 

A Smith-Ewart case 1 behavior in 1) can be observed during emulsion polymerization of monomers with a high chain-transfer rate constant such as vinyl chloride because the monomer radicals have a high tendency to escape from the particles. Especially for vinyl chloride emulsion polsunerization, Ugelstad and Hansen (113) derived the following equation 18 to calculate n, which predicts dependence, h a... 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

A series of theoretical studies of the SCV(C)P have been reported [38,40,70-74], which give valuable information on the kinetics, the molecular weights, the MWD, and the DB of the polymers obtained. Table 2 summarizes the calculated MWD and DB of hyperbranched polymers obtained by SCVP and SCVCP under various conditions. All calculations were conducted, assuming an ideal case, no cyclization (i.e., intramolecular reaction of the vinyl group with an active center), no excluded volume effects (i.e., rate constants are independent of the location of the active center or vinyl group in the macromolecule), and no side reactions (e.g., transfer or termination). 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

Fig. 19. Relationship between ethanol content, rate constant (ft), and viscosity of solution in the electron-m transfer reaction for poly(N-vinyl-3 2-methylimidazole)-Co(III) Fe-edta2 87 ...

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... 

FIGURE 6. trans-Stilbene fluorescence quenching rate constants as a function of diene (O) and vinyl ether (A) ionization potential. Dashed line is that obtained by Rehm and Weller (83a) for electron-transfer quenching. 

About 5% of the ethyl radical adduct was isolated in the reaction with r-butyl iodide and about 20% of the ethyl radical adduct was formed in the reaction with i-propyl iodide. These ratios compare quite well with those calculated by using the rate constants for halogen transfer in the above reference and the rate constants for addition of a primary radical to methyl vinyl ketone. 

Thus, a relatively large isotope effect can be expected if deuterated vinyl acetate is used for polymerization. Since the degree of polymerization for vinyl acetate equals k /k j the ratio of the rate constant for propagation to that tor chain transfer, the degree of polymerization should express the full isotope effeet. It known that in emulsion polymerization,... 

The rate of polymerization in emulsion polymerization is proportional to kg-, where kg is the fhain transfer step on the vinyl group (10). Substituting trideuterovinyl acetate for vinyl acetate raised the rate by a factor of 1.76. When the calculation for the isotope effect on rate is done accurately, taking into account the 3% H on the trideuterovinyl, we find that if the effect is purely on k3, the rate should rise by a factor of 1.69 as compared to 1.76 . 02. This is almost within the experimental error. There may be a very slight secondary isotope effect (23,24) on the propagation and re-ini tation rate constants k2 and k, but it cannot be decided from these data. 

In recent years, considerable work has been devoted to polymerization reactions of vinyl monomers at higher conversions which permit useful quantitative interpretation of the results. A useful review of studies of the gel effect, chain transfer reactions, and new theoretical postulates and studies at elevated conversions has been presented by Gladyshev and Rafikov (24). This accumulated work has demonstrated the effects of conditions at elevated conversions not only on the termination rate constant, but on initiator efficiency, propagation rate constants, and therefore, the concentration of macroradicals. A rigorous quantitative theory, however, has not yet been developed. 

The high rate constants for chemical quenching of triplet ketones by amines provide two sidelights of considerable importance. Photoinitiation of polymerization has received widespread and varied industrial applications. One problem is that many vinyl monomers quench triplet ketones very rapidly either by charge transfer or energy transfer mechanisms, without forming any radicals. Most solvents cannot compete with the olefins for the triplet ketone. However, tri-ethylamine quenches at rates close to diffusion-controlled, so that radical formation and polymerization initiation are quite efficient 158>. 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

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