Allyl acetate polymerization, chain transfer

In the polymerization of vinyl acetate, chain transfer to the acetate portion of the monomer is an important aspect of the process (see Chapter 7 of this volume). In the case of the allyl acetate polymerization, chain transfer to the acetate moiety is considered negligible as compared to the degradative chain-transfer process [14, 15]. 

An especially interesting case of inhibition is the internal or autoinhibition of allylic monomers (CH2=CH—CH2Y). Allylic monomers such as allyl acetate polymerize at abnormally low rates with the unexpected dependence of the rate on the first power of the initiator concentration. Further, the degree of polymerization, which is independent of the polymerization rate, is very low—only 14 for allyl acetate. These effects are the consequence of degradative chain transfer (case 4 in Table 3-3). The propagating radical in such a polymerization is very reactive, while the allylic C—H (the C—H bond alpha to the double bond) in the monomer is quite weak—resulting in facile chain transfer to monomer... 

Isopropenyl acetate and allyl chloride behave similarly. In the polymerization of the latter monomer degradative chain transfer occurs more readily by removal of the chlorine atom to yield the unsubstituted allyl radical CH2—CH—CH2, which manages to add monomer occasionally. This is indicated by the formation of about three polymer molecules, having an average degree of polymerization of six units, for each molecule of benzoyl peroxide decomposing. 

Anomolous results have been observed in some emulsion polymerizations—inverse dependencies of N, Rp, and Xn on surfactant concentration. Some surfactants act as inhibitors or retarders of polymerization, especially of the more highly reactive radicals from vinyl acetate and vinyl chloride [Okamura and Motoyama, 1962 Stryker et al., 1967]. This is most apparent with surfactants possessing unsaturation (e.g., certain fatty acid soaps). Degradative chain transfer through allyl hydrogens is probably quite extensive. 

In the polymerization of allyl acetate, transfer to monomer produces an unreactive radical which fails to re-initiate growth of the polymer chain. Bartlett and Tate (31) compared the rates of polymerization for the unlabelled monomer and.the deuterated monomer CH2 CH-CD2 0-CO CH3. The deuterated monomer polymerized more rapidly giving a product of higher molecular weight. These observations suggest that the rate of polymerization and the molecular weight of the polymer are controlled by the reaction... 

Russian workers have proposed that the increased activity of allyl acetate and allyl alcohol in free radical or gamma ray initiated polymerization in the presence of zinc chloride may be connected with the decreased degradative chain transfer with complexed monomer or the activation of the stabilized allyl radical in the complexed monomer—i.e., the conversion of degradative chain transfer to effective transfer (55, 87). However, these explanations have been partially rejected as inadequate. 

Deactivating chain transfer to monomer is quite common in polymerization of allyl monomers [40-42], Allyl radicals such as that of allyl acetate are resonance-stabilized, with the result that polymerization rates and molecular weights remain low. Moreover, with chain transfer as the dominant termination mechanism, the termination rate is first order in free radicals. This lets the free-radical population become proportional to the initiator concentration and leads to a polymerization rate that is first order rather half order in initiator and zero order in monomer. 

It is also to be expected that pressure will affect the rate of chain transfer reactions to monomer, polymer, and solvent. In the polymerization of allyl acetate, where degradative chain transfer to monomer occm s, the rates of the propagation and transfer reactions increase by about the same amoimt for a given increase in pressure (17). The transfer reaction becomes less degradative—i.e., the allyl acetate radicals become more reactive—as pressure is increased. 

Chain transfer to monomer was described earlier in general terms for vinyl monomers [cf. Eq. (6.138)]. Such reactions are particularly favored with allylic monomers such as allylic acetate which have the structure CH2=CH-CH2X with a C-H bond alpha to the double bond described as an allylic C-H. The propagating radical in the polymerization of such monomers is very reactive, while the allylic C-H bond in the monomer is quite weak, resulting in facile chain transfer to monomer ... 

Even though monomers like allyl acetate do not polymerize rapidly nor produce products of high molecular weight, with active chain-transfer agents good yields of addition products may be isolated [28]. Among the chain-transfer agents proposed for such systems are chloroform and carbon tetrachloride. Sakurada and Takahashi [29], formulated the reaction as follows ... 

The solution polymerization of allyl acetate was studied in an effort to determine the effect of monomer concentrations on the reaction kinetics [14]. These studies were limited to the use of benzene. The growing allyl acetate radicals formed stable adducts with the solvent much as vinyl acetate does. The stabilized adduct is terminated by combination with a growing radical. The twinning reaction is said to account for the relatively high molecular weight of the polymer despite the fact that the reaction with benzene is a chain-transfer reaction. Ethyl acetate, on the other hand, would have been a preferable solvent... 

In another paper, Zubov and co-workers [59] observed that ordinarily, radiation-initiated polymerizations of allyl acetate proceed sluggishly to low-mole-cular-weight products. However, upon the addition of phosphoric acid to the system, the rate of polymerization increases and solid polymers can be isolated. The proposed mechanism for this process involves the postulation of the formation of oligomers of allyl acetate with residual double bonds to which the phosphoric acid adds. This is thought to activate the pol5merization of the oligomer. Other possible hypotheses for the activation by phosphoric acid involves chain-transfer reactions or the formation of complexes of phosphoric acid with allyl acetate monomer and allyl acetate fiee radicals [59]. 

Compounds possessing allylic structures polymerize by free-radical mechanism only to low molecularweight oligomers. In some cases the products consist mostly of dimers and trimers. The DP for poly(allyl acetate), for instance, is only about 14. This is due to the fact that allylic monomer radicals are resonance-stabilized to such an extent that no extensive chain propagations occur. Instead, there is a large amount of chain transferring. Such chain transferring essentially terminates the reactions [151]. The resonance stabilization can be illustrated on an allyl alcohol radical ... 

Besides vinyl acetate monomer, three other components are neeessary to earry out an emulsion polymerization water, an emulsifier and/or a proteetive eolloid, and a water-soluble initiator. Most commonly, anionic long-chain alkyl sulfonates are used as surfactants in amounts up to 6%. Studies have shown that the rate of polymerization is dependent on the amoimt of emulsifier present, with the rates inereasing as the amoimt of emulsifier is increased up to a certain point and then falling olF as free-radieal ehain transfer to the surfaetant beeomes a serious competing side reaetion [240]. In general, surfactants are used in eombination with a protective colloid. Especially useful as protective colloids are poly(vinyl alcohol), hydroxyethyl cellulose, alkyl vinyl ether-maleic anhydride and styrene-allyl alcohol copolymers, and gum arable. Water-soluble initiators, particularly potassium persulfate, alkali peroxydisulfates, hydrogen peroxide, and various redox systems, are most commonly used. 

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