Degradative chain transferring

One contributing factor, which seems to have been largely ignored, is that the ring closed radical (in many cases a primary alkyl radical) is likely to be much more reactive towards double bonds than the allyl radical propagating species. This species will also have a different propensity for degradative chain transfer (a particular problem with allylamines and related monomers - see 6.2.6.4) and other processes which complicate polymerizations of the monoencs. 

Chain transfer, the reaction of a propagating radical with a non-radical substrate to produce a dead polymer chain and a new radical capable of initiating a new polymer chain, is dealt with in Chapter 6. There are also situations intermediate between chain transfer and inhibition where the radical produced is less reactive than the propagating radical but still capable of reinitiating polymerization. In this case, polymerization is slowed and the process is termed retardation or degradative chain transfer. The process is mentioned in Section 5.3 and, when relevant, in Chapter 6. 

Transfer to monomer is of particular importance during the polymerization of allyl esters (113, X=()2CR), ethers (113, X=OR), amines (113, X=NR2) and related monomcrs.iw, 8, lb2 The allylic hydrogens of these monomers arc activated towards abstraction by both the double bond and the heteroatom substituent (Scheme 6.31). These groups lend stability to the radical formed (114) and are responsible for this radical adding monomer only slowly. This, in turn, increases the likelihood of side reactions (i.e. degradative chain transfer) and causes the allyl monomers to retard polymerization. 

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. 

The free-radical crosslinking polymerization can be regarded as a special example of specific diffusion control, in which the tendency to microgel formation and decrease of apparent reactivity of Internal double bonds depends on the size of the mlcrogel which in turn depends on the molecular weight of the primary chain. Polymerization of diallyl monomers exhibits much less of these features (W) because the degree of polymerization of their primary chains is extremely low due to degradative chain transfer. 

Exchange Reactions In Hydroxylic Media. Compounds 1 and 2 (Scheme 4) Interconyert readily at room temperature under acid catalysis. The equilibrium fayors the latter. Only 4.0% of 1 (R =Me) forms from 2 In excess MeOH. Unblocked aldehyde (Scheme 4) 1s observable (GC, NMR) under certain conditions as an unstable Intermediate In the aqueous hydrolysis of 1 to 2 (R=H). It Is not detectable In the IR or NMR spectrum of 2. Although k1net1ca lly accessible, the aldehyde Is thermodynamically disfavored. As a result, the degradative chain transfer and rapid a1r oxidation observed with unblocked aldehyde containing monomers and polymers (10) 1s avoided. 

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... 

The low reactivity of a-olefins such as propylene or of 1,1-dialkyl olefins such as isobutylene toward radical polymerization is probably a consequence of degradative chain transfer with the allylic hydrogens. It should be pointed out, however, that other monomers such as methyl methacrylate and methacrylonitrile, which also contain allylic C—H bonds, do not undergo extensive degradative chain transfer. This is due to the lowered reactivity of the propagating radicals in these monomers. The ester and nitrile substituents stabilize the radicals and decrease their reactivity toward transfer. Simultaneously the reactivity of the monomer toward propagation is enhanced. These monomers, unlike the a-olefins and 1,1-dialkyl olefins, yield high polymers in radical polymerizations. 

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 diagram the main reaction is represented by A and degradative chain transfer by... 

When chain transfer occurs, xn << vn, and the molecular weight of the resulting polymer will be lowered by the chain transfer process. If the rate of the chain transfer addition process, ka, is less than the propagation rate kp then the overall polymerization rate Rp will decrease as a result of chain transfer. This is sometimes called degradative chain transfer. If ka << kp, then the degradation can become so severe as to result in inhibition (Fig. 2). 

Monoallylic compounds do not form homopolymers of high molar mass under free-radical initiation because of the low activation of the double bond and the so-called degradative chain transfer, resulting from the high reactivity of hydrogen atoms, which tends to terminate chain growth. 

The drawback of allylic, acrylic and vinylic polymerizable groups is their tendency to ho-mopolymerize. Allylic derivatives, furthermore, are susceptible to degradative chain transfer. 

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. 

Allylic transfer is also variously named degradative chain transfer, autoinhibition, or allylic termination. The stable radical derived from the monomer by reactions like (6-90) are slow to reinitiate and prone to terminate. Low-molecular-weight products are therefore formed at slow rates and small concentrations of allyl monomers can inhibit or retard the polymerization of more reactive monomers. 

The reluctance of olefins like propylene or isobutene to form high polymers in free radical reactions is a result of degradative chain transfer of allylic hydrogens. 

In 1969 Penczek and Kubisa [59] reported an exhaustive study of the kinetics and mechanism of BCMO polymerization initiated by the (i-C4H9 )j AI/H2O system and carried out in chlorobenzene solution at 55—95°C. This system produced homogeneous conditions for the polymerization and the whole process could be described as a non-stationary reaction with slow initiation, fast propagation, and slow degradative chain transfer to polymer. 

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. 

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