Propagation allyl polymers

N,N -methylenebisacrylamide and ethyleneglycoldiacrylate are both examples of crosslinkers which yield crosslinks of functionality four, if both vinyl groups are reacted. Trimethylolpropane triacrylate (TMPTA) and triallylamine have a theoretical functionality of six. However, if only two of the vinyl or allyl groups are actually used, perhaps due to steric problems in propagating a polymer chain through the crosslink as it forms [49], the functionality of the crosslink would be four. 

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. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

As may be seen from Fig. 3, there are no resonance peaks at 120-128 ppm characteristic of 1,4-microstructure in polybutadiene polymer. However, on addition of methanol to the chain live ends, resonance peaks at 120-128 ppm appear in ratios of 60% trans-1,4, 14% m-1,4, and 26% 1,2. This suggests that the protonation of the chain live ends with methanol is an independent reaction and does not relate to the actual structure of the propagating species. It may be said that the structure of the allylic lithium of polybutadiene (DP > 1) is postulated to exist in the 1,2-form (13). Yet hydrolysis of 13 gives mixed 1,4- and 1,2-microstructures. 

The polymerization of dienes with 7r-allylic complexes is of great interest since the complexes may be considered as model compounds for a propagating polymer chain. 

Methyl methacrylate (Fig. 1-4) and methacrylonitrile (6-5) are allylic-type monomers that do yield high molecular weight polymers in free radical reactions. This is probably because the propagating radicals are conjugated with and stabilized to some extent by the ester and nitrile substituents. The macroradicals are... 

A few years later, Eckard et al. studied its polymerisation by tropylium hexachlor-oantimonate and showed that only with very high salt concentrations was it possible to obtain complete conversion. Chain termination was found to be an important reaction probably because of the formation of stable, non-propagating carbenium ions as end groups. No attempt was made to characterise an addition product between monomer and initiator. The development of a red colour during the polymerisation was also reported and attributed to a stable tropylium-indene carbenium ion. This interpretation did not take into account a simpler alternative explanation which Prosser and Young put forward a year later as a feature common to all cationic polymerisations involving indene. It consists in the formation of an allylic-type carbenium ion formed in a termination reaction between an active species and an unsaturated polymer molecule (hydride-ion transfer). This type of termination is now well known in the cationic polymerisation of several monomers ... 

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. 

Cationic -allylnickel complexes polymerize 1,3-butadiene to produce the cis- 1,4-polymer. Taube investigated the polymer growth via smooth and selective insertion of the diene into the -allyl-Ni bond of the growing polymer, both from experimental and theoretical aspects. The reaction catalyzed by the cationic Ci2-allylnickel(II) complex shows kinetics that agree with a chain propagation transfer model [67]. The reaction mechanism of the cis-1,4-polymerization using technical Ni catalysts is also discussed [68]. He compared the mechanism of the reaction catalyzed by allylnickel complexes [69]. 

Kormer, Lobach and others were the first to suggest an alternative process which assigned a dominant role to allyl isomerization. In this mechanism the process begins with biden-tate coordination, followed by formation of an allyl complex. If this allyl complex has a sufficient lifetime, it will isomerize from the less stable, initially formed, anti allyl 3 to the more stable syn species, 4. As shown below, leads to trans polymer. On the other hand the propagation steps, which include coordination of diene and formation of the next bond in the chain, could be the fast steps. Pol)rmer with a cis backbone would then be the result. 

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