Degradative Initiator Transfer

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

The activation energy of the rate constant klr of reaction (1) is generally different (usually higher) from that of kpf. By lowering the polymerization temperature, transfer is relatively suppressed. Lowering of the polymerization rate is caused by the re-initiation rate constant, kitr [reaction (2)], when the latter is lower than kp. When it is much lower, we speak of degradative transfer. On the other hand, polymerization is not accelerated in practice when k tr > kp. The more rapid addition of one monomer per several... 

Under conditions where eqn. (3) holds, inhibition occurs as described in Chap. 6, Sect. 1.4. In cases described by eqn. (4), only low polymers will be formed at an undiminished rate (when very short chains are formed, the rate of oligomerization might even increase). Chains shortened by transfer will also be formed at an undiminished rate in case of eqn. (6). Perhaps the most interesting is the situation described by eqn. (5). Transfer is accompanied by slow re-initiation. A part of the active centres is present in an inactivated form. Thus, at a given moment, fewer chains are growing and monomer consumption is slower. With stationary polymerizations, the external manifestations of this situation are called retardation. For various reasons, a stationary polymerization may change to a non-stationary one due to transfer. Such cases are described as degradative transfer (see Fig. 1). 

Hydrides are reactive nucleophilic agents able to initiate growth slowly. The whole process will have the character of degradative transfer the polymerization of a-methylstyrene will be stopped at a certain conversion. The double bond at the chain end will increase the acidity of the penultimate styrene unit and the corresponding C—H bond will be weakened by resonance stabilization of the anion... 

Because the chain transfer to polymer is fast as compared with reformation of active species of propagation [Eq. (128)] and there is a reaction pathway, which due to the formation of isomerized products is irreversible [reaction (129)], continuous degradation of the already formed polythiirane chains occurs if the reaction system is kept unterminated [159]. Also isolated polymers, treated with cationic initiators degrade to low molecular weight, predominantly cyclic oligomers. Consequently, cationic polymerization of thiiranes is very strongly affected by chain transfer to polymer processes. 

The degradation reactions involved in PS include random scission (which reduces the molecular weight of the polymer), depolymerization (which yields monomer), intramolecular transfer reaction (which produces dimer, trimer, etc.), and intermolecular transfer reaction which reduces the molecular weight of the polymer. The initial degradation products from PVC are Cl radicals and HCl (Owen 1984 Ahmad and Mahmood 1996). The structure and composition of PVC and PS and the interaction of various products formed may give rise to some cross products formed from the radicals or molecules which migrate across these phase boundaries and can play an important role in the degradation of blends. 

Several recent papers refer to the effects of phosphorus compounds upon radical polymerizations. Various chlorophosphines act as degradative transfer agents. - Some compounds of quinquevalent phosphorus, notably phosphorus oxychloride (POClj), have quite pronounced and complicated effects on polymerizations initiated by AIBN they act as transfer agents, although to a lesser extent than say phosphorus trichloride. They also increase the overall rate probably by complexing with the monomer and modifying its reactivity. For styrene, AIBN, and phosphorus oxychloride, there is a contribution from a cationic polymerization initiated by the oxychloride, ... 

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