Reinitiation, polymerization

It is apparent from these reactions how chain transfer lowers the molecular weight of a chain-growth polymer. The effect of chain transfer on the rate of polymerization depends on the rate at which the new radicals reinitiate polymerization ... 

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. 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is broken. Water has an especially negative effect on polymerization, since it is an active chain-transfer agent. For example, C s is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene [Szwarc, 1960], and the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. The adventitious presence of other proton donors may not be as much of a problem. Ethanol has a transfer constant of about 10-3. Its presence in small amounts would not prevent the formation of high polymer because transfer would be slow, although the polymer would not be living. 

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

An additional termination in the trioxane polymerization is chain transfer to monomer hy hydride ion transfer, which results in terminating the propagating chain with a methoxyl group while carbocation XXII reinitiates polymerization [Kern et al., 1966 Weissermel et al., 1967]. 

With all thiiranes, addition of new monomer to a degraded reaction mixture resulted in an instantaneous start of polymerization of the newly added monomer, followed by degradation. The degradation can be stopped at any moment by adding a small amount of tetrahydrothiophene to the reaction mixture. This sulfide is known to react rapidly with sulfonium salts to form a S-alkyl tetramethylenesulfonium salt (37). The resulting reaction mixture is then no longer able to reinitiate polymerization of newly added monomer. 

In all cases, chain transfer may occur to monomer, solvent, polymer, or a chain transfer agent. For this to be a transfer reaction rather than termination, the molecule generated (A ) must be reactive enough to reinitiate polymerization [Eq. (10)]. 

The formation of stable carbenium ions can be observed visually and/ or spectroscopically. For example, styrene and a-methylstyrene polymerizations are generally colorless because the growing carbenium ions absorb at approximately 340 nm (cf., Sections II.B and IV.B.l). However, these systems may turn brown or dark red at longer reaction times due to formation of indanyl carbenium ions (A 440 nm) [14,26,325] and other delocalized carbocations similar to those in Eq. (121). The stable cyclic diaryl carbenium ions are generated by hydride transfer from the initially formed indanyl end groups [Eq. (124)] in styrene polymerizations, and by methide transfer in a-methylstyrene polymerizations. The prerequisite for this termination is therefore intramolecular transfer by Friedel-Crafts alkylation protons liberated in the first stage can then reinitiate polymerization. 

Atom transfer to form a stable radical which does not reinitiate polymerization, as in the reaction of poly(vinyl acetate) radical and diphenylamine to yield a diphenyl nitrogen radical which will not add vinyl acetate but may terminate a macroradical. 

A more detailed treatment of this problem has been given by OTooIe ( 96S)- In this treatment, allowance is made for the possibility that radicals can exit from the lod into the external phase. However, no allowance is made for the possibility that radicals which do so exit may reenter the reaction loci and reinitiate polymerization there. The transitions which afifect the numbers of loci which cross a notional boundary between states of radical occupancy i and i + I are illustrated in Fig. 12. Ihe condition for the steady state is readily found to be... 

Termination occurred when a sulphur atom of an existing polymer molecule reacted with the growing cyclic sulphonium ion to give a non-strained, branched sulphonium ion which was not capable of reinitiating polymerization, viz. 

FRRPP Physical immobilization of free radicals to reinitiate polymerization... 

The new radical A, which results from chain transfer, can reinitiate polymerization by the reaction... 

Derive suitable expressions for the rate of polymerization considering (a) an extreme case where chain termination occurs exclusively by the degradative initiator transfer and (b) a more general case where chain termination takes place by simultaneous occurrence of the degradative initiator transfer and the usual bimolecular mechanism. In both cases, assume that the radical I- formed by the chain transfer to initiator I is too inactive to reinitiate polymerization. 

Problem 6.33 Reconsider the case of degradative initiator transfer as described in Problem 6.31 but assume now that the radical formed due to chain transfer is capable of reinitiating polymerization. In addition to the normal modes of initiation, propagation, and bimolecular termination of chain radicals as in ideal polymerization, other reactions that may follow as a consequence of this degradative initiator transfer are shown in the following reaction scheme ... 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is thus broken. Water is an especially effective chain terminating agent. For example, Ctr,s is approximately 10 in the polymerization of styrene at 25° C with sodium naphthalene. Thus the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. 

In contrast to what is observed with Ziegler catalysts, H2 added to the reactor usually has no detrimental effect on the activity of Cr/silica. The tendency of H2 to lower the activities of many catalysts is thought to be caused by the formation of a metal hydride resting state, which conceivably reacts more slowly to reinitiate polymerization. 

The inhibitor radicals formed in the above reactions are stabilized by resonance to such an extent that they do not add monomer to reinitiate polymerization. 

The hydroxide ion formed is not sufficiently nucleophilic to reinitiate polymerization and the active center is thus effectively destroyed. In contrast, the Ctr,s value for ethanol being very small ( 10 ), its presence in small amounts does not limit the molecular weight. ... 

There are also situations where the reaction produces a dead polymer chain and a radical that is less reactive than the propagating radical but still capable of reinitiating polymerization. The process is then termed retardation or degradative chain transfer. 

The possibility of controlling architecture with ATRP is exemplified in the one-pot synthesis of hyperbranched polymers. These hyperbranched polymers contain n halogen atoms where n=DP. The halogen groups can be eidier activated to reinitiate polymerization or be transformed to other functional groups. 

The butyrolactonyl free-radical, then reinitiates polymerization in aqueous solution. In general initiation is postulated to take place in the aqueous phase followed by stabilization due to the adsorption of surfactants on the growing polymer chain. 

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