Chain transfer processes

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

In this section, the reactions undergone by radicals generated in the initiation or chain transfer processes are detailed. Emphasis is placed on the specificity of radical-monomer reactions and other processes likely to take place in polymerization media under typical polymerization conditions. The various factors important in determining the rate and selectivity of radicals in addition and... 

Minor (by amount) functionality is introduced into polymers as a consequence of the initiation, termination and chain transfer processes (Chapters 3, 5 and 6 respectively). These groups may either be at the chain ends (as a result of initiation, disproportionation, or chain transfer,) or they may be part of the backbone (as a consequence of termination by combination or the copolymerization of byproducts or impurities). In Section 8.2 wc consider three polymers (PS, PMMA and PVC) and discuss the types of defect structure that may be present, their origin and influence on polymer properties, and the prospects for controlling these properties through appropriate selection of polymerization conditions. 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

It is also possible that Si-Cl bonds may be consumed slowly by chain transfer process as follows41 42) ... 

Other chain transfer processes may occur. For example, the radical may abstract an atom from along the backbone of a previously formed polymer molecule, and thus initiate the growth of a branch to the main chain. There can also be chain transfer to monomer, which in the nature of the polymerisation process must be a relatively rare phenomenon. However, it can occur infrequently and give rise to a restriction in the size of the polymer molecules without ceasing the overall radical chain reaction. 

The reaction of a chain radical with a unit of a previously formed polymer represents an additional possible chain transfer process not previously considered in Chapter IV. The point of attack might again be located in the substituent X, or it might involve removal of the tertiary hydrogen on the substituted chain carbon. The following sequence of reactions, in which the latter alternative has arbitrarily been assumed, would then lead to a branched polymer molecule as indicated. ... 

The immediate result of the intervention of the chain transfer process indicated in the first step is the termination of a growing chain and the reactivation of a polymer molecule, which then adds monomer to gener-... 

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

Ethylene pressure studies have revealed a first-order dependence on ethylene for both the rate of chain propagation and the rate of chain transfer. This polymerization behavior together with X-ray analyses and DFT calculations has provided strong support for (1-11 transfer to an incoming monomer, which is responsible for the production of vinyl-terminated PEs. The calculations thus suggest that the catalysts disfavor (I-11 transfer to the Zr metal because of the extreme instability of the Zr hydride species that is produced in such a chain transfer process. 

The reasons behind this accelerated rate behavior have been attributed to a decrease in chain transfer processes (28,29) and a decreased termination rate (24,25) indicated by molecular weight measurements (26). Recently, direct evidence of decreases in the termination rate have been shown (27) and in these studies both the termination and propagation kinetic constants were determined for polymerizations exhibiting enhanced rates in a smectic phase. The propagation constant, kp, decreases slightly in the ordered phase from the isotropic polymerization. Such a decrease would be expected because of the lower temperature in the smectic phase. The termination kinetic constant, kt, however, decreases almost two orders of magnitude for the ordered polymerization, indicating a dramatically suppressed termination rate. 

Carbon dioxide has also proven to be an exemplary medium for the polymerization of TFE with perfluorinated alkylvinyl ether monomers containing sulfonyl fluoride such as CF2=CF0CF2CF(CF3)0CF2CF2S02F (PSEVPE). As seen in Table 13.2, the dramatic difference in the number of acid end groups between the commercial sample and those made in C02 indicates that chain-transfer processes stemming from vinyl ether radical arrangement are not nearly as prevalent in C02 as in conventional systems. 

M + B —Mn + B ] (termination by chain transfer) where B is the species involved in chain transfer process. 

Initiation of graft copolymerization by radical mechanisms can occur by (a) a redox process on the substrate or (bl a chain transfer process to the substrate. In addition to grafting, formation of homopolymer may occur in both cases. 

A radical chain transfer process for grafting has an initiating step ... 

In grafting reactions initiated by redox processes, there is an interaction of the intermediate products with the substrate polymer chains. There may also be a chain transfer reaction involved. It is sometimes difficult to make a clear distinction between a direct redox reaction and a chain transfer process. Three redox systems have been studied extensively in grafting onto cellulose persulfate ions 1, hydroperoxide/ferrous ions 2> 3 and cerium (IV) ions. ... 

Transfer of the free radical to another molecule serves as one of the termination steps for general polymer growth. Thus, transfer of a hydrogen atom at one end of the chain to a free radical end of another chain is a chain transfer process we dealt with in Section 6.2 under termination via disproportionation. When abstraction occurs intramolecularly or intermolecularly by a hydrogen atom some distance away from the chain end, branching results. Each chain transfer process causes the termination of one macroradical and produces another macroradical. The new radical sites serve as branch points for chain extension or branching. As noted above, such chain transfer can occur within the same chain as shown below. 

The chain fragments formed by the recombination of free radicals can be reconverted into radicals by a variety of reinitiation processes, some of which are listed in Table 1. Such reactions can occur in the gas phase via electron collision and on the polymer surface by impact of charged particles or photon absorption. Reinitiation may also be induced in both the gas phase and on the polymer surface by hydrogen transfer reactions. These last processes are similar to the chain transfer processes which occur during homogeneous polymerization. Expressions for the rates of reinitiation are given by Eqns. 20 through 23. 

In immortal polymerization, a reversible chain transfer reaction takes place much more rapidly than a propagation reaction, thereby uniformity of the molecular weight of polymer is realized. The successful high-speed immortal polymerization assisted by a Lewis acid, mentioned above, suggests that not only the propagation step (Scheme 7) but also the chain transfer process (Scheme 8) is accelerated by a Lewis acid. 

C. Chain Transfer Process Depending on Alkylaluminum Concentration. 26... 

H. Relative Importance of the Different Chain Transfer Processes. 43... 

Determinations of aluminum have been carried out on fractions of polymeric product containing isotactic chains deriving from the polymerization. These measurements were performed in an attempt to establish whether the chain transfer process, depending on the alkylaluminum concentration, leads to the formation of macromolecules which remain bound to the aluminum. The polymer has been therefore purified by physical methods from the unreacted ethylaluminum and from the heterogeneous catalyst. 

End Groups in the Isotactic Polypropylene, Deriving from the Chain Transfer Processes Depending on the Catalyst Concentration [Catalytic System a-TiCl 3-AI (CaHs) -n-Heptane)... 

Mechanism of the Chain Transfer Process Depending on the Triethylcduminum Concentration... 

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