Bimolecular chain transfer reactions

Chain transfer reactions are bimolecular or unimolecular (spontaneous). Typical bimolecular chain transfer reactions are transfer to monomer, initiator, and external chain transfer agents (especially impurities), and intermolecular chain transfer to polymer typical unimolecular chain transfer reactions are transfer to counterion in ionic polymerizations, intramolecular chain transfer to polymer, and transfer to solvent (pseudo xmimolecular). 

Chain transfer to monomer is a fundamentally important bimolecular chain transfer reaction because it usually sets the upper limit of attainable molecular weight in a chain polymerization system that has been rigorously purified of all other chain transfer agents. It consists of reaction of the propagating chain with monomer via some reaction other than the normal propagation reaction. As such, its rate equation is identical in form to that for propagation ... 

If we use initiators R-R which have very high reactivities for the chain transfer reaction to the initiator and/or primary radical termination, i.e., ordinary bimolecular termination is neglected, it is expected that a polymer will be obtained with two initiator fragments at the chain ends (Eq. 7) ... 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

Chain-transfer reactions would be expected to increase in rate with increasing pressure since transfer is a bimolecular reaction with a negative volume of activation. The variation of chain-transfer constants with pressure, however, differ depending on the relative effects of pressure on the propagation and transfer rate constants. For the case where only transfer to chain-transfer agent S is important, Cs varies with pressure according to... 

For a radical polymerization with bimolecular termination, the polymer produced contains 1.30 initiator fragments per polymer molecule. Calculate the relative extents of termination by disproportionation and coupling, assuming that no chain-transfer reactions occur. 

In comparison to carbanions, which maintain a full octet of valence electrons, carbenium ions are deficient by two electrons and are much less stable. Therefore, the controlled cationic polymerization requires specialized systems. The instability or high reactivity of the carbenium ions facilitates undesirable side reactions such as bimolecular chain transfer to monomer, /1-proton elimination, and carbenium ion rearrangement. All of that limits the control over the cationic polymerization. 

The occurrence and a limited importance of chain transfer by transacetalization cannot be doubted. We proposed this type of reaction for trioxane polymerization as early as 1959 (6) and assumed that intramolecular transacetalization produces some thermally stable macrocyclic polyoxymethylene (10). We have utilized bimolecular chain transfer by polymers to produce thermally stable block copolymers at temperatures over 100°C. [—e.g., with polyesters, polypropylene oxide, or with polyvinyl butyral)] (12). 

The chain-length-dependence of bimolecular termination reactions needs to be taken into account in order to be able to accurately estimate the MWD formed, except when very small polymer particles are formed and/or chain transfer reactions dominate over dead polymer chain formation. 

All the bimolecular chain reactions or chain-transfer reactions given by reactions (6)—(12) have the form... 

For ethylene polymerization with lithium catalysts the most probable chain transfer reactions include (1) monomolecular elimination of LiH, (2) bimolecular displacement of the polymer by monomer, (3) metalation of monomer, and (4) metalation of solvent as shown below. 

A propagating radical may add a monomer molecule to continue propagation or it may undergo chain transfer or bimolecular chain termination reaction. The probability that itVill add monomer is then (Rudin, 1982) ... 

Just as the equilibrium constants of dissociative reactions tend toward large numbers of atmospheres as temperature is increased, those of equimolecular reactions, like the mutually bimolecular atom transfer reactions which occur in the H2-O2 chain mechanism, tend toward values near unity. The effects of energy differences are diminished, and the residual entropy differences are only a few cal mole degree". When combined with the substantial chain centre concentrations which accumulate by the nonsteady branching, this effect makes the equilibrium position of the chain propagation steps a much more significant aspect of the chain reaction course than under lower-temperature or quasisteady reaction conditions. 

The conversion ranges of state I and state II depended on the structures and properties of the polymer network formed. At the beginning, the polymer radicals disappeared via bimolecular reactions according to the hjrperbolic law (equation (4.9)) and then by radical trapping according to an exponential law (equation (4.15)). The relative importance of these two processes depended mainly on radical mobilities, monomer functionality and chain transfer reaction. 

Kinetic studies on the radiation-induced polymerization and post-polymerization of TFE were carried out using chlorofluorohydrocarbons as solvents. The remarkable postpolymerization is again explained by the unusually slow rate of the bimolecular chain termination. A chain transfer reaction was also discussed by Hisasue et al. [721]. Suwa et al. [679] discussed the emulsion polymerization of TFE by radiation with ammonium perfluorooctanoate (FC-143) as the emulsifier. The polymerization rate is proportional to the 0.8 power of the dose rate and is almost independent of the emulsifier concentration (up to 2wt% in water). Molecular weights between 10 and 10 were observed, which increases with reaction time but decreases with the emulsifier concentration. In general, the molecular weight of PTFE prepared by radiation-induced polymerization in solution and in emulsion is relatively low compared with commercial PTFE. However, it is also possible to produce molecular weight of up to 3 x 10 if an emulsifier-free polymerization are carried out [677,678,723]. 

The fact that two distinct pathways exist for the radical termination reaction of course immediately raises questions about the relative importance of each of these modes. In early kinetic studies, as far as fifty years ago [e.g. 25-27], the number of initiator endgroups per polymer molecule was determined. At that time, it was already concluded that termination by combination was the dominant mechanism in the majority of bimolecular termination reactions (although chain transfer reactions were neglected in drawing this conclusion). The first monomer system studied for which evidence was found that significant amounts of disproportionation could occur during a free-radical polymerization, was methyl methacrylate [28]. 

On the other hand, if chain-transfer reactions are predominant over bimolecular termination, the instantaneous molecular weights are given by Eqs. (52). 

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