Bimolecular chain transfer

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). 

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 ... 

Usually, reactions 1 and 2 take place in the aqueous jiiase, yttiile all the other kinetic events can occur both in the aqueous and in the polymer phases. Note that Pj,n indicates the concentration of active polymer chains with nTronaner units and tenninal unit of type j (i. e. of monomer j) Hi is the concentration of monomer i and T is the concentration of the chain transfer agent. Reactions 4 and 5 are responsible for chain desorption from the polymer pjarticles reactions 6 and 7 describe bimolecular temination by conJoination and disproportionation, respiectively. All the kinetic constants are depsendent upon the last monomer unit in the chain, i. e. terminal model is assumed. 

Reciprocal degrees of polymerization of polystyrenes prepared by thermal polymerization at 100°C in hydrocarbon solvents are plotted against [>8]/[itf] in Fig. 16. Conversions were sufficiently low to permit the assumption of constancy in this ratio, which is taken equal to its initial value. The linearity of plots such as these, including those for numerous other monomer-solvent pairs which have been investigated, affords the best confirmation for the widespread occurrence of chain transfer and for the bimolecular mechanisms assumed. It is... 

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 polymerization of MA with 7 was carried out in the presence of 13, i.e., 7 and 13 were used as two-component iniferters [175]. When an identical amount of 13 to 7 was added to the system, the polymerization proceeded according to a mechanism close to the ideal living radical polymerization mechanism. Similar results were also obtained for the polymerization of VAc. These results indicate that the chain end of the polymer was formed by the competition of primary radical termination and/or chain transfer to bimolecular termination, and that it could be controlled by the addition of 13. 

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. 

The rate of chain-breaking is made up of the rates of unimolecular termination and monomer transfer, kt + kml = J0 say, and the rate of bimolecular chain-breaking by various reagents, / . Thus... 

Investigations conducted by the same group using laser flash photolysis techniques elucidated details of the PET-reductive activation of selenosilanes and the application of this chemistry to a bimolecular group-transfer radical reaction and intermolecular radical chain-transfer addition [59], Based on this new concept, a catalytic procedure utilizing PhSeSiRs for radical reactions such as cycliza-tion, intermolecular addition and tandem anellation was designed (Scheme 39) [60],... 

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. 

It is reasonable to expect that in a viscous monomer such as trimethylol-propane triacrylate (>/ = 65 cp), bimolecular termination reactions proceed more slowly than in monofunctional monomers. However, considering the long lifetime observed for the polymer radicals in these monomers, caution must be exercised in the interpretation of the linear intensity dependence. Long-lived radicals are more likely to terminate by chain transfer and... 

N is active only toward P, while P is active not only toward N and the monomer (propagation, Eq. (2.87)) but also toward P (irreversible bimolecular termination, Eq. (2.89)) and toward neutral molecules (chain transfer, Eq. (2.88)). When the last two reactions are unimportant compared with the first two, the system may be viewed as a living (controlled) polymerization. 

The Time Evolution of the Doubly Distinguished Particles. In the 0-1-2 system, all events associated with the doubly distinguished particles lead to the loss of the particles. Entry, bimolecular combination, transfer (from either distinguished chain) and exit (again from either distinguished chain) all may occur ... 

First Order Stoppage Alone. If stoppage is determined solely by a first order process, such as transfer, the foregoing analysis predicts a nearly exponential distribution function. The polydispersity index must then be very close to 2.00. The same result is obtained for bulk and solution polymerizations dominated by chain transfer. Compartmentalization thus has no major effect on the polydispersity of the polymer produced, as was recognized by Gerrens (11), if the stoppage process is dominated by chain transfer. This contrasts with the significant effects of compartmentalization if bimolecular events dominate termination. 

These chains cannot undergo bimolecular termination and so grow unhindered until a second free radical enters the latex particle. This environment is manifestly different from that in other growing latex particles and from that in the bulk system. This argument also explains why compartmentalization has no effect on the MWD if termination is by chain transfer because the chains containing one free radical can still undergo the transfer process, just as they do in the bulk system. 

Note that if it is possible to cause free radical entry events to occur much more frequently than chain transfer to monomer events (e.g., for styrene, at a rate greater than 1 per second), then the polymer will be controlled primarily by bimolecular termination. Note, too, that if chain stoppage is dominated by chain transfer, the molecular weight of the polymer produced would be independent of particle volume. Morton et al. (16) have obtained data for styrene that support this conclusion. 

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