Molecular Weight by Chain Transfer

This group usually leads to anionic or coordinate polymerization which are not covered by this review. Nevertheless, polymerizations of oxetanes and THF, known to proceed exclusively by a cationic mechanism, have also been induced by various organometallic initiators. In many cases, these initiators lead to higher molecular weight polymers, probably reacting fast and first with impurities that could, if not destroyed, lower the molecular weight by chain transfer. 

Compared to conventional catalysts, (diphenylcarbene)pentacar-bonyltungsten has three advantages (a) it converts cycloalkenes stereo-selectively into cw-polyalkenamers (b) it contains no metal alkyl bonds, and therefore no acyclic olefins can form to decrease molecular weights by chain transfer and (c) it contains no metal halide to induce side reactions or corrode equipment. 

In the more general case of joint control of molecular weight by both transfer and radical termination, it is appropriate to consider that two distributions are formed simultaneously. One of these distributions consists of molecules terminated by chain transfer the other of pairs of chains joined by the combination of radicals. For any conversion increment, the two coexisting distributions will depend on the same parameter p representing the probability of continuation of the growth of any chain, i.e. 

It was this molecular-weight limiting chain transfer which led us to seek synthetic approaches to building a crosslinked rubber structure from poly(propylene oxide) chains. This was successfully accomplished in 1949 by building branched chain... 

Glass-Transition Temperature. The T of PVP is sensitive to residual moisture (75) and unreacted monomer. It is even sensitive to how the polymer was prepared, suggesting that MWD, branching, and cross-linking may play a part (76). Polymers presumably with the same molecular weight prepared by bulk polymerization exhibit lower T s compared to samples prepared by aqueous solution polymerization, lending credence to an example, in this case, of branching caused by chain-transfer to monomer. 

Although solution polymerisation is able to control and retards auto acceleration, the solvent is rarely intent and the product of lower Molecular weight gets obtained by chain transfer with the solvent. 

All reactions involved in polymer chain growth are equilibrium reactions and consequently, their reverse reactions lead to chain degradation. The equilibrium constants are rather small and thus, the low-molecular-weight by-products have to be removed efficiently to shift the reaction to the product side. In industrial reactors, the overall esterification, as well as the polycondensation rate, is controlled by mass transport. Limitations of the latter arise mainly from the low solubility of TPA in EG, the diffusion of EG and water in the molten polymer and the mass transfer at the phase boundary between molten polymer and the gas phase. The importance of diffusion for the overall reaction rate has been demonstrated in experiments with thin polymer films [10]. 

Finally, chain transfer is undesirable except when it is used intentionally to limit molecular weight by adding good chain transfer agents such as carbon tetrachloride. Here transfer of a chlorine atom limits the size of one chain and at the same time initiates formation of a new chain by the trichloromethyl radical. Instead of (3), (3), (3), etc., we get (3), (3), (7), (8), (3), (3), (7), (8), etc., with a lower average chain length. 

---