Living polymerization mechanism changes

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

Recently, Khosravi et al. reported on the change in polymerization mechanism from living anionic polymerization to ROMP. For this purpose, ethylene oxide was polymerized with Ph2CFl and terminated with 4-chloromethylstyrene. The thus-obtained vinyl-terminated poly(ethylene oxide) (PEO) was then reacted with RuCl2(PCy3)2(CHPh) and used for the ROMP of 2,3-difunctional norborn-2-enes to yield the desired poly(PEO-b-NBE) block copolymer (Scheme 19.15) [208]. 

For a monodisperse polymer sample, d = 1. The ranges of d values change drastically with the different mechanisms of polymerization. The values of d are 1.01-1.05 in living polymerization (anionic, cationic, living free radical, etc.), around 1.5 in condensation polymerization or coupling termination of polymerization, around 2 in disproportionation reactions on polymerization, 2-5 for high-conversion olefins, 5-10 in self-acceleration on common free radical polymerization, 8-30 in coordination polymerization, and 20-50 in branching reactions on polymerization. 

More specific topics, such as block copolymer synthesis by changing the polymerization mechanism [18], by step-growth polymerization [19], via macroinitiators [20], living free-radical polymerization [21, 22] or ionic polymerization [23] were reviewed later on, as well as the synthesis of selected block copolymer types, for example hydrophilic-hydrophilic copolymers [24], copolymers based on PEO [10,16]. 

Block copolymers from cyclooolelins have been prepared by various experimental techniques [52]. Some interesting methods use living ROMP catalysts, which allow ready synthesis of new products having controllable structures and properties. Other methods apply cross-metathesis between unsaturated polymers and/or polyalkenamers [3], polymerization of cycloolelins in the presence of unsaturated polymers [4], polymerization of two or more cycloolelins of quite different reactivities with classical ROMP catalysts [4], and copolymerization of cycloolefins with other monomers, effected by changing the polymerization mechanism from ROMP to anionic, cationic, Ziegler Natta, and group transfer, and vice versa [6-8, 52]. 

Our picture of the transitions between centres is very incomplete so far, based on studies of distribution curve shapes in the products. When a monomer is polymerized by a living mechanism on two or more centres of widely differing reactivity, chains of characteristic legth are produced on each centre type. In a strictly living medium where centres of one type are not transformed to another, a product with a bi- or multimodal distribution curve of degrees of polymerization is formed. When the various centre types are in a dynamic equilibrium where the centre type changes in the course of propagation, the distribution curve of the product will be broader than the width of either of the peaks in the previous case, but narrower than the overall... 

The nickel(II) complex 14 is also known as a catalyst for the polymerization of 1,3-butadiene. The versatility of this nickel catalyst was used in the synthesis of butadiene-isocyanide block copolymers [27]. In the so-called change of mechanism block copolymerization, 1,3-butadiene was initially polymerized with 14, yielding a butadiene polymer bearing living 773-allyl-nickel termini (Scheme 20). Addition of terf-butyl isocyanide to this living... 

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