Polymerization chain transfer during cationic

The cationic polymerization of propylene, 1-butene, and higher 1-alkenes yields only very low molecular weight polymers DP < 10 - 20) with highly complicated strucmres that arise due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer, and elimination, besides chain transfer during polymerization. In the polymerization of ethylene, initiation involving protonation and ethylation is quickly followed by energetically favorable isomerization ... 

The free-radical polymerization of NVP is rather complex but its liability to be polymerized by its own peroxide is well documented as well as its strong tendency for chain transfer during polymerization However, retardation by oxygen has also been claimed Alternatively, the formation of a donor-acceptor complex has been proposed which could yield anionic and cationic species,... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

For PIB, a method resulting in end-reactive polymers, however, based on chain transfer reactions during polymerization, was addressed as the cationic inifer method (54). 

Back-biting reaction occurring during cationic polymerization of lactams is detrimental to preparation of block copolymers of two different lactams by sequential polymerization. Block copolymers can be obtained only in those systems in which the rate of polymerization of the second monomer is much higher than the rate of chain transfer to polymer resulting in transamidation [219]. 

Analogously to water, alcohols (used in the form of polyols) also cause a chain-transfer reaction during the cationic polymerization of epoxides. Polyols (polyvalent alcohols with an average molecular weight from several hundred to several thousand grams/mole) are often utihzed in technical formulations. These serve to reduce the network density of the polymerized epoxide and therefore make the material less brittle. At the same time the reactivity also is influenced and the susceptibiHty of the polymerization rate and polymer properties to the influence of air humidity is reduced. This is important, as the influence of air humidity is difficult to reproduce in technical appHcations. 

Since the discovery of living polymerizations by Swarc in 1956 [1], the area of synthesis and application of well-defined polymer structures has been developed. The livingness of a polymerization is defined as the absence of termination and transfer reactions during the course of the polymerization. If there is also fast initiation and chain-end fidelity, which are prerequisites for the so-called controlled polymerization, well-defined polymers are obtained that have a narrow molar mass distribution as well as defined end groups. Such well-defined polymers can be prepared by various types of living and controlled polymerization techniques, including anionic polymerization [2], controlled radical polymerization [3-5], and cationic polymerization [6, 7]. 

Scheme 8.13 Chain transfer by indane formation during the uncontrolled cationic polymerization of styrene.

The preceding sections have dealt with polymerization by either insertion or GTP mechanisms. Of course, vinyl monomers are also polymerizable by radical, anionic, or cationic mechanisms. In this short section, we summarize the processes which are reasonably well understood from a mechanistic viewpoint, and which involve the intervention of transition metal alkyls (or hydrides), either during initiation, propagation, or chain transfer/termination. A much larger class of polymerization reactions where redox-active transition metal complexes are used to mediate radical polymerizations by reversible atom transfer (ATRP) or other means has been extensively and recently reviewed from a mechanistic perspective and will only be briefly mentioned here. 

As illustrated in Scheme 10.8, the amino group-terminated polypropylene is probably obtained through a special chain transfer mechanism that may involve a dormant species (iii) formed after insertion of the alkylaluminum-treated allylamine. The nitrogen atom of the inserted masked allylamine is located near the Zr cation and results in a stable complex (iii), to which nucleophilic attack from A1(CH3 )3 can occur at the chain end to produce a polymeric intermediate (iv). With HCl treatment during workup, this intermediate (iv) is converted into iPP-NH2 (v). ... 

Not long ago criteria for the generation of living cationic polymerizations have been developed [443-445]. These criteria include an almost instantaneous and quantitative initiation and the absence of chain transfer and termination reactions. These requirements were satisfied when iodine was used to polymerize NVK in dichloromethane solution containing tetrabutylammonium iodide at low temperatures [446,447]. Polymer molecular weights increased in a linear fashion with monomer consumption during the reaction and upon the addition of successive aliquots of monomer. 

In another study [96], MALDI-TOF MS end-group characterization was carried out at different time points for the cationic polymerization of oxazoUne monomers containing thioethers in order to verify a proposed mechanism of the chain transfer events occurring during the polymerization. The use of time-dependent MALDI-TOF mass spectral analysis provided strong evidence to support the prevalence of chain transfer via nucleophilic addition/elimination of the thioether functional group, confirming theoretical predictions. 

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