Living polymerization systems cationic

The capability of ROMP of polymerizing even more complex functional monomers was demonstrated by the fact that the cationic NHC precursor l,3-di(l-mesityl)-4- [(bicydo[2.2.1 ]hept-5-en-2-ylcarbonyl)oxy]methyl -4,5-dihydro-lH-imidazol-3-ium tetrafluoroborate can be polymerized with the Schrock initiator Mo(N-2,6-f-Pr2-C6H3) (CHCMe2Ph)(OCMe(CF3)2)2 at ambient temperature in CH2CI2. The observed theoretical DP of 7 was in excellent accordance with a DP of 7 1 found via end-group analysis using H-NMR. The polymerization system fulfilled at least the requirements of a class-V living polymerization system. 

The success of the HI/I, system suggested the effectiveness of a combination of a nudeophilic counteranion and a relatively mild Lewis add for living polymerization. Thus, cationic polymerization of alkyl VEs was examined using various protonic adds (or an addud of a VE with a protonic acid) with weak Lewis adds. Living polymerization was achieved using hydrogen halides " and acetic adds in conjunction with zinc... 

Since the discovery of living cationic systems, cationic polymerization has progressed to a new stage where the synthesis of designed materials is now possible. The rapid advances in this field will lead to useful new polymeric materials and processes that will greatiy increase the economic impact of cationic initiation. 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

Quantitative aspects of photopolymerization have been described in Sec. 3-4c. There are some differences between radical and cationic photopolymerizations. The dependence of Rp on light intensify is half-order for radical polymerization, but first-order for cationic polymerization. Radical photopolymerizations stop immediately on cessation of irradiation. Most cationic photopolymerizations, once initiated, continue in the absence of light because most of the reaction systems chosen are living polymerizations (Sec. 5-2g). 

As was stated above, the interpretation that the field affects the dis-sodation state of the growing chain ends was not uniquely substantiated by the experimental data, except those on copolymerizations. Thus it is interesting to investigate the field influence on much simpler systems than cationic homopolymerizations. For this purpose we have chosen living anionic systems in which only propagation steps are involved. The system first studied was a living anionic polymerization of styrene with n-butyllithium in the binary mixtures of benzene and tetrahydrofuran (17,24) and in the binary mixtures of benzene and dimethoxyethane (15). 

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