Living single-component catalysts

M-CHj HM=CH2. Well-defined, single-component catalysts initiate living poly-... 

Considerable progress has been achieved in development of catalyst systems for living polymerization of various substituted acetylenes during the last 10 or 15 years [69]. Nowadays, there are available single-component catalysts based on stable carbene complexes and multicompmient catalysts based on MoOCLj and WOCI4, both operating in metathesis mode, as well as Rh(diene)... 

Unlike in radical or anionic polymerizations, in ROMP with single-component metathesis catalysts the growing polymer chain remains able to further grow even after consumption of the monomer. This enables the manufacture of block copolymers with interesting physicochemical properties by sequential addition of different monomers to such living systems. 

Living polymerization by single-component metal carbene catalysts... 

Well-controlled polymerization of substituted acetylenes was also reported. A tetracoordinate organorhodium complex induces the stereospecific living polymerization of phenylacetylene.600 The polymerization proceeds via a 2-1 -insertion mechanism to provide stereoregular poly(phenylacetylene) with m-transoidal backbone structure. Rh complexes were also used in the same process in supercritical C02601 and in the polymerization of terminal alkyl- and arylacetylenes.602 Single-component transition-metal catalysts based on Ni acetylides603 and Pd acet-ylides604 were used in the polymerization of p-diethynylbenzene. 

A Ta carbene complex (2 in Table 15.2) is the first example of a single-component metathesis catalyst that induces the living polymerization of substituted acetylene, where the monomer used is 2-butyne [37]. The initiation efficiency is quantitative. 

In recent years metallocene complexes have also been successfully used as polymerization catalysts for methyl methacrylate. Collins et al. and Soga et al. reported that cationic zirconocene complexes catalyse the polymerization of MMA [217-220]. But these metallocene complexes consisted of more components than the only metallocene complex. Hocker et al. investigated some novel single-component zirconocene complexes as catalysts for the stereospecific polymerization of MMA [221]. MMA was polymerized by the cationic bridged zirconocene complex [rPr(Cp)(Ind)Zr(Me)(THF)][BPh4] at temperatures between -20 and 20 °C. The polymerization led to mainly isotactic PMMA due to an enantiomorphic site mechanism and a low polydispersity index (1.12-2.33). Also it has been assumed that the polymerization mechanism is of living character. 

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