Catalysts living polymerization

Mo and W alkylidene complexes 4, the so-called Schrock carbenes, have explosively evolved the polymerization chemistry of substituted acetylenes. Although the preparation of these catalysts is relatively difficult because of their low stability, in other words, high reactivity, they elegantly act as living polymerization catalysts for substituted... 

Figure 11 Living-polymerization catalysts for propylene and higher a-olefins.

It has also been demonstrated that a PP-fc-EPR block copolymer was prepared with a living polymerization catalyst composed of V(acac)3/AlEt2Cl at -78 °C [ 145]. Also, PP-fo-EPR was produced with metallocene catalyst systems at -78 °C [146],... 

As a complement to the stable metallacyclobutane catalysts, a series of stable alkylidene catalysts have been prepared and shown to be active living polymerization catalysts. The complex W(CHt-Bu)(NAr)(Ot-Bu)2 [47,48] (Ar = 2,6-diisopro-pylphenyl) (35) was reacted with 50-200 equivalents of norbornene in toluene at 25 °C, followed by end-capping with benzaldehyde, yielding polymers in which the major component has a molecular weight proportional to the number of equivalents of norbornene consumed, with dispersities of approximately 1.05,... 

The polymerization of 1-chlo o-l ctyne by MoCl5- 4Sn-EtOH (27) also produces polymers with narrow MWDs (M /Mn 1.2-1.3). The Mn increases in proportion to the monomer conversion. Hence this polymerization is concluded to be living, although 26 (with M0OCI4) is superior to 27 (with M0CI5) as a living polymerization catalyst. 

CHBu CH(C5HgCH) ] (structurally characterized) formed from norbomene acts as a living polymerization catalyst for the formation of polynorbomene." Kinetic studies show that the rate determining step is ring opening of the metallacycle. 

The general formula of metal halide-based living polymerization catalysts is expressed as MO,jCl ,-co-catalyst-ROH (M = Mo or W, = 0 or 1, m = 5 or 4). The most important feature of these catalysts is ease of preparation, but their initiation efficiency is low. A typical living polymerization by metal halide-based catalysts has been achieved with 1-chloro-l-octyne as monomer and using MoOCl -w-Bu Sn-EtOH (1 in Table 15.2) as catalyst [41]. Specifically, poly(l-chloro-l-octyne) with a narrow MWD < 1.2) is obtained, and the... 

Scheme 4 Catalytic synthesis of monodisperse polymers with highly active living polymerization catalyst...

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