Living substituted acetylenes

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... 

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. 

Metal carbenes 19 and 20 have been reported to be effective in the polymerization of substituted acetylenes. Since 19 has an olefin ligand that can be removed when an acetylene monomer approaches, it is more active than 18. Metal carbenes 20 and 21a induce living polymerizations of substituted acetylenes(see below). In general, metal carbene catalysts are not very active, but the initiation reaction thereby is simple, and hence they are useful for the investigation of kinetics, etc. 

With catalyst 26, 1-chloro-l-alkynes with different alkyl lengths also undergo living polymerizations. Consequently, the sequential addition of 1-chloro-l-hexadecyne (A), 1-chloro-l-hexyne (B) and 1-chloro-l-hexadecyne (A) in this order provides an A-B-A-type triblock copolymer. Similarly, one can obtain a B-A-B-type triblock copolymer. These are the first examples of block copolymers from substituted acetylenes. 

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)... 

In these two decades remarkable progress has been made in the development of excellent catalysts for living and stereospecific acetylene polymerizations (10,26-28). The r-conjugated polymers prepared by the sequential polsrmerization are strictly limited to polyacetylenes, except for only a few examples. Thus, synthesis of tailor-made conjugated macromolecules such as end-functionalized polymers, block copolymers, star-shaped polymers is possible only in the case of substituted acetylenes. 

Table 3. Mo-Based Carbene Catalysts ((7)) for the Living Polymerization of Substituted Acetylenes...

Recently, many examples of catalysts for living polymerization of substituted acetylenes have been reported. These catalysts are useful for the elucidation of the polymerization mechanism as well as precision synthesis of various polymer architectures such as block copolymers and star polymers. Table 15.2. 

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 general, Ru carbenes do not show high catalytic activity for the polymerization of substituted acetylenes, and therefore have not been studied in detail. Nuyken and coworkers [63,64] developed a well-defined Grubbs-type Ru carbene for living polymerization of diethyl dipropargylmalonate (5, Table 15.2). 

Metallocene and half-metallocene complexes also work as catalysts for the polymerization of substituted acetylenes for example, a metallocene catalyst, CP2M0CI2 (Cp = cydopentadienyl), in conjunction with EtAICl2 (1 3 mole ratio), polymerizes PA into a polymer with Mn = 4 X 10 . A half-metallocene-based ternary catalyst system, CpMoCU-EtMgBr-EtOH (1 2 2), polymerizes 0-CF3-PA in a living fashion to give a polymer whose Mw/M is 1.06 a... 

Figure 10 Catalysts for living polymerization of substituted acetylenes.

In contrast to unsubstituted acetylene, the polymerization of differently substituted 1-alkynes and di-l-alkynes may be carried out conveniently using Schrock-type catalysts. Dipropargylmalonate and derivatives thereof may be cyclopolymer-ized in a living manner using Mo(N-2,6-i-Pr2-CgH3)(CH-t-Bu)[OCMe(CF3)2]2. The... 

The oligomerization of propene on zeolite H-Y has been studied [33,37] by variable-temperature MAS NMR. Alkoxy species formed between protonated alkenes and oxygens of the zeolitic framework were found to be important long-lived intermediates in these reactions. Simple secondary or tertiary carbocations are either absent in the zeolite at low temperatures, or are so transient as to be undetectable by NMR even at temperatures as low as 163 K. There was, however, evidence for long-lived alkyl-substituted cyclopentenyl carbocations, which are formed as free ions in the zeolite at room temperature. At 503 K the oligomers crack to form branched butanes, pentanes and other alkanes. The final product was highly aromatic coke. The structure, dynamics and reactivity of an alkoxy intermediate formed from acetylene on zeolite catalysts have been investigated by Lazo et. al. [32]. 

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