Living polymerization acetylenes

Living anionic ring-opening polymerization, strained metallocenophanes, 12, 326 Living polymerization acetylenes... 

Some transition metal catalysts induce the living polymerization of various acetylenic compounds.68,69 Such polymerizations of phenylacetylene catalyzed by rhodium complexes are used in conjunction with a quantitative initiation and introduction of functional groups at the initiating chain end (Scheme 16).70 The catalyst is prepared from an [RhCl(nbd)]2/Ph2C=C(Ph)Li/PPh3 mixture and proceeds smoothly to give quantitatively the polymer 54 with a low polydispersity ratio. 

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... 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

A Ta vinylalkylidene complex 6, confirmed by a single crystal X-ray analysis, was revealed to polymerize 2-butyne in a manner of living polymerization.The initiation efficiency is quantitative, and the living end can be end-capped with aromatic aldehydes. As polymers from symmetric acetylenes are generally insoluble, soluble poly(2-butyne) is accessible if the degree of polymerization is suppressed below 200. The NMR analysis of living oligomers of 2-butyne clearly indicates that both cis- and 7ra r-structures exist in the main chain. 

A number of Mo carbene catalysts, bearing various modified ligands, have been reported and proven to elegantly induce living polymerization of acetylene monomers. The first example is the cyclopolymerization of 1,6-heptadiynes catalyzed by Mo carbenes Mo carbenes ligated by bulky imido and alkoxy groups are quite effective. In... 

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. 

Keywords Living polymerization, Living copolymerization, Rare earth metal complexes, Alkyl methacrylate, Alkyl acrylates, Lactones, Ethylene, 1-Olefins, Conjugated dienes, Acetylene... 

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. 

Potentially important advantages of controlled living polymerization reactions of acetylene derivatives are greater tolerance of functionalities, control over the nature of the capping groups, and the ability to prepare block or random copolymers that contain other monomers that can be polymerized by the well-defined alkylidene complexes. ... 

It was reported that the alkylidene mechanism for the living polymerization of 2-butyne and acetylene itselP was operative in two systems that contained well-defined alkylidene complexes, although controlled living acetylene polymerization by well-defined initiators is still relatively rare. 

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)... 

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

Rh-catalyzed living polymerization was first accomplished in 1994. The excellent ability of preisolated, well-defined catalyst to produce quantitative yields of poly(phenylacetylenes) with narrow polydispersities was demonstrated. The catalyst used, (nbd)(PPh3)2Rh-C=C-Ph (9, Figure 21.9), has been completely characterized by single-crystal X-ray analysis. It has been disclosed that the initiation reaction of acetylene polymerization with this catalyst proceeds not through direct insertion... 

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