Side living polymerization

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

Table 1 lists monomers which may undergo anionic living polymerization without side reactions. A limitation is that a number of important monomers cannot be polymerized anionically. 

The use of single-site initiators for the polymerization of acrylates is attractive, since steric protection of the metal center should eliminate the unwanted side reactions described above, allowing living polymerization systems to be developed. Further, stereocontrol may be achievable by appropriate ligand selection. 

Metals have been incorporated into polyenes, either in the side chain as a consequence of the living polymerization of ethynylferrocene and ethynylruthen-ocene [ 179], or in the main chain as a consequence of polymerization of poly(fer-rocenylenedivinylene) and poly(ferrocenylenebutenylene) [180]. The former polymers were prepared in a living manner polymers containing as many as 40 equivalents of monomer were still soluble. In the latter only relatively low molecular weight materials were obtained. 

An extremely favorable consequence of both strategies is the presence of significant amounts of covalent, or inactive, chain ends. This substantially lowers the overall concentration of reactive chain ends which results in a decrease in the occurrence of unwanted side reactions such as termination, disproportionation, or combination. This enables the polymer chain to grow in a controlled fashion, exhibiting many of the attributes typically associated with a living polymerization. However, it should be pointed out that the occurrence of these side reactions is not eliminated and in the strictest sense, the polymerizations are not truly living. 

Polymerization by trialkylamines is useful for synthesizing polypeptides of molecular weights of up to —0.5 million, and the polymerization has many characteristics similar to those of living polymerizations. Polymerizations by the most powerful bases, especially organometallic compounds, are not as useful for polymerizations to such high molecular weights because of side reactions [Imanishi, 1984 Kricheldorf, 1989]. 

AM polymerization offers the potential for suppressing side reactions and achieving living polymerization with the ability to control MW and achieve high molecular weights. 

There are two strategies for constructing a polymer with benzyl ester in the middle of the skeleton. One is to make polymer skeletons with the same molecular weight, and then combine two skeletons with benzyl ester. The other is to synthesize a chemical with propagation sites of polymerization at both sides of benzyl ester, and use the chemical as an initiator of living polymerization. Since the former strategy did not work well, probably due to low reactivity of polymer molecules with benzyl esters, the latter approach will be mentioned. 

Reetz [13, 65] and also Sivaram [66] have shown that nucleophilic tetrabuty-lammonium salts will initiate living polymerization of acrylates at room temperature. Molecular weights 1500-25,000 are obtained with MWDs of 1.1-1.4. Methyl and ethyl acrylates don t work as well as the more bulky acrylates. Side reactions are end group cyclization and Hoffmann elimination [13] (Scheme 29). 

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