Living anionic polymerization, polypeptides

A variety of important monomers are polymerized via anionic polymerization including cyclic ones like ethylene oxide as base for poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) and A-carboxyanhydrides (NCAs) used for making polypeptides and caprolactam, the monomer of nylon 6 (Eig. 3.9). Whereas in all cases rather stringent conditions have to be used to allow at least an efficient polymerization, for technical products often not fully living conditions are achieved, which leads to broader molar mass distributions. 

In the first way, the polyvinyl block is obtained by anionic polymerization as described before except that the monofunctional initiator (cumylpotassium) is replaced by a bifunctional initiator (dimer dianion of a-methyl-styrene/K). Then the amination of the two living ends and the synthesis of the polypeptide blocks are performed as described before for AB copolymers. 

We find it particularly remarkable that the living anionic and cationic polymerization of unsaturated compounds was a huge success, despite the relatively difficult preparative conditions, while the living character of the Sar-NCA polymerization, which is much easier to handle, went virtually unnoticed by the community (or at least this is our perception 50 years later). Only a handful of research groups studied this class of monomers during the following decades, and mainly to better understand polypeptides [34, 35]. Even less studied was polysarcosine as a material [36 ]. 

Not all monomers are anionically polymerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use such end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymers with polyvinyl and polylactone blocks19 and of copolymers with polyvinyl and polypeptide blocks20-2S). One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene2. ... 

Quite recently,the anionic homopolymerization of a few substituted p-lactams and the copolymerization of some of the above pairs have been smdied in order to prepare polyamide 3-derived polypeptides displaying biological properties. The solution polymerization or copolymerization, initiated by li amide disubstituted with trrmethylsilyl groups and activated with 4-tert-butylbenzoyl chloride, does not have living character. From that study, some insights emerged into the reactivity of the above p-lactams in terms of their acidities, as well as electrophUidty of the imide end groups. 

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