Monofunctional living chain ends

In a preliminary study (13) of the reaction betwen monofunctional living polystyrene and AIBN, we have shown that a coupling reaction occurs leading to the incorporation of azo groups either in the middle of the chain or at the end. 

The same authors chose another very reactive nucleophilic function, the silyl enol ether group, which upon reaction with living cationic chain ends of poly(vinyl ether)s, also leads to a carbon-carbon bond with formation of a ketone (Scheme 4). Model reactions of living poly(IBVE) with various monofunctional silyl enol ethers [47] showed that the a-substituent R should have electron-donating properties in order to increase the electron density on the double bond. 

Most interesting from the standpoint of commercial development is the formation of block copolymers by the living polymer method. Sequential addition of monomers to a living anionic polymerization system is at present the most useful method of synthesizing well-defined block copolymers. Depending on whether monofunctional or difunctional initiators are used, one or both chain ends remain active after monomer A has completely reacted. Monomer B is then added, and its polymerization is initiated by the living polymeric carbanion of polymer A. This method of sequential monomer addition can be used to produce block copolymers of several different types. 

On the other hand, a functionality of 1 on each chain end will be related to telechelic compounds (Scheme 1). This includes diols, diamines, and diacides. Of course it also comprises diolefin compounds that usually lead to gels or networks. We can also note that when the G and G functional groups are different at each chain end, the appropriate term becomes heterotelechelic (Table 1). It is also necessary to specify the particular case of macromolecules bearing a well-identified G functionality at one chain end and a thermally reactivated group at the G chain end. These groups can be nitroxides, an iodine atom, xanthate, etc.and are commonly used in living radical polymerizations (LRP). These compounds may be classified as monofunctional oligomers (Table 1). 

More complicated architectures can also be prepared by such a strategy. A dumbbell-shaped polymer with five PI chains on the ends of a PS chain was obtained via difunctional initiation (the product of the reaction of MDDPE and sBuLi) of S. The living chain was reacted (using the titration technique[3]) with a hexafunctional chlorosilane followed by coupling with PILi. Monofunctional instead of difunctional initiation lead to PS(PI)5 hexa-arm stars (76 < Mn,Ps-connector/(kg/mol) < 168 Mn,arm = (8 2)kg/mol, 1.02 <... 

Both methods were successfully applied to functionalize one chain end with diamino groups. The utility of monofunctional and bifunctional polymers in the preparation of block and graft copolymers was also demonstrated by using a dianhydride and a diamine as comonomers in a manner similar to that described in Section I. No homopolystyrene was detected, indicating the accurate functionalization of the chain end by living radical polymerization. 

In all arm-first methods a living monofunctional polymer of known length and low polymolecularity serves as a precursor. Subsequently, the active sites located at chain end can be used in one of two different ways ... 

Monofunctionalized PTHF is thus obtained by end capping living polymers with monofunctional initiators. Bifunctional initiators must be used to synthesize bifunctional telechelics. Yamashita has described the synthesis of bis(dioxolan-2-ylium) cations (60) and their use as initiators for the polymerization of THF. Bi- and tri-functional initiators (61)-(63) were synthesized by Penczek and co-workers. Chains reportedly grow independently at all sites, and copolymers can be formed if 10-20% of THF is replaced by methyl oxirane. ... 

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