Living alternating copolymerizations

The successive insertion of CO and olefin into a Pd—C bond leads to an alternating copolymerization of CO and olefins [ 100]. When the reaction is carried out with an isolated, cationic acetylpalladium catalyst, living polymerization giving a singly dispersed polymer proceeds (eq (72)) [lOI]. 

The stepwise alternating insertions of CO and an alkene into Pd-C bonds comprise important steps in living catalysts for the alternating copolymerization [65]. The olefin insertion into acyl complexes provides cationic alkyl species in which a carbonyl oxygen is coordinated to the palladium center as shown in Eq. 7.4 The chelating alkyl complexes, whose presence has been confirmed by several research groups, would give extra stabilization to prevent occurrence of /I-elimination. 

Polymerization of ROZI monomers behaved in almost all cases similarly to ROZO monomers in many respects, except for the product polymer structures with a trimethylenimine chain of PROZI instead of an FI chain of PROZO. Many similar behaviors in both monomers of ROZI and ROZO include a living character of CROP, various copolymer syntheses, DIP, and an M monomer in zwitterionic alternating copolymerizations. It may be important to mention, however, that ROZI is a little more basic than ROZO from the initiation reaction studies for example, pKa value for iPrOZI and iPrOZO are 7.1 and 5.4 in water at 25 °C, respectively. 

An alternative route for the preparation of styrenic macromonomers is the reaction of living chains with 4-(chlorodimethylsilyl)styrene (CDMSS) [192]. The key parameter for the successful synthesis of the macromonomers is the faster reaction of the living anionic chain with the chlorosilane group rather than with the double bond of the CDMSS. Anionic in situ copolymerization of the above macromonomers (without isolation) with conventional monomers leads, under appropriate conditions, to well-defined comb-like chains with a variety of structures. 

All authors accept the alternating incorporation of epoxide and anhydride into the macromolecular chain 36 39.40.45 52.73-74). However, the mechanisms of termination and chain transfer have not yet been elucidated. Although the lability of the nitrogen atom is obvious 39 40 44> and its salts or associates are readily thermally decomposed 89), Fischer 39 detected its presence in precipitated polyesters by elemental analysis. A simple calculation confirms the presence of the nitrogen atom in almost every tenth macromolecule. In this case, the isolated polyester might be a living polymer and, on the addition of monomers, it might initiate another copolymerization. Similar experiments have not been reported so far. 

Frechet and coworkers recently described how living free radical polymerization can be used to make dendrigrafts. Either 2,2,6,6-tetramethylpiperidine oxide (TEMPO) modified polymerization or atom transfer radical polymerization (ATRP) can be used [96] (see Scheme 10). The method requires two alternating steps. In each polymerization step a copolymer is formed that contains some benzyl chloride functionality introduced by copolymerization with a small amount of p-(4-chloromethylbenzyloxymethyl) styrene. This unit is transformed into a TEMPO derivative. The TEMPO derivative initiates the polymerization of the next generation monomer or comonomer mixture. Alternatively, the chloromethyl groups on the polymer initiate an ATRP polymerization in the presence of CulCl or CuICl-4,4T dipyridyl complex. This was shown to be the case for styrene and n-butylmethacrylate. SEC shows clearly the increase in molecu-... 

Additionally, graft copolymers can be prepared by a grafting onto method that involves coupling living polymers to reactive side groups on a prepolymer. An alternative approach to the preparation of graft copolymers involves the use of macromonomers. A macromonomer is a prepolymer with terminal polymerizable C=C bond. In this method, graft copolymers are produced by copolymerization of the macromonomer with another olefinic monomer as shown in Fig. 14.26. 

An elegant alternative to living polymerization for the preparation of block polymers is to use functionalized Grignard initiators. The polymerization of methyl methacrylate to isotactic (in toluene at — 78"C) or syndiotactic polymers (in THF at — llO C) can be initiated by o-, m-, and p-vinylbenzylmagnesium chloride. The polymers had a low polydispersity and contained one vinylbenzyl group at the chain end, by H-NMR. The poly(methylmethacrylate) macromers thus obtained were polymerized or copolymerized with styrene to give graft and block polymers of controlled architecture [50,51]. 

---