Cationic graft copolymer Polymerization

The completely cationic synthesis of comb or graft copolymers have yet to be realized [103]. However, numerous backbone polymers, branches and macromonomers have been prepared separately via cationic polymerization and these have been combined with other grafting and polymerization processes to prepare (co)polymers that cannot easily be prepared otherwise [103]. 

The metallocene catalyst with cationic nature and spatially opened active site provides favorable condition for the incorporation of p-alkylstyrene (p-ms) to polyolefins. The p-ms groups can be easily metallated to produce "stable" polymeric anions for graft-from polymerization. With the coexist of anion-polymerizable monomers, we have prepared many graft copolymers, such as PE-g-PS, PE-g-PMMA, PE-g-PAN, PP-g-PS, PP-g-PB, PP-g-PI and PP-g-PMMA. 

Table 4. Graft copolymers obtained by grafting from and grafting onto techniques (names of the polymers obtained by cationic polymerization are italicized)...

The homopolymer of the pyridinium salt monomer readily initiates the cationic polymerization of BOE upon heating to around 120 °C, to yield grafted copolymers. The copolymer with styrene similarly catalyzed the polymerization to form the corresponding grafted copolymers. The initiation activity of the copolymer in the polymerization was higher than that of the homopolymer but lower than that of the low-molar mass analogue, N-benzyl-p-cyanopyridinium hexafluoroantimonate. 

N-Benzyl and iV-alkoxy pyridinium salts are suitable thermal and photochemical initiators for cationic polymerization, respectively. Attractive features of these salts are the concept of latency, easy synthetic procedures, their chemical stability and ease of handling owing to their low hygroscopicity. Besides their use as initiators, the applications of these salts in polymer synthesis are of interest. As shown in this article, a wide range of block and graft copolymer built from monomers with different chemical natures are accessible through their latency. 

As already mentioned, cationic ring-opening polymerization offers several possibilities of preparing useful high molecular weight polymers, which may find applications as such. The main interest, however, is in preparation of reactive polymers, which are used in synthesis of block or graft copolymers. 

Several graft copolymers were prepared by copolymerization of macromonomers described in the previous section. Another route-to-graft copolymer is based on conversion of suitable groups along the chain of preformed polymer into species capable of initiating the cationic ringopening polymerization of suitable monomer. Thus, for example, polym-... 

The use of polymeric initiators or coinitiators to induce the polymerisation of a second monomer by a cationic mechanism is a particularly attractive possibility, since it would permit the synthesis of block and graft copolymers. The search for adequate systems in this context has been intensive, but only very recently has it met with some success, and this is far from being as spectacular as the achievements obtained in the same area with anionic systems. The main difficulties to be surmcwntedhave been discussed in the general introduction to this review (see Chap. I), and have to do with the ubiquity of transfer and termination reactions in cationic polymerisation. Nevertheless, the advances of the last few years seem encouraging and one would expect that the near future will provide considerable progress, both quantitative and qualitative. 

Trimethylsilyl triflate can initiate the cationic polymerization of different vinyl and heterocyclic monomers (28). The triflated poly silane can react in the same way and can induce growth of a large number of chains. In this way, comblike graft copolymers can be prepared. 

The synthesis of polystyrene-g-polytetrahydrofurane [188] was achieved by ATR copolymerization of methacrylic PTHF macromonomer, MA-PTHF, with styrene (Scheme 105). The PTHF macromonomer was synthesized by cationic ring opening polymerization of THF with acrylate ions, formed by the reaction of methacryloyl chloride and AgC104. The polydispersity indices of the graft copolymers determined by SEC ranged between 1.3-1.4. Kinetic studies revealed that the relative reactivity ratio of the macromonomer to St was independent of the molecular weight of PTHF. 

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