Copolymerization coupling, termination

In copolymerization, several different combinations of initiation and termination mechanisms are possible, giving rise to a variety of different polymerization rate equations. Only two cases will be singled out here free-radical copolymerization with termination by coupling, and ionic polymerization with termination by chain transfer to a deactivating agent or impurity. For other combinations, the derivation of rate equations follows along the same lines. 

Coupling of GTP living polymers with halide-terminating agents to form star polymers has been achieved [Hertler, 1996 Webster and Sogah, 1989]. Star polymers are also synthesized by using polyfunctional initiators or by copolymerization with dimethacrylate monomers. 

The slow rate of copolymerization in acetone was related to the ease of termination of macroradicals by coupling. This coupling was hindered by the coiling of the macroradical chains in benzene, but propagation continued to take place since the monomers were able to pene-... 

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 inverted sequence of the same procedure has also been used [139] to prepare the same three-block copolymers. Indeed, thermal polymerization of MMA by ABME gives rise to a polymer mainly containing only one benzoin methyl ether moiety per macromolecule, since growing MMA radicals terminate mostly by disproportionation. Thus, terminally photoactive poly(MMA) is used to obtain the photoinitiated block copolymerization of styrene. In this case, a 90% yield of block copolymers is obtained, appreciably higher than in the preceding method, fully consistent with the usual assumption that the termination in styrene polymers occurs by combination. In fact, coupling of the growing styryl radicals with the less reactive poly(MMA)-bound methoxy benzyl radicals also contributes to the formation of block copolymers. 

Neutral catalysts or catalyst precursors based on fluorinated ligand systems have been applied in compressed CO2 to a broad range of transformations such as Zn- and Cr-catalyzed copolymerization of epoxides and CO2 [53, 54], Mo-catalyzed olefin metathesis [9], Pd-catalyzed coupling reactions [43, 55, 56] and Pd-catalyzed hydrogen peroxide synthesis [57]. Rhodium complexes with perfluoroalkyl-substituted P ligands proved successful in hydroformylation of terminal alkenes [28, 42, 44, 58], enantioselective hydroformylation [18, 59, 60], hydrogenation [61], hydroboration [62], and polymerization of phenylacetylene... 

While it is assumed that termination by coupling takes place when maleic anhydride and styrene are copolymerized in a good solvent such as acetone, insoluble macroradicals precipitate when these monomers are copolymerized in a poor solvent such as benzene (7). Insoluble macroradicals obtained by bulk polymerization of acrylonitrile (1, 11) and the solution copolymerization of maleic anhydride and styrene in benzene (7) have been used as seeds for the preparation of block copolymers. 

Also, it has been reported that poly(styrene-co-maleic anhydride-b-styrene) could be obtained either by addition of styrene monomer to the styrene-maleic anhydride macroradicals or by the free-radical-initiated copolymerization of maleic anhydride with more than an equimolar proportion of styrene in benzene solution (7). However, the maximum amount of styrene present in these block copolymers was less than 35% of the weight of the original macroradical. This limitation on the yield of the block copolymer is now assumed to be related to the increased solubility of the styrene block in the benzene solvent, which permits termination of the new macroradicals by coupling. 

With the purpose of conferring thermosensitivity to chitosan-based hydrogels. Park et al. proposed the grafting of carboxylic acid-terminated poly(ethylene oxide-h-propylene oxide) block copolymer (Pluronic) onto the primary amine of chitosan, mediated by EDC coupling agent [102]. With the same purpose, Wang et al. grafted poly(N-isopropyl acrylamide) (NiPAM) chains onto chitosan by the copolymerization of acrylic acid-derivatized chitosan and N-isopropylacrylamide (NIPAAm) in aqueous solution [103]. 

Won et al. [19], have reported synthesis of polyesters with valine, leucine, isoleucine, methionine, and phenylalanine (Table 12.1). This three-step process involves synthesis of a diester and a dinitro compound that are copolymerized [19], An amino acid is first coupled with a diol (with 3, 4, or 6 methylene groups) in the presence of tosyl to yield a diester with acid salts of diamine at the terminal ends. The second monomer, di-p-nitrophenyl ester of carboxylic acids, is synthesized by a condensation reaction of adipoyl or se-bacoyl chloride with p-nitro phenol. The final polymerization step involves an arduous condensation reaction in the presence of a strong proton abstractor between acid salt of bis(amino acid-alkyne diester) and di-p-nitrophenyl ester of dicarboxylic acids. Following along the same lines, Chu and Guo [22] have copolymerized a mixture of nitro phenyl ester of succinate, adipate, or sebacate and nitrophenyl fumarate with toluenesulfonic acid salt of phenylalanine butane-1,4-diester. The addition of fumarate derivative to the monomer mixture provides an unsaturated double bond in the polymer backbone that can be functionalized for specific biomedical... 

Another example of chitosan-based pDNA carriers derived by copolymerization was chitosan-NIPAAm/vinyl laurate (VL) copolymer, which was reported by Sun et al. (2005), and was prepared by coupling a carboxyl-terminated A-isopropylacrylamide/VL copolymer with chitosan (Mw = 2kDa). Despite its transfectability (at N/P ratios ranging from 2 1 to 20 1) being superior to naked DNA in C2C12 mouse skeletal muscle cells, its maximal efficacy obtained was only 50% of that of Lipofectamine 2000. For the practical application of the vector in NA therapy, a further boost to its NA delivery efficiency is required. 

Copolymerization of TFE with perfluoroalkyl vinyl ethers proved to be quite facile, either by dispersion techniques similar to that employed with FEP [7] or by a newly developed process ploying a fluorocarbon solvent. However, from the earliest studies it was evident that some complications had to be overcome. The most significant of these was a tendency for the alkyl vinyl ethers to rearrange when exposed to free radicals. In the extreme case a chain reaction could be initiated which would result in incomplete rearrangement to the isomeric acid fluoride. During polymerization at temperatures low enough to prevent excessive reaction by this route, the process, nevertheless, competes effectively with free radical coupling as a termination mechanism. 

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