Chemoselective polymerization

Fig. 29 Functionalized lactones chemoselectively polymerized by aluminum and tin aUcoxides [124-135]...

Uyama H, Lohavisavapanich C, Ikeda R et al (1998) Chemoselective polymerization of a phenol derivative having a methacryl group by peroxidase catalyst. Macromolecules 31 554-556... 

Polymerization of vinyl monomers proceeds much slower than the natural polymerization of for example, phenolic compounds. This can be exploited for the chemoselective polymerization of vinyl-substituted phenols and aminophe-nols (see Chapter 7) [41, 43]. Kobayashi and coworkers reported the selective polymerization of methacrylate-esters [41]. Thus, phenol-esters were selectively polymerized via the phenolic moieties leaving the methacrylate functionality unmodified (Figure 6.9). The latter was only attacked in the absence of polymerizable phenols. 

Figure 6.9 Chemoselective polymerization of bifunctional (phenol/vinyl) monomer units using HRP [41],...

Regarding the production of polymers (Kobayashi and Higashimura 2003) oxidases have been used to catalyze the chemoselective polymerization of phenolic monomers having a reactive functional group like methacryloyl and also the induced polymerization of syringic acid that cannot be polymerized by conventional metal catalysis. Other example is the use of tyrosinase to produce the coupling of phenoxy radicals from 2- and/or 6-unsubstituted phenols. For example, crystalline... 

Scheme 23.3 Chemoselective polymerization of a phenol derivative having a methacryloyl group by peroxidase catalyst.

In the chemoselective polymerization of monomer 1 sequencing of siloxane units in the polymer chain depends exclusively on the way in the which monomer is opened and added to the end of the growing chain. There are three non-equivalent places of opening of 1 marked by a, b, c in equation 4, which lead to three different arrangements of siloxane groups in the open chain monomer units. 

Fig.1 Chemoselective polymerization of phenol derivative by horseradish peroxidase...

Uyama, H., Lohavisavapanich, C., Ikeda, R., Kobayashi, S. (1998) Chemoselective Polymerization of a Phenol Derivative Having a Methacryl Group by Peroxidase Catalyst, Macromol, 31,554— 556. 

The tendency of nitrones to react with radicals has been widely used in new synthetic routes to well-defined polymers with low polydispersity. The recent progress in controlled radical polymerization (CRP), mainly nitroxide-mediated polymerization (NMP) (695), is based on the direct transformation of nitrones to nitroxides and alkoxyamines in the polymerization medium (696, 697). In polymer chemistry, NMP has become popular as a method for preparing living polymers (698) under mild, chemoselective conditions with good control over both, the polydispersity and molecular weight. 

The stereoselectivity mechanisms for polymerizations of dienes present several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of allylic type rather than of o type, as for the monoalkene polymerizations. There is experimental evidence, also supported by molecular modeling studies, that a relevant role for chemoselectivity and stereoselectivity is also played by the chirality of the back-biting coordination to the metal of the double bond of the polydienyl chain closest to the coordinated allyl group. 

Last but not least, enzymatic polymerization is more chemoselective than chemical polymerization as witnessed, for instance, by the successful polymerization of functionalized lactones bearing unsaturations and epoxides (Fig. 31) [150]. 

Enzymatic polymerizations are an emerging research area with not only enormous scientific and technological promise, but also a tremendous impact on environmental issues. Biocatalytic synthetic pathways are very attractive as they have many advantages, such as mild reaction conditions, high enantio-, regio- and chemoselectivity, and the use of nontoxic natural catalysts. 

Also, Kerep and Ritter reported a radical chain transfer agent as a dual initiator, FRP-1 [45]. The first step builds on the fact that hydroxyl groups are much better nucleophiles in enzymatic ROP than thiols. Due to the chemoselectivity of the enzyme, PCLs with predominantly thiol endgroups were obtained, which were subsequently used as macroinitiator for styrene. The authors report that the reaction yield can be further increased by microwave irradiation. Although thiols provide less control over the radical polymerization than RAFT agents, the subsequent radical polymerization successfully leads to the synthesis of PCL-Z -PS. 

In a related approach, Padovani et al. prepared copolymers of styrene and a styrene derivative containing two pendant ester bonds using free-radical polymerization (Scheme 15) [108], Transesterification reactions were conducted with Novozym 435 as the catalyst and benzyl alcohol or (rac)-l-phenylethanol as the nucleophile. Interestingly, the ester bond closest to the polymer backbone (position A in Scheme 15) remained unaffected, whereas ester bond B reacted in up to 98% to the corresponding benzyl ester. The transesterification was not only highly chemoselective but also enantioselective. Conversion of (rac)-l-phenylethanol in the transesterification reaction amounted to a maximum conversion of 47.9% of the (/ )-alcohol, and only at the ester position B. 

Other authors have described the lipase-catalyzed chemoselective acylation of alcohols in the presence of phenolic moities [14], the protease-catalyzed acylation of the 17-amino moiety of an estradiol derivative [15], the chemoselectivity in the aminolysis reaction of methyl acrylate (amide formation vs the favored Michael addition) catalyzed by Candida antarctica lipase (Novozym 435) [16], and the lipase preference for the O-esterification in the presence of thiol moieties, as, for instance, in 2-mercaptoethanol and dithiotreitol [17]. This last finding was recently exploited for the synthesis of thiol end-functionalized polyesters by enzymatic polymerization of e-caprolactone initiated by 2-mercaptoethanol (Figure 6.2)... 

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