End-functionalized polyesters

Last but not least, some of us have recently synthesized polyimide-aliphatic polyester triblock and graft copolymers in collaboration with Hedrick and his coworkers [ 97,98]. Well-defined aminophenyl or diaminophenyl end-functional polyester oligomers have been synthesized on purpose and used as end-cappers or macromonomers leading to the aforementioned triblock or graft copolymers, respectively. The polyimide-polyester copolymers so obtained proved to be highly efficient promoters of polyimide nanofoams (for more details see Sect. 4.2). 

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)... 

Lipase catalysts have been used for functionalization of polymers. A terminal hydroxy group of poly-(e-CL) was reacted with carboxylic acids using lipase CA catalyst to give end-functionalized polyesters.231 Lipase MM catalyzed the regioselective transesterification of the terminal ester group of oligo (methyl methacrylate) with allyl alcohol.232 In the PPL-catalyzed reaction of racemic 2,2,2-trichloroethyl 3,4-epoxybutanoate with hydroxy-terminated PEG, the... 

Figure 3. End-functionalized polyesters a) poly(ethylene glycol)-b polyester, b) phenyl azide end-capped, and c) phosphoryl choline end-capped...

Liquid organic rubbers with reactive functionality can be prepared by several methods. End-functional oligomers are preferred. Chains attached to the network at only one end do not contribute as much strength to the network as those attached at both ends [34], Urethane chemistry is a handy route to such molecules. A hydroxy-terminated oligomer (commonly a polyester or a polyether) can be reacted with excess diisocyanate, and then with a hydroxy methacrylate to form a reactive toughener [35]. The methacrylate ends undergo copolymerization with the rest of the acrylic monomers. The resulting adhesive is especially effective on poIy(vinyl chloride) shown in Scheme 2. 

End-functional polymers were also synthesized by lipase-catalyzed polymerization of DDL in the presence of vinyl esters [103,104]. The vinyl ester acted as terminator ( terminator method ). In using vinyl methacrylate (12.5 mol % or 15 mol % based on DDL) and lipase PF as terminator and catalyst, respectively, the quantitative introduction of methacryloyl group at the polymer terminal was achieved to give the methacryl-type macromonomer (Fig. 12). By the addition of divinyl sebacate, the telechelic polyester having a carboxylic acid group at both ends was obtained. 

Amongst the first studies presenting the use of dendritic polymers for thermoset applications was the work of Hult et al. [62]. They modified hyperbranched hydroxy functional polyesters with various ratios of maleate-allyl ether/alkyl ester end groups. Dependent on this ratio, resins with different vis-... 

Fig. 20. Plot of the exponent ras a function of the chain lengths between two branching points. Open symbols results by Colby et al. [118,119] for branched polyesters. The variation of this length was achieved by co-condensation of trifunctional monomers with increasing fraction of bifunctional units. Filled symbols refer to polydicyanurates (N Nj ), anhydride cured phenyl monoglycidyl ether with a small fraction of bisphenol A diglyci-dylether as crosslinker (N=8Nj ) and end-functionalized 3-arm polystyrene stars crosslinked by diisocyanate (N 40 N ). N denotes the number of repeating units between two branching points...

The next step concerned the synthesis of the polyester-polyimide copolymers. Triblock copolymers have been prepared by a step-growth copolymerization of stoichiometric amounts of an aromatic diamine and dianhydride (e.g., PMDA and 3FDA, as depicted in Scheme 41a) added with the single oo-amino polyesters as chain end-cappers. Graft copolymers can be prepared as well. In this case, the diamine end-functionalized oligomeric macromonomers are copolymerized with the polyimide condensation comonomers (Scheme 41b). 

Functional polyesters have been synthesized by utilizing characteristic catalysis of lipase. An optically pure polyester was synthesized by PPL-catalyzed enantioselective polymerization of bis(2,2,2-trichlo-roethyl) trans-3,4-epoxyadipate with 1,4-butanediol in diethyl ether (Scheme 26).212 The molar ratio of the diester to the diol was adjusted to 2 1 to produce the (-)-polymer with an enantiomeric purity of >96%. From end group analysis, the molecular weight was calculated to be 5.3 x 103. 

Finally, the polyesters, regardless of the synthesis route applied, can be modified after polymerisation to introduce reactive end-functionalities or alter the physical properties of the material [97,98]. 

---