Lipases pure monomers

Conventional ring-opening polymerization of cyclic anhydrides, carbonates, lactones, and lactides require extremely pure monomers and anhydrous conditions as well as metallic catalysts, which must be completely removed before use, particularly for medical applications. To avoid these difficult restrictions, an enzymatic polymerization may be one of the more feasible methods to obtain the polyesters. This method was first reported by two independent groups (Kobayashi [152] and Gutman [153]) who showed that lipases, enzymes capable of catalyzing the hydrolysis of fatty acid esters, can polymerize various medium-sized lactones. 

Lipase-catalyzed Synthesis and Polymerization of Optically Pure Monomers... 

One of the easiest ways to prepare chiral polymers is the polymerization of optically pure monomers. These monomers can be derived or isolated from the chiral pool, be synthesized from prochiral substrates using asymmetric catalysis, or be obtained by lipase-catalyzed resolution of a racemate followed by further synthetic manipulation. 

The use of lipase-catalyzed kinetic resolution as key step in monomer synthesis is a powerful approach for the synthesis of optically-enriched chiral polymers. An alternative route towards the synthesis of chiral polymers consists of the enzymatic polymerization of optically pure monomers. 

Figure 11.2 Examples of lipase-catalyzed kinetic resolution for the preparation of optically pure monomers that were subsequently chemically polymerized.

Polycarbonates have attracted attention in recent years because of their potential use in biomedical applications based on their biodegradability, biocompatibility, low toxicity and good mechanical properties [67]. These polymers can be prepared by the ROP of cyclic carbonate monomers by anionic, cationic, and coordination catalysts. However, lipase-catalyzed polymerization seems to be a feasible alternative to prepare polycarbonates as chemical methods often suffer from partial elimination of carbon dioxide (resulting in ether linkages), require extremely pure monomers and anhydrous conditions. 

Polymers derived from natural sources such as proteins, DNA, and polyhy-droxyalkanoates are optically pure, making the biocatalysts responsible for their synthesis highly appealing for the preparation of chiral synthetic polymers. In recent years, enzymes have been explored successfully as catalysts for the preparation of polymers from natural or synthetic monomers. Moreover, the extraordinary enantioselectivity of lipases is exploited on an industrial scale for kinetic resolutions of secondary alcohols and amines, affording chiral intermediates for the pharmaceutical and agrochemical industry. It is therefore not surprising that more recent research has focused on the use of lipases for synthesis of chiral polymers from racemic monomers. 

The apparent kinetic constants of the molecularly pure cyclic BA monomer to trimer for lipase CA were determined according to the literature using Lineweaver-Burk plots for the enzymatic ring-opening reaction of the cyclic oligomers (13,14). The apparent and Froa/Am of lipase CA for the... 

Enantiomerically pure functional polycarbonate, having many potential biomedical uses, was synthesised from a novel seven-membered cyclic carbonate monomer derived from naturally occurring L-tartaric acid, using four commercially available lipases from different sources at 80 ° C, in bulk. The highest number-average MW, Mn = 15,500 g/mol, polydispersity index (PDI) = 1.7, [a]o ° = +77.8 and melting temperature (Tm) = 58.8 °C, optically active polycarbonate was obtained with lipase CALB [61]. 

---