Lipase Catalysis. A Direct Route to Linear Aliphatic Copolyesters of Bis(hydroxymethyl)butyric Acid with Pendant Carboxylic Acid Groups

Enzymatic Synthesis and Characterization of Hydrophilic Sugar Based Polyesters and Their Modification with Stearic Acid

Biodegradable and hydrophilic functional polyesters were synthesized enzymatically using xylitol or d-sorbitol together with divinyl adipate and lipase B from Candida antartica (CAL-B). The resulting polyesters had pendant OH-groups from their sugar units which were esterified to different degrees with stearic acid chloride. The structure and the degrees of polymerization of the resulting graft copolymers based on poly(xylitol adipate) and poly(d-sorbitol adipate) were characterized by ¹H NMR spectroscopy and SEC. DSC, WAXS and SAXS measurements indicated that a phase separation between polymer backbone and stearoyl side chains occurred in the graft copolymers, and, additionally, the side chains were able to crystallize which resulted in the formation of a lamellar morphology. Additionally, nanoparticles of the graft copolymers in an aqueous environment were studied by DLS and negative stain TEM.

Lipases in polymer chemistry

Lipases are highly active in the polymerization of a range of monomers. Both ring-opening polymerization of cyclic monomers such as lactones and carbonates as well as polycondensation reactions have been investigated in great detail. Moreover, in combination with other (chemical) polymerization techniques, lipase-catalyzed polymerization has been employed to synthesize a variety of polymer materials. Major advantages of enzymatic catalysts are the often-observed excellent regio-, chemo- and enantioselectivity that allows for the direct preparation of functional materials. In particular, the application of techniques such as Dynamic Kinetic Resolution (DKR) in the lipase-catalyzed polymerization of racemic monomers is a new development in enzymatic polymerization. This paper reviews selected examples of the application of lipases in polymer chemistry covering the synthesis of linear polymers, chemoenzymatic polymerization and applications of enantioselective techniques for the synthesis and modification of polymers.

Lipase-catalyzed synthesis of azido-functionalized aliphatic polyesters towards acid-degradable amphiphilic graft copolymers

A series of novel aliphatic polyesters with azido functional groups were synthesized via the direct lipase-catalyzed polycondensation of dialkyl diester, diol and 2-azido-1,3-propanediol (azido glycerol) using immobilized lipase B from Candida antarctica (CALB). The effects of polymerization conditions including reaction time, temperature, enzyme amount, substrates and monomer feed ratio on the molecular weights of the products were studied. The polyesters with pendant azido groups were characterized by (1)H NMR, (13)C NMR, 2D NMR, FTIR, GPC and DSC. Alkyne end-functionalized poly(ethylene glycol) containing a cleavable acetal group was then grafted onto the polyester backbone by copper-catalyzed azide-alkyne cycloaddition (CuAAC, click chemistry). Using fluorescence spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM), these amphiphilic graft copolymers were found to readily self-assemble into nanosized micelles in aqueous solution with critical micelle concentrations between 0.70 and 1.97 mg L(-1), and micelle sizes from 20-70 nm. The degradation of these polymers under acidic conditions was investigated by GPC and (1)H NMR spectroscopy. Cell cytotoxicity tests indicated that the micelles had no apparent cytotoxicity to Bel-7402 cells, suggesting their potential as carriers for controlled drug delivery.

Synthesis and Characterization of Branched Polymers from Lipase-Catalyzed Trimethylolpropane Copolymerizations

Lipase-catalyzed terpolymerizations were performed with the monomers trimethylolpropane (B3), 1,8-octanediol (B2), and adipic acid (A2). Polymerizations were performed in bulk, at 70 degrees C, for 42 h, using immobilized lipase B from Candida antartica (Novozyme-435) as a catalyst. To determine the substitution pattern of trimethylolpropane (TMP) in copolymers, model compounds with variable degrees of acetylation were synthesized. Inverse-gated 13C NMR spectra were recorded to first determine the chemical shift positions for mono-, di-, and trisubstituted TMP units and, subsequently, to determine substitution of TMP units along chains. Variation of TMP in the monomer feed gave copolymers with degrees of branching (DB) from 20% to 67%. In one example, a hyperbranched copolyester with 53 mol % TMP adipate units was formed in 80% yield, with Mw 14 100 (relative to polystyrene standards), Mw/Mn 5.3, and DB 36%. Thermal and crystalline properties of the copolyesters were studied by thermogravimetric analysis and differential scanning calorimetry.

Influence of PEG Endgroup and Molecular Weight on Its Reactivity for Lipase-Catalyzed Polyester Synthesis

Polycondensations were performed at 70 degrees C in bulk using physically immobilized lipase B from Candida antarctica (CAL-B) as catalyst. Study of copolymerizations between sebacic acid and PEG diols of differing Mn values (200, 400, 600, 1000, 2000, and 10 000) showed that PEG 400 and 600 were most reactive (DP(avg) up to about 6). Increasing the PEG diol chain length from 600 to 1000, 2000, and 10 000 resulted in large decreases in copolymer DP(avg) values. PEG200 diacids (i.e., HOOC-(CH2)x-O-(CH2CH2O)n-(CH2)x-COOH) were successfully synthesized where x was 1, 4, 5, 7, 9, and 11. Study of copolymerizations of these diacids with 1,8-octanediol showed that, by introduction of a five-carbon methylene spacer (x = 5), remarkable increases in the reactivity of PEG200 diacids were achieved. In addition, introduction of this spacer was also effective for increasing the reactivity of PEG diacids of higher molecular weight (i.e., PEG400, 600, and 1000). This work verified the hypothesis that, by conversion of PEG chain ends to structures more closely resembling fatty acids, modified PEG building blocks are obtained that are better recognized as substrates by CAL-B during condensation reactions.