The polymerization of e-caprolactone, (e-CL) using porcine pancreatic lipase (PPL) as the catalyst was studied. Polymerization reactions (4 days, 65 °C) of e-CL at ~10% (w/v) concentrations in dioxane, toluene, and heptane using butanol as an initiating species (monomer/butanol ratio = 14.7) gave poly(e-caprolactone) (PCL) with Mn values (by GPC) of 313, 753, and 1600, respectively. Monomer conversion to PCL for these polymerizations was 33, 55, and 100%, respectively. Mn measurements of PCL products by NMR end group analyses were slightly lower (by a factor of -0.9) than the values obtained by GPC. Polymerizations conducted in heptane at 37, 45, 55, and 65 °C showed the highest extent of monomer conversion at 65 °C. Therefore, subsequent studies were conducted at 65 °C in heptane. For a polymerization carried out with a 15/1 monomer/butanol ratio and ~0.29 mmol of water, ~70 and ~100% of the monomer had been converted to PCL by reaction times of 24 and 96 h, respectively. Polymer molecular weight increased slowly with conversion, suggesting that this is a chain polymerization with rapid initiation and slow propagation. Increases in the e-CL/butanol ratio from 15/1 up to where no butanol was added showed only a modest increase in product molecular weight from 1600 to 2700. This was explained by the fact that the water present in polymerizations was active in chain initiation. Variation in the monomer/butanol ratio at constant water concentration resulted in PCL chains with 0-0.65 mol fraction of butyl ester and 0.33-0.86 mol fraction of carboxylic acid chain end groups (by NMR analyses). The presence of water concentrations in polymerization reactions above that which is strongly enzyme bound is believed to be an important factor which limited the formation of PCL chains of significantly higher molecular weight.

Starting from L-tyrosine (Tyr) and its metabolites desaminotyrosine (Dat) and tyramine (Tym), four structurally related model dipeptides were prepared: Dat-Tym (neither N- or C-terminus present), Z-Tyr-Tym (N-terminus protected by benzyloxycarbonyl), Dat-Tyr-Hex (C-terminus protected by a hexyl ester group), and Z-Tyr-Tyr-Hex (both N- and C-termini present, protected by benzyloxycarbonyl and hexyl ester, respectively). The model dipeptides were used as monomers in the synthesis of polycarbonates. The polymerization reaction in the presence of either phosgene or triphosgene proceeded via the phenolic hydroxyl groups. Polymers with molecular weights of 105,000-400,000 da (by gel permeation chromatography, relative to polystyrene standards) were obtained. The physicomechanical properties (solubility, mechanical strength, glass transition and decomposition temperature, processibility) of the polymers were determined, and an attempt was made to correlate the polymer properties with the nature of the N- and C-terminus protecting groups. The presence of the urethane bond at the N-terminus protecting group was found to reduce solubility, ductility, and processibility, probably due to interchain hydrogen bonding. The presence of a C-terminus alkyl ester group increased solubility and processibility. Thus, the most promising candidate polymer for biomedical applications was obtained from Dat-Tyr-Hex, the monomer carrying a C-terminus protecting group only. Since very similar results had recently been obtained for a series of structurally related polyiminocarbonates, the structure property correlations seem to be generally valid.

The gradual shift from biostable prostheses to degradable, temporary implants represents one of the most significant trends in biomaterials research. In view of this trend, medical applications of degradable implant materials were reviewed with special emphasis on orthopedic polymeric implants. Among the polymeric implant materials derived from natural sources, collagen, various polysaccharides such as cellulose, and microbial polyesters have been intensively investigated. Among the synthetic, degradable polymers, aliphatic polyesters such as poly(glycolic acid), poly(lactic acid), poly(caprolactone) and polydioxanone, are most commonly investigated. Only recently, several new classes of polymers such as poly(ortho esters), polyanhydrides, and degradable polycarbonates have been introduced as potential implant materials. A particularly versatile group of new biomaterials with promising engineering properties are the "pseudo"-poly(amino acids), amino acid derived polymers in which conventional peptide bonds have been replaced by various chemical linkages.

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