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Two types of mixtures were prepared by solution blending: high molecular weight polyhydroxybutyrate (PHB)/poly(e-caprolactone) (PCL) and PHB/low molecular weight chemically modified PCLs (mPCL). The morphology, crystallization, and enzymatic degradation of the blends were studied by differential scanning calorimetry, polarized light optical microscopy, scanning electron microscopy, 1 H NMR, and weight loss measurements. In addition, enzymatic degradation studies were performed by an exposure to Aspergillus flavus. High molecular weight PHB/PCL blends were found to be immiscible in the entire composition range. Phenomena such as PCL fractionated crystallization and a decrease in PHB nucleation density were detected. When PHB was blended with mPCLs, the blends were partially miscible; two phases were formed, but the PHB-rich phase exhibited clear signs of miscibility through a depression of both the T m and the Tg of the PHB component (which was stronger with lower molecular weight mPCL), and an increase in the growth rate of PHB spherulites in the blends as compared to neat PHB or to the PHB component in the PHB/PCL blends. The biodegradation by a exposure to A. flavus showed that the blends are synergistically attacked in comparison to the homopolymers. Two factors may influence the improved degradation rate of the blends: the dispersion of the components and their crystallinity that was reduced in view of the fractionated crystallization and impurities transfer. In the case of the PHB/mPCL blends, the increased miscibility between the components caused a reduction in the degradation rate.

The hydrogenation of polystyrene-block-polybutadiene-block-poly(£-caprolactone) SBC triblock copolymers was performed in the presence of the Wilkinson catalyst RhCl(P(C 6 H 5 ) 3 ) 3 . Reaction conditions (hydrogen pressure, temperature and reaction time) were varied to ensure quantitative hydrogenation without detectable side reactions. Gel permeation chromatography showed no broadening of the molecular weight distribution during hydrogenation. The efficiency of the catalyst is markedly influenced by the molecular weight of the copolymer. Due to the presence of the polyethylene (PE) block, the resulting polymers exhibit a reduced solubility in comparison to the starting materials. Using differential scanning calorimetry (DSC), preliminary results about the crystallization and melting behavior of the PE-block were obtained. In the triblock copolymers, the PE-block showed a marked depression of the melting point and crystallinity when compared to pure hydrogenated polybutadiene of equivalent molecular weight and microstructure or to a comparable PE-block within a polyethylene-block-poly(e-caprolactone) diblock copolymer. A fractionated crystallization process of the PCL-block was observed when the PCL component in the hydrogenated triblock copolymers was present as a minor phase.

The dynamic crystallization and the melting behavior of polystyrene-block-poly(e-caprolactone) (PS-b-PCL, short notation SC), polybutadiene-block-poly(e-caprolactone) (PB-b-PCL, BC) and polystyrene-block-polybutadiene-block-poly(e-caprolactone) (PS-b-PB-b-PCL, SBC) have been studied using differential scanning calorimetry. The copolymers with high molecular weight exhibit microphase separation into microphases consisting of polystyrene, polybutadiene and poly(e-caprolactone) and partial crystallization of the corresponding PCL block. The crystallization occurs at temperatures below the PS glass transition. Depending on the block copolymer composition, crystallization takes place through a combination of heterogeneous and homogeneous nucleation. Isothermal crystallization was studied in order to determine the equilibrium melting point, T m 0 , of the PCL block, which depends on the weight ratio w PB :w PCL . The crystallization kinetics was analyzed in terms of Avrami equation. A general decrease in the overall crystallization rate in the block copolymers relative to the equivalent PCL homopolymer was found. Additionally, the growth rate of the spherulites was followed using polarized optical microscopy. Depending on the composition and flexibility of the block connected to PCL, some copolymers showed ring-banded spherulites.

The sequential anionic polymerization of polystyrene-block-polybutadiene-block-poly(e-caprolactone) (SBC) triblock copolymers as well as polystyrene-block-poly(e-caprolactone) (SC) and polybutadiene-block-poly(e-caprolactone) (BC) diblock copolymers was achieved in benzene. To initiate the polymerization of the highly reactive e-caprolactone, the nucleophilicity of the carbanion has to be reduced. For this purpose 1,1-diphenylethylene (DPE) was used. To avoid inter- and intramolecular transesterification reactions of the growing caprolactone block, the reaction time of this monomer in the block copolymers was strictly controlled. The reaction between polybutadienyl anions and DPE is too slow, and an incomplete reaction results in side reactions when e-caprolactone is added. An alternative method was developed, which consisted in the addition of a small quantity of styrene, after the polymerization of butadiene, to diminish the total reaction time and reduce the probability of chain transfer.