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Natural-origin (R)-polyOS-hydroxybutyrate) (PHB) was degraded by acid-catalyzed methanolysis to form low molecular weight (average DP = 26) stereoisomerically pure PHB chains. The hydroxyl terminus of these polymers was then reacted with AlEta to form a PHB-O-AlEti macroinitiator species. These macroinitiators were then used to carry out the ring-opening polymerizations of c-caprolactone and lactide monomers to prepare PHB-PCL, PHB-d,l-PLA, and PHB-l-PLA A-B diblock copolymers of variable chain segment lengths. The various A-B diblocks were characterized by VPO to determine the Mn values, and therefore the respective chain segment lengths. Analysis by GPC showed that, in all cases, the diblocks gave unimodal peaks. The identification of the NMR methyl doublet which corresponds to the hydroxyl end group repeat unit for the PHB prepolymer, its subsequent disappearance after the formation of the PHBl-PLA diblock, and the identification of the resultant *H NMR methyl doublet due to the new hydroxyl terminal L-lactide repeat unit were used in combination with the VPO and GPC results to confirm that we did indeed form these new PHB diblock copolymers. The diblock copolymers were analyzed by DSC and X-ray diffraction to determine their thermal and crystalline properties, respectively. It was shown that for diblocks (PHB DP = 26 in all cases) with short PCL (DP = 12) and l-PLA (DP = 13) chain segments, PCL and PLA crystalline phases did not form. With increased chain lengths of the B chain segment (PCL DP = 38 and 51, l-PLA DP = 23) the crystallization of both components of the diblock was observed as a superposition of the respective X-ray diffraction patterns. The PHB-l-PLA diblock (PHB DP = 26, l-PLA DP -23) when melted and then rapidly quenched from the melt was kinetically frozen into a solid-state morphology such that miscibility of the two component chain segments resulted (a new T, at approximately 20 °C was observed). Phase separation occurred when the diblock copolymer was annealed at 55 °C for 24 h prior to DSC analysis.

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