Synthesis and Rheological Characterization of Star-Shaped and Linear Poly(hydroxybutyrate)

Indium and zinc complexes, [(NNO tBu )InCl] 2 (μ-Cl)(μ-OTHMB) ( 2 ) and (NN i O tBu )Zn(CH 2 CH 3 ) ( 3 ), were used to produce monodispersed three- and six-armed star-shaped PHBs using tris(hydroxymethyl)benzene (THMB) and dipentaerythritol as the chain transfer agents. Reactions catalyzed by complex 2 were highly controlled, with THMB:catalyst ratios of up to 590:1, resulting in star-shaped PHBs with predictable molecular weights ( M n = 1.25–219 kDa) and narrow dispersities ( Đ = 1.02–1.08). The zinc-based catalyst, 3, was less controlled than the indium analogue but nevertheless generated moderately syndiotactic PHBs with maximum M n values of ∼100 kDa. Importantly, 3 allowed the formation of previously unknown 6-armed star PHBs, allowing us to compare the effects of the different PHB architectures on the rheological behavior of the materials. High molecular weight linear and star polymers were characterized using solution and melt viscoelastic studies. Zero-shear viscosity of linear PHBs exhibited a power law relationship with the span molecular weight; however, it scaled exponentially for star polymers with slightly higher values for the 6-armed star PHBs. This was attributed to the moderately syndiotactic microstructure of these polymers. The absence of a district arm retraction relaxation in the dynamic master curves, and overshoot in the transient viscosity for the 6-armed star PHBs, are due to the lower entanglement density and slightly broader molecular weight distribution of these polymers.