Synthesis and Rheological Characterization of Star-Shaped
and Linear Poly(hydroxybutyrate)
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Abstract
Indium and zinc complexes, [(NNO<sub>tBu</sub>)InCl]<sub>2</sub>(μ-Cl)(μ-OTHMB) (<b>2</b>) and (NN<sub>i</sub>O<sub>tBu</sub>)Zn(CH<sub>2</sub>CH<sub>3</sub>) (<b>3</b>), 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 <b>2</b> were highly controlled, with THMB:catalyst ratios of up to 590:1,
resulting in star-shaped PHBs with predictable molecular weights (<i>M</i><sub>n</sub> = 1.25–219 kDa) and narrow dispersities
(<i>Đ</i> = 1.02–1.08). The zinc-based catalyst, <b>3</b>, was less controlled than the indium analogue but nevertheless
generated moderately syndiotactic PHBs with maximum <i>M</i><sub>n</sub> values of ∼100 kDa. Importantly, <b>3</b> 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