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

Indium and zinc complexes, [(NNO_tBu)­InCl]₂(μ-Cl)­(μ-OTHMB) (<b>2</b>) and (NN_iO_tBu)­Zn­(CH₂CH₃) (<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>_n = 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>_n 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

Highly Active Chiral Zinc Catalysts for Immortal Polymerization of β‑Butyrolactone Form Melt Processable Syndio-Rich Poly(hydroxybutyrate)

Highly crystalline poly­(hydroxybutyrate) suffers from high melting point and entanglement molecular weight. This leads to low melt strength, limits processing through regular techniques, and precludes many applications. In this work we report a series of racemic and enantiopure zinc catalysts supported by variously substituted diaminophenolate ancillary ligands which form high melt strength PHBs with different molecular weights and microstructure. These complexes are active for the highly controlled polymerization of β-butyrolactone (BBL); some can polymerize 2000 equiv of BBL in less than 30 min. Changing the steric bulk of the ligand forms PHBs of varied syndiotacticity (<i>P</i>_r = 0.75 to 0.55). These are highly robust systems capable of polymerizing an unprecedented 20000 equiv of BBL in the presence of 5000 equiv of benzyl alcohol. Thermorheological investigations reveal that the synthesized PHBs have surprisingly high melt strength at above the melting point. For processable PHBs, high density of entanglements and relatively low crystallinity are crucial. We show that the best PHBs should have high molecular weight and moderate syndiotacticity

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines

Air- and Moisture-Stable Indium Salan Catalysts for Living Multiblock PLA Formation in Air

We introduce an air- and moisture-stable hydroxy-bridged indium salan complex as a highly active and controlled catalyst for the ring-opening polymerization of cyclic esters in air. The reversible activation of this complex with linear and branched alcohols leads to immortal polymerization, allowing the controlled formation of block copolymers in air. It is the only reported example of a living catalyst that remains controlled after multiple exposures to ambient air at high temperatures. Although the prevalent catalyst for ring-opening polymerization, tin octanoate, is robust, it does not promote controlled polymerization. Our indium catalyst is exceptional in being both robust and controlled

Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (<b>1</b>), along with its zinc analogue (<b>2</b>), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis­(diamino)­phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (<b>7</b>) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (<i>rac</i>-LA) to afford highly heterotactic poly­(lactic acid) (PLA; <i>P</i>_r > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from <i>meso</i>-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed

Catalytic Synthesis of Secondary Amine-Containing Polymers: Variable Hydrogen Bonding for Tunable Rheological Properties

A synthetic protocol using atom-economic, catalytic hydroaminoalkylation and ring-opening metathesis polymerization (ROMP) has been developed for the versatile synthesis of a new class of aryl-substituted secondary amine-containing polymers. This catalytic route minimizes waste generation and avoids protection/deprotection protocols, postpolymerization modification, and byproduct formation. Different amines can be readily incorporated to access variable hydrogen-bonding characteristics. Thermal and melt rheological characterization has shown the profound effect of hydrogen bonding on the bulk properties of these amine-containing norbornene polymers

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

The impact of the metal size and Lewis acidity on the polymerization activity of group 13 metal complexes was studied, and it was shown that, within the same ligand family, indium complexes are far more reactive and selective than their gallium analogues. To this end, gallium and aluminum complexes supported by a tridentate diaminophenolate ligand, as well as gallium complexes supported by <i>N</i>,<i>N</i>′-ethylenebis­(salicylimine)­(salen) ligands, were synthesized and compared to their indium analogues. Using the tridentate ligand set, it was possible to isolate the gallium chloride complexes <b>3</b> and (±)-<b>4</b> and the aluminum analogues <b>5</b> and (±)-<b>6</b>. The alkoxygallium complex (±)-<b>2</b>, supported by a salen ligand, was also prepared and characterized and, along with the three-component system GaCl₃/BnOH/NEt₃, was tested for the ring-opening polymerization of lactide and ε-caprolactone. The polymerization rates and selectivities of both systems were significantly lower than those for the indium analogues. The reaction of (±)-<b>2</b> with 1 equiv of lactide forms the first insertion product, which is stable in solution and can be characterized at room temperature. In order to understand the differences of the reactivity within the group 13 metal complexes, a Lewis acidity study using triethylphosphine oxide (the Gutmann–Beckett method) was undertaken for a series of aluminum, gallium, and indium halide complexes; this study shows that indium halide complexes are less Lewis acidic than their aluminum and gallium analogues. Density functional theory calculations show that the Mulliken charges for the indium complexes are higher than those for the gallium analogues. These data suggest that the impact of ligands on the reactivity is more significant than that of the metal Lewis acidity