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

Mechanism of Living Lactide Polymerization by Dinuclear Indium Catalysts and Its Impact on Isoselectivity

A family of racemic and enantiopure indium complexes <b>1</b>–<b>11</b> bearing bulky chiral diaminoaryloxy ligands, H­(NNO_R), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO_R)­InX]₂[μ-Y]­[μ-OEt] (<b>5</b>, R = ^<i>t</i>Bu, X = Y = Cl; <b>8</b>, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly­(lactic acid) (PLA) at room temperature. In particular, complex <b>5</b> showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex <b>8</b>. Importantly, the selectivities obtained for the ROP of racemic LA with (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> and (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>8</b> are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes

Mechanism of Living Lactide Polymerization by Dinuclear Indium Catalysts and Its Impact on Isoselectivity

A family of racemic and enantiopure indium complexes <b>1</b>–<b>11</b> bearing bulky chiral diaminoaryloxy ligands, H­(NNO_R), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO_R)­InX]₂[μ-Y]­[μ-OEt] (<b>5</b>, R = ^<i>t</i>Bu, X = Y = Cl; <b>8</b>, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly­(lactic acid) (PLA) at room temperature. In particular, complex <b>5</b> showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex <b>8</b>. Importantly, the selectivities obtained for the ROP of racemic LA with (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> and (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>8</b> are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes

A Comparison of the Rheological and Mechanical Properties of Isotactic, Syndiotactic, and Heterotactic Poly(lactide)

A series of poly­(lactide) (PLA) samples, exhibiting various levels of syndiotactic enrichment, were formed via the ring-opening polymerization of <i>meso</i>-lactide using two families of dinuclear indium catalysts: (<i>RR</i>/<i>RR</i>)-[(NNO)­InCl]₂(μ-Cl)­(μ-OEt) (<b>1</b>) and (<i>RR</i>/<i>RR</i>)-[(ONNO)­In­(μ-OEt)]₂ (<b>2</b>). Isotactic and heterotactic PLAs were also synthesized using known methodologies, and the thermal and rheological behaviors of these PLAs with different microstructures were compared. Solution rheological studies showed that the values of intrinsic viscosities and hydrodynamic radii as functions of molecular weight (<i>M</i>_w) were highest for iso-PLAs, followed by hetero and then syndio-PLAs. The viscosities of the heterotactically enriched PLAs were in agreement with literature values reported for atactic PLAs. The molecular weight between entanglements (<i>M</i>_e) was greatest for the syndiotactically enriched PLAs, giving rise to the lowest zero-shear viscosity. In addition, hetero- and isotactically enriched PLA had higher flow activation energies (<i>E</i>_a,flow) than syndiotactic variants, implying the inclusion of transient aggregate regions within these polymers due to enhanced L- and D-interactions. Although strain hardening was observed for all types of PLAs, it was more dominant for isotactic PLAs due to stronger L- and D-interactions possibly leading to a small degree of stereocomplex microcrystallites

Theoretical Investigation of Lactide Ring-Opening Polymerization Induced by a Dinuclear Indium Catalyst

A DFT study of the ring-opening polymerization of lactide (LA) induced by a dinuclear indium catalyst supported by a chiral diamino phenoxy ligand, [(NN_HO)­InCl]₂(μ-Cl)­(μ-OEt) (<b>1</b>), is reported. The nature of the active catalyst, mononuclear vs dinuclear, was investigated and was shown to be dinuclear because of the high energetic cost of its dissociation. The selectivity of the system was investigated for the polymerization of LA with the dinuclear (<i>R,R</i>/<i>R,R</i>)-<b>1</b> catalyst. In complete agreement with experimental results we observed that (1) selectivity is controlled by the nucleophilic addition of LA to the alcoholate, resulting in the chain-end control of polymerization, (2) a slight kinetic preference for the polymerization of l-LA over d-LA is found that translates to a <i>k</i>_rel value of ∼14, which is identical with the experimental value, and (3) when <i>rac</i>-LA is used, no clear preference for d- vs l-LA insertion is found, leading to isotactic PLA

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

Probing the Role of Secondary versus Tertiary Amine Donor Ligands for Indium Catalysts in Lactide Polymerization

The role of the central amine donor in a previously reported dinuclear indium catalyst, [N_Me2N_HO)­InCl]₂­(μ-Cl)­(μ-OEt) (<b>1</b>), for the polymerization of lactide was investigated through experimental methods. The solid state structural data of a series of dimeric complexes related to <b>1</b>, including the previously reported bromide derivative [(N_Me2N_HO)­InBr]­(μ-Br)­(μ-OEt) (<b>2</b>) and the newly synthesized methylated derivative [(N_Me2N_MeO)­InCl]₂­(μ-Cl)­(μ-OEt) (<b>6</b>), showed that weak hydrogen bonding may be present within some of these complexes in the solid state. The polymerization of <i>rac</i>-lactide with <b>2</b>, <b>6</b>, and a related achiral complex [(L_H)­InCl]₂­(μ-Cl)­(μ-OEt) (<b>8</b>) synthesized for this study indicates that hydrogen bonding may not influence the reactivity of these compounds. The nature of the central amine donor may play a role in tuning the reactivity of these types of catalysts. Catalysts with central secondary amine donors, such as complexes <b>1</b>, <b>2</b>, and <b>8</b>, are 2 orders of magnitude more reactive than those with central tertiary amine donors, such as complex <b>6</b>

Probing the Role of Secondary versus Tertiary Amine Donor Ligands for Indium Catalysts in Lactide Polymerization

The role of the central amine donor in a previously reported dinuclear indium catalyst, [N_Me2N_HO)­InCl]₂­(μ-Cl)­(μ-OEt) (<b>1</b>), for the polymerization of lactide was investigated through experimental methods. The solid state structural data of a series of dimeric complexes related to <b>1</b>, including the previously reported bromide derivative [(N_Me2N_HO)­InBr]­(μ-Br)­(μ-OEt) (<b>2</b>) and the newly synthesized methylated derivative [(N_Me2N_MeO)­InCl]₂­(μ-Cl)­(μ-OEt) (<b>6</b>), showed that weak hydrogen bonding may be present within some of these complexes in the solid state. The polymerization of <i>rac</i>-lactide with <b>2</b>, <b>6</b>, and a related achiral complex [(L_H)­InCl]₂­(μ-Cl)­(μ-OEt) (<b>8</b>) synthesized for this study indicates that hydrogen bonding may not influence the reactivity of these compounds. The nature of the central amine donor may play a role in tuning the reactivity of these types of catalysts. Catalysts with central secondary amine donors, such as complexes <b>1</b>, <b>2</b>, and <b>8</b>, are 2 orders of magnitude more reactive than those with central tertiary amine donors, such as complex <b>6</b>

Role of Aggregation in the Synthesis and Polymerization Activity of SalBinap Indium Alkoxide Complexes

The reaction of racemic SalBinap ligand, (±)-H₂(<b>ONN</b>*<b>O</b>_<b>Me</b>), with InCl₃ and excess NaOEt generated a mixture of two dinuclear compounds [(μ–κ²-ONN*O_Me)­In­(μ-OEt)]₂ (<b>1a</b>) and [κ⁴-ONN*O_Me)­In­(μ-OEt)]₂ (<b>1b</b>), which were isolated and fully characterized. Polymerization of racemic lactide with <b>1a</b> and <b>1b</b> was slow in refluxing THF and showed only modest stereoselectivity. Catalyst <b>1b</b> displayed better control than <b>1a</b>, with the experimental molecular weights of the resulting poly­(lactic acid) in agreement with the expected values. The higher-than-expected molecular weights observed in polymers formed by <b>1a</b> were due to partial initiation of the catalyst. The reaction of (±)-H₂(<b>ONN</b>*<b>O</b>_<b>tBu</b>) with InCl₃ yielded (κ⁴-ONN*O_tBu)­InCl (<b>2</b>); however, further reactivity of the compound formed a mixture of products. An attempt to prevent aggregation by reacting (±)-H₂(<b>ONN</b>*<b>O</b>_<b>Me</b>) with InCl₃ and excess NaO^<i>i</i>Pr yielded an intractable mixture, including [(μ–κ²-ONN*O_Me)­In]₂­(μ-Cl)­(μ-OH) (<b>3</b>). The thermal stabilities of compounds <b>1a</b> and <b>1b</b> under polymerization conditions were investigated. Examination of the polymerization behavior of complexes <b>1a</b> and <b>1b</b> and the reaction equilibrium between the two illustrates the importance of aggregation in indium salen complexes compared to their aluminum counterparts

Synthesis and Thermorheological Analysis of Biobased Lignin-<i>graft</i>-poly(lactide) Copolymers and Their Blends

Despite numerous accounts of biobased composite materials through blending and copolymerization of lignin and other polymers, there are no systematic studies connecting the synthetic methodology, molecular structure, and polymer topology with the rheological properties of these materials. In this report lignin-<i>graft</i>-poly­(lactide) copolymers are synthesized via three routes (indium and organocatalyzed “graft-from” methods as well as a “graft-to” method) and the resulting reaction products (shown to include linear PLAs, cyclic PLAs, and star-shaped lignin-<i>graft</i>-PLA copolymers) are investigated using chemical and rheological methods. The topology of the products of the graft-from methods is affected by the initial lignin concentration; polymerizations with low lignin loading generate cyclic PLAs, which can be identified by 10-fold lower viscosities compared to linear PLAs of the same molecular weight. Under higher lignin loadings, star-shaped lignin-<i>graft</i>-PLA copolymers are formed which show viscosities 2 orders of magnitude lower than those of comparable linear PLAs. Rheological studies show that cyclic PLAs lack a well-defined rubber plateau, whereas star-shaped lignin-<i>graft</i>-PLAs lack a significant <i>G</i>′ to <i>G</i>′′ cross-over. The rheological results coupled with thermogravimetric analysis give an indication to the structure of star-shaped lignin-<i>graft</i>-PLA copolymers, which are estimated to contain a small lignin core surrounded by PLA segments with molecular weights from 2.0 to 20 kg mol^–1