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>

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

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>

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