Precise construction of weather-sensitive poly(ester-alt-thioesters) from phthalic thioanhydride and oxetane

Structure-Activity Relationships for s-Block Metal/Co(III) Heterodinuclear Catalysts in Cyclohexene Oxide Ring-Opening Copolymerizations

In homogeneous catalysis, uncovering structure-activity relationships remains very rare but invaluable to understand and rationally improve performances. Here, generalizable structure-activity relationships apply to a series of heterodinuclear polymerization catalysts featuring Co(III) and s-block metals M(I/II) (M=Na(I), K(I), Ca(II), Sr(II), Ba(II)). These are shown to apply to polycarbonate production by the ring-opening copolymerizations (ROCOP) of cyclohexene oxide (CHO) and carbon dioxide (CO2), conducted at high (20 bar) and low (1 bar) CO2 pressures, and to polyester production by copolymerization of cyclohexene oxide and phthalic anhydride (PA). For the CHO/PA and high-pressure CHO/CO2 copolymerizations, activity increases exponentially with s-block metal acidity peaking at the Co(III)K(I) catalyst, whilst for the low-pressure CHO/CO2 copolymerization it increases linearly to the same metal combination. The polymerization kinetics fit second order rate laws and the correlations support dinuclear metallate mechanistic hypotheses. These relationships help understand and identify key metal complex structural features in synergic polymerization catalysis.

Precise placement of thioester bonds into sequence-controlled polymers containing ABAC-type units

The precise placement of thioester bonds into sequence-controlled polymers remains a grand challenge. Here, we demonstrate the versatile synthesis of sequence-controlled polymers from the step polymerization of cyclic thioanhydrides (A), diacrylates (B), and diols/diamines (C). In addition to easily accessible diverse monomers, the method is metal-free/catalyst-free, atom-economical, and wide in monomer scope, yielding 107 polymers with >90% yields and weight-average molecular weights of up to 175.4 kDa. The obtained polymers contain ABAC-type repeating units and precisely distributed in-chain thioester and ester (and amide) groups. The chemoselectivity of the polymerization is revealed by density functional theory calculations. The polymer library exhibits considerably tunable performance: glass-transition temperatures of -36-72 °C, melting temperatures of 43-133 °C, degradability, thermoplastics/elastomers, and thioester-based functions. This study furnishes a facile method to precisely incorporate thioester bonds into sequence-controlled polymers.

Monomer centred selectivity guidelines for sulfurated ring-opening copolymerisations

Sulfur-containing polymers, such as thioesters and thiocarbonates, offer sustainability advantages, including enhanced degradability and chemical recyclability. However, their synthesis remains underdeveloped compared to that of their oxygen-containing counterparts. Although catalytic ring-opening copolymerization (ROCOP) can provide access to sulfur-containing polymers, these materials often exhibit uncontrolled microstructures and unpredictable properties. A comprehensive model that elucidates the factors determining selectivity in these catalytic reactions is still lacking, despite its central importance for advancing these polymerizations into widely applicable methodologies. In this study, we investigate the factors that lead to selectivity in sulfurated ROCOP across various monomer combinations, including thioanhydrides or carbon disulfide with epoxides, thiiranes, and oxetanes. We find that unwanted by-products primarily arise from backbiting reactions of catalyst-bound alkoxide chain ends, which can be mitigated by (i) selecting monomers that form primary alkoxide of thiolate chain ends, (ii) maximizing ring strain in the backbiting step, and (iii) timely termination of the polymerization. By applying these strategies, the selectivity of the catalytic ROCOP can be controlled and we successfully synthesized perfectly alternating poly(esters-alt-thioesters) from various oxetanes and the highly industrially relevant ethylene oxide. Our study thereby shifts the focus for achieving selectivity from catalyst to monomer choice providing valuable mechanistic insights for the development of future selective polymerizations, paving the way for sulfurated polymers as potential alternatives to current commodity materials.

Easy Synthetic Access to High-Melting Sulfurated Copolymers and their Self-Assembling Diblock Copolymers from Phenylisothiocyanate and Oxetane

Although sulfurated polymers promise unique properties, their controlled synthesis, particularly when it comes to complex and functional architectures, remains challenging. Here, we show that the copolymerization of oxetane and phenyl isothiocyanate selectively yields polythioimidocarbonates as a new class of sulfur containing polymers, with narrow molecular weight distributions (Mn=5-80 kg/mol with Đ≤1.2; Mn,max=124 kg/mol) and high melting points of up to 181 °C. The method tolerates different substituent patterns on both the oxetane and the isothiocyanate. Self-nucleation experiments reveal that π-stacking of phenyl substituents, the presence of unsubstituted polymer backbones, and the kinetically controlled linkage selectivity are key factors in maximising melting points. The increased tolerance to macro-chain transfer agents and the controlled propagation allows the synthesis of double crystalline and amphiphilic diblock copolymers, which can be assembled into micellar- and worm-like structures with amorphous cores in water. In contrast, crystallization driven self-assembly in ethanol gives cylindrical micelles or platelets.

Improved access to polythioesters by heterobimetallic aluminium catalysis

Bimetallic Al(III) catalysis mediates thioanhydride/epoxide copolymerisation at greatly improved rates and monomer tolerance than analogous Cr(III) catalysis. Moving to sulfurated monomers furthermore generally improves rates and selectivites.