Claudio De Rosa

Polymer properties are profoundly impacted by tacticity, necessitating an understanding of the origins of stereocontrol to make high-performance materials. Recently, we reported that controlled incorporation of stereoerrors into trans-poly(3-hydroxy-2-methylbutyrate) (trans-PHMB) through the isoselective polymerization of rac-trans-3,4-dimethylpropiolactone (trans-DMPL) using racemic “sandwich” (BDI)Zn alkoxide complexes (where BDI = β-diketiminate) enables tuning of mechanical properties to match those of low-density polyethylene. However, the mechanism of stereocontrol of these catalysts was not explored. Herein, we report a thorough experimental and computational investigation uncovering a distinct manifestation of cooperative enantiomorphic-site and chain-end control, which we describe as site-mediated asymmetric chain-end control, where catalyst (Δ and Λ) and chain-end chiralities (R,R or S,S) work synergistically to influence the reaction’s stereoselectivity and rate. Experimental and computational data converge remarkably well, revealing that a Δ-catalyst with an (R,R)-chain end (Δ-RR) is isoselective 90% of the time. In contrast, a Δ-catalyst with an (S,S)-chain end (Δ-SS) is unselective. Detailed Density Functional Theory (DFT) investigations suggest molecular-level rationale for this mechanism of stereocontrol, demonstrating that chain positioning, monomer distortion, and ligand interactions work in conjunction to foster isoselectivity. Finally, DFT and kinetic modeling reveal that when polymerizing rac-trans-DMPL, the populations of Δ-RR and Δ-SS are equal and the high overall isoselectivity (mm% = 71–73%; where mm% is the percent of triads with a meso configuration across both ester linkages) of the catalyst is attributed to Δ-RR being faster than Δ-SS.

Polyolefins with melting temperatures above 200°C are rare, as their synthesis typically relies on the polymerization of highly sterically congested α‐olefins, which is challenging for both industrial Ziegler–Natta and homogeneous catalysts. Pyridylamido‐Hf complexes are molecular catalysts known for their high activity and ability to incorporate sterically demanding monomers, for example, allyltrimethylsilane (ATMS) giving isotactic poly(allyltrimethylsilane) with a melting temperature of 310°C and high turnover frequency. However, they are unable to polymerize efficiently monomers such as vinylcyclohexane (VCH) or 4,4‐dimethyl‐1‐pentene (44DMP). In this study, we report a novel synthetic strategy that enables the polymerization of such sterically challenging monomers, based on a pre‐activation of the Hf catalyst by using ATMS as ligand modifier in situ. We show that ATMS acts as an effective pre‐activator of the Hf complex, enabling controlled formation of highly active species and unlocking the polymerization of sterically demanding α‐olefins. This afforded the first synthesis of isotactic poly(4,4‐dimethyl‐1‐pentene) with an unprecedented melting temperature of 420°C, and efficient polymerization of VCH and 3‐methyl‐1‐butene. These findings demonstrate how the tuning of the pyridylamido‐Hf active species through rational choice of the activating monomer can be leveraged to expand the access to ultrahigh melting temperature polyolefins.

Polyolefins with melting temperatures above 200°C are rare, as their synthesis typically relies on the polymerization of highly sterically congested α‐olefins, which is challenging for both industrial Ziegler–Natta and homogeneous catalysts. Pyridylamido‐Hf complexes are molecular catalysts known for their high activity and ability to incorporate sterically demanding monomers, for example, allyltrimethylsilane (ATMS) giving isotactic poly(allyltrimethylsilane) with a melting temperature of 310°C and high turnover frequency. However, they are unable to polymerize efficiently monomers such as vinylcyclohexane (VCH) or 4,4‐dimethyl‐1‐pentene (44DMP). In this study, we report a novel synthetic strategy that enables the polymerization of such sterically challenging monomers, based on a pre‐activation of the Hf catalyst by using ATMS as ligand modifier in situ. We show that ATMS acts as an effective pre‐activator of the Hf complex, enabling controlled formation of highly active species and unlocking the polymerization of sterically demanding α‐olefins. This afforded the first synthesis of isotactic poly(4,4‐dimethyl‐1‐pentene) with an unprecedented melting temperature of 420°C, and efficient polymerization of VCH and 3‐methyl‐1‐butene. These findings demonstrate how the tuning of the pyridylamido‐Hf active species through rational choice of the activating monomer can be leveraged to expand the access to ultrahigh melting temperature polyolefins.

The crystal structure of form I of poly[tris(ethylene sulfide)-alt-tris(ethylene oxide)] was determined by combining X-ray and electron diffraction with molecular force field calculations. The polymer was synthesized by thiol-ene emulsion polymerization. The electron diffraction pattern from flat-on single crystals revealed a two-dimensional orthorhombic unit cell with parameters a = 8.60 Å and b = 4.95 Å. The value of the c axis of 18.7 Å was determined from the electron diffraction pattern of tilted single crystals. Molecular force field calculations optimized the chain conformation of the sequence −(CH2−S−CH2−CH2−S−CH2−CH2−S−CH2)−(CH2−O−CH2−CH2−O−CH2−CH2−O−CH2)− to a succession of torsion angles (G−G− T G+G+ T G−G−) T (TT G+ TT G+ TT) exhibiting repetition after one monomeric unit with a chain axis of 18.7 Å, in agreement with the experimental c axis determined by tilted electron diffraction. The main features of the experimental diffraction patterns are well reproduced with a model of the crystal structure characterized by the packing of chains in this low-energy conformation packed in an orthorhombic unit cell with axes a = 8.60 Å, b = 4.95 Å, and c = 18.7 Å, according to the space group Pa (β = 90°). Statistical disorder in the orientation of the chains about their axes and disorder in the conformation of the chains are present in the crystal structure. The conformational disorder has been modeled assuming a statistical succession of monomeric units having the tris(ethylene sulfide) segment always in the conformation G−G− T G+G+ T G−G− T and the tris(ethylene oxide) segment in the helical conformation (TTG+)3 or in the trans-planar conformation (TTT)3. The real crystal structure is intermediate between the limit-ordered models and the limit statistical disordered models.

Selective ring-opening polymerization (ROP) of chiral lactones enables access to biodegradable polyesters with precisely controlled microstructures. Here, combined DFT modeling and experimental validation elucidate how fine-tuning of enantiopure SalBinam aluminum catalysts (through introduction of bromine atoms and tert-butyl groups, respectively, in ortho- and para-positions) modulates regio- and stereocontrol in the ROP of methyl glycolide (MeG) and lactide (LA). Computations reveal that regioselectivity in MeG polymerization arises mainly from steric repulsion, with a small contribution from weak stabilizing C-H···Br interactions that favor ring-opening at the glycolic site, consistent with the experimentally enhanced regioselectivity for (R)-MeG. In contrast, the same steric congestion at the ligand’s ortho positions destabilizes the key transition states in rac-LA polymerization, reducing the calculated stereoselectivity. Experiments confirm the predicted loss of stereocontrol, yielding nearly atactic PLA under standard conditions. Extension of the computational framework to rac-MeG polymerization promoted by racemic catalyst identified a low-barrier, stepwise polymer chain exchange pathway that rationalizes the experimentally observed syndiotacticity of poly(lactic-co-glycolic acid).

Crystallization, a phase transition that is prevalent in nature, provides unique molecular arrangements that impact the structure-dependent properties of a material. Owing to thermodynamic and kinetic factors, synthetic polymeric materials show only partial ordering. Indeed, usually polymers that possess sufficient stereo- and regioregularity can partially crystallize. Atactic polymers that do not meet the minimum regularity in the chemical structure needed to pack chains within a crystal unit cell are completely amorphous. However, there are unusual cases where stereoirregular, atactic polymers can crystallize; this happens when the structural features of the atactic polymers provide specific ordering so that their irregular chains can pack into crystalline structures. This Review not only highlights the general relationship between the structures required in atactic polymers to induce crystallization but also provides researchers with the tools to design polymer systems with crystallinity without requiring exquisite stereocontrol in their synthesis.