Mechanochemical synthesis of poly(trimethylene carbonate)s: an example of rate acceleration

Anionic ring-opening polymerization of functional epoxide monomers in the solid state

Despite recent advancements in mechanochemical polymerization, understanding the unique mechanochemical reactivity during the ball milling polymerization process still requires extensive investigations. Herein, solid-state anionic ring-opening polymerization is used to synthesize polyethers from various functional epoxide monomers. The critical parameters of the monomers are investigated to elucidate the unique reactivity of ball milling polymerization. The controllable syntheses of the desired polyethers are characterized via NMR, GPC, and MALDI-ToF analyses. Interestingly, bulky monomers exhibit faster conversions in the solid-state in clear contrast to that observed for solution polymerization. Particularly, a close linear correlation is observed between the conversion of the ball milling polymerization and melting point of the functional epoxide monomers, indicating melting point as a critical predictor of mechanochemical polymerization reactivity. This study provides insights into the efficient design and understanding of mechanochemical polymerization.

Mechanochemical solid-state vinyl polymerization with anionic initiator

Mechanochemistry has been extended to various polymer syntheses to achieve efficiency, greenness, and new products. However, many fundamental polymerization reactions have not been explored, although anionic polymerization of vinyl compounds has been pursued under mechanochemical conditions. Two solid monomers, 4-biphenyl methacrylate and 4-vinyl biphenyl, representing methacrylate and styrenic classes, respectively, were reacted with secondary butyl lithium under high-speed ball-milling. The alkyl-anion-promoted polymerization process was established by excluding radical initiation and producing the expected polymers with good efficiency. However, the generally expected features of anionic polymerization, such as molecular weight control and narrow dispersity, were not observed. Analysis of the milling parameters, reaction monitoring, and microstructural analysis revealed that the mechanism of the mechanochemical process differs from that of conventional anionic polymerizations. The mechanical force fractured the newly formed polymer chains via anionic initiation and generated macroradicals, which participated in the polymerization process. The anionic process governs the initiation step and the radical process becomes dominant during the propagation step.

Site-Isolated Main-Group Tris(2-pyridyl)borate Complexes by Pyridine Substitution and Their Ring-Opening Polymerization Catalysis

Tris(2-pyridyl)borates are an emerging class of scorpionate ligands, distinguished as exceptionally robust and electron-donating. However, the rapid formation of inert homoleptic complexes with divalent metals has so far limited their catalytic utility. We report site-isolating tris(2-pyridyl)borate ligands, bearing isopropyl, tert-butyl, and mesityl substituents at the pyridine 6-position to suppress the formation of inert homoleptic complexes. These ligands form the first 1:1 complexes between tris(2-pyridyl)borates and Mg²⁺, Zn²⁺, or Ca²⁺, with isopropyl-substituted Tpy^iPrH showing the most generality. Single-crystal X-ray diffraction analysis of the resulting complexes and comparison to density functional theory (DFT) models showed geometric distortions driven by steric repulsion between the pyridine 6-substituents and the hexamethyldisilazide (HMDS^-, ^-N(SiMe₃)₂) anion. We show that this steric profile is a feature of the six-membered pyridine ring and contrasts with more established tris(pyrazolyl)borate and tris(imidazoline)borate scorpionate complexes. Tpy^iPrMg(HMDS) (1) and its zinc analogue are moderately active for the controlled polymerization of l-lactide, ε-caprolactone, and trimethylene carbonate. Furthermore, 1 gives controlled polymerization under more demanding melt-phase polymerization conditions at 100 °C, and block copolymerization of ε-caprolactone and trimethylene carbonate. These results will enable useful catalysis and coordination chemistry studies with tris(2-pyridyl)borates, and characterizes their structural complementarity to more familiar scorpionate ligands.

The mechanochemical synthesis of polymers

Mechanochemistry - the utilization of mechanical forces to induce chemical reactions - is a rarely considered tool for polymer synthesis. It offers numerous advantages such as reduced solvent consumption, accessibility of novel structures, and the avoidance of problems posed by low monomer solubility and fast precipitation. Consequently, the development of new high-performance materials based on mechanochemically synthesised polymers has drawn much interest, particularly from the perspective of green chemistry. This review covers the constructive mechanochemical synthesis of polymers, starting from early examples and progressing to the current state of the art while emphasising linear and porous polymers as well as post-polymerisation modifications.

Mechanochemical ring-opening metathesis polymerization: development, scope, and mechano-exclusive polymer synthesis

Ruthenium-alkylidene initiated ring-opening metathesis polymerization has been realized under solid-state conditions by employing a mechanochemical ball milling method. This method promotes greenness and broadens the scope to include mechano-exclusive products. The carbene- and pyridine-based Grubbs 3^rd-generation complex outperformed other catalysts and maintained similar mechanistic features of solution-phase reactions. High-speed ball milling provides sufficient mixing and energy to the solid reaction mixture, which is composed of an initiator and monomers, to minimize or eliminate the use of solvents. Therefore, the solubility and miscibility of monomers and Ru-initiators are not limiting factors in solid-state ball milling. A wide variety of solid monomers, including ionomers, fluorous monomers, and macromonomers, were successfully polymerized under ball milling conditions. Importantly, direct copolymerization of immiscible (ionic/hydrophobic) monomers exemplifies the synthesis of mechano-exclusive polymers that are difficult to make using traditional solution procedures. Finally, the addition of a small amount of a liquid additive (i.e., liquid-assisted grinding) minimized chain-degradation, enabling high-molecular-weight polymer synthesis.

Mechanochemical ball-milling ring-opening metathesis polymerization minimized solvent use and produced previously inaccessible polymers in solution.

Simple and Rapid Mechanochemical Synthesis of Lactide and 3S-(Isobutyl)morpholine-2,5-dione-Based Random Copolymers Using DBU and Thiourea

There is a growing interest surrounding morpholine-2,5-dione-based materials due to their impressive biocompatibility as well as their capacity to break down by hydrolytic and enzymatic pathways. In this study, the ring-opening (co)polymerization of leucine-derived 3S-(isobutyl)morpholine-2,5-dione (MD) and lactide (LA) was performed via ball-milling using a catalytic system composed of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 3-[3,5-bis(trifluoromethyl)phenyl]-1-cyclohexylthiourea (TU). Once the homopolymerizations of MD and LA optimized and numerous parameters were studied, the mechanochemical ring-opening copolymerization of these monomers was explored. The feasibility of ring-opening copolymerizations in mechanochemical systems was demonstrated and a range of P(MD-co-LA) copolymers were produced with varying proportions of MD (23%, 48%, and 69%). Furthermore, the beneficial cocatalytic effects of TU with regards to ROP control were found to be operative within mechanochemical systems. Further parallels were observed between solution- and mechanochemical-based ROPs.

Atom Transfer Radical Polymerization in the Solid-State

Poly(2-vinylnaphthalene) was synthesized in the solid-state by ball milling a mixture of the corresponding monomer, a Cu-based catalyst, and an activated haloalkane as the polymerization initiator. Various reaction conditions, including milling time, milling frequency and added reductant to accelerate the polymerization were optimized. Monomer conversion and the evolution of polymer molecular weight were monitored over time using 1 H NMR spectroscopy and size exclusion chromatography, respectively, and linear correlations were observed. While the polymer molecular weight was effectively tuned by changing the initial monomer-to-initiator ratio, the experimentally measured values were found to be lower than their theoretical values. The difference was attributed to premature mechanical decomposition and modeled to accurately account for the decrement. Random copolymers of two monomers with orthogonal solubilities, sodium styrene sulfonate and 2-vinylnaphthalene, were also synthesized in the solid-state. Inspection of the data revealed that the solid-state polymerization reaction was controlled, followed a mechanism similar to that described for solution-state atom transfer radical polymerizations, and may be used to prepare polymers that are inaccessible via solution-state methods.

Chemical recycling of poly(bisphenol A carbonate)

This review covers the current status of chemical recycling and upcycling of poly(bisphenol A carbonate), a leading engineering plastic of great economic and environmental interest.

Stereoselective deconjugation of macrocyclic α,β-unsaturated esters by sequential amidation and olefin transposition: application to enantioselective phase-transfer catalysis

The stereoselective synthesis of chiral macrocycles bearing two aliphatic amide functional groups is reported. After the amidation mediated by TBD, a guanidine derivative, the olefin transposition step is performed with a slight excess of t-BuOK. The products are afforded in moderate to good combined yields (up to 59%) and with an excellent syn diastereoselectivity (dr > 49 : 1). Introducing enantiopure α-branched substituents was possible and it resulted in mixtures of diastereomers, which could be tested as phase-transfer catalysts using the formation of a phenylalanine analog as a test reaction (up to 43% ee). A clear matched-mismatched situation was observed in the two diastereomeric series.