Tunable zinc benzamidinate complexes: coordination modes and catalytic activity in the ring-opening polymerization of L-lactide

Seven asymmetric zinc benzamidinate complexes featuring or lacking side-arm functionalities were synthesized. Using equimolar zinc reagent produced distinct dinuclear motifs [(C6H5-C = NC6H5)ZnEt]2 (R = tBu, 1; (CH2)2OMe, 2; (CH2)2NMe2, 3). Half the zinc reagent yielded dinuclear [(C6H5-C = NC6H5)2Zn]2 (R = tBu, 4) or mononuclear zinc bis(chelate) complexes (R = (CH2)2OMe, 5; (CH2)2NMe2, 6; CH2Py, 7). Molecular structures of 1-4 and 7 were determined via single-crystal X-ray diffraction. Altering benzamidinate substituents modifies both coordination modes and catalytic activities in ring-opening polymerization of L-lactide. Specifically, complex 7 exhibits enhanced catalytic activity at 25 °C using 100 equivalents of L-lactide with a turnover frequency of 1820 h-1.

Recyclable cyclic bio-based acrylic polymer via pairwise monomer enchainment by a trifunctional Lewis pair

The existing catalyst/initiator systems and methodologies used for the synthesis of polymers can access only a few cyclic polymers composed entirely of a single monomer type, and the synthesis of such authentic cyclic polar vinyl polymers (acrylics) devoid of any foreign motifs remains a challenge. Here we report that a tethered B-P-B trifunctional, intramolecular frustrated Lewis pair catalyst enables the synthesis of an authentic cyclic acrylic polymer, cyclic poly(γ-methyl-α-methylene-γ-butyrolactone) (c-PMMBL), from the bio-based monomer MMBL. Detailed studies have revealed an initiation and propagation mechanism through pairwise monomer enchainment enabled by the cooperative and synergistic initiator/catalyst sites of the trifunctional catalyst. We propose that macrocyclic intermediates and transition states comprising two catalyst molecules are involved in the catalyst-regulated ring expansion and eventual cyclization, forming authentic c-PMMBL rings and concurrently regenerating the catalyst. The cyclic topology of the c-PMMBL polymers imparts an ~50 °C higher onset decomposition temperature and a much narrower degradation window compared with their linear counterparts of similar molecular weight and dispersity, while maintaining high chemical recyclability.

Synthesis of Linear to Cyclic Polylactide via a One-Pot Step-Wise Ring-Opening Polymerization and Back-Biting Reaction of Ring Closure Using Magnesium Complexes

The controllable synthesis of cyclic polylactide remains a challenging topic so far. In this work, a new strategy of one-pot step-wise ring-opening polymerization (ROP) followed by a back-biting reaction of ring closure was reported, in which one magnesium atrane-like complex {N,N-bis[3,5-di-cumyl-2-benzyloxy]-[2-(2-aminoethoxy)ethoxy]magnesium} was utilized to initiate the ROP of lactide using 4-dimethylaminopyridine as a co-catalyst; then, macrocyclic polylactides were liberated out via increasing temperature after complete depletion of the monomer in which a back-biting reaction was utilized as a ring-closure method. The living feature at the first ROP stage can be proved well by the controllable molecular weights ranging from 3.10 to 34.70 kDa and narrow molecular weight distributions of linear polylactides obtained after quenching the reaction. The final cyclic polylactides with molecular weights (vs polystyrene) ranging from 2.50 to 16.10 kDa can be achieved too after the back-biting reaction of ring closure. Although a shoulder peak at the gel permeation chromatography profile appears when the ratio of monomer:initiator is high up to 100:1 or 200:1, this system is suitable for the controllable syntheses of cyclic polylactides with desirable modest molecular weights.

Mechanism of Spatial and Temporal Control in Precision Cyclic Vinyl Polymer Synthesis by Lewis Pair Polymerization

In typical cyclic polymer synthesis via ring-closure, chain growth and cyclization events are competing with each other, thus affording cyclic polymers with uncontrolled molecular weight or ring size and high dispersity. Here we uncover a mechanism by which Lewis pair polymerization (LPP) operates on polar vinyl monomers that allows the control of where and when cyclization takes place, thereby achieving spatial and temporal control to afford precision cyclic vinyl polymers or block copolymers with predictable molecular weight and low dispersity (≈1.03). A combined experimental and theoretical study demonstrates that cyclization occurs only after all monomers have been consumed (when) via conjugate addition of the propagating chain end to the specific site of the initiating chain end (where), allowing the cyclic polymer formation steps to be regulated and executed with precision in space and time.

The versatile roles of neutral donor ligands in tuning catalyst performance for the ring-opening polymerization of cyclic esters

The use of neutral donor ligands is an effective strategy to modify catalyst structure and performance in the synthesis of sustainable polymers through the ring-opening polymerization (ROP) of cyclic esters.

Highly Tunable Metal-Free Ring Opening Polymerization of Glycidol into Various Controlled Topologies Catalyzed by Frustrated Lewis Pairs

Controlling the topology of a polymer is essential in determining its physical properties and processing. Even after numerous studies, obtaining a diverse array of topologies, particularly within the framework of...

Synchronous Control of Chain Length/Sequence/Topology for Precision Synthesis of Cyclic Block Copolymers from Monomer Mixtures

Precision synthesis of cyclic polymers with predictable molecular weight and low dispersity is a challenging task, particularly concerning cyclic polar vinyl polymers through a rapid chain-growth mechanism and without high dilution. Harder yet is the precision synthesis of cyclic block copolymers (cBCPs), ideally from comonomer mixtures. Here we report that Lewis pair polymerization (LPP) capable of thermodynamically and kinetically compounded sequence control successfully addressed this longstanding challenge. Thus, LPP of acrylate/methacrylate mixtures under ambient temperature and normal concentration conditions rapidly and selectively affords well-defined cBCPs with high molecular weight (M_n = 247 kg/mol) and low dispersity (Đ = 1.04) in one step. Such cBCPs have been characterized by multiple techniques, including direct structural observation by imaging.

Compounded Sequence Control in Polymerization of One-Pot Mixtures of Highly Reactive Acrylates by Differentiating Lewis Pairs

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Synthesis of macrocyclic poly(α-hydroxyl acids) via DABCO-mediated ROP of O-carboxylanhydrides derived from l-phenylalanine even in the presence of an alcohol

On exploration of a catalytic system including simple 1,4-diazabicyclo-[2.2.2]octane (DABCO), triethylboron (TEB) and benzylalcohol (BnOH), a new pathway to achieve cyclic PAHAs was developed via ROP of OCAs.

A computational study of the reactivity of rare-earth/phosphorus Lewis pairs toward polymerization of conjugated polar alkenes

The polymerization mechanism of methyl methacrylate (MMA) catalyzed by rare-earth/phosphorus (RE/P) Lewis pairs has been systematically studied through density functional theory (DFT) calculations.

A new, simple, and efficient strategy for the preparation of active antifungal biodegradable materials via ring-opening polymerization of l-lactide with zinc aryloxides

In this work, zinc aryloxides supported by monodentate hydroxybiphenyls [ArOH: ortho-phenylphenol (o-XenOH), meta-phenylphenol (m-XenOH), or para-phenylphenol (p-XenOH] and N,N,N',N'-tetramethylethylenediamine (TMEDA) were used to develop active polymeric materials for antifungal agents for agricultural use. The direct reaction of ligand precursor ArOH with ZnEt2 (1 : 2) in a toluene/TMEDA mixture (1 : 10) afforded a series of three isostructural monomeric compounds, namely [Zn(o-XenO)2(TMEDA)] (1), [Zn(m-XenO)2(TMEDA)] (2), and [Zn(p-XenO)2(TMEDA)] (3). These were characterized by single-crystal X-ray diffraction, and spectroscopic and other analytical methods. The results show that 1-3 are effective initiators for the ring-opening polymerization (ROP) of l-lactide (l-LA) via bifunctional activation of the monomer with Lewis pairs to give polymers terminated with TMEDA and Zn(OAr)2 as the α- and ω-chain ends, respectively. Combinations of ZnEt2 with two molar equivalents of ArOH proligands were used to synthesize polylactides containing fungicide molecules covalently bonded via ester linkers. The ROP of l-LA initiated by these zinc-based systems could be used for the preparation of polyesters with promising antifungal activities.

Ultra-High-Molecular-Weight Polymers Produced by the Immortal Phosphine-Based Catalyst System

A strong organophosphorus superbase, N-(diphenylphosphanyl)-1,3-diisopropyl-4,5-dimethyl-1,3-dihydro-2H-imidazol-2-imine (IAP3) was combined with a sterically encumbered but modestly acidic Lewis acid (LA), (4-Me-2,6-^t Bu₂ -C₆ H₂ O)Alⁱ Bu₂ ((BHT)Alⁱ Bu₂ ), to synergistically promote the frustrated Lewis pair (FLP)-catalyzed living polymerization of methyl methacrylate (MMA), achieving ultrahigh molecular weight (UHMW) poly(methyl methacrylate) (PMMA) with M_n up to 1927 kg mol^-1 and narrow molecular weight distribution (MWD) at room temperature (RT). This FLP catalyst system exhibits exceptionally long lifetime polymerization performance even in the absence of free MMA, which could reinitiate the desired living polymerization after the resulting system was held at RT for 24 h.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Zn(OAc)₂-Catalyzing Ring-Opening Polymerization of N-Carboxyanhydrides for the Synthesis of Well-Defined Polypeptides

Despite notable progress, the fabrication of well-defined polypeptides via controlled ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCAs) using convenient catalysts under mild conditions in a relatively short polymerization time is still challenging. Herein, an easily obtained catalyst system composed of zinc acetate and aniline was explored to mediate the fast ROP of γ-benzyl-l-glutamate-N-carboxyanhydride (BLG-NCA) monomer, to produce poly(γ-benzyl-l-glutamates) (PBLGs) with controllable molecular weights and narrow dispersity. Considering the excellent cooperative action of zinc acetate and a broad scope of aniline derivatives with different functional groups to control ROP of BLG-NCA, this method may offer a useful platform enabling the rapid generation of end-functionalized PBLG and block copolymers for numerous biomedical applications.

Controlled and Efficient Polymerization of Conjugated Polar Alkenes by Lewis Pairs Based on Sterically Hindered Aryloxide-Substituted Alkylaluminumitle

Reported herein is the development of an effective strategy for controlled and efficient Lewis pair polymerization of conjugated polar alkenes, including methyl methacrylate (MMA), n-butyl methacrylate (nBuMA), and γ-methyl-α-methylene-γ-butyrolactone (γMMBL), by the utilization of sterically encumbered Al(BHT)₂Me (BHT: 2,6-di-tert-butyl-4-methylphenol) as a Lewis acid that shuts down intramolecular backbiting termination. In combination with a selected N-heterocyclic carbene (NHC) as a Lewis base, the polymerization of MMA exhibited activity up to 3000 h-1 TOF and an acceptable initiation efficiency of 60.6%, producing polymers with high molecular weight (Mn up to 130 kg/mol) and extremely narrow dispersity (Đ = 1.06~1.13). This controlled polymerization with a living characteristic has been evidenced by chain-extension experiments and chain-end analysis, and enabled the synthesis of well-defined diblock copolymers.

Lewis Pair Catalysts in the Polymerization of Lactide and Related Cyclic Esters

Polyesters, especially poly(lactide) (PLA), are used widely as biodegradable and biocompatible materials, yet their controllable synthesis, especially the stereoselective synthesis of polyesters, is still a challenge. Recently some excellent Lewis pair catalysts for ring-opening polymerization (ROP) of lactide and related cyclic esters have emerged. This review article will highlight the key advances in the ROP catalyzed by Lewis pair compounds with the aim of encouraging the wider application of Lewis pair catalysts in the polymerization of lactide and related cyclic esters.

An efficient strategy for achieving controlled ring-opening polymerization of O-carboxyanhydrides via amine initiation in collaboration with metal-alkoxide catalysis

Amines and Zn(C₆F₅)₂ for polymerization initiation and functionality transfer from Zn(C₆F₅)₂ to a metal-alkoxide species for promoting chain propagation to achieve well controlled ring-opening polymerization of O-carboxyanhydrides.