A comparison of zwitterionic and anionic mechanisms in the dual-catalytic polymerization of lactide

Two different polymerization mechanisms for lactide are selectivity addressed to illuminate the respective role of organobase and Lewis acid component.

A simplified approach for the metal-free polymerization of propylene oxide

Triethyl borane (Et₃B), in combination with phosphazene-type superbases, has recently emerged as a powerful co-catalyst for the anionic polymerization of epoxides. Here, it is demonstrated that the monomer-activating property of Et₃B can also compensate for the application of much gentler organobases. This not only results in simpler setups, but also significantly reduces nucleophilicity/basicity-derived side reactions. Notably, this principle applies to such a degree that simple 4-dimethylaminopyridine (DMAP) or 1,4-diazabicyclo[2.2.2]octane (DABCO) can serve to polymerize propylene oxide (PO). With suitable initiators, this results for example in very well-defined block copolyethers (Ð_M ≤ 1.03) without requiring work-up to remove side products such as PPO homopolymer. Performance correlates nicely with the corresponding organobase proton affinities (PAs), and a limiting PA of 220–230 kcal mol⁻¹ was identified for successful PO polymerization.

Mild organobases and Et₃B constitute an operationally simple, user-friendly catalyst setup for polymerizing propylene oxide.

Simple Lewis pairs of zinc salts and organobases as bifunctional catalysts for controlled ring-opening polymerization of O-carboxyanhydrides

Simple Lewis pairs consisting of organobases and zinc salts were explored to promote ring-opening polymerization of phenyl O-carboxyanhydride via bifunctional catalysis, producing well-defined poly(α-hydroxyalkanoic acid)s with good isotacticity (P_m = 0.88).

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.

The Lewis Pair Polymerization of Lactones Using Metal Halides and N-Heterocyclic Olefins: Theoretical Insights

Lewis pair polymerization employing N-Heterocyclic olefins (NHOs) and simple metal halides as co-catalysts has emerged as a useful tool to polymerize diverse lactones. To elucidate some of the mechanistic aspects that remain unclear to date and to better understand the impact of the metal species, computational methods have been applied. Several key aspects have been considered: (1) the formation of NHO-metal halide adducts has been evaluated for eight different NHOs and three different Lewis acids, (2) the coordination of four lactones to MgCl₂ was studied and (3) the deprotonation of an initiator (butanol) was investigated in the presence and absence of metal halide for one specific Lewis pair. It was found that the propensity for adduct formation can be influenced, perhaps even designed, by varying both organic and metallic components. Apart from the NHO backbone, the substituents on the exocyclic, olefinic carbon have emerged as interesting tuning site. The tendency to form adducts is ZnCl₂ > MgCl₂ > LiCl. If lactones coordinate to MgCl₂, the most likely binding mode is via the carbonyl oxygen. A chelating coordination cannot be ruled out and seems to gain importance upon increasing ring-size of the lactone. For a representative NHO, it is demonstrated that in a metal-free setting an initiating alcohol cannot be deprotonated, while in the presence of MgCl₂ the same process is exothermic with a low barrier.

Sterically Bulky NHC Adducts of GaMe3 and InMe3 for H2 Activation and Lactide Polymerization

The sterically bulky Ga(III) and In(III) (IPr*)MMe3 adducts (1 and 2) and (SItBu)MMe3 adducts (3 and 4) (M = Ga, In; IPr* = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}-1,3-dihydro- imidazol-2-ylidene; SItBu = 1,3-bis(1,1-dimethylethyl)-imidazolidin-2-ylidene) were prepared and structurally characterized, allowing an estimation of the steric hindrance of such Lewis pairs (yields in 1–4: 92%, 90%, 73%, and 42%, respectively). While the IPr* adducts 1 and 2 are robust species, the more severely congested SItBu adducts 3 and 4 are more reactive and exhibit a limited stability in solution. Adduct (SItBu)GaMe3 (3) reacts quickly with H2 at room temperature to afford the corresponding aminal product, 1,3-di-tert-butylimidazolidine (5), along with free GaMe3. Such Frustrated Lewis Pair (FLP) reactivity constitutes the first instance of a H2 activation involving a simple trialkyl GaR3 species. Adduct 3 also mediates the ring-opening polymerization (ROP) of rac-lactide at room temperature to afford cyclic polylactide (PLA).

Metallic organophosphate catalyzed bulk ring-opening polymerization

Metallic organophosphates have catalyzed the bulk ring-opening polymerization of cyclic esters and cyclic carbonates through a bifunctional mode.

Computational Studies on the Selective Polymerization of Lactide Catalyzed by Bifunctional Yttrium NHC Catalyst

A theoretical investigation of the ring-opening polymerization (ROP) mechanism of rac-lactide (LA) with an yttrium complex featuring a N-heterocyclic carbine (NHC) tethered moiety is reported. It was found that the carbonyl of lactide is attacked by N(SiMe3)2 group rather than NHC species at the chain initiation step. The polymerization selectivity was further investigated via two consecutive insertions of lactide monomer molecules. The insertion of the second monomer in different assembly modes indicated that the steric interactions between the last enchained monomer unit and the incoming monomer together with the repulsion between the incoming monomer and the ligand framework are the primary factors determining the stereoselectivity. The interaction energy between the monomer and the metal center could also play an important role in the stereocontrol.

Investigation of the dinuclear effect of aluminum complexes in the ring-opening polymerization of ε-caprolactone

Al complexes bearing hydrazine-bridging Schiff base ligands showed the best catalytic activity, approximately 3- to 11-fold higher than that of dinuclear Al complexes bearing Salen ligands and mononuclear Al complexes bearing Schiff base ligands.

P,O-Phosphinophenolate zinc(II) species: synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate

The P,O-type phosphinophenol proligands (1·H, 2-PPh2-4-Me-6-Me-C6H2OH; 2·H, 2-PPh2-4-Me-6-(t)Bu-C6H2OH) readily react with one equiv. of ZnEt2 to afford in high yields the corresponding Zn(II)-ethyl dimers of the type [(κ(2)-P,O)Zn-Et]2 (3 and 4) with two μ-OPh bridging oxygens connecting the two Zn(II) centers, as determined by X-ray diffraction (XRD) studies in the case of 3. Based on diffusion-ordered NMR spectroscopy (DOSY), both species 3 and 4 retain their dimeric structures in solution. The alcoholysis reaction of Zn(II) alkyls 3 and 4 with BnOH led to the high yield formation of the corresponding Zn(II) benzyloxide species [(κ(2)-P,O)Zn-OBn]2 (5 and 6), isolated in a pure form as colorless solids. The centrosymmetric and dimeric nature of Zn(II) alkoxides 5 and 6 in solution was deduced from DOSY NMR experiments and multinuclear NMR data. Though the heteroleptic species 5 is stable in solution, its analogue 6 is instable in CH2Cl2 solution at room temperature to slowly decompose to the corresponding homoleptic species 8via the transient formation of (κ(2)-P,O)2Zn2(μ-OBn)(μ–κ(1):κ(1)-P,O) (6′). Crystallization of compound 6 led to crystals of 6′, as established by XRD analysis. The reaction of ZnEt2 with two equiv. of 1·H and 2·H allowed access to the corresponding homoleptic species of the type [Zn(P,O)2] (7 and 8). All gathered data are consistent with compound 7 being a dinuclear species in the solid state and in solution. Data for species 8, which bears a sterically demanding P,O-ligand, are consistent with a mononuclear species in solution. The Zn(II) alkoxide species 5 and the [Zn(P,O)2]-type compounds 7 and 8 were evaluated as initiators of the ring-opening polymerization (ROP) of lactide (LA), ε-caprolactone (ε-CL) and trimethylene carbonate (TMC). Species 5 is a well-behaved ROP initiator for the homo-, co- and ter-polymerization of all three monomers with the production of narrow disperse materials under living and immortal conditions. Though species 7 and 8 are ROP inactive on their own, they readily polymerize LA in the presence of a nucleophile such as BnOH to produce narrow disperse PLA, presumably via an activated-monomer ROP mechanism.

The ring-opening polymerization of ε-caprolactone and l-lactide using aluminum complexes bearing benzothiazole ligands as catalysts

A series of aluminum complexes bearing benzothiazole ligands was synthesized and the ring-opening polymerization of ε-caprolactone (CL) and l-lactide (LA) using these aluminum complexes as catalysts was studied.

N-Heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers

This review spotlights current areas of high interest for NHC-based polymerization research.

Controlled Divinyl Monomer Polymerization Mediated by Lewis Pairs: A Powerful Synthetic Strategy for Functional Polymers

Lewis pair cooperation provides a facile approach for polymerizing dissymmetric divinyl polar monomers such as 4-vinylbenzyl methacrylate in excellent regioselectivity and high reactivity at mild conditions, affording soluble polymers bearing pendant active vinyl groups with high molecular weight (up to 6.4 × 10⁵ g/mol) and narrow polydispersity (PDI < 1.5). ESI-TOF MS study demonstrated that the polymerization process only concerned the methacrylic double bond and selectively remained the pendant allylic or styrene C═C bond.

Zwitterionic polymerization of glycidyl monomers to cyclic polyethers with B(C6F5)3

Glycidyl monomers react with B(C₆F₅)₃ to generate cyclic polyethers decorated with functional groups.

Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo- and polymerization catalysis

The thermally triggered release of catalytically active, free NHCs from various heat-sensitive progenitors is discussed.