Chain-end Control Research Articles

Highly Active and Stereoselective Ring‐Opening Polymerization of Racemic Lactide Over HTGCP Catalysts

ABSTRACTDespite the extraordinary success has been achieved in metal catalyst‐promoted stereoselective ring‐opening polymerization (ROP) of rac‐lactide (rac‐LA), well‐controlled stereoselective rac‐LA ROP by organic catalyst still remains a scientific challenge. In this study, we showcase the remarkable ability of the phosphazene base 2,2,4,4,6,6‐hexa(tetramethyl‐guanidyl)‐1,3,5,2λ5,4λ5,6λ5‐cyclictri(phosphazene) (HTGCP) to facilitate the stereoselective ROP of rac‐LA at low temperatures, yielding isotactic stereoblock poly (lactic acid) (PLA) with a stereoselectivity index (Pm) up to 0.86. Furthermore, by harnessing the synergy between HTGCP and a (thio)urea, both high stereoselectivity (Pm up to 0.85) and remarkable catalytic activity (> 99% monomer conversion within an hour) can be achieved at ambient temperature. Notably, upon decreasing the temperature to −20°C, an even higher degree of regularity is attained (Pm up to 0.91). The direct linear correlation between molecular weights (Mn), monomer conversions, and the narrow dispersity (Đ) of the resulting polymers, coupled with their high end‐group fidelity, underscores the meticulous control exerted over the polymerization. Based on these findings, we propose that the chain end control mechanism underlies the synthesis of isotactic stereoblock PLA from rac‐LA in the presence of HTGCP, offering a novel and efficient pathway to produce high‐performance PLA materials.

Mechanistic Insights into the cis-Selective Catalytic Ring-Opening Metathesis Polymerization.

Cis-selective ring-opening metathesis polymerization (ROMP) with the commercial Grubbs "nitrato catalyst" has shown promise for synthesizing stereoregular materials, but it comes with the drawback of losing control over the molecular weight due to the poor initiation rate of the catalyst and the need for stoichiometric ruthenium complex loading. To address these issues, we developed a chain transfer polymerization method that allows for the catalytic synthesis of polymers while controlling the degree of polymerization. This allowed us to produce shorter polymers with exceptional chain-end control. Analysis of the polymers revealed a novel double monomer addition mechanism for this catalyst. MALDI-ToF mass spectrometric measurements showed that when using small monomers like norbornene, the polymer chains contained only odd numbers of monomers. In contrast, the polymerization of norbornene-imide-type monomers shows a major distribution with odd numbers of monomers along with a minor distribution of even numbers. This unique distribution of polymer chain types had not been previously observed in ROMP. We explain this phenomenon by the chiral nature of the catalyst that yields two isomeric catalytic species with dissimilar reactivities toward monomer and chain transfer agents.

Enantiomorphic Site-Assisted Chain End Control Stereospecific Alternating Copolymerization of Chiral Cyclic Diesters.

Stereospecific alternating copolymerization of different chiral cyclic esters is one feasible approach to enrich the structural diversity of copolyesters and tailor their properties. However, dramatically different reactivities of different cyclic esters let a perfectly stereospecific alternating polymerization of these cyclic esters be a challenge, thus the catalyst is required to balance their reactivities. Herein, a remarkable enantiomorphic site effect on chain end control was discovered and successfully utilized to balance the reactivities of highly reactive S, S-lactide (S, S-LA) and low reactive R, R-ethylglycolide (R, R-EG)/R, R-propylglycolide (R, R-PG) during their heterospecific alternating copolymerization. The enantiomorphic site of R, R-SalenAl complex can increase the relative reactivity of R, R-EG/R, R-PG and suppress that of S, S-LA, then a perfectly alternating sequence of the copolymer of S, S-LA and R, R-EG/R, R-PG can be achieved (Palt = 0.96/0.91); inversely, using S, S-SalenAl complex, the significant enantiomorphic site effect enlarges the reactivity difference of two monomers, the alternating level was just 0.70/0.68 even to 0.61. Poly(S, S-LA-alt-R, R-EG) with a high alternating regularity exhibits lower glass transition temperatures and a dramatically higher elongation at break (εB = 449 ± 51% (Palt = 0.96) vs εB = 6 ± 1% (Palt = 0.70)).

Enantiomorphic Site‐Assisted Chain End Control Stereospecific Alternating Copolymerization of Chiral Cyclic Diesters

Stereospecific alternating copolymerization of different chiral cyclic esters is one feasible approach to enrich the structural diversity of copolyesters and tailor their properties. However, dramatically different reactivities of different cyclic esters let a perfectly stereospecific alternating polymerization of these cyclic esters be a challenge, thus the catalyst is required to balance their reactivities. Herein, a remarkable enantiomorphic site effect on chain end control was discovered and successfully utilized to balance the reactivities of highly reactive S, S‐lactide (S, S‐LA) and low reactive R, R‐ethylglycolide (R, R‐EG)/R, R‐propylglycolide (R, R‐PG) during their heterospecific alternating copolymerization. The enantiomorphic site of R, R‐SalenAl complex can increase the relative reactivity of R, R‐EG/R, R‐PG and suppress that of S, S‐LA, then a perfectly alternating sequence of the copolymer of S, S‐LA and R, R‐EG/R, R‐PG can be achieved (Palt = 0.96/0.91); inversely, using S, S‐SalenAl complex, the significant enantiomorphic site effect enlarges the reactivity difference of two monomers, the alternating level was just 0.70/0.68 even to 0.61. Poly(S, S‐LA‐alt‐R, R‐EG) with a high alternating regularity exhibits lower glass transition temperatures and a dramatically higher elongation at break (εB = 449 ± 51% (Palt = 0.96) vs εB = 6 ± 1% (Palt = 0.70)).

Stereoselectivity Control Interplay in Racemic Lactide Polymerization by Achiral Al-Salen Complexes.

The origin of stereocontrol in ring opening polymerization (ROP) of racemic lactide (rac-LA) promoted by achiral aluminium-based catalysts has been explained through DFT calculations combined with a molecular descriptor (%VBur) and the activation strain model (ASM-NEDA) analysis. The proposed chain end control (CEC) model suggests that the ligand framework adopts a chiral configuration mimicking the enantiomorphic site control (ESC) while also incorporating control of the last inserted monomer unit. It is found that the ligand wrapping mode around the aluminium centre is dictated by the monomer configuration (R,R-LA and S,S-LA). A good correlation with experimental data is achieved only when accounting for the ligand dynamic features and its steric influences, as highlighted by %VBur steric maps and ASM-NEDA analysis. Understanding the ESC and CEC interplay is an important target for obtaining stereoselective ROP polymerization for the synthesis of biodegradable materials with tailored properties.

Organocatalytic ring-opening polymerization of lactide by bis(thiourea) H-bonding donating cocatalysts with binaphthyl-amine framework

Development of H-bonding organo-catalysts with high catalytic activity and high degree of control is significantly important for the metal-free polyesters that are potentially suitable for biomedical usage. The concomitant use of base and commercially available thiourea generally suffered from prolonged reaction time and resulted in incomplete monomer conversion in some cases. Introducing one or more (thio)urea H-bonding donating arms to the parent thiourea has been proved to be an effective method for substantially increasing the activity of thiourea H-bonding donors. Consequently, we synthesized bis-thioureas derived from binaphthyl-amine bearing different substituents and investigated their catalytic performance in ROPs of lactide in this work. The density functional theory (DFT) calculations suggested that the “activated-thiourea” mode is more preferred for the bis(thiourea) containing binaphthyl-amine framework. The bis(thiourea)/base binary systems could effectively promote the LA polymerization using benzyl alcohol as initiator. The polymerization rates and the degree of control over the polymerization are highly dependent on the structures of the bis(thiourea) and base. Bis(thiourea)/1,5,7-triazabicyclo[4.4.0]dec‑5-ene pairs exhibited highest catalytic activity compared to other bis(thiourea)/base pairs, and the turnover frequency is high up to 1980 h–1 at 75 °C. In addition, the bulky hindrance and axial chirality of the binaphthyl-amine framework could enable stereoselective ROP of rac-LA to produce isotactic-rich and crystalline PLAs at relatively low temperature (0 °C). Mechanistic studies indicated that both enantiomorphic site control (ESC) and chain end control (CEC) concurrently occurred in the rac-LA polymerization catalyzed by bis(thiourea)/base binary systems. This work will inspire future design of H-bonding donors with high catalytic performance.

A comprehensive picture on chain walking olefin polymerization

α-Diimine nickel and palladium catalyzed α-olefin polymerization could produce polyolefins with complicated branching structures due to the chain walking feature. The thorough understanding of the polymer microstructure could provide a general picture of monomer enchainment including the insertion selectivity and chain walking pathways. By utilizing α-diimine nickel (DBB-Ipty-Ni) and palladium (DBB-Ipty-Pd) catalysts reported previously by us that show an excellent selectivity in ethylene polymerization, homopolymerizations of linear α-olefin such as 1-pentene, 1-hexene and 1-octene, and non-linear α-olefin 4-methyl-1-pentene (4M1P) were conducted. In-depth studies of the polyolefin microstructures revealed the enchainment of α-olefin monomers, in which the enchainment of 4M1P indicated a chain-end control on the insertion regio-selectivity. Moreover, internal olefins including cis-2-hexene and cis-3-hexene, and non-conjugated diolefin 1,7-octadiene were successfully polymerized by the α-diimine palladium catalyst. It is worth noting that the isomerization of internal olefins to 1-alkenes in polymerization was found. Chain walking cyclopolymerization of 1,7-octadiene occurred to produce an alternating cyclic copolymer bearing cis-five- and cis-six-membered rings on the polymer backbone. A rational enchainment mechanism was proposed to explain the formation of the special alternating cyclic chain structure. In addition, mechanical properties of poly(α-olefin)s were studied, among which poly(1-octene) exhibited the highest tensile stress-at-break (17.9 MPa) and strain-at-break (1480%), while poly(1-pentene) showed the best strain recovery (81%).

Defining Stereochemistry in the Polymerization of Lactide by Aluminum Catalysts: Insights into the Dual-Stereocontrol Mechanism.

Aspects of the proposed pathway combining chain-end and enantiomorphic site control for the stereospecific polymerization of lactide (LA) were investigated through studies of aluminum complexes supported by enantiopure and racemic bipyrrolidine-based salan ligands, Lig1AlOBn and Lig2AlOBn. Spectroscopic analysis of stoichiometric initiation reactions and the definition of the stereochemistry of the selective formation of the "match" single-insertion products by X-ray crystallography led to key conclusions about the observed stereocontrol. Notably, it was determined to rely heavily on the preference for the trio of stereocenters around the metal to have a "match" formation (RR-ligand + S-polymer), which works synergistically with the enantiomorphic site preference of the catalyst to ring-open next to a stereocenter of a monomer of the same chirality as that of the ligand, resulting in highly heterotactic or syndiotactic PLA from rac- or meso-LA, respectively.

Highly Active Organocatalysts for Stereoselective Ring-Opening Polymerization of Racemic Lactide at Room Temperature.

Although significant advances have been achieved, highly stereocontrolled polymerization using organocatalysts is still a great challenge, such as ring-opening polymerization of racemic lactide (rac-LA) for the synthesis of stereoregular polylactide (PLA). In this context, a series of binary organocatalysts consisting of different phosphazenes (CTPB, C3N3-Me-P3, C3N3-Py-P3, t-BuP2, and t-BuP4) and achiral ureas (U1-U6) were applied for the stereocontrolled ROP of rac-LA under mild conditions. It is remarkable that C3N3-Py-P3/U4 with the compatible basicity/acidity showed both high activity (92% conversion within 10 min) and great stereoselectivity (Pm = 0.92) at room temperature (20 °C), and the resultant stereoblock PLA had predictable molar mass, narrow distribution (Đ = 1.07), and high melting temperature (Tm = 190 °C). Interactions involved among phosphazene, urea, and initiator were investigated by an in situ NMR technique. It was found that C3N3-Py-P3 reacted with benzyl alcohol (BnOH) to form a relatively loose ion pair in the presence of U4, which accounted for both high activity and stereoselectivity. Kinetics studies for different LA monomers at 20 °C showed kobs-1 (rac-LA) = 0.212 min-1, kobs-1 (D-LA) = 0.311 min-1, and kobs-1 (L-LA) = 0.317 min-1, indicative of the chain end control mechanism for stereocontrolled ROP.

Isoselective mechanism for asymmetric kinetic resolution polymerization of rac-lactide catalyzed by chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes

Chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes were synthesized and utilized for the asymmetric kinetic resolution polymerization of rac-lactide. These polymerizations catalyzed by chiral catalysts were performed efficiently, resulting in isotactic polylactide with stereogradient microstructure and narrow dispersity. The asymmetric kinetic resolution polymerization and block polymerization experiments indicated that the combination of enantiomorphic site control and chain-end control should be responsible for the stereocontrol mechanism during the polymerization process.

Lactide polymerization using a sterically encumbered, flexible zinc complex

4-(tert-Butyl)-2-trityl-6-(di-(2-picolyl)amine)phenol, LH, was prepared from paraformaldehyde, 4-(tert-butyl)-2-tritylphenol, and di-(2-picolyl)amine. Reaction with Zn(N(SiMe3)2)2 gave LZnN(SiMe3)2. The complex was shown by X-ray diffraction study, variable temperature NMR, and DFT calculations to coordinate only one pyridine ligand, which allows for fast and facile complex isomerisation. LZnN(SiMe3)2 was active in rac-lactide polymerization, but in contrast to previous complexes of this type, it did not show any evidence for isotactic monomer enchainment via a catalytic-site mediated chain-end control mechanism. Addition of alcohol led to increased activity, but the complex was unstable in the presence of free alcohol.

Iron-catalysed synthesis and chemical recycling of telechelic 1,3-enchained oligocyclobutanes.

Closed-loop recycling offers the opportunity to mitigate plastic waste through reversible polymer construction and deconstruction. Although examples of chemical recycling of polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers, which are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition/oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n'-divinyl)oligocyclobutane. This material is thermally stable, has stereoregular segments arising from chain-end control, and exhibits high crystallinity even at low molecular weight. Exposure of the oligocyclobutane to vacuum in the presence of the pyridine(diimine) iron precatalyst used to synthesize it resulted in deoligomerization to generate pristine butadiene, demonstrating a rare example of closed-loop chemical recycling of an oligomeric material derived from a commodity hydrocarbon feedstock.

Isoselective Ring-opening Polymerization of Racemic Lactide Catalyzed by N-heterocyclic Olefin/(Thio)urea Organocatalysts

The isoselective ring-opening polymerization of racemic lactide was achieved by combining N-heterocyclic olefin (NHO) with mono(thio)ureas or bis(thio)ureas as catalytic systems. The polymerization process shows high stereoselectivity, controllability and reactivity, delivering multi-block isotactic polylactides with high chain-end fidelity and narrow molecular weight distributions. The enhancement of catalytic performance was observed in the following order: bisthiourea (DTU) < monothiourea (TU) < bisurea (DU) < urea (U). The highest Pm (probability of forming a meso dyad) = 0.91 was observed at −70 °C when using NHO/U1 catalytic system and the high stereoselectivity was attributed to chain-end control mechanism.

Chiral 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-Catalyzed Stereoselective Ring-Opening Polymerization of rac-Lactide: High Reactivity for Isotactic Enriched Polylactides (PLAs).

Chiral 4,8-diphenyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (DiPh-TBD) was synthesized and applied to a ring-opening polymerization of rac-lactide (rac-LA). The chiral DiPh-TBD promoted the synthesis of isotactic enriched polylactides (PLAs) with controlled molecular weight and narrow molecular weight distributions under mild, metal-free conditions. When the [rac-LA]/[Cat.] ratio was 100/1, full monomer conversion was achieved within only 1 min and a moderate probability of 0.67 meso dyads (Pm) was obtained at room temperature. A chain-end control mechanism (CEC) was found to be responsible for the isoselectivity based on the homodecoupled 1H NMR spectrum, the chiral HPLC measurement, and kinetic studies.

Isospecific Group-Transfer Polymerization of Diethyl Vinylphosphonate and Multidimensional NMR Analysis of the Polymer Microstructure

Control of stereoregularity is an integral part of a precision polymerization method and for the development of functional materials. Yttrium- and aluminum-based catalysts are known for converting diethyl vinylphosphonate (DEVP) into its stimuli-responsive polymer in a precise but stereoirregular way. Herein, we present Y- and Al-based constrained geometry complexes (CGCs) to induce isotacticity without losing control over other macromolecular parameters. After having established convenient synthesis routes and detailed structural analyses, these CGCs showed exceedingly high turn-over frequencies (up to 45 000 h–1) in the group-transfer polymerization of DEVP. The initiator efficiencies (≤99%) and dispersities (≤1.02) strongly depended on the substitution pattern of the applied ligands. An analysis of the microstructure using multidimensional NMR (1H–1H and 1H–13C(−31P)) correlation experiments demonstrated significant disparities for the stereospecificity of the CGCs and enabled a reliable signal assignment. The yttrium catalysts produced highly isotactic poly(diethyl vinylphosphonate), likely following a chain-end control mechanism, whereas the aluminum complexes produced less defined polymers.

Isoselective Lactide Ring Opening Polymerisation using [2]Rotaxane Catalysts.

Polylactide (PLA) is a fully biodegradable and recyclable plastic, produced from a bio‐derived monomer: it is a circular economy plastic. Its properties depend upon its stereochemistry and isotactic PLA shows superior thermal‐mechanical performances. Here, a new means to control tacticity by exploiting rotaxane conformational dynamism is described. Dynamic achiral [2]rotaxanes can show high isoselectivity (Pi=0.8, 298 K) without requiring any chiral additives and enchain by a chain end control mechanism. The organocatalytic dynamic stereoselectivity is likely applicable to other small‐molecule and polymerization catalyses.

Isoselective Lactide Ring Opening Polymerisation using [2]Rotaxane Catalysts

AbstractPolylactide (PLA) is a fully biodegradable and recyclable plastic, produced from a bio‐derived monomer: it is a circular economy plastic. Its properties depend upon its stereochemistry and isotactic PLA shows superior thermal‐mechanical performances. Here, a new means to control tacticity by exploiting rotaxane conformational dynamism is described. Dynamic achiral [2]rotaxanes can show high isoselectivity (Pi=0.8, 298 K) without requiring any chiral additives and enchain by a chain end control mechanism. The organocatalytic dynamic stereoselectivity is likely applicable to other small‐molecule and polymerization catalyses.

Isoselective Ring-Opening Polymerization of rac-Lactide from Chiral Takemoto's Organocatalysts: Elucidation of Stereocontrol.

Despite significant advances in organocatalysis, stereoselective polymerization reactions utilizing chiral organocatalysts have received very little attention, and much about the underlying mechanisms remains unknown. Here, we report that both commercially available (R,R)- and (S,S)-enantiomers of chiral thiourea-amine Takemoto's organocatalysts promote efficient control and high isoselectivity at room temperature of the ring-opening polymerization (ROP) of racemic lactide by kinetic resolution, yielding highly isotactic, semicrystalline and metal-free polylactide (PLA). Kinetic investigations and combined analyses of the resulting PLAs have allowed the stereocontrol mechanism, which eventually involves both enantiomorphic site control and chain-end control, to be determined. Moreover, epimerization of rac-LA to meso-LA is identified as being responsible for the introduction of some stereoerrors during the ROP process.

Exploring Steric Effects of Zinc Complexes Bearing Achiral Benzoxazolyl Aminophenolate Ligands in Isoselective Polymerization of rac-Lactide.

A series of tridentate achiral benzoxazolyl-based aminophenolate zinc complexes, LZnN(SiMe3)2 (L = 2-{[benzoxazoly-CH2N(R3)-]CH2}-6-R1-4-R2-C6H2O, R1 = R2 = Cl, R3 = Bn (1); R1 = R2 = tBu, R3 = Bn (2); R1 = trityl, R2 = Me: R3 = Bn (3); R3 = phenethyl (4); R3 = 3-methylbutyl (7); R3 = n-hexyl (8); R3 = cyclopentyl (9); R3 = cyclooctyl (11); R3 = 1-adamantyl (12)), was synthesized via the reactions of Zn[N(SiMe3)2]2 and 1 equiv of the corresponding aminophenol proligands. All of the complexes were obtained as racemates, and the X-ray diffraction studies confirmed the monomeric structures of typical complexes 11 and 12, where the metal center is tetra-coordinated by three donors of the aminophenolate ligand and one silylamido group. All of the complexes proved to be efficient initiators for the ring-opening polymerization of rac-lactide ( rac-LA) at ambient temperature, and the polymerizations were better controlled in the presence of 2-propanol. The substituents on the ortho-position of the phenoxide unit of the ligand and the skeleton nitrogen atom show significant influences on the stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of heterotactic biased polylactide (PLA) by complexes 1 and 2 ( Pm = 0.40-0.44) and moderately to highly isotactic PLA by complexes 3-12 ( Pm = 0.74-0.89). Detailed mechanism studies and microstructure analysis of typical PLA samples revealed that these zinc initiators afforded isotactic stereoblock PLAs via a chain-end control mechanism, and there is no obvious polymer exchange process during the polymerization process.

Catalytic-Site-Mediated Chain-End Control in the Polymerization of rac-Lactide with Copper Iminopyrrolide Complexes

Reaction of copper(II) methoxide with N-R-2-iminopyrroles (LH) and pyridylmethanol (R′OH) provided the dinuclear complexes {LCu(μ-OR′)}2 (R = naphtyl, CHPh2, 2,6-xylyl, 2,6-diisopropylphenyl (diip), or p-bromobenzyl). All complexes crystallized as dinuclear compounds with a square-pyramidal coordination geometry around copper and either imine or pyridine (for R = diip) in the apical position. The naphthyl-substituted complex was inactive in rac-lactide polymerization at room temperature in benzene. All other complexes showed good activity with apparent rate constants of kobs = 0.16(1)–1.89(8) h–1 at 2 mM catalyst concentration. All complexes showed a preference for slight isotactic monomer enchainment with Pm = 0.60–0.68. Stereoerror analysis indicate that the chain-end determines stereocontrol. A dependance of stereocontrol on the steric bulk of the ligand, on the initial monomer concentration and on the symmetry of the catalytic site support that the chiral information on the chain-end is mediated via t...