Stereospecific Polymerization Research Articles

Simultaneous and in situ syntheses of an enantiomeric pair of homochiral polymers as their perfect stereocomplex in a crystal

Circumventing the issues of conventional stereocomplexation of preformed polymers, herein, we synthesize two enantiopure polymers of opposite chirality simultaneously and in situ as their 1:1 stereocomplex via topochemical polymerization. We design and synthesize an inositol-based achiral monomer for topochemical ene-azide cycloaddition (TEAC) polymerization. In the crystal, the monomer exhibits conformational enantiomerism, and its conformational enantiomers are self-sorted in an arrangement for TEAC polymerization to yield two enantiopure polymers of opposite chirality. Upon heating the monomer crystals, each self-sorted set of conformational enantiomers undergoes regio- and stereospecific polymerization in a single-crystal-to-single-crystal fashion, generating two 1, 4-triazolinyl-linked polymers of opposite chirality simultaneously. The new chiral carbons in all the triazoline rings of a particular polymer chain have the same absolute configuration. These homochiral polymer strands align parallelly, forming a layer, and such enantiopure layers of opposite chirality stack alternately, forming a perfect 1:1 stereocomplex, which we confirmed using single-crystal XRD analysis.

Enhanced Control of Isoprene Polymerization with Trialkyl Rare Earth Metal Complexes through Neutral Donor Support.

The development of catalysts for stereospecific polymerization of 1,3-dienes is an area of interest due to the robust nature of poly(1,3-diene)s' physical and mechanical properties, as well as the material's versatility in many applications. Dialkyl rare earth metal complexes supported by a diverse cast of ligand frameworks are selective for the polymerization of 1,3-dienes and are an exciting option for examination. However, development in this area has been hampered by the focus on complex catalyst systems that are costly to make. In this study, we synthesize a series of simple homoleptic trialkyl rare earth metal precatalysts and highlight their efficacy for isoprene polymerization using 1 or 2 equiv of [Ph3C][B(C6F5)4] activator. We investigated the addition of commercially available in situ donors, leading to the identification of triphenylphosphine as an ideal support to enhance the dispersity control and prevent loss of catalyst activity. We demonstrated how the activation and reaction conditions, including the order/time of reagent addition and donor electronics, had a major impact on the rate, control, and selectivity for the polymerization of 1,3-dienes. Further interrogation of the catalyst system signals the crucial role of triphenylphosphine in providing enhanced stability and control in this living catalyst system.

Iminopyridine Copper Complex‐Based Catalysts for 1,3‐Dienes Stereospecific Polymerization

AbstractSome novel copper complexes are synthesized by reacting copper(II) dichloride and copper(II) dibromide with a series of pyridyl‐imine ligands differing in the nature of the substituents at the iminic carbon and at the imino nitrogen atom. All the complexes are characterized by analytical and infrared data: for some of them single crystals are obtained, and their molecular structure is determined by X‐ray diffraction. The complexes are combined with methylaluminoxane (MAO) for the polymerization of 1,3‐dienes (i.e., 1,3‐butadiene and isoprene) obtaining active and selective catalysts and, in case of isoprene, polymers with quite unusual structures, i.e., predominantly alternating cis‐1,4‐alt‐3,4 structures containing cis‐1,4 units sequences within the polymer chain.

Metal Catalyzed Polymerization: From Stereoregular Poly(α‐olefins) to Tailor‐Made Biodegradable/Biorenewable Polymers and Copolymers

AbstractMetal catalyzed polymerizations are among the most important chemical reactions, accounting for the production of about 400 million tons per year of polymeric materials, 50 % of which are polyolefins. The CIRCC research units at the University of Salerno, founded by the late Professor Adolfo Zambelli, a coworker of Giulio Natta and a pioneer in the studies of stereospecific polymerization catalysts, has a consolidated expertise in this field. Although often considered a “mature” area of research, olefin polymerization catalysis continues to drive great interest of both industrial and academic scientists. On the other hand, strong political and economic pressure toward the development of “green” and possibly biodegradable alternatives to olefin‐based polymers stimulated our group to direct increasing research efforts in the area of sustainable polymers. In this perspective, we focus on the most recent work from the CIRCC research units involved in homogeneous catalysis for polymerization of a variety of monomers, with the aim to address how the concepts and the expertise developed for olefin polymerization can be applied to the development of different metal‐catalyzed polymerizations and copolymerizations. Of course, although the results are discussed in the frame of the most important literature contributions, a comprehensive review of such a wide and diversified topic is out of the scope of the paper. References to reviews covering the different types of metal catalyzed polymerizations are provided in each chapter.

Dichloro(2,2′-bipyridine)copper/MAO: An Active and Stereospecific Catalyst for 1,3-Diene Polymerization

Dichloro(2,2'-bipyridine)copper was synthesized by reacting copper dichloride with bypyridine, and its behavior, in combination with methylaluminoxane (MAO), in the polymerization of butadiene, isoprene, 2,3-dimethyl-1,3 butadiene, and 3-methyl-1,3-pentadiene was examined. The purpose of this study is to find catalytic systems that are more sustainable than those currently used for the polymerization of butadiene and isoprene (e.g., Co and Ni), but that are comparable in terms of catalytic activity and selectivity. Predominantly, syndiotactic 1,2 polybutadiene, crystalline syndiotactic 3,4 polyisoprene, crystalline syndiotactic 1,2 poly(3-methyl-1,3-pentadiene), and crystalline cis-1,4 poly(2,3-dimethyl-1,3-butadiene) were obtained in a manner similar to that observed with the analogous iron complex. As far as we know, the investigated catalytic system represents the first example of a copper-based catalyst in the field of stereospecific polymerization. Given the great availability of copper, its extremely low toxicity (and therefore high sustainability), and the similarity of its behavior to that of iron, the result obtained seems to us of considerable interest and worthy of further investigation.

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.

Stereospecific Polymerization of Bulky Methacrylates Using Organocatalyst in Strong Donating Solvent via Self-Controlled Mechanism

Stereospecific Polymerization of Bulky Methacrylates Using Organocatalyst in Strong Donating Solvent via Self-Controlled Mechanism

Single-crystal-to-single-crystal translation of a helical supramolecular polymer to a helical covalent polymer

Polymers possessing helical conformation in the solid state are in high demand. We report a helical peptide-polymer via the topochemical ene-azide cycloaddition (TEAC) polymerization. The molecules of the designed Gly-Phe-based dipeptide, decorated with ene and azide, assemble in its crystals as β-sheets and as supramolecular helices in two mutually perpendicular directions. While the NH…O H-bonding facilitates β-sheet-like stacking along one direction, weak CH…N H-bonding between the azide-nitrogen and vinylic-hydrogen of molecules belonging to the adjacent stacks arranges them in a head-to-tail manner as supramolecular helices. In the crystal lattice, the azide and alkene of adjacent molecules in the supramolecular helix are suitably preorganized for their TEAC reaction. The dipeptide underwent regio- and stereospecific polymerization upon mild heating in a single-crystal-to-single-crystal fashion, yielding a triazoline-linked helical covalent polymer that could be characterized by single-crystal X-ray diffraction studies. Upon heating, the triazoline-linked polymer undergoes denitrogenation to aziridine-linked polymer, as evidenced by differential scanning calorimetry, thermogravimetric analysis, and solid-state NMR analyses.

Dienes Polymerization: Where We Are and What Lies Ahead

Recently, the research activity in the field of the stereospecific polymerization of conjugated dienes has focused on the use of catalysts based on organometallic complexes having a well-defined structure and containing ligands of various types (e.g., phosphines, bis-imines, and pyridylimines), since it has been observed that the nature of the ligand, as expected on the basis of the diene polymerization mechanism proposed several years ago by Porri, is able to exert a strong influence on the polymerization regio- and stereoselectivity, on the polymer molecular weights and their distribution, and in some cases to impart a living feature to the catalysts themselves. In this Perspective we highlight the most recent results obtained in this field and discuss possible future developments in this area.

Synthesis of Polymers Bearing a Chiral Backbone via Stereospecific Ionic Ring-Opening Polymerization of Chiral Donor-Acceptor Cyclopropanes.

The stereospecific ionic ring-opening polymerization of various donor-acceptor cyclopropanes is reported. The chiral cyclopropane monomers are readily prepared with established methodology and stereospecific polymerization is best conducted with a catalytic amount of MgBr2 serving as a Lewis acid and as an initiator. Polymers with molecular masses of up to 7800g mol-1 containing a stereocenter in every repeating unit are obtained and the substituents of the monomers can be readily varied to access a novel class of chiral polymers.

Stereospecific polymerization of 1,3-butadiene catalyzed by iron (III) Salen complexes combined with exogenous electron donor

Iron Salen complexes varied with steric effect were designed and examined in polymerization of butadiene coupled with triphenylphosphate as electron donor. The results demonstrate they are able to serve as unique catalysts, yielding a family of crystalline 1,2 syndiotactic polybutadienes as thermoplastic resin materials. The incorporation of sterically hindered group into the iron catalyst provides a high degree of control over the stereospecificity with certain adverse effect on activity. The necessity of donor during the stereopolymerization of butadiene is also highlighted by concomitant increase in activity and selectivity at higher temperature. The formed stereo active sites are very uniform in nature irrelative to the alkylaluminum and monomer feeding, resulting in polymers having constant microstructures and stable properties. The highly stable catalytic performances by the catalyst formed by accessible catalyst component could be industrially favored for production of syndiotactic 1,2 polybutadiene.

Neodymium catalysts for polymerization of dienes, vinyl monomers, and ε-caprolactone

This review discusses various neodymium catalysts for stereospecific polymerization of dienes, vinyl monomers, and ε-caprolactone.

Stereospecific coordination polymerization of vinyl monomers mediated by rare earth metal complexes

The stereospecific polymerization is one of the most critical topics in polymer sciences, due to the significant influence of tacticity on the properties of polymers. This review summarizes recent progress in stereospecific coordination polymerization mediated by rare earth metal complexes. The stereospecific coordination polymerization is achieved by optimizing the structures of ligands, the rare earth metal ions, and the polymerization conditions, while styrene, polar substituted styrenes, 2-vinylpyridine and other olefins are used as monomers. The properties of resulting polymers are significantly influenced by the structure of monomers, and the tacticity of polymers.

Biobased Cycloolefin Polymers: Carvone-Derived Cyclic Conjugated Diene with Reactive exo-Methylene Group for Regioselective and Stereospecific Living Cationic Polymerization.

Carvone, a naturally abundant chiral cyclic α,β-unsaturated carbonyl compound, was chemically transformed into cyclic exo-methylene conjugated dienes. The exo-methylene group had high reactivity in cationic polymerization and was efficiently polymerized in a controlled manner via regioselective 1,4-conjugated additions using initiating systems effective for living cationic polymerization of vinyl ethers. The obtained polymers with 1,3-cyclohexenyl units and tetra-substituted olefins in the main chain showed high glass transition temperatures over 110 °C. The chiral monomer underwent stereospecific polymerization to result in polymers with low solubility and weak packing of the rigid main chain in the lamellar layers. The racemic mixture resulted in soluble amorphous polymers, which were subsequently hydrogenated into cycloolefin polymers with enhanced thermal properties.

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

Self-assisted stereospecific polymerization of unmasked polar 4-methylthio-1-butene

Stereospecific polymerization of polar olefins has always been an attractive but challenging project because the Lewis basic polar groups of monomers are usually poisonous to the Lewis acidic metal centers of the catalysts. In this contribution, thiophene-fused cyclopentadienyl scandium complexes 1—3 were successfully synthesized. Combined with alkylaluminium and organoborate, these complexes showed extremely low activity and no selectivity for 1-hexene polymerization. Surprisingly, highly stereo-selective coordination polymerization of unprotected polar 4-methylthio-1-butene has been achieved in high activity for the first time under the same polymerization conditions. High-molecular-weight (Mn=110×103) and perfectly syn-diotactic (rrrr>99%) poly(4-methylthio-1-butene) (P(MTB)) was afforded. Thus the methylthio-group-assisted mechanism that the unmasked methylthio group promoted the polymerization through σ-π chelation to the active scandium center together with the vinyl group was proposed. Moreover, the methylsulfonyl functionalized syndiotactic poly(1-butene) was also easily prepared by the oxidation of P(MTB). These results provided a new route for the synthesis of functionalized stereo-regular polyolefins.

Rhodium(I)‐α‐Phenylvinylfluorenyl Complexes: Synthesis, Characterization, and Evaluation as Initiators in the Stereospecific Polymerization of Phenylacetylene

The synthesis, characterization and use, as initiators for phenylacetylene polymerizations, of three new rhodium(I)‐vinyl complexes containing fluorenyl functionality with fluorine‐functionalized phosphine ligands is described. Rh(nbd)(CPh=CFlu)P(4‐FC6H4)3, Rh(nbd)(CPh=CFlu)P(4‐CF3C6H4)3, and Rh(nbd)(CPh=CFlu)P[3,5‐(CF3)2C6H3]3 (nbd: 2,5‐norbornadiene; Flu: fluorenyl) were prepared and isolated as discrete, orange compounds and were readily recrystallized yielding X‐ray quality crystals. All complexes were characterized by a combination of 1H, 31P, 19F, 103Rh NMR spectroscopy and 2D 31P‐103Rh/31P‐103Rh{103Rh} heteronuclear multiple‐quantum correlation (HMQC) experiments, elemental analysis, and single‐crystal X‐ray analysis. The complexes were active as initiators in the co‐ordination insertion polymerization of phenylacetylene, with initiation efficiencies spanning the range 13–56 %, and yielded polyphenylacetylenes of low dispersity (Ð = Mw/Mn) with high cis‐transoidal stereoregularity.

A (2-(naphthalen-2-yl)phenyl)rhodium(i) complex formed by a proposed intramolecular 1,4-ortho-to-ortho' Rh metal-atom migration and its efficacy as an initiator in the controlled stereospecific polymerisation of phenylacetylene.

The synthesis of a novel Rh(i)-aryl complex is detailed and its ability to serve as an initiator in the stereospecific polymerisation of phenylacetylene evaluated. Targeting the Rh(i) species, (2-phenylnaphthalen-1-yl)rhodium(i)(2,5-norbornadiene)tris(para-fluorophenylphosphine), Rh(nbd)(P(4-FC6H4)3)(2-PhNapth), following recrystallization we obtained the isomeric (2-(naphthalen-2-yl)phenyl)rhodium(i) complex, Rh(nbd)(P(4-FC6H4)3)(2-NapthPh), as determined by X-ray single-crystal structure analysis, and confirmed by X-ray powder diffraction. The isolation of the latter species was proposed to occur from the target (2-PhNapth) derivative via an intramolecular 1,4-Rh atom migration. This supposition was supported by density functional theory (DFT) calculations that indicated the isolated (2-NapthPh) derivative has lower energy (-19 kJ mol-1) than the targeted complex. The structure of the isolated (2-NapthPh) species was confirmed by multinuclear NMR spectroscopy including 2D 31P-103Rh{1H, 103Rh}, heteronuclear multiple-quantum correlation (HMQC) experiments; however, NMR analysis indicated the presence of a second, minor species in solution in an approximate 1 : 4 ratio with the 2-NapthPh complex. The minor species was identified as a second structural isomer, the 3-phenylnaphthyl derivative, proposed to be formed under a dynamic equilibrium with the 2-NapthPh derivative via a second 1,4-Rh atom migration. DFT calculations indicate that this 1,4-migration proceeds through a low-energy pathway involved in the oxidative addition of a C-H bond to Rh followed by a reductive elimination with the distribution of the products being thermodynamically controlled. The recrystallized Rh(nbd)(P(4-FC6H4)3)(2-NapthPh) complex was subsequently evaluated as an initiator in the polymerisation of phenylacetylene (PA); gratifyingly, the Rh(i) species was an active initiating species with the pseudo-first-order kinetic and molecular weight evolution vs time plots both linear implying a controlled polymerisation while yielding (co)polymers with low dispersities (Đ = Mw/Mn typically ≤1.25) and high cis-transoidal stereoregularity (>95%). Typical initiation efficiencies, while not quantitative (as judged by size exclusion chromatography), were nonetheless high at ca. 0.8. The presence of the minor 3-phenylnaphthyl species when in solution is proposed to be the cause of the observed non-quantitative initiation.

Polymerization of phenylacetylene by cationic acetylacetonate palladium complexes

A series of cationic acetylacetonate palladium complexes were demonstrated to be active for the polymerization and oligomerization of phenylacetylene. Catalyst screening and optimization have determined the superior activity of complex [(acac)Pd(TOMPP)2]BF4 (TOMPP – tris(2-methoxyphenyl)phosphine) for the stereospecific polymerization of phenylacetylene in non-coordinating solvents as well as in an aqueous emulsion. Catalyst turnover frequencies of 100–160 h−1 were observed (Mw = (10–27)·104 with PDI = 1.7–2.3, cis double bond content in PPA up to 95%). The effect of temperature, solvent nature, and the use of cocatalyst, BF3·OEt2, have been studied.

Trans-1,4- stereospecific polymerization of isoprene with MgCl2-supported Ziegler-Natta catalyst I. Initial polymerization kinetic and polymerization mechanism

In order to understand the trans-1,4 stereospecific polymerization mechanism of conjugated dienes with the heterogeneous Ziegler-Natta catalyst, isoprene polymerization kinetics at different polymerization temperature in the initial polymerization stage and the microstructures of polymers and its fractions were studied in this work. Both polymerization temperature and time influenced the polymerization behavior greatly. The nascent polyisoprene (30s) with unusual high cis-1,4-units was observed and then the trans-1,4/cis-1,4 ratio of the polymers increased obviously to finally trans-1,4-dominated (96 mol%) with the increase in polymerization time. Fractions of the products based on the different crystallizability at different temperature were isolated and characterized by 1H-NMR, 13C-NMR, GPC and DSC. The nascent polyisoprene under 30s polymerization time was mainly composed by fractions A and G with 30–60 mol% trans-1,4-units and 2–26 mol % trans-cis dyad linkages. Active species with low stereoregularity may contribute to theses fractions. While fractions D and E with high trans-1,4 unit (>96 mol%) from active species with high stereoregularity were growing to the predominated components with increased conversion. At least two kinds of active species with different stereospecificity and lifetime were proposed to explain the isoprene polymerization with heterogeneous TiCl4/MgCl2-Al(i-Bu)3 catalyst. Effects of temperature on cis/trans-1,4 structure were discussed. Hopely, this research would provide a further understanding about trans-1,4 stereospecific polymerization of conjugated dienes with heterogeneous Ziegler-Natta type catalysts.