Polymer Stereochemistry Research Articles

Solubility-Equilibrium-Assisted Kinetic Resolution Polymerization toward Isotactic Polyesters Containing Axial Chirality.

High-level control over polymer stereochemistry leverages the fine-tuning of material properties, but it is still a formidable challenge in synthetic polymer chemistry. Herein we prepared a new class of salph yttrium catalysts bearing axially chiral binaphthyl moieties for axially stereocontrolled polymerization of rac-Me-DBO. (S)-Y3-bearing bulkier binaphthyl units accomplished moderate isoselectivity via kinetic resolution polymerization, affording P(Me-BDO) with a Pm of up to 0.80. Remarkably, exploiting the solubility equilibrium to maintain a constant for the concentration of two enantiopure monomer pairs in the solution state contributed to a boost in polymerization isoselectivity and furnished isotactic P(Me-DBO) products with a Pm of up to 0.93. Detailed mechanistic investigations supported our solubility-equilibrium shifting hypothesis. This solubility-equilibrium-assisted kinetic resolution polymerization strategy was expected to become a versatile platform to improve stereocontrol without de novo catalyst design.

Stereoselective Polymerization of 3,6-Disubstituted N-Vinylcarbazoles.

Poly(N-vinylcarbazole) (PNVC-H) is a valuable nonconjugated photoconductive polymer, but the free radical polymerization conditions typically used for its synthesis do not control polymer stereochemistry and are not tolerant to many substituted N-vinylcarbazoles. Here, we report the stereoselective cationic polymerization of a series of 3,6-disubtituted N-vinylcarbazole derivatives using a chiral scandium-bis(oxazoline) Lewis acid catalyst. The combination of asymmetric ion-pairing catalysis and inherent monomer stereoelectronics facilitated stereoselective polymerization at room temperature, which enabled the polymerization of less soluble 3,6-disubstituted-N-vinylcarbazole derivatives. Isotactic halogen-substituted PNVCs demonstrated self-assembly in solution through halogen-halogen bonding, which was not observed in their atactic counterparts. Initial spectral characterization displayed a wide range of excitation-emission profiles for substituted PNVCs, which demonstrate the promise of these materials as a new class of nonconjugated photoconductive polymers for optoelectronic applications. Overall, these results showcase a diverse class of isotactic poly(N-vinylcarbazoles), highlight the benefits of identifying alternative stereocontrol mechanisms for polymerization, and expand the suite of accessible nonconjugated hole-transport materials.

NMR Analyses and Statistical Modeling of Biobased Polymer Microstructures-A Selected Review.

NMR analysis combined with statistical modeling offers a useful approach to investigate the microstructures of polymers. This article provides a selective review of the developments in both the NMR analysis of biobased polymers and the statistical models that can be used to characterize these materials. The information obtained from NMR and statistical models can provide insights into the microstructure and stereochemistry of appropriate biobased polymers and establish a systematic approach to their analysis. In suitable cases, the analysis can help optimize the synthetic procedures and facilitate the development of new or modified polymeric materials for various applications. Examples are given of the studies of poly(hydroxyalkanoates), poly(lactic acid), and selected polysaccharides, e.g., alginate, pectin, and chitosan. This article may serve as both a reference and a guide for future workers interested in the NMR sequence analysis of biobased materials.

Ring-Opening Metathesis Polymerization of the Dewar Isomer of 1,2-Azaborinine, a B-N Isostere of Benzene.

The successful polymerization of the Dewar isomer of an azaborinine heterocycle is reported. Controlled ring-opening metathesis polymerization was accomplished with Grubbs and Hoveya-Grubbs second generation catalysts (G2, HG2), as well as a Z-selective Ru catalyst (HGM2001). The structure of the polymers containing 4-membered B-N heterocycles was verified by GPC and multinuclear and 2D NMR. Differences in stereochemistry of polymers derived from G2/HG2 versus the Z-selective catalyst HGM2001 were substantiated by 2D NOESY, FT-IR, and Raman analyses. The incorporation of B-N heterocycles into these polymer structures is promising as a route to functional polymers that contain polar side groups.

Effect of stereo-chemistry and hydrophilic nature of synthetic carboxylic acid polymers on conformation, intermolecular structure and hydration at air-water interface

Conformation, intermolecular structure and hydration of different stereo-isomers of poly(methacrylic acid) and poly(acrylic acid), specifically isotactic-poly(methacrylic acid) (i-PMA), syndiotactic-poly(methacrylic acid) (s-PMA) and syndiotactic-poly(acrylic acid) (s-PAA) at vacuum-water interface were studied using molecular dynamics simulations. The adsorbed chain conformation was analyzed by radius-of-gyration components parallel and perpendicular to the interface, relative shape anisotropy parameter, distribution and autocorrelation function of dihedral angles, and orientation distribution of different groups relative to interface plane. i-PMA and s-PAA adsorb in planar conformation (i-PMA is curved compared to s-PAA) whereas s-PMA adsorbs in random coil conformation. Different chemical groups of s-PAA and i-PMA show preferential orientation whereas it is absent in the case of s-PMA. The random coil conformation of s-PMA exhibits greater intra-chain hydrogen bonding whereas extended chain of s-PAA exhibits greater polymer-water hydrogen bonding. The overall contribution to polymer-water hydrogen bonds by carbonyl oxygen atoms is greater than that by hydroxyl hydrogen atom. Water molecules hydrogen bonded to s-PAA show faster dynamics compared to those bonded to i-PMA and s-PMA. The slower dynamics of polymer-water hydrogen bonds is correlated to the slower dynamics of polymer conformations. Knowledge of orientation of chemical groups is useful for understanding binding of adsorbed polymer with other polymers and ligands.

Asymmetric Ion-Pairing in Stereoselective Vinyl Polymerization

Controlling polymer tacticity is a key consideration in macromolecular synthesis due to the impact of stereochemistry on a material’s thermomechanical and optical properties. Recently, inspired by work in small molecule catalysis, asymmetric ion-pairing has emerged as a valuable approach to control the stereochemistry of polymers made through chain growth polymerization of vinyl monomers. This Perspective outlines some of the challenges inherent to polymer stereocontrol as well as highlights recent catalyst development in the area of asymmetric ion-pairing that has enabled control of both the configuration and conformation of vinyl polymers. Several synthetic opportunities and mechanistic questions have been identified that will expand the impact of asymmetric ion-pairing in polymer synthesis.

Sugar-Based Polymers with Stereochemistry-Dependent Degradability and Mechanical Properties.

Stereochemistry in polymers can be used as an effective tool to control the mechanical and physical properties of the resulting materials. Typically, though, in synthetic polymers, differences among polymer stereoisomers leads to incremental property variation, i.e., no changes to the baseline plastic or elastic behavior. Here we show that stereochemical differences in sugar-based monomers yield a family of nonsegmented, alternating polyurethanes that can be either strong amorphous thermoplastic elastomers with properties that exceed most cross-linked rubbers or robust, semicrystalline thermoplastics with properties comparable to commercial plastics. The stereochemical differences in the monomers direct distinct intra- and interchain supramolecular hydrogen-bonding interactions in the bulk materials to define their behavior. The chemical similarity among these isohexide-based polymers enables both statistical copolymerization and blending, which each afford independent control over degradability and mechanical properties. The modular molecular design of the polymers provides an opportunity to create a family of materials with divergent properties that possess inherently built degradability and outstanding mechanical performance.

Using Stereochemistry to Control Mechanical Properties in Thiol-Yne Click-Hydrogels.

The stereochemistry of polymers has a profound impact on their mechanical properties. While this has been observed in thermoplastics, studies on how stereochemistry affects the bulk properties of swollen networks, such as hydrogels, are limited. Typically, changing the stiffness of a hydrogel is achieved at the cost of changing another parameter, that in turn affects the physical properties of the material and ultimately influences the cellular response. Herein, we report that by manipulating the stereochemistry of a double bond, formed in situ during gelation, materials with diverse mechanical properties but comparable physical properties can be obtained. Click‐hydrogels that possess a high % trans content are stiffer than their high % cis analogues by almost a factor of 3. Human mesenchymal stem cells acted as a substrate stiffness cell reporter demonstrating the potential of these platforms to study mechanotransduction without the influence of other external factors.

Using Stereochemistry to Control Mechanical Properties in Thiol-Yne Click-Hydrogels.

The stereochemistry of polymers has a profound impact on their mechanical properties. While this has been observed in thermoplastics, studies on how stereochemistry affects the bulk properties of swollen networks, such as hydrogels, are limited. Typically, changing the stiffness of a hydrogel is achieved at the cost of changing another parameter, that in turn affects the physical properties of the material and ultimately influences the cellular response. Herein, we report that by manipulating the stereochemistry of a double bond, formed in situ during gelation, materials with diverse mechanical properties but comparable physical properties can be obtained. Click-hydrogels that possess a high % trans content are stiffer than their high % cis analogues by almost a factor of 3. Human mesenchymal stem cells acted as a substrate stiffness cell reporter demonstrating the potential of these platforms to study mechanotransduction without the influence of other external factors.

Stereochemistry-Controlled Mechanical Properties and Degradation in 3D-Printable Photosets.

Stereochemistry provides an appealing handle by which to control the properties of small molecules and polymers. While it is established that stereochemistry in linear polymers affects their bulk mechanical properties, the application of this concept to photocurable networks could allow for resins that can accommodate the increasing demand for mechanically diverse materials without the need to significantly change their formulation. Herein, we exploit cis and trans stereochemistry in pre-resin oligomers to create photoset materials with mechanical properties and degradation rates that are controlled by their stereochemistry and molecular weight. Both the synthesis of stereopure (cis or trans) acrylate-terminated pre-polymers and the subsequent UV-triggered cross-linking occurred with a retention of stereochemistry, close to 100%. The stereochemistry of a 4 kDa oligomer within the resin enabled the tuning of the formulation to either a fast eroding, soft cis elastomer or a stiff trans plastic that is more resistant to degradation. These results demonstrate that stereochemistry is a powerful tool to modify the stiffness, toughness, and degradability of high-resolution, three-dimensional printed scaffolds from the same formulated ratio of components.

Control of Crystallinity and Stereocomplexation of Synthetic Carbohydrate Polymers from d‐ and l‐Xylose

AbstractManipulating the stereochemistry of polymers is a powerful method to alter their physical properties. Despite the chirality of monosaccharides, reports on the impact of stereochemistry in natural polysaccharides and synthetic carbohydrate polymers remain absent. Herein, we report the cocrystallisation of regio‐ and stereoregular polyethers derived from d‐ and l‐xylose, leading to enhanced thermal properties compared to the enantiopure polymers. To the best of our knowledge, this is the first example of a stereocomplex between carbohydrate polymers of opposite chirality. In contrast, atactic polymers obtained from a racemic mixture of monomers are amorphous. We also show that the polymer hydroxyl groups are amenable to post‐polymerisation functionalization. These strategies afford a family of carbohydrate polyethers, the physical and chemical properties of which can both be controlled, and which opens new possibilities for polysaccharide mimics in biomedical applications or as advanced materials.

Control of Crystallinity and Stereocomplexation of Synthetic Carbohydrate Polymers from d- and l-Xylose.

Manipulating the stereochemistry of polymers is a powerful method to alter their physical properties. Despite the chirality of monosaccharides, reports on the impact of stereochemistry in natural polysaccharides and synthetic carbohydrate polymers remain absent. Herein, we report the cocrystallisation of regio‐ and stereoregular polyethers derived from d‐ and l‐xylose, leading to enhanced thermal properties compared to the enantiopure polymers. To the best of our knowledge, this is the first example of a stereocomplex between carbohydrate polymers of opposite chirality. In contrast, atactic polymers obtained from a racemic mixture of monomers are amorphous. We also show that the polymer hydroxyl groups are amenable to post‐polymerisation functionalization. These strategies afford a family of carbohydrate polyethers, the physical and chemical properties of which can both be controlled, and which opens new possibilities for polysaccharide mimics in biomedical applications or as advanced materials.

Controlling polymer stereochemistry in ring-opening polymerization: a decade of advances shaping the future of biodegradable polyesters.

This review highlights recent developments in the field of biodegradable polymeric materials intended to replace non-degradable conventional plastics, focusing on studies from the last ten years involving the stereoselective ring-opening polymerization of cyclic esters. This encompasses exciting advances in both catalyst design and monomer scope. Notably, the last decade has seen the emergence of metal-free stereocontrolled ROP for instance, as well as the synthesis and stereocontrolled polymerization of new types of chiral monomers. This study will emphasize recent stereoselective polymerization catalysts and chiral monomers and will focus on stereocontrol quantification, the mechanisms of stereocontrol and their differentiation if reported and studied for a specific catalyst system.

Stereoselectivity Inversion: Isospecific Propylene Polymerization Catalyzed by Rigid Cyclic Bis(phenoxyaldimine) Titanium Complexes

Precise control over olefin polymerization, especially polymer stereochemistry, through artful catalyst design is attractive and highly challenging. A rigid cyclic framework was first introduced in...

Stereochemical enhancement of polymer properties

The importance of stereochemistry to the function of molecules is generally well understood. However, to date, control over stereochemistry and its potential to influence properties of the resulting polymers are, as yet, not fully realized. This Review focuses on the state of the art with respect to how stereochemistry in polymers has been used to influence and control their physical and mechanical properties, as well as begin to control their function. A brief overview of the synthetic methodology by which to access these materials is included, with the main focus directed towards the effect of stereochemistry on mechanical properties, biodegradation and conductivity. In addition, advances in applications of stereodefined polymers for enantioseparation and as supports for catalysts in asymmetric transformations are discussed. Finally, we consider the opportunities that the rich stereochemistry of sustainably sourced monomers might offer in this field. Where possible, we have drawn parallels between design principles in order to identify opportunities and limitations that these approaches may present in their effects on materials properties, performance and function. Changes in the stereochemistry of polymer chains result in changes to the mechanical and physical behaviour of the resulting materials. By harnessing synthetic methods to create stereocontrolled polymers, a new parameter can be accessed to control the behaviour of bulk material.

Competition between Liquid-liquid De-mixing, Crystallization, and Glass Transition in Solutions of PLA of Different Stereochemistry and DEET

Liquid-liquid (L-L) de-mixing and vitrification of solutions of either crystallizable poly(L-lactic acid) (PLLA) or non-crystallizable poly(D/L-lactic acid) (PDLLA) with 50 m% N, N-diethyl-3-methylbenzamide (DEET) were analyzed by calorimetry and cloud-point measurements, which allows drawing conclusions about the effect of polymer stereochemistry on the phase behavior. Regardless of the PLA stereochemistry, vitrification of the solutions on fast cooling, hindering crystallization of PLLA, occurred below −20 °C and suppressed prior L-L de-mixing. The experimental results prove that crystallization in samples containing crystallizable PLLA, observed at around 55 °C on slow cooling, is not preceded by L-L de-mixing.

NMR Analysis of Poly(Lactic Acid) via Statistical Models

The physical properties of poly(lactic acid) (PLA) are influenced by its stereoregularity and stereosequence distribution, and its polymer stereochemistry can be effectively studied by NMR spectroscopy. In previously published NMR studies of PLA tacticity, the NMR data were fitted to pair-addition Bernoullian models. In this work, we prepared several PLA samples with a tin catalyst at different L,L-lactide and D,D-lactide ratios. Upon analysis of the tetrad intensities with the pair-addition Bernoullian model, we found substantial deviations between observed and calculated intensities due to the presence of transesterification and racemization during the polymerization processes. We formulated a two-state (pair-addition Bernoullian and single-addition Bernoullian) model, and it gave a better fit to the observed data. The use of the two-state model provides a quantitative measure of the extent of transesterification and racemization, and potentially yields useful information on the polymerization mechanism.

Solvent and Polymer Stereochemistry Play Key Roles in Deuterium Exchange and Partial Racemization of Polypropylenes

Recent reports have described deuterium exchange reactions on polyolefins; such isotopic labeling enables powerful analytical techniques, but exchange on isotactic polypropylene was previously unsuccessful. Here, we report successful exchange reactions on isotactic and syndiotactic polypropylenes, revealing the key role that the polymer stereochemistry and reaction solvent play in the mechanism of adsorption and exchange. Alteration of the polymer stereochemistry accompanies exchange in some cases, though molecular weight is preserved in all reactions.

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Well-defined carbohydrate-based polymers in calcium carbonate crystallization: Influence of stereochemistry in the polymer side chain on polymorphism and morphology

In this work we demonstrate the remarkable phase control on the crystallization of calcium carbonate by the stereochemistry of carbohydrate-based polymers. The polymers (poly(2-(2,3,4,6-tetra-O-acetyl-β-d-glucosyloxy)ethyl methacrylate) and poly(2-(2,3,4,6-tetra-O-acetyl-β-d-galactosyloxy)ethyl methacrylate)) have been synthesized from the respective glucose or galactose containing monomers (3 step synthesis) by RAFT polymerization leading to well-defined carbohydrate-based polymers with number averages of the molecular weights (Mw) of 10,000–18,000g/mol and a dispersities (Đ) from 1.1 to 1.2. For the deprotected polymers we found differences in the phase selection of calcium carbonate. We found that this effect is based on the chelating character of the hydroxyl groups of the pyranoses and their individual orientation, as demonstrated by comparison of the protected and unprotected polymers in crystallization experiments as well as computer assisted simulations.