Urethane Chemistry Research Articles

Non-Isocyanate Synthesis of Aliphatic Polyurethane by BiCl3-Catalyzed Transurethanization Polycondensation.

Non-isocyanate polyurethane synthesis by non-Sn catalysis is an essential challenge toward green polyurethane synthesis. Bismuth compounds are attractive candidates due to their low cost, low toxicity, and availability to urethane chemistry. This work applied various Bi catalysts to the self-polycondensation of a bishydroxyurethane monomer and found BiCl3 to be an excellent catalyst through optimization. The catalytic activity and price of BiCl3 are comparable to those of Bu2SnO, while its toxicity is significantly low. BiCl3 is, therefore, a promising alternative to Sn-based catalysts in non-isocyanate polyurethane synthesis.

Pegylated dendritic polyurethane as unimolecular micelles for tumor chemotherapy: Effect of molecular architecture

Unimolecular micelles have attracted intense interests as drug carriers for tumor chemotherapy owing to their superior stability in comparison with the self-assembled supramolecular ones. Among them, the dendritic polymers with the polar frameworks could favour the loading of chemotherapeutic drugs rather than the hyperbranched polymers via radical polymerization, by enhancing the interaction with drugs. While the tedious synthesis procedure for dendritic polymers could be simplified with the construction principle on urethane chemistry. Here, the PEGylated dendritic polyurethanes, Ph-DPUGly-PEG and Ph-DPUTEA-PEG, were designed with glycerol or triethanolamine as monomer, respectively. The effect of the molecular architecture of the Ph-DPU-PEGs unimolecular micelles on the controlled drug releasing performance was compared. It was found that the Ph-DPUTEA-PEG with tertiary amine as branching points could efficiently endow the pH-triggered drug release, due to its protonation.

Sustainable design of vanillin-based vitrimers using vinylogous urethane chemistry

A vanillin derivative containing β-hydroxy-amine functionalities was introduced into vitrimer materials using vinylogous urethane chemistry. These new materials have a high bio-based content and show fast reprocessability.

Polyaddition Synthesis Using Alkyne Esters for the Design of Vinylogous Urethane Vitrimers

Vitrimers are a subclass of covalent adaptable networks which introduce reshapeability and recyclability in thermoset materials while maintaining a high degree of chemical resistance and dimensional stability. Vitrimer materials based on vinylogous urethane (VU) chemistry have drawn a lot of attention in this area. Classically, these are obtained by the polycondensation polymerization of acetoacetate and amine monomers. Unfortunately, this also releases water, often leading to porosity defects in the initially obtained non-reprocessed cross-linked materials. Here, we demonstrate that alkyne esters (AE) can be used as alternative building blocks for VU vitrimers by a polyaddition polymerization with amines, leading to water-free formulations and straightforward access to defect-free cured VU vitrimer materials. The bond formation and dynamic bond exchange was also studied by small molecule reactions, further rationalized by a computational (DFT) approach. The resulting water-free VU vitrimers display similar material properties compared to vitrimers based on acetoacetates, although also some differences are seen, which can be related to a minor amide-bond forming side reaction. Furthermore, by use of this novel AE approach, polyaddition curing of VU epoxy vitrimers can easily be prepared in a one-pot three-component method, combining AE, amine, and epoxy monomers. This study shows that AE monomers can be used as an easy drop-in method to obtain processable epoxy materials with tunable viscoelastic properties, where the viscous flow behavior can in principle be fully tuned by varying the monomers’ ratios and compositions.

An Ultra-Low-Temperature Elastomer with Excellent Mechanical Performance and Solvent Resistance.

Elastomers presenting good elasticity, ductility, and chemical resistance at low temperatures can serve as superior performers for explorations in extremely cold environments. However, no commercially available elastomer to date can comprehensively fulfill those demands. Here, a perfluoropolyether (PFPE)-based network crosslinked by dynamic urethane chemistry is demonstrated, which may satisfy the demands of application in ultracold environments. As the crucial constitute in such a crosslinked network, PFPE provides the elastomer with excellent elasticity at a temperature down to -110 °Cand outstanding ductility within the cryogenic temperature range. Importantly, the high proportion of fluorocarbon segment also provides wonderful compatibility to most organic solvents, accounting for the low-swelling characteristics of the elastomer in sealing applications. Furthermore, the dynamic crosslinking feature allows the cured elastomer to be reprocessed like thermoplastic polymers, which affords great promise to recycle and reuse the elastomer after its disposal. Inherently, this elastomer would inspire a worldwide interest in the design of elastic devices that are adaptable to extremely low temperature.

Surface Modification of (Non)-Fluorinated Vitrimers through Dynamic Transamination.

Surface modifications are typically permanent in shape and chemistry. Herein, vinylogous urethane (VU) chemistry is presented as an easily accessible and versatile platform for rapid, facile, and reworkable surface modification. It is demonstrated that both physical and chemical post-modification of permanent, yet dynamic elastic polymer networks are achieved. Surface patterns with high regularity are created, both via a straightforward replication process using a polydimethylsiloxane stamp (resolution ca. 10-100µm) as well as using thermally activated nano-imprint lithography (NIL) to form hole, pillar, or line patterns (ca. 300nm) in elastic VU-based vitrimers. The tunable, rapid exchange allows patterning at 130°C in less than 15min, resulting in an increased water contact angle and surface-structure induced light reflection. Moreover, it is also demonstrated that the use of a single dynamic covalent chemistry makes it possible to strongly adhere to fluorinated and non-fluorinated materials based on incompatible matrices, causing cohesive failure in a peel test. In a topography scan, the visibly transparent interface is shown to possess a continuous phase without a gap, while maintaining distinctively separated (non)-fluorinated domains. Finally, this approach allowed for a straightforward coating of a non-fluorinated material with a fluorinated monomer to minimize the overall fluorinated content.

Tuning the size selectivity of isoporous membranes for protein fractionation via two scalable post treatment approaches

The molecular design of block copolymer membranes has been widely used as a toolbox to explore the functionality and pore size of the membranes. However, the study of membrane performance as a function of systematic change of the pore size has not been widely explored so far. We prepared an integral asymmetric isoporous membrane on the example of polystyrene-block-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA) membranes using an easily scalable method which combines the block copolymer self-assembly and nonsolvent induced phase separation. The pore size of the PS-b-PHEMA membrane was tuned via two post treatment methods, a chemical postmodification by urethane chemistry and a physical post treatment by thermal annealing, respectively. At pH 7.4 bovine serum albumin (BSA), hemoglobin (Hb) and catalase (Cat) do not adsorb on the prepared membranes. The retention of BSA, Hb and Cat from their aqueous solution was investigated and the ideal selectivities of three protein pairs ψBSA/Hb, ψBSA/Cat and ψHb/Cat were calculated. The ψBSA/Cat and ψHb/Cat of the postmodified membranes were significantly higher than those of the PS-b-PHEMA membrane. This study is the first to demonstrate that the size selectivity of a SNIPS membrane can be enhanced by controlled physical and chemical post treatment of a SNIPS membrane.

Dynamic Covalent Polyurethane Networks with Excellent Property and Cross-Link Density Recovery after Recycling and Potential for Monomer Recovery

Dynamic urethane chemistry, which involves both associative and dissociative mechanisms, can be used to achieve reprocessability in cross-linked polyurethane (PU) networks. We discovered that prope...

Dual-Layer Approach toward Self-Healing and Self-Cleaning Polyurethane Thermosets.

There is an urgent need for coatings that exhibit both self-healing as well as self-cleaning properties as they can be used for a wide range of applications. Herein we report a novel approach toward fabricating polyurethane thermosets possessing both self-cleaning and self-healing properties. The desired coating was achieved via casting a bottom layer of self-healable polyurethanes comprised of reversible phenolic urethane bonds followed by a subsequent dip-coating of the prepared layer in a solution of bis(3-aminopropyl)-terminated polydimethylsiloxane (PDMS-NH2). The PDMS was used to impart self-cleaning properties to the coating. While the self-healing behavior of the bottom polyurethane layer is achieved through phenolic urethane chemistry, via the exchange of phenolic urethane moieties. The prepared coatings were tested for their optical, mechanical, self-healing, and self-cleaning properties using a variety of characterization methods, which confirmed the successful fabrication of novel self-cleaning and self-healing clear urethane coatings.

Hierarchical Machine Learning Model for Mechanical Property Predictions of Polyurethane Elastomers From Small Datasets

Polyurethanes are a broad class of material that finds application in coatings, foams, and solid elastomers. The urethane chemistry allows a diversity of monomers to be used, and prediction of mechanical properties, which are determined by complex interplay between monomer chemistry and chain architecture, is an unresolved challenge. Urethanes are based on aromatic or cyclic isocyanates and linear or branched polyols, and polymerization results in linear chains for bifunctional monomers or branched chains for multifunctional monomers. Strong intermolecular interactions between aromatic groups result in the formation of hard-segment domains that generate physical crosslinks between disorganized rubbery domains and anchor the material microstructure, contributing to resistance to deformation. Here, a general hierarchical machine learning (HML) model for predicting the stress-at-break, strain-at-break, and Tan δ for thermoplastic and thermoset polyurethanes is presented. The algorithm was trained on a library of 18 polymers with different diisocyanates, bifunctional or trifunctional polyols, and NCO:OH index. HML reduces data requirements through robust embedding of domain knowledge and surrogate data in a middle layer that bridges input variables (composition) and output responses (mechanical properties). In this work, the middle layer included information on overall polymer composition, predictions of chain architecture derived from Monte Carlo simulations of polymerization, information on interchain interactions from empirically derived molecular potentials and shifts in infrared (IR) spectroscopy absorbances. The HML predictions are shown to be more accurate than those from a Random Forest model directly relating composition and properties, suggesting that embedding domain knowledge provides significant advantages in predicting the properties of complex material systems based on small datasets.

Hydroxylation of EPDM as a means for ambient temperature vulcanisation via urethane chemistry

This paper describes a single-step, quantitative method for hydroxylation of unsaturated rubbers like EPDM. The resultant polyhydroxy elastomer was vulcanised using a diisocyanate under relatively milder conditions. The vulcanised product showed increased modulus in contrast to the sulfur- or peroxide-crosslinked systems. The enhanced cohesion originates from the urethane linkages which can be controlled either by regulating the concentration of the oxidant during hydroxylation or through the isocyanate –to- hydroxyl ratio (NCO/OH) during vulcanisation. This is the first ever report on the mid chain hydroxylation of unsaturated polymers and their polyurethane networks.

Novel Post-Treatment Approaches to Tailor the Pore Size of PS-b-PHEMA Isoporous Membranes.

Using the example of an integral-asymmetric isoporous membrane prepared from polystyrene-block-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA), physical and chemical ways of post-treatment are introduced with the aim to tailor the pore size. These post-treatments are i) thermal annealing and ii) urethane chemistry of ethyl isocyanate (EI) in the presence of perfluoro(methyl cyclohexane). Via these approaches, the pore size of PS-b-PHEMA membranes is successfully tailored in the range of 10-20 nm with narrow pore size distribution by controlling the duration of thermal annealing and chemical reaction, respectively. The excellent hydrophilicity of the PS-b-PHEMA membrane is not changed by thermal annealing. The chemical postmodification using EI is associated with a loss of hydrophilicity with increasing conversion.

Tailoring vinylogous urethane chemistry for the cross-linked polybutadiene: Wide freedom design, multiple recycling methods, good shape memory behavior

Vinylogous urethane chemistry can provide the sustainable way to address the demands for reprocessing the cross-linked polymers. In this paper, we extended this method to prepare the existing commercially available cross-linked polybutadiene (PB) with multiple reprocessing ways. We firstly attach acetoacetyl group to the chains of PB and then use diamines as cross-linkers to form vinylogous urethanes involved network, which could be tailored with great freedom, including via the content of attached acetoacetyl group, the ratio of amine to acetoacetyl group and different types of amine. Furthermore, the dynamic nature of the cross-linked PB is studied by stress relaxation. Based on the Arrhenius equation, the activation energy for chain re-arrangement is calculated about 48 ± 5 kJ mol−1 and the network could be efficiently reshuffled at the temperature between 120 °C and 150 °C. More important, the cross-linked PB could be effectively remolded by hot pressing or solvent assisted welding methods via catalyst-free transamination of vinylogous urethanes. We envisage that this strategy may provide an eco-environmental method to dispose the waste rubbers and prolong their life time.

Renewable High-Performance Polyurethane Bioplastics Derived from Lignin–Poly(ε-caprolactone)

Here, we report novel lignin-poly(e-caprolactone)-based polyurethane bioplastics with high performance. The poly(e-caprolactone) (PCL) was incorporated as a biodegradable soft segment to the lignin by the bridge of hexamethylene diisocyanate (HDI) with long flexible aliphatic chains and high reactivity. The effects of -NCO/-OH molar ratio, content of lignin, and molecular weight of the PCL on the properties of the resultant polyurethane plastics were thoroughly evaluated. It is important that the polyurethane film still possessed high performance in the tensile strength, breaking elongation, and tear strength, which could reach 19.35 MPa, 188.36%, and 38.94 kN/m, respectively, when the content of lignin reached as high as 37.3%; moreover, it was very stable at 340.8 °C and presented excellent solvent-resistance. The results demonstrated that the modification of the lignin based on the urethane chemistry represents an effective strategy for developing lignin-based high-performance sustainable materials.

ChemInform Abstract: Cooperative Catalysis of Cyclic Carbonate Ring Opening: Application Towards Non‐Isocyanate Polyurethane Materials.

AbstractThe reaction between cyclic carbonates and amines to produce hydroxyurethanes is an important alternative to current urethane chemistry.

Hydroxypropyl tannin from Pinus pinaster bark as polyol source in urethane chemistry

Novel thermosetting polyphenol-based polyurethane (PU) films were prepared using condensed tannin (CTs) and hydroxypropyl tannin derivatives (HPTs) from Pinus pinaster bark and diisocyanates. Four HPTs with a degree of substitution (DS) rising stepwise from 1 to 4 were crosslinked with either an aromatic (polymeric methylene diphenyl diisocyanate (pMDI)) or an aliphatic isocyanate (hexamethylene diisocyanate (HDI)). Chemical structure, moisture sorption, thermal, and morphological properties of the films were studied. Properties varies with NCO:OH ratio (0.5–4.0), DS, diisocyanate type, and cross-link density (CD). Partial tannin hydroxypropylation (DS 2, 3) based/films showed a flexible-type character in term of Young’s modulus, elongation and tensile strain. Significant relationships were established between CD and physicochemical properties. HPTs constitute suitable macro building-block polyols for urethane chemistry.

Cooperative Catalysis of Cyclic Carbonate Ring Opening: Application Towards Non‐Isocyanate Polyurethane Materials

AbstractThe reaction between cyclic carbonates and amines to produce hydroxyurethanes is an important alternative to current urethane chemistry. In order to address the issue of slow reaction rates, an efficient ring opening of cyclic carbonates with amines has been achieved utilizing cooperative catalysis. A new Lewis acid/Lewis base combination substantially decreases the reaction times for small molecule systems to reach complete conversion. Although triazabicyclodecene (TBD) has a substantial impact on the reaction rate, the addition of lithium triflate (LiOTf) as a co‐catalyst allows for the fastest ring opening reported in the current literature. Cooperative catalysis is also applied to the synthesis of polymers containing hydroxyurethane linkages and is able to achieve rapid conversion of the bis‐cyclic carbonate and diamine precursors when compared with the uncatalyzed reaction.

Long-living, stress- and pH-tolerant superhydrophobic silica particles via fast and efficient urethane chemistry; facile preparation of self-recoverable SH coatings

A facile preparation of long-lived, stress- and pH-tolerant superhydrophobic (SH) particles is presented. The generated coatings are resistant to a wide pH range and display self-recoverable SH behavior after multiple tests.

PU elastomers comprising spherical nanosilicas: Balancing rheology and properties

For polyurethane (PU) engineering elastomers, although urethane chemistry allows tailoring of elastomer properties, there is an inherent correlation between increasing strength and decreasing elongation-to-break. The ability to break this correlation provides great potential for creating high-performance materials that possess attractive combined properties. The incorporation of nanofillers is one of the promising approaches. However, while property enhancement is achieved, nanocomposite systems often experience significant increase in viscosity leading to process difficulty. In this paper, the use of nanospherical particles to mitigate the rheology was explored. We were able to achieve significant property enhancement in the ultimate mechanical properties and thermal stability while maintaining low viscosity during process. By first forming well dispersed nanosilica in polyol, followed by elastomer casting, material with significantly higher strength, elongation-to-break, thus, toughness, and degradation temperature than one without nanosilica was obtained. The performance improvement is attributed to the ability to form a low viscosity, stable nanofluid where the nanosilicas are present as primary nanoparticles in the polyol. The morphology of the formed elastomers was studied. It is hypothesized that nanoparticles reduce defects that contribute to premature failure of the elastomer, thus improving ultimate properties.

Bulk superhydrophobic materials: a facile and efficient approach to access superhydrophobicity by silane and urethane chemistries

CaCO3-based bulk superhydrophobic material is synthesised by sequential silane and urethane chemistry. The cross-linked material displays a static contact angle of ~155° and a water-roll-off angle of 5–8° throughout the thickness which is attributed to 3D- hydrogen bonding.