Diglycerol-based polyesters: melt polymerization with hydrophobic anhydrides

Al(III)/K(I) Heterodinuclear Polymerization Catalysts Showing Fast Rates and High Selectivity for Polyester Polyols

Low molar mass, hydroxyl end-capped polymers, often termed "polyols," are widely used to make polyurethanes, resins, and coatings and as surfactants in liquid formulations. Epoxide/anhydride ring-opening copolymerization (ROCOP) is a controlled polymerization route to make them, and its viability depends upon catalyst selection. In the catalysis, the polyester polyol molar masses and end-groups are controlled by adding specific but excess quantities of diols (vs catalyst), known as the chain transfer agent (CTA), to the polymerizations, but many of the best current catalysts are inhibited or even deactivated by alcohols. Herein, a series of air-stable Al(III)/K(I) heterodinuclear polymerization catalysts show rates and selectivity at the upper end of the field. They also show remarkable increases in activity, with good selectivity and control, as quantities of diol are increased from 10-400 equiv. The reactions are accelerated by alcohols, and simultaneously, their use allows for the production of hydroxy telechelic poly/oligoesters (400 < Mn (g mol-1) < 20,400, Đ < 1.19). For example, cyclohexene oxide (CHO)/phthalic anhydride (PA) ROCOP, using the best Al(III)/K(I) catalyst with 200 equiv of diol, shows a turnover frequency (TOF) of 1890 h-1, which is 4.4× higher than equivalent reactions without any diol (Catalyst/Diol/PA/CHO = 1:10-400:400:2000, 100 °C). In all cases, the catalysis is well controlled and highly ester linkage selective (ester linkages >99%) and operates effectively using bicyclic and/or biobased anhydrides with bicyclic or flexible alkylene epoxides. These catalysts are recommended for future production and application development using polyester polyols.

Sustainable Polymers: Our Evolving Understanding

ConspectusThis Account discusses the evolution of our strategy to conduct environmentally responsible research in the field of polymer chemistry. To contextualize our work, we begin with a broad historical overview of the modern environmental movement, the rise of sustainability as a concept, and how chemistry has responded to these forces, which were often sharply critical of our field. We then trace our own responses, from graduate school onward, chronicling a series of experiences and research projects that molded, challenged, and reshaped how we think about sustainability in polymer science.Since beginning our independent careers in 2004, we have recognized and worked to resolve the tension between designing synthetic polymers for specific desired thermomechanical properties and minimizing environmental impact. In our early years, we were most strongly guided by the 12 Principles of Green Chemistry (12PGC), which had only recently been proposed. The authors' early research agendas had a rather narrow focus on two areas, specifically catalysis and biobased monomers, which we saw as strongly linked to sustainability. Over time, we found these areas to be too narrow in their focus, ignoring important considerations such as the capacity of monomer supply to support scale-up and the impact polymers have at the end of their usage lifetimes. With respect to monomers and catalysts, we consider descriptive metrics that quantify waste production and the toxicity of compounds used during synthesis. In terms of polymer end-of-life, we discuss hydrophobicity as a tool to help understand susceptibility to degradation in the environment as well as some of the concerns with design for degradation, a critical component of 12PGC.Now, after nearly two decades of investigation, we believe that achieving sustainability in polymer science will require us to move beyond the qualitative use of the 12PGC to a portfolio of metrics. We note a heartening increase in the availability and use of such metrics and tools across the field. These include items that provide limited insight but are relatively trivial to integrate into existing workflows such as E factor or the Toxicity Estimation Software Tool. We also appreciate the increased use of Life Cycle Assessment (LCA), which is both dramatically more thorough and difficult to deploy. Finally, we propose the creation of a national LCA center, similar to instrumental core facilities. Such a resource would enable the use of this tool across multiple phases of research and we hope would more effectively guide us to a sustainable future.

Intermolecular Diels-Alder Cycloadditions of Furfural-Based Chemicals from Renewable Resources: A Focus on the Regio- and Diastereoselectivity in the Reaction with Alkenes

A recent strong trend toward green and sustainable chemistry has promoted the intensive use of renewable carbon sources for the production of polymers, biofuels, chemicals, monomers and other valuable products. The Diels-Alder reaction is of great importance in the chemistry of renewable resources and provides an atom-economic pathway for fine chemical synthesis and for the production of materials. The biobased furans furfural and 5-(hydroxymethyl)furfural, which can be easily obtained from the carbohydrate part of plant biomass, were recognized as "platform chemicals" that will help to replace the existing oil-based refining to biorefining. Diels-Alder cycloaddition of furanic dienes with various dienophiles represents the ideal example of a "green" process characterized by a 100% atom economy and a reasonable E-factor. In this review, we first summarize the literature data on the regio- and diastereoselectivity of intermolecular Diels-Alder reactions of furfural derivatives with alkenes with the aim of establishing the current progress in the efficient production of practically important low-molecular-weight products. The information provided here will be useful and relevant to scientists in many fields, including medical and pharmaceutical research, polymer development and materials science.

Mg(ii) heterodinuclear catalysts delivering carbon dioxide derived multi-block polymers

Carbon dioxide derived polymers are emerging as useful materials for applications spanning packaging, construction, house-hold goods and automotive components. To accelerate and broaden their uptake requires both more active and selective catalysts and greater structural diversity for the carbon dioxide derived polymers. Here, highly active catalysts show controllable selectivity for the enchainment of mixtures of epoxide, anhydride, carbon dioxide and lactone. Firstly, metal dependent selectivity differences are uncovered using a series of dinuclear catalysts, Mg(ii)Mg(ii), Zn(ii)Zn(ii), Mg(ii)Zn(ii), and Mg(ii)Co(ii), each exposed to mixtures of bio-derived tricyclic anhydride, cyclohexene oxide and carbon dioxide (1 bar). Depending upon the metal combinations, different block structures are possible with Zn(ii)Zn(ii) yielding poly(ester-b-carbonate); Mg(ii)Mg(ii) or Mg(ii)Co(ii) catalysts delivering poly(carbonate-b-ester); and Mg(ii)Zn(ii) furnishing a random copolymer. These results indicate that carbon dioxide insertion reactions follow the order Co(ii) > Mg(ii) > Zn(ii). Using the most active and selective catalyst, Mg(ii)Co(ii), and exploiting reversible on/off switches between carbon dioxide/nitrogen at 1 bar delivers precision triblock (ABA), pentablock (BABAB) and heptablock (ABABABA) polymers (where A = poly(cyclohexylene oxide-alt-tricyclic anhydride), PE; B = poly(cyclohexene carbonate), PCHC). The Mg(ii)Co(ii) catalyst also selectively polymerizes a mixture of anhydride, carbon dioxide, cyclohexene oxide and ε-caprolactone to deliver a CBABC pentablock copolymer (A = PE, B = PCHC C = poly(caprolactone), PCL). The catalysts combine high activity and selectivity to deliver new polymers featuring regularly placed carbon dioxide and biomass derived linkages.

Bio-based and Degradable Block Polyester Pressure-Sensitive Adhesives

A new class of bio-based fully degradable block polyesters are pressure-sensitive adhesives. Bio-derived monomers are efficiently polymerized to make block polyesters with controlled compositions. They show moderate to high peel adhesions (4-13 N cm-1 ) and controllable storage and loss moduli, and they are removed by adhesive failure. Their properties compare favorably with commercial adhesives or bio-based polyester formulations but without the need for tackifier or additives.

Easy access to oxygenated block polymers via switchable catalysis

Oxygenated block polyols are versatile, potentially bio-based and/or degradable materials widely applied in the manufacture of coatings, resins, polyurethanes and other products. Typical preparations involve multistep syntheses and/or macroinitiator approaches. Here, a straightforward and well-controlled one-pot synthesis of ABA triblocks, namely poly(ether-b-ester-b-ether), and ABCBA pentablocks, of the form poly(ester-b-ether-b-ester’-b-ether-b-ester), using a commercial chromium catalyst system is described. The polymerization catalysis exploits mechanistic switches between anhydride/epoxide ring-opening copolymerization, epoxide ring-opening polymerization and lactone ring-opening polymerization without requiring any external stimuli. Testing a range of anhydrides, epoxides and chain-transfer agents reveals some of the requirements and guidelines for successful catalysis. Following these rules of switch catalysis with multiple monomer additions allows the preparation of multiblock polymers of the form (ABA)_n up to 15 blocks. Overall, this switchable catalysis delivers polyols in a straightforward and highly controlled manner. As proof of potential for the materials, methods to post-functionalize and/or couple the polyols to make higher polymers are demonstrated.

Multiblock oxygenated polyols often show better properties than the constituent polyols, but their synthesis can be complex and difficult. Here a switchable catalysis concept is described which allows for the efficient preparation of multiblock poly(ether-b-ester) materials starting from mixtures of common monomers.

Continuous flow processing as a tool for the generation of terpene-derived monomer libraries

A Diels–Alder reaction employing terpenes for rapid synthesis of monomer libraries under flow conditions is presented.

Predicting Monomers for Use in Polymerization-Induced Self-Assembly

We report an in silico method to predict monomers suitable for use in polymerization-induced self-assembly (PISA). By calculating the dependence of LogP_oct  /surface area (SA) on the length of the growing polymer chain, the change in hydrophobicity during polymerization was determined. This allowed for evaluation of the capability of a monomer to polymerize to form self-assembled structures during chain extension. Using this method, we identified five new monomers for use in aqueous PISA via reversible addition-fragmentation chain transfer (RAFT) polymerization, and confirmed that these all successfully underwent PISA to produce nanostructures of various morphologies. The results obtained using this method correlated well with and predicted the differences in morphology obtained from the PISA of block copolymers of similar molecular weight but different chemical structures. Thus, we propose this method can be utilized for the discovery of new monomers for PISA and also the prediction of their self-assembly behavior.

Controlling the Size of Two-Dimensional Polymer Platelets for Water-in-Water Emulsifiers

A wide range of biorelevant applications, particularly in pharmaceutical formulations and the food and cosmetic industries, require the stabilization of two water-soluble blended components which would otherwise form incompatible biphasic mixtures. Such water-in-water emulsions can be achieved using Pickering stabilization, where two-dimensional (2D) nanomaterials are particularly effective due to their high surface area. However, control over the shape and size of the 2D nanomaterials is challenging, where it has not yet been possible to examine chemically identical nanostructures with the same thickness but different surface areas to probe the size-effect on emulsion stabilization ability. Hence, the rationale design and realization of the full potential of Pickering water-in-water emulsion stabilization have not yet been achieved. Herein, we report for the first time 2D poly(lactide) platelets with tunable sizes (with varying coronal chemistry) and of uniform shape using a crystallization-driven self-assembly methodology. We have used this series of nanostructures to explore the effect of 2D platelet size and chemistry on the stabilization of a water-in-water emulsion of a poly(ethylene glycol) (PEG)/dextran mixture. We have demonstrated that cationic, zwitterionic, and neutral large platelets (ca. 3.7 × 106 nm2) all attain smaller droplet sizes and more stable emulsions than their respective smaller platelets (ca. 1.2 × 105 nm2). This series of 2D platelets of controlled dimensions provides an excellent exemplar system for the investigation of the effect of just the surface area on the potential effectiveness in a particular application.

1D vs. 2D shape selectivity in the crystallization-driven self-assembly of polylactide block copolymers

2D materials such as graphene, LAPONITE® clays or molybdenum disulfide nanosheets are of extremely high interest to the materials community as a result of their high surface area and controllable surface properties. While several methods to access 2D inorganic materials are known, the investigation of 2D organic nanomaterials is less well developed on account of the lack of ready synthetic accessibility. Crystallization-driven self-assembly (CDSA) has become a powerful method to access a wide range of complex but precisely-defined nanostructures. The preparation of 2D structures, however, particularly those aimed towards biomedical applications, is limited, with few offering biocompatible and biodegradable characteristics as well as control over self-assembly in two dimensions. Herein, in contrast to conventional self-assembly rules, we show that the solubility of polylactide (PLLA)-based amphiphiles in alcohols results in unprecedented shape selectivity based on unimer solubility. We use log Poct analysis to drive solvent selection for the formation of large uniform 2D diamond-shaped platelets, up to several microns in size, using long, soluble coronal blocks. By contrast, less soluble PLLA-containing block copolymers yield cylindrical micelles and mixed morphologies. The methods developed in this work provide a simple and consistently reproducible protocol for the preparation of well-defined 2D organic nanomaterials, whose size and morphology are expected to facilitate potential applications in drug delivery, tissue engineering and in nanocomposites.

Zinc versus Magnesium: Orthogonal Catalyst Reactivity in Selective Polymerizations of Epoxides, Bio-derived Anhydrides and Carbon Dioxide

Developing selective polymerizations from complex monomer mixtures is an important challenge. Here, dinuclear catalysts allow selective polymerization from mixtures of sterically hindered tricyclic anhydrides, carbon dioxide and epoxides to yield well-controlled copoly(ester-carbonates). Surprisingly, two very similar homogeneous catalysts differing only in the central metal, zinc versus magnesium, show very high but diametrically opposite monomer selectivity. The selectivity is attributed to different polymerization kinetics and to steric factors associated with the anhydrides.

Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure-Property Relationships

Polyesters synthesized through the alternating copolymerization of epoxides and cyclic anhydrides compose a growing class of polymers that exhibit an impressive array of chemical and physical properties. Because they are synthesized through the chain-growth polymerization of two variable monomers, their syntheses can be controlled by discrete metal complexes, and the resulting materials vary widely in their functionality and physical properties. This polymer-focused review gives a perspective on the current state of the field of epoxide/anhydride copolymerization mediated by discrete catalysts and the relationships between the structures and properties of these polyesters.

Post polymer modification of polyethylenimine with citrate esters: selectivity and hydrophobicity

Hydrophobic modification of water soluble PEI with citrate esters is selective and increases log P_oct.

Localized delivery and enhanced osteogenic differentiation with biodegradable galactitol polyester elastomers

Cytocompatible galactitol based polyesters showed variations in physical properties, degradation, dye release and ability to direct cells towards bone lineage.

Maltitol-based biodegradable polyesters with tailored degradation and controlled release for bone regeneration

Polyesters based on maltitol and different dicarboxylic acids promoted mineral deposition and directed cells towards osteogenic lineage.

Recent progress in sustainable polymers obtained from cyclic terpenes: synthesis, properties, and application potential

The functionalization and polymerization of biobased monocyclic terpenes and their derivatives for the synthesis of sustainable polymers is described, especially in view of the synthetic routes and properties of the obtained macromolecular architectures. Comparison of these procedures and the obtained materials with "classical" oil-based approaches, and also with alternative biobased routes, gives interesting insights into the potential of these small terpene building-block structures for modern polymer science and technology.

New insights into synthesis and oligomerization of ε-lactams derived from the terpenoid ketone (−)-menthone

We present convenient ring-opening polymerizations of two lactams derived from menthone for the synthesis of sustainable oligo-, polyamides.