Synergy

High-density polyethylene (HDPE) is a semicrystalline thermoplastic that can be processed into a versatile range of commodity materials such as storage containers, plastic bags, pipes, electrical coatings, and much more. Due to its chemical resistance, high tensile strength, and light weight, HDPE is a prominent commodity plastic with over 50 million metric tons produced annually at a value of 63 billion US dollars. Thus, development of new methods to control the properties and processability of HDPE is vital for reducing the energy costs of polyolefin processing and minimizing the quantity of plastic used in commercial grade materials. A team of researchers from the Fors and Coates groups at Cornell investigated the effects of MWD shape on the properties and processability of HDPE. By adding a titanium phenoxyimine initiator at different rates and times into a solution of ethylene, the MWD shape of HDPE can be controlled. Rheological and tensile testing of these samples revealed that MWD shape had a profound influence on complex viscosity but did not impact tensile strength; leading to future promise in lowering processing energies of HDPE production without compromising material properties.

Isosorbide is an inexpensive feedstock derived from sorbitol and has been studied in polymers for nearly half a century. Its excellent properties have made it an attractive, sustainable building block for a wide variety of high-performance applications such as packaging, electronic displays, and biomedical applications. A team of CSP researchers from the Reineke, LaPointe, Cramer, and Dauenhauer groups have recently demonstrated the first platform for tailored polymer architectures from isosorbide via ring-opening polymerization (ROP). This work reports a scalable synthesis to concurrently produce and purify an annulated isosorbide derivative. High-throughput screening by the team proved to be invaluable for rapid assessment of a wide range of catalysts and conditions for ROP. In addition, density functional theory calculations offered critical insight into competing cationic and quasizwitterionic polymerization mechanisms. Ultimately, catalytic ROP  has been achieved via initiation with an activated epoxide, which also induces ring-opening selectivity of this complex cyclic ether. Moreover, the polymerization is selectively directed towards different macromolecular architectures (linear vs. cyclic) through simple variations in reaction conditions.

A team comprising Grant Fahnhorst, Daniel Stasiw, Professor Bill Tolman, and Professor Tom Hoye demonstrated an unprecedented isomerization pathway by which a preformed linear polymer [linear poly(CMVL) from the monomer CMVL] is converted to a highly branched architecture [hyperbranched poly(CMVL)] using a metal-alkoxide catalyst. The process is a direct ramification of the use of lactone monomer and/or polyester containing a pendant sidechain ester substituent. The team also demonstrated that the isomeric lactone monomer, isoCMVL, and its linear polyester poly(isoCMVL), proceeds through a directly analogous isomerization pathway to give the same polymer as that from CMVL, namely the hyperbranched poly(CMVL). The A scheme showing the polymerization and isomerization of both CMVL and isoCMVL to the same hyperbranched polymer, hyperbranched poly(CMVL). ROTEP = ringopening transesterification polymerization. Fundamental concepts exposed through this investigation could also allow for the introduction of branching into other families of degradable polyesters, a potential strategy for management of aliphatic polyester properties.

Cross-linked polymers are extensively used due to their high thermal stability and solvent resistance. However, this class of materials creates significant challenges in the arena of sustainability. One approach to address these issues has been to develop new, high-performance cross-linked materials with more sustainable characteristics. In particular, polyesters have attracted notable interest in the field of sustainable polymers; polyesters can be synthesized from various chemicals derived from renewable feedstocks (i.e., biomass), and the ester bonds in the polymer backbone enables degradation on more reasonable timescales. After synthesizing elastomers derived from lactone monomers that could be obtained from biomass, CSP graduate student, Guilhem De Hoe, traveled to Switzerland to study the enzymatic hydrolysis of the elastomers, which is key for their degradation. This holistic study of  the preparation, characterization, and biodegradation potential of these polyester elastomers was pursued by researchers under CSP investigators Marc Hillmyer and Geoff Coates along with collaborators Kris McNeill and Michael Sander of ETH Zürich.

Following political upheaval and limited access to rubber tree plantations in the 20th century, the solution was to obtain isoprene, the key C-5 monomer in rubber, from petroleum. Thermal cracking of the naptha fraction of petroleum (gasoline-range) produced a small fraction of isoprene that could be separated and refined before polymerizing to make “synthetic rubber”. This process remains dominant today in the manufacture of automobile tires. Sugar-derived isoprene has been a major research challenge for the last decade, but only limited yields have ever been achieved by one-step fermentation. Moreover, a direct catalytic process from glucose to isoprene is not viable. Glucose contains a straight carbon chain, while isoprene requires introduction of methyl branching at the C2 position, which is not currently possible using heterogeneous catalysts. A team of researchers under CSP investigators Kechun Zhang and Paul Dauenhauer collaborated to address this challenge.

Polyethylene (PE) and isotactic polypropylene (iPP) are the two most abundant plastics worldwide and account for about two-thirds of all plastic production. Despite the similar hydrocarbon makeup, these polymers phase separate, which erodes the mechanical properties of melt blends and creates challenges for recycling the materials. Given the abundance, utility, and environmental impact of PE and iPP, there is a compelling need to improve the recyclability and associated mechanical properties of these repurposed materials. A CSP research team of synthetic chemists as well as chemical and materials engineers discovered that as little as 1% by weight of a newly created PE/iPP multiblock polymer could combine commercial PE and iPP into remarkably tough composite blends.

A key way that renewable polyesters are prepared is through ring-opening transesterification polymerization (ROTEP) catalyzed by metal-alkoxide complexes. While a mechanism involving coordination of the lactone monomer followed by ring-opening by the alkoxide is generally accepted, a detailed understanding of individual reaction steps and catalyst structural influences on reaction rates that would be potentially useful for future catalyst development is lacking. In research aimed at gaining insight into ligand structural effects on the mechanism of ROTEP, synergistic experimental and theoretical studies of ε-caprolactone (CL) polymerization by aluminum-alkoxide complexes were performed.

Thermosets are a class of polymeric materials used in everyday life for applications requiring mechanically strong and durable materials; however, nearly all commodity thermosets are non-repairable, thus, they must be disposed of after failure and are generally considered nonsustainable materials. Therefore, the drive to develop materials with similar strength that can be recycled represents a significant challenge in the field of polymer science. To develop materials that satisfy these challenging demands, researchers in the NSF Center for Sustainable Polymers have developed a class of cross-linked polyhydroxyurethanes that demonstrate mechanical properties competitive with traditional thermoset polymers and can be recycled simply via compression molding at elevated temperatures.

Developing bio-based plastics that outperform those made from fossil resources is a complex scientific puzzle, the solution for which requires the diversity of expertise embodied by the CSP’s researchers. For example, Prof. Kechun Zhang and Postdoctoral Associate Mingyong Xiong used their expertise in biotechnology and bioengineering to design a biosynthetic route to produce β- methyl-δ-valerolactone (MVL), a new biobased monomer. Profs. Marc Hillmyer and Frank Bates and Graduate Student Deborah Schneiderman controllably copolymerized the monomer designed to make a soft, amorphous, aliphatic polyester.