Depolymerization and Polymerization Chemistry

The push to recycle plastics, which already comprise 10% (> 30 vol %) of municipal solid waste, has increased dramatically in recent years as environmental concerns over landfill capacity continued to grow. Converting waste polymers to valuable chemicals would be desirable, but the lack of a comprehensive understanding of pyrolysis - a set of reactions, which involve high temperatures, mixed feedstocks, messy radical transformations, and a very complex product distribution - presents a significant obstacle. Mechanistic modeling of the decomposition of individual polymers and polymer mixtures during pyrolysis should allow for objective design of polymer recycling procedures. Another strategy is to used mixed plastic waste for new applications through compatibilization. Gradient copolymers are a new class of materials that have been shown to create more stable interfaces between dissimilar polymers. The design of the backbone topology of these copolymers is critical in determining their properties.

Tertiary Resource Recovery through Pyrolysis

We have successfully simulated pyrolysis of polystyrene (temperature 310-420°C) using a model that tracked 64 polymeric and low molecular weight (LMW) products and incorporated 2700 individual reactions. Similarly, the researchers have been able to model the yields of the ten main LMW products from the pyrolysis of polypropylene (temperature 350-420°C) using a model that tracked 130 polymeric and LMW species and incorporated 9100 individual reactions. The individual components of the models for polystyrene and polypropylene are combined to create a binary degradation model for polystyrene and polypropylene mixtures, which allows to capture the 270% enhancement in the propylene degradation observed experimentally in polystyrene/polypropylene mixtures.

We have recently extended this framework to create a detailed, mechanistic model for high-density polyethylene pyrolysis that was used to study the time evolution of low molecular weight products formed during the pyrolysis. Specifically the role of the unzipping, backbiting, and random scission general reaction pathways play in the evolution of low molecular weight species was probed. The model tracked 151 species and utilized over 11,000 reactions. Rate parameters were adapted from our previous work, literature values, and regression against experimental data. The model results were found to be in excellent agreement with experimental data for the evolution of condensable low molecular weight products and fair agreement for the evolution of the gaseous products. The time evolution curves of specific low molecular weight products indicated that the random scission pathway was important for all species, while the backbiting pathway played a complementary role. Net rate analysis was used to further elucidate the competition between the pathways. Net rate analysis of end-chain radicals showed that the unzipping pathway was not competitive with the other pathways, as expected based on experimental yields of ethylene. The random scission pathway was found to be controlling with the backbiting pathway playing a more minor role for product formation. By comparing the net rates for formation of specific position mid-chain radical via intramolecular hydrogen shift reactions the importance of the backbiting pathway was shown to vary based on the facility of the reactions. The products formed from scission of mid-chain radicals that are formed from x,x+4 intramolecular hydrogen transfer were favored.

Our detailed mechanistic models are able to predict the changes in molecular weight polymers undergo as they are degraded and the yields of specific low molecular weight products that have use as fuels and chemicals.

Levine, S.E. and Broadbelt, L.J., "Reaction Pathways to Dimer in Polystyrene Pyrolysis: A Mechanistic Modeling Study", Polym. Deg. Stab., 2008, 93(5), 941-951.

Kruse, T.M., Levine, S.E., Wong, H.-W., Duoss, E., Lebovitz, A.H., Torkelson, J.M., and Broadbelt, L.J., "Binary Mixture Pyrolysis of Polypropylene and Polystyrene: A Modeling and Experimental Study", Journal of Analytical and Applied Pyrolysis, 2005, 73, 342-354.

Kruse, T.M., Woo, O.S., Wong, H.-W., Khan, S.S., and Broadbelt, L.J., "Mechanistic Modeling of Polymer Degradation: A Comprehensive Study of Polystyrene", Macromolecules, 2002, 35(20), 7830-7844.

Prediction of Explicit Sequence of Copolymers Using Kinetic Monte Carlo

There has been increasing need for polymeric materials engineered at the microscale for use in highly specialized applications. Polymeric materials that incorporate two or more types of monomers are of particular interest because unique macroscopic properties may result depending on microscopic properties such as composition, degree of polymerization, and arrangement. Alteration of any of these properties can potentially yield materials with significantly different physical properties. Of particular importance is the arrangement, or sequencing, of monomers along each chain. It is possible to have copolymeric materials with similar molecular weight and overall composition, but different arrangements of monomers along each chain, which affects intrachain attractive and repulsive forces and thus macroscopic physical behavior. An example of this would be random and block copolymers possessing similar compositions and size but significantly different chain sequencing; while a random block copolymer would exhibit a single glass transition temperature with a value that is between those of its two component polymers, a block copolymer possesses two glass transition temperatures corresponding to its respective polymer blocks. Therefore, it is of key importance to not only measure size and composition of these materials, but also to determine the relative arrangement of monomers along each chain. We have developed a stochastic modeling framework to capture key features of gradient copolymerization systems. Our modeling methodology provides an unprecedented level of detail that enables predictions of composition and sequence distributions at the molecular level.

Kinetic Monte Carlo tracks the reactions of individual chains explicitly, allowing detailed information about composition and sequence to be obtained. The chemical composition distribution for a styrene/4-acetoxystyrene gradient copolymer as a function of reaction time is shown. The variation in the chemical composition as the chains grow is a fingerprint of the gradient character of the copolymer.