New biocompatible and biodegradable materials are continually needed in several areas for medical use, for example, in drug delivery, therapeutic devices and gene therapy/delivery. The most common materials currently used for such applications have been degradable polyesters, including poly (L-lactic acid), poly(glycolic acid), and poly(lactic-co-glycolic acid). However, these polyesters have quite slow degradation rates, and often lack many properties necessary for medical applications such as homogeneous bulk degradation, which is detrimental to the long-term mechanical properties of the material. Surface eroding polymers, such as polyanhydrides, maintain their mechanical integrity during degradation and exhibit a gradual loss in size.

In this project we have shown that thiol-ene chemistry, a step-growth mechanism of polymerization, can be applied to make materials that are elastomeric, photocurable and have controllable degradation rates, starting from only several hours. Thiol-ene chemistry is also quite simple and has readily available monomers, and the degradable functionality resides in the main chain, rather than a side chain, which reduces the molecular weight of the degradation products compared to chain growth polymerizations. Thus, using thiol-ene chemistry to make polyanhydride network polymers provides significant flexibility in tailoring characteristics such as crosslink density, functionality and hydrophilicity.

We are interested in the design, synthesis and structural/functional analysis of self-healing, shape-memory polyanhydrides, which are materials of significant interest in several applications, in particular medical, defense and aerospace industries.

The collaboration between the Shipp and Mather (formally Syracuse U., now Bucknell U.) groups recently showed that dynamic covalent exchange reactions at elevated temperatures (> 50 °C) among the network chains of the anhydride elastomer allow near-complete reconfiguration of the permanent shape in the solid state. This exciting discovery was born from collaboration between the Shipp and Mather groups, which synergistically couples the expertise in synthesis and polyanhydrides of the Shipp group with the shape memory polymer (SMP) and self-healing (SH) expertise of the Mather group. Several years ago the Shipp laboratory discovered that thiol-ene/yne polymerization, often referred to as one of the ‘click’ chemistries, can be reliably applied to the rapid production of polyanhydrides that are surface-eroding in aqueous environments.

The group has recently begun two different projects in the general area of polyurethanes. One is the development of novel polyurethanes for the chemical-mechanical planarization (CMP) process, which is widely used in semiconductor manufacturing and other fields that require sub-nanometer planarity. The photo (at left) is of Dr Shipp and two former students who now work in CMP-related fields at the International Conference on Planarization Technologies (ICPT) held in Portland, OR, in September 2022.

The goals of the second research project are to determine structure-performance relationships of bio-sourced thermoplastic non-isocyanate polyurethane (NIPUs), and develop NIPU up-cycling processes. The aim is to determine compositions that provide NIPUs that will have properties similar to PUs commonly used in industry.

There has been several new polymerization techniques that have been developed in recent years, including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization and nitroxide-mediated polymerization (NMP). Each of these methods are free radical based polymerizations, thus making them amenable to the polymerization of a variety of monomers under non-strenuous reaction conditions. They also have the capability of synthesizing a large range of polymer architectures. However, there are many questions that remain about mechanisms and reaction rates. Work in our group is examining what factors affect the mechanism and kinetic parameters, as well as looking into alternate methods of controlling polymerizations.

The primary goal of the proposed research is to develop new polymer nanocomposite materials. While polymer nanocomposites have been investigated over the past 20 years a major restriction has been in the degree of control that can be achieved in terms of the polymer molecular weight, polydispersity, functionality, composition and topology. We are developing procedures to produce nanocomposites that are well-defined in each of these aspects. These novel methods will not only allow the development of new and improved materials, but also allow for an in depth investigation into the dispersion of filler (often clay or silica nanoparticles) within the polymer matrix.