(385ac) Effects of Capping Agents on Deblocking Temperatures of MDI-Based Blocked Isocyanates: A Study of Nucleophilicity, Pka, and Chain Length Trends

Polyurethanes are the 6^th most used synthetic polymer in the world with an annual production of 26 MMT in the year 2022. Thermoplastic polyurethanes (TPU) are a class of polyurethanes that distinguishes itself by its capacity to be melted and reshaped, unlike thermoset polyurethanes. The widespread use of TPU has resulted in substantial waste in landfills and oceans, necessitating effective recycling solutions to address environmental concerns. Chemical recycling of TPUs presents a sustainable approach towards circular re-use of waste polyurethanes.

The present work focuses on utilizing blocked isocyanate chemistry as a strategic approach for TPU depolymerization, involving the capping of reactive isocyanate groups during depolymerization in order to passivate them for separation, followed by thermal deblocking to regain their reactivity for repolymerization within a four-step chemical recycling process that involves 1) catalytic depolymerization and capping of TPU, 2) separation of the capped hard and soft segments, 3) thermal dissociation to recover the capping agent and (uncapped) hard segment segments, and 4) repolymerization of fresh TPU from the recovered hard and soft segments. In order to make this scheme feasible, efficient, and economic, the selection of capping agents is critical. A suitable capping agent needs to deblock at low temperature to minimize the energy intensity of the process while remaining stable at the temperature of the depolymerization step. Understanding the deblocking temperatures of different capping agents is hence critical. Based on a thorough literature review of potential capping agents, we synthesized capped 4,4'-methylene diphenyl diisocyanate (MDI) using select capping agents, determined deblocking temperatures of these model compounds using thermogravimetric analysis (TGA), and derived correlations between deblocking temperature and physicochemical properties of the capping agents, such as nucleophilicity, pKa, and chain length. The results reveal clear trends, with specific functional groups and molecular characteristics affecting the deblocking temperature. The results hence yield a guideline for the molecular design of effective capping agents for blocked isocyanate chemistry, enhancing the feasibility of TPU depolymerization and promoting circular economy practices in polymer recycling.