Effects of crosslinking density on the in situ formation of gold-polymer composite particles and their catalytic properties

Abstract

This work demonstrates the in situ formation of gold nanoparticles (AuNPs) within a series of poly(N-isopropylacrylamide), PNIPAM, particles containing various amounts of crosslinkers from 2.5 wt% to 40 wt%. As the networks of these host PNIPAM particles possess different levels of crosslinking density and free volume, the in situ reduction of gold ions greatly influences the structural features (e.g., size and distribution) of the guest AuNPs while maintaining comparable loading efficiency. After thorough characterization of their physicochemical properties, the resulting composite particles are tested as catalysts in C-C forming homocoupling reactions to understand the relationships between the crosslinking density and catalytic reactivity. Interestingly, decreasing the crosslinking density of PNIPAM particles results in the integration of gradually smaller and more uniform AuNPs whose catalytic activities with turnover frequency (TOF) values of ~38/h are comparatively superior to other AuNP-based systems (e.g., ~20/h). The enhanced catalytic property could also be contributed by the loosely crosslinked polymer particle networks with increased free volume, resulting in efficient mass transfer environments (e.g., faster diffusion of molecules). It is apparent that the crosslinking degree of the PNIPAM particles greatly impacts the overall formation of the guest AuNPs and their catalytic reactivity and recyclability in the homocoupling reaction. As such, examining the influence of host polymer networks on the structural and catalytic properties of the guest nanoparticles provides valuable information for the development of novel composite particles in versatile catalytic applications