Exfoliation of layered silicates through in situ controlled free radical polymerization mediated by a silicate-anchored initiator

Abstract

In this study, a new type of controlled free radical polymerization (CFRP) initiators from azetidine-2,4-dione functional groups were synthesized and then embedded within the interlayer spaces of layered silicates to prepare a series of polymer-layered silicate nanohybrids. Kinetics analyses and determination of the molecular weights and distributions of the polymerization products revealed that these CFRP initiators were viable species for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated stable free radical polymerization. Anchoring the CFRP initiator within the galleries of montmorillonite (MMT) enlarged the d-spacing from 15 to 38 Å. The incorporated cationic CFRP initiator (a silicate-anchored TEMPO derivative) provides the initiation functionality for the controlled free radical polymerization within the interlayers. The molecular weights of the resulting polystyrenes varied from 6800 to 85 200 g mol⁻¹, depending on the initiator–MMT/styrene weight ratio, with polydispersities of 1.14–1.45 for the lower-molecular-weight samples and 1.96–2.43 for the higher-molecular-weight samples. X-Ray diffraction patterns and transmission electron microscopy revealed that the growing polymer chains separated the layered silicates, thereby leading to the formation of exfoliated individual nanoplatelets. Thus, in situ controlled polymerization within the two-dimensional MMT platform, mediated by silicate-anchored initiators, is an effective dispersion/exfoliation route toward polymer-layered silicate nanohybrids.