Controllable Reversible Addition–Fragmentation Termination Monomers for Advances in Photochemically Controlled Covalent Adaptable Networks

Abstract

With the advent of systematically designed controllable reversible addition–fragmentation termination (CRAFT) compounds, we have identified structure–property relationships related to the RAFT compositional structure as it impacts photoplasticity in covalent adaptable networks (CANs). In this study, we have expanded the range and functional capabilities of addition–fragmentation capable network forming monomers by synthesizing and evaluating systematically varying CRAFT monomers with the general formula ABCBA. Subsequent assessment of the impact of these monomers on photoplasticity and stress relaxation was performed. Structural variation of the A and B segments, in particular, imparts increased efficiency and efficacy in stress relaxation and photoplasticity. The CRAFT monomers employed have highly efficient stress relaxation properties demonstrating stress reduction of up to 54% and 75%, respectively, in postpolymerization network photoplasticity experiments. Furthermore, polymerization stress reduction in purely acrylate and acrylate–thiol networks with CRAFT monomers shows a remarkably enhanced efficacy with the inclusion of relatively small amounts of the monomers. With a loading of only 1.5 wt % of the alkene trithiocarbonate monomer in each system more than 75% stress reduction was achieved.

RAFT Polymerization – Then and Now

“…This provides capacity for self-healing and for stress relaxation within the network (168)(169)(170)(171). RAFT agent functionality can also be incorporated into polymer networks formed by thiol-ene chemistry (172) or other processes to form similar structures. The presence of the thiocarbonylthio linkages within the network also means that the network is intrinsically unstable and can be degraded by reagents that cleave the thiocarbonylthio linkage (83).…”

Reconfigurable surface patterns on covalent adaptive network polymers using nanoimprint lithography

“…This smart material behavior has generated substantial interest in utilizing the photoplasticity [1,2], stress relaxation [3], and self-healing [4] capabilities of similar materials [5e10]. These materials, known as covalent adaptive networks (CANs) differ from the permanent and static networks associated with traditional thermosets in that their internal covalent bonds can be broken and reformed.…”

Effects of directed architecture in epoxy functionalized prepolymers for photocurable thin films

“…For example, reversible addition-fragmentation chain transfer (RAFT) polymerization is a popular means to produce controlled polymer structure; however, the common trithiocarbonate and dithioethers RAFT agents remain reactive after the polymerization reaction allowing network rearrangements during the photopolymerization. 15 Nitroxide-mediated polymerization (NMP), another method of CRP, utilizes thermally reversible termination to control the propagation mechanism thereby providing control over molecular weight. In contrast to RAFT, the thermally reversible termination of NMP is not susceptible to network rearrangement in the presence of other radical sources, as the majority of nitroxides are only active at elevated temperatures.…”