Development of Initiation Strategies for the Optimization and Stabilization of Brushes Formed by Surface-Initiated Radical Polymerization

Three challenges facing the continued development of the surface-initiated (SI) polymerization field are addressed. The first challenge is that surface-initiated free radical polymerization (SI-FRP) is currently under-represented in literature reports, despite the fact that SI-FRP offers several important advantages over surface-initiated controlled radical polymerization (SI-CRP) and represents a complementary approach. Recognizing that simple, efficient, and safer approaches to synthesizing effective surface free radical initiators are needed, we present a novel synthetic route to an azo type free radical initiator that forms self-assembled monolayers on oxide coated surfaces such as silicon and glass. The yield of the novel synthetic approach is approximately double that of the previous method, and the usage of cyanide is reduced dramatically. We characterize the decomposition kinetics of the initiator using differential scanning calorimetry, comparing it with other similar initiators. We characterize the surface properties of initiator layers on silicon using ellipsometry, water contact angle measurements, and x-ray photoelectron spectroscopy (XPS). We demonstrate the use of the initiator for SI-FRP as well as SI-CRP mechanisms including reverse atom transfer radical polymerization (RATRP) and reversible addition-fragmentation transfer (RAFT) polymerization.;A second challenge facing the field of SI polymerization involves the stability of grafted polymer brushes in aqueous environments and under tension, both of which are commonly encountered in many applications. We hypothesize that the mechanism of brush degrafting involves hydrolysis of ester backbone units in the initiator, which are ubiquitous in surface-bound polymerization initiator systems. The novel initiator described herein is unique in that its backbone consists of a chemically stable alkane chain, free of esters or other easily hydrolysable groups. We present results of experiments demonstrating the stability of polymer brushes formed from the novel initiator compared against brushes formed from ester-based initiators. The ester-free brushes are found to be more stable under a variety of conditions including p-toluenesulfonic acid, which is commonly used to cleave grafted polymers for analysis, as well in as aqueous systems at low, high, and neutral pH. Therefore the novel initiator constitutes a helpful contribution to the problem of brush stability.;The third challenge addressed herein concerns the need for a more robust understanding of SI polymerization mechanisms and their effect on resultant polymer properties. SI polymerizations often differ significantly from corresponding bulk processes due to confinement effects. We conduct a detailed study of the thickness of polystyrene brushes formed by SI-FRP as a function of temperature and initiator conversion. These results are compared against computer simulations of bulk FRP conducted using Gillespie's stochastic simulation algorithm (GSSA). The Arrhenius behavior of the brush thickness is found to deviate significantly from that expected for bulk polymerization due to a two-dimensional Trommsdorff effect in the brush layer. We investigate the strengths and weaknesses of GSSA applied to radical polymerizations in bulk and under confinement, suggesting ideas for improving the accuracy of SI polymerization models. Furthermore, the conditions necessary for free and controlled radical polymerizations are investigated. We describe efforts to achieve SI-CRP by physical means, without chemical additives, using microwave irradiation of conductive substrates. We analyze the heat transfer from microwave absorbing substrates mathematically and we employ a modified CSSA algorithm to gain further insight into the parameter space affecting control over molecular weight and molecular weight distribution in radical polymerization.