Polymers at interfaces: Using atom transfer radical polymerization in the controlled growth of homopolymers and block copolymers from silicon surfaces in the absence of untethered sacrificial initiator

The atom transfer radical polymerization (ATRP) of styrene and acrylates from silicon wafers modified with an initiator layer composed of 2-bromoisobutyrate fragments is described. In the presence of the proper ratio of activating and deactivating transition-metal species, controlled radical polymerizations of styrene were observed such that the thickness of the layer consisting of chains grown from the surface increased linearly with the molecular weight of chains polymerized in solution in identical, yet separate, experiments. The layer thickness increased linearly with reaction time for ATRP of styrene and methyl acrylate due to both the extremely low initiator concentration relative to monomer and the low monomer conversion. Further evidence for control was observed by the polymerization of blocks of either methyl or tert-butyl acrylate from the polystyrene layer. Modification of the hydrophilicity of the surface layer was achieved by hydrolysis of the poly(styrene-b-tert-butyl acrylate) to poly(styrene-b-acrylic acid) and confirmed by decrease in water contact angle from 86 degrees to 18 degrees. The mechanistic aspects of ATRP in the polymerization process were confirmed by the growth of very thick polystyrene films in the presence of a pure copper(II) complex. Since no deactivator. was present, the metal complex served only to facilitate initiation by a redox process. Attempts to extend chain with methyl acrylate under controlled conditions were unsuccessful in those films. The simulation of polymerization of surface layers suggests broader molecular weight and chain end distributions, confirming XPS results on the progressive decrease of Br absorption intensity.