Polymer surface engineering via thiol-mediated reactions

Ryan Matthew Hensarling

Abstract

Synthesis of polymer brushes to decorate a surface with desired functionality typically involves surface-initiated polymerization (SIP) of functional, but non-reactive monomers. This approach suffers major drawbacks associated with synthesizing sufficiently thick polymer brushes containing surface-attached polymer chains of high molecular weight at high grafting density (i.e. cost, synthetic effort and functional group intolerance during polymerization). The research herein seeks to circumvent these limitations by the decoration of surfaces with polymer chains bearing specific pendent functional groups amenable to post-polymerization modification (PPM). In particular, this dissertation leverages PPM via a specific class of click reactions - thiol-click - that 1) enables the rapid generation of a diverse library of functional surfaces from a single substrates precursor, 2) utilizes a structurally diverse range of commercially available or easily attainable reagents, 3) proceeds rapidly to quantitative conversions under mild conditions and 4) opens the door to orthogonal and site-selective functionalization. In the first two studies, radical-mediated thiol-yne and base-catalyzed thiol-isocyanate reactions are demonstrated as modular platforms for the rapid and practical fabrication of highly functional, multicomponent surfaces under ambient conditions. Brush surfaces expressing a three-dimensional configuration of alkyne or isocyanate functionalities were modified with high efficiency and short reaction times using a library of commercially available thiols. In the third study, two routes to multifunctional brush surfaces were demonstrated utilizing orthogonal thiol-click reactions. In the first approach, alkyne-functionalized homopolymer brushes were modified with multiple thiols via a statistical, radical-mediated thiol-yne co-click reaction; and in the second approach, statistical copolymer brushes carrying two distinctly-addressable reactive moieties were sequentially modified via orthogonal base-catalyzed thiol-X (where X represents an isocyanate, epoxy, or α-bromoester) and radical-mediated thiol-yne reactions. In the fourth study thiol-click PPMs are investigated in depth to determine how surface constraints affect the modification process by probing the penetration depth of functional thiol modifiers into pendent isocyanate-containing polymer brushes via neutron reflectivity studies. Also, the synthesis of tapered block copolymer brush surfaces was demonstrated by exploiting the inherent mass transport limitations of post-polymerization modification processes on reactive brush surfaces. In the fifth study a post-polymerization surface modification approach providing pendent thiol functionality along the polymer brush backbone using the photolabile protection chemistry of both o -nitrobenzyl and p -methoxyphenacyl thioethers was developed. Addressing the protecting groups with light not only affords spatial control of reactive thiol functionality but enables a plethora of thiol-mediated transformations with isocyanates and maleimides providing a modular route to create functional polymer surfaces.