Catechols as versatile platforms in polymer chemistry

Aqueous fabrication of pH-gated, polymer-brush-modified alumina hybrid membranes

In this Article, we studied the surface immobilization of five organic-acid-modified atom-transfer radical polymerization (ATRP) initiators based on salicylic acid, catechol, phthalic acid, and m- and p-benzoic acid on alumina, and we also investigated the growth of hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(poly(ethylene glycol)methycrylate) (PPEGMA6) brushes from the resulting initiator-modified substrates. Whereas the surface immobilization of phthalic acid- and benzoic acid-based initiators results in only very thin brushes or no brush growth at all, SI-ATRP of HEMA and PEGMA6 from alumina surfaces modified with salicylate or catechol generates brushes with thicknesses comparable to those obtained using organosilane-based initiators. Most interestingly, the surface immobilization of the catechol- and salicylate based-initiators was found to be pH-dependent, which allowed facile variation of the ATRP initiator surface concentration and, concomitantly, the polymer brush grafting density by adjusting the pH of the aqueous solution that was used to immobilize the initiator. This is in contrast to organosilane-based initiators, where the variation of the grafting density is usually accomplished using mixtures of the ATRP initiator and an ATRP inactive "dummy". Another difference between the organosilane-based initiators and the organic acid analogues is the stability of hydrophilic brushes grown from alumina. After a certain threshold thickness was exceeded, organosilane-tethered PPEGMA6 brushes were observed to detach from the substrate, in contrast to brushes grown from catechol or salicylate initiators, which did not show signs of degradation. Finally, as a first proof-of-concept, the salicylate-based initiator was used to develop an all-aqueous protocol for the modification of alumina membranes with hydrophilic PHEMA and succinic anhydride post-modified polymer brushes. The water permeation properties of these hybrid membranes can be controlled by adjusting the brush thickness in the case of the neutral PHEMA brush coating or can be pH-gated after post-polymerization modification to introduce carboxylic acid groups.

Molecular weight effects upon the adhesive bonding of a mussel mimetic polymer

Characterization of marine biological adhesives are teaching us how nature makes materials and providing new ideas for synthetic systems. One of the most widely studied adhering animals is the marine mussel. This mollusk bonds to wet rocks by producing an adhesive from cross-linked proteins. Several laboratories are now making synthetic mimics of mussel adhesive proteins, with 3,4-dihydroxyphenylalanine (DOPA) or similar molecules pendant from polymer chains. In select cases, appreciable bulk bonding results, with strengths as high as commercial glues. Polymer molecular weight is amongst several parameters that need to be examined in order to both understand biomimetic adhesion as well as to maximize performance. Experiments presented here explore how the bulk adhesion of a mussel mimetic polymer varies as a function of molecular weight. Systematic structure-function studies were carried out both with and without the presence of an oxidative cross-linker. Without cross-linking, higher molecular weights generally afforded higher adhesion. When a [N(C4H9)4](IO4) cross-linker was added, adhesion peaked at molecular weights of ~50,000-65,000 g/mol. These data help to illustrate how changes to the balance of cohesion versus adhesion influence bulk bonding. Mussel adhesive plaques achieve this balance by incorporating several proteins with molecular weights ranging from 6000 to 110,000 g/mol. To mimic these varied proteins we made a blend of polymers containing a range of molecular weights. Interestingly, this blend adhered more strongly than any of the individual polymers when cross-linked with [N(C4H9)4](IO4). These results are helping us to both understand the origins of biological materials as well as design high performance polymers.