Layer‐by‐Layer Assembly of Reactive Ultrathin Films Mediated by Click‐Type Reactions of Poly(2‐Alkenyl Azlactone)s

Abstract

Methods that permit the deposition or assembly of reactive polymer films on topologically complex substrates are useful for the patterning or chemical modification of surfaces of interest in a broad range of applications. [1][2][3][4][5] Here, we report a layer-by-layer approach to the assembly of covalently-crosslinked ultrathin films that makes use of fast and efficient 'click'-type interfacial reactions between poly(2-alkenyl azlactone)s and appropriately functionalized polyamines. In contrast to conventional, aqueous methods for the layer-by-layer fabrication of multilayered assemblies composed of polyelectrolytes, [6][7][8][9] fabrication of these materials occurs in organic solvents and is driven by rapid interfacial formation of covalent bonds during assembly. These methods permit precise, nanometer-scale control over the thicknesses and compositions of covalently crosslinked thin films. In addition, we demonstrate that it is possible to chemically tailor the properties of surfaces coated with these ultrathin films post-fabrication by exploiting the accessibility and reactivity of residual 'spring-loaded' azlactone functionality. These results suggest the basis of methods for the post-fabrication modification of curved or topologically complex surfaces coated with multilayered films and the patterning or passivation of surfaces with chemical or biological functionality of interest in the contexts of catalysis, medicine, and other areas of biotechnology. Aqueous methods for the layer-by-layer deposition of oppositely charged polyelectrolytes on surfaces are used widely for the bottom-up assembly of nanostructured polymer films. [6][7][8][9] These methods generally take advantage of multivalent weak interactions (e.g., electrostatic or hydrogen bonding interactions) between polyelectrolytes and oppositely charged surfaces and allow precise control over the thicknesses, compositions, and morphologies of thin films fabricated from a broad range of water-soluble polymers.

Post-polymerization Modification of Surface-Bound Polymers

“…To improve the system, Lynn and coworkers developed "reactive" LbL assembly of thin polymer multilayers containing azlactones [67,[69][70][71][72][73]. Azlactone-functionalized polymers can react with polymers containing primary amines through ring-opening reactions [70]. Consequently, The group of Lynn focused mostly on films constituted of poly(2-vinyl-4,4-dimethylazlactone) (PVDMA) and poly(ethylene imine) (PEI) for reactive LbL assembly [3,[74][75][76][77][78][79][80][81][82].…”

“…The method is technically straightforward and inexpensive; however, the LbL assembly of charged polyelectrolytes only involves weak intramolecular interactions within the ionically crosslinked multilayers. To improve the system, Lynn and coworkers developed "reactive" LbL assembly of thin polymer multilayers containing azlactones [67,[69][70][71][72][73]. Azlactone-functionalized polymers can react with polymers containing primary amines through ring-opening reactions [70].…”

Covalent Layer‐by‐Layer Assembly Using Reactive Polymers

“…First, as noted above, reactions between azlactones and primary amines occur very rapidly in the absence of a catalyst [135,136] (dipping times required for azlactone-based LbL assembly processes are typically between 15 and 30 s, and times as short as 1 s can also be used [54]). While the need for copper catalysts in click-based LbL protocols has not yet proved to be a limitation, catalyst-free processes can, in general, decrease the complexity of fabrication procedures and eliminate the potential need to remove catalyst material for certain applications.…”

“…In an initial report, Buck et al [54] demonstrated that multilayers could be fabricated by the alternating immersion of silicon substrates into organic solutions of branched PEI and the azlactone-functionalized polymer poly(2-vinyl-4,4-dimethylazlactone) (PVDMA; see structures in Figure 15.10). Characterization using ellipsometry demonstrated that film growth occurred in a linear manner to yield films ∼100 nm thick after the deposition of 10 PEI/PVDMA layer pairs (or ''bilayers'') ( Figure 15.12a).…”

Post‐polymerization Modification of Polymer Brushes

“…Several classes of polymeric active esters such as NHS [27][28][29][30][31][32][33][34][35], pentafluorophenol [36][37][38][39][40], p-nitrophenol [41][42][43][44], and dicarboxyimide [45] functionalized esters have previously been investigated. NHS esters are commonly used for biological post-polymerization modification, where the conjugation of biomolecules occurs between the amine functional groups on peptides and the activated ester [46][47][48][49][50][51][52][53][54][55]. Minimization of hydrolysis and side reactions of the activated ester can be achieved by using aqueous solutions with a pH of 7 or by performing the aminolysis reaction in an organic solvent in the presence of a proton acceptor [46].…”