Itaconate Based Dual-reactive Polymeric Coatings and Photo-cross-stitching Studies of Periodically Grafted Amphiphilic Systems

Abstract

Dual-reactive coatings are those that covalently anchor to the substrate while at the same time permit installation of desired segments in a post-coating step; several approaches have been developed to design such polymers with dual reactivity for surface modification. In the first part of my presentation, I will be discussing about a simple approach that we have developed to prepare surface-anchored coatings that can be functionalized with desired segments in a post-coating step; to achieve this, a hetero-diester of itaconic acid (IA), namely 2-hydroxyethyl, propargyl itaconate (HEPI) was prepared, which upon free-radical initiated polymerization yielded a polymer, PHEPI. This polymer is endowed with three important features: i) the pendant hydroxyl groups can react with Si-OH groups on suitably treated glass/silicon surfaces; ii) the pendant propargyl groups can help install chosen segments onto the coating, via azide-alkyne reaction, to render the desired properties to the surface, and iii) thermally induced crosslinking of the coating, that occurs at ~140°C, helps create robust surface-anchored coatings of desired thickness. Derivatization of PHEPI coatings was easily done by treatment with a variety of organic azides, such as PEG-azide, cetyl azide, fluoroalkyl azide, etc., to yield surfaces with distinct characteristics; the derivatized surfaces were characterized by contact angle measurements, IR and XPS. Additionally, the surface characteristics were fine-tuned by co-clicking different proportions of mixtures of azides with different polarity, in the post-coating step; this revealed the versatility of the dual-reactive primer approach. Further, I will demonstrate that this polymer can also be spray-coated onto filter paper, permitting further modification of their surface properties at a late stage, in a process that was termed spray-click-bake. Finally, I will also show that the propargyl groups in Si-coated PHEPI can be clicked using photoinitiated thiol-yne reaction; this demonstrated the potential of the polymer for photo-patterning of coated substrates.1 Periodically grafted amphiphilic polymers (PGAPs) are systems carrying hydrophobic alkylene backbone segments and hydrophilic polyoxyethylene (PEG) pendant segments that are located at precisely intervals. It was earlier established in our lab that these PGAPs, although adopt a random coil conformation in good solvent, it undergoes zigzag folding upon dissolution in a PEG-segment selective solvent, such as water, to collocate the hydrophobic alkylene segments at the centre protected by PEG segments which form the outer corona.2 In the second part of the presentation, a strategy to crosslink the folded form of suitably designed PGAPs will be demonstrated; to achieve this, a photo-reactive unit, namely a dicinnamoyl diester (DCDE) unit, was centrally located within the hydrophobic alkylene segment. Thus, when the chain is zigzag folded in water, the DCDE units come in close proximity to permit their photodimerization and generate covalently crosslinked single-chain nanoparticles (SCNP) from the folded chain. The modular synthetic strategy adopted for the polymer synthesis, permitted the installation of PEG segments of different sizes (PEG750, PEG1000 and PEG2000) in a post-polymerization step; it was observed that when PEG2000 segments are installed, the zigzag folded chains are effectively enveloped by the solvated PEG segments permitting the generation of crosslinked SCNPs. Another aspect that I will be discussing concerns with the number-density of photoreactive DCDE units, within the zigzag folded region, required for effective generation of crosslinked SCNPs. To examine this, copolymers were prepared, that helped insert simple hydrophobic alkylene segments of similar length, instead of the DCDE units; it was noticed that while the rate of photoreaction was affected, the photo-cross-stitching does occur even when the density of DCDE units were as low as 35 mole-percent. This observation is instructive for the development of functional crosslinked SCNPs, wherein non-dimerizable segments will need to be incorporated without significantly impacting the intra-chain photo-crosslinking process.