57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

This grant supports our efforts on developing new polymeric hydrogels based on the reversible covalent crosslinking of hydrophilic polymer chains using sequence-specific molecular duplexes that are equipped with both multiple hydrogen-bonding and dynamic covalent interactions. These duplexes, derived from H-bonded duplexes we developed, are capable of mediating highly specific association and covalent crosslinking in both nonpolar and polar (including aqueous) media. Our objectives are (1) modified commercially available, biocompatible polymer chains such as poly(lactic acids) (PLA) and poly(ethylene glycol)(PEG) chains with our duplex-forming units, (2) to form libraries of amphiphilic dynamic covalent diblock, triblock and multi-block copolymers with various, readily adjustable PLA/PEG ratios mediated by the introduced duplex units, and (3) to correlate the effect of PLA/PEG (thus the hydrophobic/hydrophilic) ratios on gelating ability of the resultant amphiphilic block copolymers.

In the first year, we have established the efficient modification of PLA and PEG chains. PLA and PEG chains with molecular weights between 1,000 to 5,000 have been end-modified with our duplex-forming units. The modified polymer chains have been characterized by using techniques including ¹H NMR, GPC, and VPO, results from which have demonstrated the efficiency of our chemistry.

We have started probing the conditions of ligating the modified PEG and PLA chains into linear amphiphilic block copolymers.  Three PLA/PEG pairs have been tested under mild conditions (in organic or aqueous media in the presence of iodine) that allow the formation of covalently linked duplexes, which in turn led to nearly quantitative ligation of the PLA and PEG blocks. The formed amphiphilic block copolymers have been fully characterized by NMR, GPC, MALDI, DLS, and AFM, results from which confirmed the high efficiency of our crosslinking/ligation methods.

In addition to linear block copolymers, we are also developing branched crosslinkers which allow crosslinked networks of hydrophilic polymer chains to be formed. By adjusting the ratio between linear (PEG) blocks and the branched crosslinkers, the crosslinking densities of the corresponding polymer networks can be systematically tuned, which will lead to the tuning of the mechanical strength of the corresponding hydrogels

On another front, we have recently incorporated a stimuli-responsive structural motif into the design of our H-bonded duplexes. It was found that duplexes units containing a structural motif derived from 2, 6-diacylaminopyridine underwent drastic structural change in the presence and absence of acids. Stably associated, sequence-complementary duplex strands were found to complete dissociate in the presence of added acid(s). The observation have pave the way based on which responsive amphiphilic block copolymers and their corresponding hydrogels are being developed.