59th Annual Report on Research 2014

With the support of ACS PRF (PRF# 51827-ND7, 01/01/12-08/31/15), we have recently demonstrated that the new carbonyl- and hydroxyl-PPGs (as in 3 and 6) developed in our lab can be converted to the robust photocleavable linkers (7a-c). The di-block copolymers (8a-c) with these incorporated photolinkers can be readily cleaved with various convenient light sources (including sunlight) to provide terminal-functionalized polystyrene. Selective cleavage of different BCPs by changing the irradiation wavelength has been demonstrated. The new linkers are structurally simple, easy to prepare and use, and can be cleaved in high efficiency under various conditions without excluding air. They are stable under indoor lighting and withstand various chemical treatments, especially the polymerization conditions. These photoresponsive BCPs should be potentially useful in a variety of applications. These results were published in Chem. Commun., 2013, and this PRF grant was acknowledged exclusively for the support of the work.

Encouraged by these results, in the past year, we continued to develop new photolinkers and significant progress has been achieved (Scheme 2). For example, starting from the commercially available inexpensive 3-aminobenzoic acid, reduction with LAH led to the 3-aminobenzyl alcohol which is also commercially available. Diethylation of 11 led to the PPG reagent 12, i.e., 3-(N, N-diethylamino)benzyl alcohol. Under normal esterification conditions, the photolabile ester 14 was prepared. Irradiation of 14 led to release of the carboxylic acid 13 in high efficiency, along with recovery of the benzyl alcohol 12. This new discovery is exciting in that unlike the widely used ortho-nitrobenzyl PPG, the PPG chromophore in 14 was not destroyed in the process of substrate release. Based on this new discovery, starting from the commercially available dimethyl aminoterephthalate 15, we synthesized the diol 16 as a new PPG reagent in only two routine steps. The diol coupled with the acid 13 to provide the photocleavable diester 17. Upon UV-irradiation, the acid 13 was cleanly released from 17! (to be published)

Our new photochemical results shown in Scheme 2 provide a rapid access to a photocleavable joint to be incorporated into polymer skeletons. Synthesis of the joint 16 takes only two steps by using commercially available inexpensive starting materials, which allow its preparation in large scale at a low cost. We demonstrated that the diol linker 18, prepared in a way similar to the preparation of 16, can be converted to the mono ester 19 by coupling with 2-bromoisobutyryl bromide, and then to the diester 20 by coupling with 5-hexynoic acid. This difunctionalized photocleavable linker can be used in a one-pot copper (I)-catalyzed simultaneous atom transfer radical polymerization (ATRP) reaction to prepare the PEG-l-PS block copolymer 21. The last step of making 21 is currently under optimization. Therefore, with the new photocleavable linker, new BCPs can be synthesized even faster than our previously reported BCPs.

The joint 16 can also couple with pre-prepared polymer segments (e.g., carboxyl-terminated PS and PEG) or copolymerize in making photocleavable polycarbonate. Research along these directions is currently under investigation in our lab.