Tuning the Temperature Response of Branched Poly(N-isopropylacrylamide) Prepared by RAFT Polymerization

Abstract

Reversible addition-fragmentation chain transfer (RAFT) polymerization in the presence of a compound capable of both reversible chain transfer through a thiocarbonylthio moiety and propagation via a vinyl group led to highly branched copolymers by a method analogous to self-condensing vinyl copolymerization. An acryloyl trithiocarbonate prepared by copper-catalyzed azide-alkyne cycloaddition was copolymerized with N-isopropylacrylamide (NIPAM) in ratios selected to tune the distribution and length of branches in the resulting thermoresponsive polymers. The degree of branching increased with chain transfer agent (CTA) concentration, as proven by NMR spectroscopy, size exclusion chromatography, and viscometry. Retention of the thiocarbonylthio compound during the polymerization was evidenced by successful chain extension of a branched N-isopropylacrylamide (PNIPAM) macroCTA by RAFT polymerization of N,N-dimethylacrylamide. The branched polymers led to reduced lower critical solution temperatures as compared to linear PNIPAM, an effect attributed primarily to an increased contribution of hydrophobic end groups. End group cleavage by radical-induced reduction resulted in an increased transition temperature more similar to that expected for linear PNIPAM.

Building-block design

“…Trithiocarbonates are known to constitute potent control agents for radical polymerizations of NIPAm [189][190][191][192] (as opposed to, for example, dithiocarbamates). [193] All polymerizations were carried out at atmospheric pressure and 60-70°C in DMF [189] .…”

“…To then isolate the polymer from the solvent and the extant (solid) monomer, it was first precipitated from the polymerization mixture by addition of diethyl ether, [192,198] which is a solvent for the monomer. The polymer was collected by centrifugation and removal of the supernatant solution.…”

Combining RAFT Radical Polymerization and Click/Highly Efficient Coupling Chemistries: A Powerful Strategy for the Preparation of Novel Materials

“…Li et al also described the synthesis of PNIPAM-based polymer-protein conjugates with bovine serum albumin (BSA) [37] likewise employing CTA-AZIDE1 as a means of obtaining azido-end-functionalized PNIPAM that was subsequently clicked with a propargyl modified BSA. Vogt et al [36,46] formation of highly branched polymers with the average degree of branching and branch length being readily manipulated via the reaction stoichiometry. Qiu et al [40] reported a route to cyclic PNIPAM from a precursor linear polymer prepared with CTA-AZIDE4 .…”

Sequential thiol‐ene/thiol‐ene and thiol‐ene/thiol‐yne reactions as a route to well‐defined mono and bis end‐functionalized poly(N‐isopropylacrylamide)

“…[36][37][38][39][40][41][42][43][44][45][46] As a consequence of the degenerative chain transfer mechanism, (co)polymers prepared by RAFT bear very specific end-groups, the chemical nature of which is dependent on the structure of the chain transfer agent (CTA) and, to a lesser extent, the CTA/initiator pair. Assuming no undesirable side reactions then the a terminus of a (co)polymer chain will be chemically identical to the R-group of the RAFT CTA while the x terminus bears a thiocarbonylthio functional group, the exact chemical structure of which will be dependent of the class of RAFT CTA used, that is, dithioester versus xanthate versus trithiocarbonate etc.…”