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

Abstract

Sequential thiol-ene/thiol-ene and thiol-ene/thiol-yne reactions have been used as a facile and quantitative method for modifying end-groups on an N-isopro-pylacrylamide (NIPAm) homopolymer. A well-defined precursor of polyNIPAm (PNIPAm) was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization in DMF at 70 °C using the 1-cyano-1-methylethyl dithiobenzoate/2,2'-azobis(2-methylpropionitrile) chain transfer agent/initiator combination yielding a homopolymer with an absolute molecular weight of 5880 and polydispersity index of 1.18. The dithiobenzoate end-groups were modified in a one-pot process via primary amine cleavage followed by phosphine-mediated nucleophilic thiol-ene click reactions with either allyl methacrylate or propargyl acrylate yielding ene and yne terminal PNIPAm homopolymers quantitatively. The ene and yne groups were then modified, quantitatively as determined by 1H NMR spectroscopy, via radical thiol-ene and radical thiol-yne reactions with three representative commercially available thiols yielding the mono and bis end functional NIPAm homopolymers. This is the first time such sequential thiol-ene/thiol-ene and thiol-ene/thiol-yne reactions have been used in polymer synthesis/end-group modification. The lower critical solution temperatures (LCST) were then determined for all PNIPAm homopolymers using a combination of optical measurements and dynamic light scattering". It is shown that the LCST varies depending" on the chemical nature of the end-groups with measured values lying in the range 26-35 °C. © 2009 Wiley Periodicals. Inc.