Synthesis of thermally responsive cylindrical molecular brushes via a combination of nitroxide-mediated radical polymerization and “grafting onto” strategy

Loosely grafted amphiphilic molecular brushes consisting of a hydrophobic polystyrene backbone and hydrophilic poly(ethylene glycol) monomethyl ether (MPEG) side chains, PS-MPEG, were synthesized by a novel two step method. In the first step, well-defined linear poly(p-chloromethylstyrene) PCMS with the degree of polymerization DP ≈ 200 and polydispersity index, M_w/M_n = 1.4 was prepared by bulk nitroxide (TEMPO)-mediated radical polymerization (NMP). In the second step, pendant p-chloromethyl units were coupled with activated chains of poly(ethylene glycol) monomethyl ether (MPEG) during the course of the Williamson etherification reaction. Using MPEG with M_n = 1100 g/mol over 50% repeating units on PCMS underwent nucleophilic substitution yielding a densely grafted brush with theoretical molecular weight M_n,th ≈ 123,000 g/mol. The molecular brush PS-MPEG assumed conformation of a stiff cylinder in both dilute toluene (non-selective good solvent for PS backbone and MPEG grafts) and water (selective for MPEG grafts) solutions as determined by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). The contour length L = 570 Å and L = 694 Å obtained by fitting the scattering data using model of Sharp-Bloomfield, Pedersen-Schurtenberger and Kholodenko worm revealed dimensions corresponding to theoretically estimated size of a single molecular brush. It was found that PS-MPEG molecular brush in dilute aqueous solutions exhibited lower critical solution temperature (LCST) in the physiological range.