Synthesis of star polymers of styrene and alkyl (meth)acrylates from a porphyrin initiator core via ATRP

A free-base tetrabromoporphyrin, 15,20-tetrakis(4-(2-methyl-2-bromopropoxy)phenyl)-21H,23H-porphine (2), was synthesized in high yield (91%) by the esterification of 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H, 23H-porphine (1) with 2-bromo-2-methylpropanoyl bromide. The free-base porphyrin (2) was demonstrated to be suitable as an initiator for atom transfer radical polymerization (ATRP) of methyl methacrylate giving porphyrin-core star-poly(methyl methacrylate) with conversions of up to 98% ((CuBr)-Br-I, N-(n-propyl)-2-pyridyl-methanimine, toluene, 90 degrees C). UV-vis spectroscopic analysis demonstrated that a degree of complexation of Cu(II) by the porphyrin core occurred during the polymerization. To avoid Cu(11) complexation, zinc(II) 10,15,20-tetrakis(4-(2-methyl-2-bromopropoxy)phenyl)-21H,23H-porphine (4) was synthesized from the free-base porphyrin (2) and employed as an initiator in the ATRP of MMA, giving the corresponding Zn porphyrin-core star-PMMA. The free-base porphyrin (2) was also employed as an initiator for the polymerization of styrene, methyl acrylate, butyl methacrylate, octadecyl acrylate and the copolymerization of isobutyl methacrylate (IBMA) and trifluoroethyl methacrylate (TFEMA), in all cases giving star polymers with conversions of 33-87%. Basic hydrolysis of a porphyrin-core star-polystyrene polymer cleaved the ester linkages about the porphyrin, liberating the individual polystyrene chains which had a number-average molecular weight approximately one-fourth that of the precursor star polymer and a narrow polydispersity index (M-w/M-n, = 1.15) thereby demonstrating efficient initiation from the porphyrin core. Palladium(II) 10,15,20-tetrakis(4-(2-methyl-2-bromopropoxy)phenyl)-21H,23H-porphine (3) was synthesized from the free-base porphyrin (2) and employed as an initiator in the ATRP of MMA but the polymerization was completely inhibited. Pd(II) was introduced into the star polymer cores by heating either a solution of the porphyrin-core star-PMMA or the Zn porphyrin-core star-PMMA with (PdCl2)-Cl-II in benzonitrile. Pt(II) was introduced into a star polymer core by heating a solution of the Zn porphyrin-core star-PMMA-co-TFEMA with (PtCl2)-Cl-II in benzonitrile. UV-vis spectroscopic analysis confirmed the synthesis of Pd(11) and Pt(II) porphyrin complexes and photoluminescent spectroscopy confirmed the luminescent properties of the materials