Study On Thermal Initiation In Two Molecular Initiating Stable Free Radical Polymerization Of Styrene

A novel radical initiator,4,4'-azobis(4-cyanopentyl)-α-bromoisobutyrate (ABCBIB),having active groups for ATRP,was synthesized.The stable free radical polymerization(SFRP)of polystyrene was carried out in bulk with ABCBIB as initiator in the presence of 4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy(TEMPO-OH). ATRP of styrene was performed with the resulted polystyrene as macroinitiator.The thermal initiation was investigated by GPC quantitatively.First,the SFRP of styrene with ABCBIB as initiator in the presence of TEMPO-OH was approved to be living radical polymerization.A series of polymerizations of styrene were carried out with different ratios of ABCBIB to TEMPO-OH.It was approved that the molecular weight of polymer was determined by the concentration of TEMPO-OH rather than that of ABCBIB.Second,four kinds of polystyrenes,PSt-1,PSt-2,PSt-3 and PSt-4 were prepared through changing[ABCBIB]:[TEMPO-OH]to be 1:1,1:2,1:3 and 1:4 respectively.Using polystyrenes obtained above as macroinitiators, CuBr/PMDETA as catalyst,bulk atom transfer radical polymerization(ATRP) of styrene was carried out at 80℃.The GPC curves of the polystyrenes obtained in ATRP showed double peaks.These results imply that the terminals of macroinitiators were not uniform.The polymer chains generated by the decomposition of ABCBIB hadα-bromoisobutyrate at theirα-ends.Theα-bromoisobutyrate is efficient initiator for ATRP of styrene,while the polymer chains formed by thermal initiation are inert for ATRP.Therefore,if polystyrenes prepared by SFRP are used as macroinitiator for ATRP of styrene, some polystyrene chains will grow to higher molecular weights,but the molecular weight of the other part of polystyrene chains will keep constant. Double peaks were divided into two peaks whose molecular weights were calculated using GPC software.The amount of polymer chains generated by ABCBIB are calculated by equation N_Br=Xm(M₀)/(M_n1-M_n2).It was found that the ratio of polymer chains generated by thermal initiation increased with the increasing ratio of TEMPO-OH to ABCBIB.It was also found that when the ratio of TEMPO-OH to ABCBIB was low,the biomolecular termination could be observed in the SFRP,however,with the ratio of TEMPO-OH to ABCBIB increased,the biomolecular termination turned to be diminished till vanished.