Amphiphilic heteroarm star-shaped polymer was synthesized by the use of polyfunctional chain transfer agent via sequential free radical polymerization in three steps.Firstly

conventional free radical polymerization of methyl methacrylate (MMA) initiated by 2

2′-azobis(isobutyronitrile) (AIBN) was carried out at 70℃ in toluene in the presence of polyfunctional chain transfer agent

pentaerythritol-tetrakis (3-mercaptopropinate) (PETMP)

with [MMA]0:[PETMP]0:[AIBN]0 to be 262.5:4:1 and[MMA]0=5 mol/L.After polymerized for 2 h

the monomer conversion reached 43.61%

and HS-PMMA with numberaverage molecular weight to be 4000 determined by gel permeation chromatography (GPC) and polydispersity index 1.37 was obtained.The 1H-NMR spectrum of HS-PMMA confirmed that there were two unreacted thiol groups still existed at the center of PMMA chain.Secondly

the HS-PMMA obtained above was used as macro-chain-transfer agent for free radical polymerization of tert-butyl methacrylate (tBMA).The polymerization of tBMA was carried out at 72℃ in toluene with [tBMA]0:[HS-PMMA]0:[AIBN]0 to be 655:5:1 and [tBMA]0=1.57 mol/L.With the increase of tBMA conversion

the number-average molecular weights of the crude products shifted to higher molecular weight direction.Furthermore

their molecular weight distribution curves were unimodal through out the polymerization process.When tBMA conversion reached 70.00%

its number-average molecular weight increased from 4000 of HS-PMMA to 10600.Its 1H-NMR spectrum showed that the residual thiol groups of the obtained heteroarm polymer decreased from 2.0 of the HS-PMMA to 0.4.That is to say

on average

the heteroarm polymer had two PMMA arms and 1.6 P tBMA arms.Finally

cleavage of the tert-butyl ester groups was achieved by treating the heteroarm star-shaped polymers in 1H-NMR spectrum of hydrolyzed product showed that the tert-butyl protons of the PtBMA disappeared completely

indicating that the PtBMA chains had been successfully transformed into poly(methylacrylic acid) (PMAA) chains.The self-assembly behavior of the amphiphilic heteroarm star-shaped polymer in 1% KOH were investigated.Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to characterize the sizes and size distributions of the aggregates of the hydrolyzed product.TEM showed that the amphiphilic heteroarm star-shaped polymer aggregates took spherical shape and that its diameter was about 50~100 nm.DLS results showed that the average particle diameter was 273.6 nm and the diameter distribution index was 0.114.