A novel amphiphilic and thermosensitive triblock copolymer of poly(N-isopropylacrylamide)-b-poly(3-benzyloxycarbonyl-L-lysine)-b-poly(ethylene glycol) (PNIPAM-b-PZLL-b-mPEG) with designed molecular weight and narrow molecular weight distribution was prepared.Synthetic procedure included the following steps.(1) Amino-terminated PNIPAM was synthesized via the free radical polymerization of NIPAM in methanol using 2

2′-azobisisobutyronitrile (AIBN) and 2-aminoethanethiol hydrochloride (AET#8226;HCl) as the initiator and chain transfer agent

respectively.Its molecular weight was calculated by the 1H-NMR-spectra in D2O.(2) The block copolymers

PNIPAM-b-PZLL were synthesized by the ringopening polymerization (ROP) of 3-benzyloxycarbonyl-L-lysine N-carboxyanhydride (Z-Lys-NCA) with PNIPAMNH2 as a macroinitiator.Their structure was confirmed by the 1H-NMR spectra.The molecular weight and molecular weight distribution of PNIPAM-b-PZLL were characterized by GPC analysis in THF.The relatively narrow polydispersity of the obtained PNIPAM-b-PZLL could be explained by the living character of the ROP of NCAs initiated by primary amine hydrochlorides which avoided the side reaction called “active monomer" mechanism.Generally

the molecular weights could be controlled by the feed molar ratios of monomer to maroinitiator.(3) For preparing the triblock polymers

PNIPAM-b-PZLL-b-mPEG

it was necessary to convert the terminal hydroxyl group of mPEG of different molecular weight (2000 or 5000) into carboxyl group by succinic anhydride.Its 1H-NMR spectra showed this was a high efficiency reaction.Then the triblock copolymers were obtained by the condensation reaction between carboxyl-capped mPEG and aminocapped PNIPAM-b-PZLL in the presence of dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP).The 1H-NMR confirmed their structure.Based on the integration ratio

their relative amount in the copolymer was calculated.And their Mn(NMR) agreed well with the theoretical numberaverage molecular weight.GPC analysis in THF revealed a monomodal and symmetric peak which showed the successful synthesis of target copolymers.All the molecular weight distributions (Mw/Mn=1.11～1.20) were relatively narrow.The triblock copolymer could selfassemble into micelles in aqueous solution with hydrophobic PZLL as the core- and hydrophilic PNIPAM and mPEG as the corona.Its critical micelle concentration (CMC) was measured by a fluorescence spectra technique using pyrene as a probe.The CMC decreased with the increase of PZLL content and increased with the increase of PEG content.Dynamic light scattering (DLS) analyses of triblock copolymer micelles revealed their size of less than 100 nm and narrow size distribution.It is well-known that PNIPAM homopolymer dissolved in cold and dilute aqueous solution but gets insoluble at ca. 32℃ due to its lower critical solution temperature (LCST) phase behavior.When the temperature was above the LCST

the PNIPAM undergoes a reversible phase transition caused by the coil-to-globule transition and becomes more hydrophobic.So changing temperature will influence the stability of micelles.As anticipated

the micelles aggregated at temperatures above LCST when the Mn of mPEG was 2000.That is probably that the mPEG2000 could not prevent the intermicellar hydrophobic interaction.However

no obvious aggregation of micelles was found at temperatures above LCST when the Mn of mPEG was 5000 because the hydrophobic interaction between the micelles was prevented by mPEG5000 effectively.