Characterization And Differentiation Of Copolymrs With Different Types Of Gradient Structures

The copolymerization of 4-Chlorostyrene(4ClSt)and methyl acrylate(MA)via atom transfer radical polymerization(ATRP)was attempted using a cuprous bromide(CuBr)/ N,N,N',N",N".-pentamethyldiethylenetriamine(PMDETA)catalyst system with an ethyl-2-bromoisobutanoate(Eib-Br)as the initiator to obtain a series of gradient copolymers with gradient structures along the chain.The homopolymers of 4ClSt and MA were also gained through above ATRP procedure.Their relative low polydispersity values(＜1.2)demonstrated a controlled polymerization.Free radical polymerization (FRP)of the above two monomers was performed using benzoyl peroxide(BPO)as the initiator to produce copolymers with gradient structures among the chains.Copolymers obtained by FRP just had an average polydispersity value of 1.6,which accorded with characters of FRP.The nuclear magnetic resonance(NMR)spectra were used to character the gradient structures of copolymers and to reflect the cumulative composition of monomer S and A. (S and A denote 4ClSt and MA units for short respectively).Monomer reactivity ratios (r_S=1.2,r_A=0.09)were obtained by ¹H NMR spectra,and the microstructures of copolymers,which were manifested by the concentrations of A-centered triad(SAS,SAA/AAS and AAA)and S-centered triad(ASA,ASS/SSA and SSS)for gradient copolymers were obtained by ¹³C NMR.In order to characterize gradient structure of copolymers,gradient index was introduced into our reasearch.The result showed gradient copolymers with the same feed ratio and conversion but different types of gradient structures had the same corresponding triad concentrations.Our study is on the differentiation of gradient structures along the chain and among the chains,but no obvious differentiation was achieved by NMR.In thin layer chromatography(TLC),dichloromethane/acetone mixed solvents were,chosen to be developing solvents through investigating the performances of P4ClSt and PMA on thin layer plates.The performances of homopolymers with different molecular weights under the same developing condition were investigated,and the result showed moleclar weight had no effect on R_f values of homopolymers,and under this developing condition, developing mechanisms of both homopolymers and copolymers were adsorption-desorption.Good differentiation of gradient structures along the chain and among the chains was achieved by developing in dichloromethane/acetone.By this procedure,gradient structure along the chain was developed to be a spot,and gradient structure among the chains was developed as a band.Their different performances on chromatogram agreed well with their different types of gradient structures root in different preparations.There are different segment lengths of gradient copolymers with the same type gradient structure due to different feed ratios and conversions in the preparation.The TLC performances of gradient copolymers with gradient structure along the chain but different segment lengths of S were investigated,and TLC performances of gradient copolymers with gradient structure among the chain but different segment lengths of A were also investigated.