In the present research thiol-terminated polyethylene oxides of different molecular weight were prepared and applied to synthesize polyethylene oxide/polyethylene block copolymers.
Thiol-terminated polyethylene oxide (HS-(PEO)-SH) was prepared as following: Polyethylene glycol was first converted to its chlorohydrin derivative by reacting with epichlorohydrin in the presence of $BF_3(C_2H_5)_2O$ and was then reacted with sodium trithio-carbonate to obtain the corresponding thiol-terminated polyethylene oxide. In another synthetic route polyethylene glycol was converted to its chloride derivative by reacting with thionyl chloride and was then reacted with sodium trithiocarbonate to obtain the corresponding thiol-terminated polyethylene oxide.
To synthesize block copolymers through free radical chain transfer mechanism styrene was polymerized with 2,2'-azobisisobutyronitrile (AIBN) in the presence of thiol-terminated polyethylene oxide. The polymerization product was fractionated by selective extraction, first with cyclohexane/acetone mixture to remove polystyrene homopolymer and second with isopropanol/water mixture to remove unreacted thiol-terminated polyethylene oxide. The yield of block copolymers was small, but the formation of block copolymer was confirmed by TLC and NMR spectra.
Attempts were also made to prepare block copolymers through redox mechanism. Ammonium persulfate was used in methanol/water mixture to form a redox system with terminal thiols, thus generating terminal thiyl radical which in turn polymerize styrene forming block copolymers.
Compared with AIBN-initiated polymerization, redox system was found more effective in producing block copolymers. A block copolymer obtained from monomer feed ratio, HS-(PEO)-SH/styrene $\frac{1}{2}$ showed a composition of 50 polyethylene oxide to 50 polystyrene. The composition was confirmed by NMR.
말단에 thiol 기를 갖는 polyethylene oxide (HS ← PEO → SH)를 합성하고 이를 이용하여 PEO / Styrene 블록 공중합체를 합성 하였다.
Thiol 말단기를 갖는 prepolymer는 다음과 같이 합성 하였다. $BF_3·(C_2H_5)_2O$를 촉매로 polyethylene glycol(PEG) 과 epichlorohydrin을 반응시켜 PEG의 chlorohydrin 중간체를 얻은 후 이를 sodium trithiocarbonate 와 반응시킨다음 산성화하여 HS ← PEO → SH 를 얻었다. 또한 PEG와 $SOCl_2$ 를 반응시켜 염화물을 만든후 이들 sodium trithio carbonate와 반응시켜서 HS ← PEO → SH 를 얻을 수 있었다.
Free radical chain transfer mechanism에 의한 공중합물 생성의 기능성을 알아보기 위하여 개시제로 2.2' - azobisiso-butyronitrile (AIBN) 을 사용하였고, redox 중합에 의한 공중합물 생성의 가능성을 알아보기 위하여 개시제로 ammonium persulfate(APS)를 사용하였다. AIBN을 사용한 경우에는 연쇄이동에 의하여 공중합체가 생성되었고 redox 계의 경우 APS가 산화제로 작용하고 thiol prepolymer 가 환원제로 작용 thiyl 라디칼을 생성하여 중합반응이 개시되어서 PEO / Styrene 블록 공중합체가 얻어졌다.
공중합물을 분리하기 위하여 cyclohexane / acetone, isopropanol / water의 혼합용매를 사용하였으며 분리의 선호도를 알아보기 위하여 TLC 에의한 방법을 이용하였다.
AIBN 개시제에 의한 중합반응의 경우 용매에 따라서 큰 차이를 보였으며 monomer의 양, 개시제의 양에 대하여는 큰영향이 없었다. 또한 redox 계에 의한 중합반응은 AIBN 개시에 의한 반응과 비교하여 좋은 대조를 보였고 온도의존성을 보였으며, styrene / prepolyer 비율 2/1을 반응시켰을 경우 블럭공중합체내의 Styrene / PEO 비율이 1/1 정도 된다는 것을 NMR 스펙트럼으로 알 수 있었다.