In the present research thiol-terminated polyethylene glycols of different molecular weight were prepared and applied to synthesize polyethylene glycol/polyacrylonitrile block copolymers.
Thiol-terminated polyethylene glycols (hereafter called (PEG)-SH) was prepared as following; polyethylene glycol was first converted to its chlorohydrin derivative by reacting with epichlorohydrin in the presence of $BF_3\cdot(C_2H_5)_2O$ and then reacted with sodium trithiocarbonate to obtain the corresponding thiol-terminated polyethylene glycol ((PEG)-SH).
Redox polymerization of acrylonitrile in aqueous medium was carried out with potassium persulfate as an oxidizing agent at various temperatures. Macromeric thiol acted as an reducing agent generating thiyl radicals upon the reaction with oxidizing agent and subsequently polymerized acrylonitrile, thereby resulting polyethylne glycol/polyacrylonitrile block copolymers. The polymerization proceeded well even at 10℃ indicating that a redox system is operative.
Various polymerization conditions were applied and their effects were investigated. The increase of (PEG)-SH concentration relative to acrylonitrile in feed generally decreased the yield of copolymers and lowered molecular weight of the block copolymers, but increased the polyethylene glycol content in block copolymers.
Viscosity measurements indicated that the molecular weights of block copolymers obtained in the present polymerizations were rather low being in the range of 30,000 and 50,000 when(PEG)-SH of molecular weight 7,500 were used. This molecular weight indicated that each copolymer molecule contains on average one polyethylene glycol block.
Polyethylene glycol/polyacrylonitrile block copolymers showed increased moisture regain at different relative humidity(RH) : The copolymer containing 23% polyethylene glycol regained 10% of moisture at 65% RH and 22% at 95% RH.
Differential scanning calorimeter measurement failed to any significant melting transition of polyethylene glycol and thermogravimetric analysis indicated that the block copolymers begin to degrade at the temperature lower than the degradation temperature of homo polyacrylonitrile.
말단에 thiol 기가 포함된 polyethlene glycol ((PEG)-SH)을 합성하고, 이를 이용하여 PEG/PAN block 공중합체를 얻었다.
(PEG)-SH 는 다음과 같이 합성하였다. $BF_3\cdot(C_2H_5)_2O$를 촉매로 하여 epichlorohydrin 과 반응시켜 PEG 의 chlorohydrin 중간체를 얻은 다음 이를 sodium trithiocarbonate 와 반응시킨후 산성화하여 (PEG)-SH 를 얻었다.
수용액 중에서 acrylonitrile 을 모노머로 KPS 를 산화제로 사용하여서 redox 중합을 행하였다. 이때 macromeric thiol 이 환원제로 작용 thiyl 라디칼을 생성하여 중합반응이 개시되어서 PEG/PAN block 공중합체가 얻어졌다. 반응은 10℃ 정도의 낮은 온도에서도 진행되었으며 유발시간은 보통 5분이 넘지 않았다.
AN 농도에 비해 (PEG)-SH 의 농도가 증가될수록 전체수율과 분자량은 감소하였으나 block 공중합체내의 PEG 함량은 증가하였다. 점도 측정에 의한 분자량계산 결과로 미루어 보아 block 공중합체 1분자당 평균 1개의 PEG block 이 포함되어 있는 것으로 볼 수 있다. PEG/PAN block 공중합체는 PEG 함량에 비례하여 흡습성이 증가하는데 23% 의 PEG 함량을 가진 공중합체 경우 상대습도 65% 에서는 10%, 95% 상대습도에서는 22%의 흡습률을 가졌다.
DSC 로는 PEG 에 관한 정보를 얻을 수 없었으나 TGA 사용결과 PAN homo 중합체보다 열적성질이 나빠진 것을 알수 있었다.
