Abstract
The molar optical rotation of poly(cis-5-methylproline) was measured in solvent mixtures of chloroform and chloroethanol. After proper allowance for time-dependent mutarota-tions, equilibrium states between form A and form B were observed to occur with a solvent composition of 0.5~10 % chloroethanol in chloroform by volume. From the equilibrium constants, which were calculated by optical rotations at equilibrium measured at three different temperatures (5, 25, and 45 $^{circ}$C), the thermodynamic parameters-free enthalpy, enthalpy and entropy changes for the mutarotation-were evaluated. It was found that starting with equimolar concentrations of form A and form B, the forward mutarotation occurred in the solvent compositions of chloroethanol greater than 3 % by volume, whereas the reverse mutarotation resulted in solvent compositions of chloroethanol less than 3 % by volume. The changes in enthalpy and entropy for the forward mutarotation were found to be positive, while those were for the reverse mutarotation were negative. The driving forces for the forward mutarotation were found to be the increase in entropy, whereas that for the reverse mutarotation was the negative enthalpy change. The thermodynamic data were explained by the interaction between polymer and solvent, i.e., preferential hydrogen bonding of chloroethanol with the carbonyl group in form B over form A, and by difference in conformational energies between form A and form B.
ChIoroform과 chloroethanol 혼합용매속에서 Poly(cis-5-methylproIine)의 광회전을 측정하였다. 이 혼합용매속에 chloroethanol이 0.5∼10부피%일때 형태 A와 B사이에는 평형상태가 이루어졌다. 광회전을 이용하여 5, 25 및 45$^{circ}$C에서 평형정수를 측정하고 자유엔탈피, 엔탈피 및 엔트로피 변화를 계산하였다. 같은 몰수의 형태 A와 B에서 출발하면 chloroethanol이 3부피% 이상일때는 정방향 변광회전이 일어나고 이하일때는 역방향 회전이 일어났다. 엔탈피와 엔트로피 변화는 정방향 변광회전일때는 양수이고 역방향 변광회전일 때는 음수였다. 반응 추진역은 정방향 회전일때는 엔트로피의 증가이고 역방향 회전일때는 엔탈피의 감소였다. 측정된 열역학적 자료들은 폴리머와 용매사이의 상호작용 즉 용매중의 chloroethanol이 형태 B분자내의 카보닐기와 선택적으로 수소결합을 형성하며, 형태 A와 B의 헝태 에너지 차이에 있다는것을 설명한다