POLY(TETRAHYDROFURAN) UREA ADDUCT - A STRUCTURAL INVESTIGATION

被引:20
作者
CHENITE, A [1 ]
BRISSE, F [1 ]
机构
[1] UNIV MONTREAL,DEPT CHIM,CP 6128,SUCC A,MONTREAL H3C 3J7,QUEBEC,CANADA
关键词
D O I
10.1021/ma00028a042
中图分类号
O63 [高分子化学(高聚物)];
学科分类号
070305 ; 080501 ; 081704 ;
摘要
PTHF and urea form a crystalline complex of the host-guest type whose structure has been established from three-dimensional X-ray data. The crystal belongs to the hexagonal system, space group P6(1), and has a unit cell of dimensions a = b = 8.199 (1) angstrom and c = 11.032 (2) angstrom. The crystal structure was established by direct methods from 410 unique reflections. The refinement of the atomic coordinates and the temperature factors coverged to an agreement index, R, of 0.046. The stoichiometry of the adduct is [(CH2)4O]18[urea]6- In this structure, the urea molecules (the host) are linked to one another through hydrogen bonds, and the resulting network consists of hexagonal channels much like a honeycomb. The PTHF chain, the guest, fits within the hexagonal channel. The C and O atoms are indistinguishable, and there is a crystallographic disorder about the chain axis. The PTHF chain is in the trans conformation, but there is a small twist of 24-degrees about each bond so that the chain is coiled. The morphology of the crystals, as revealed by scanning electron microscopy, consists of hexagonal prisms, for low M(w). When M(w) increases, the crystal consists of stacks of hexagonal platelets. DSC measurements indicate that there is no phase change as the temperature is lowered to -150-degrees-C. The melting temperature, which varies with M(w), is slightly higher than that of pure urea. The enthalpy of melting, DELTA-H(m), has a value of 14.4 kJ/mol [(CH2)4O]. The values of M(w)BAR, M(w)BAR, and the M(w)BAR/M(n)BAR ratio were measured before and after complex formation. It was found that the polymolecularity index improves significantly, from 1.59 to 1.20 for PTHF of M(w)BAR = 1000 and from 1.82 to 1.39 for PTHF whose M(w) is 2000.
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页码:776 / 782
页数:7
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