CONFORMATIONAL CHARACTERISTICS AND UNPERTURBED CHAIN DIMENSIONS OF THE OXYGEN-CONTAINING POLYMERS [-OCH(2)CR(2)CH(2)-](N) WITH R=H, CH3, AND CH2CH3

MM3 molecular mechanics calculations reveal that poly(3,3'-diethyloxet ane) (PDEO) differs sharply from poly(3,3'-dimethyloxetane) (PDMO) and poly(trimethylene oxide) (POM(3)) in terms of conformational preferen ces, notwithstanding the similarity of these three polymers as success ive members of the homologous series [OCH(2)CR(2)CH(2)](n) where R = C H2CH3, CH3, and H, respectively. Within the four-bond repeat unit OCH( 2)CR(2)CH(2), POM(3) and PDMO both prefer ttgg while PDEO prefers tttt . These conformational differences are reflected in the predicted valu es of the characteristic ratio CR = (r(2))(0)/nl(2) for the unperturbe d chain dimensions (after allowance for the oxygen gauche effect) obta ined from rotational-isomeric-state (RIS) calculations: 3.9 for POM(3) , 4.7 for PDMO, and 23 for PDEO. These CR values for POM(3) and PDMO a re consistent with experiment (3.9 and 4.3, respectively) and with pre vious theoretical treatments. The larger R substituent of PDEO compare d with PDMO and POM(3) imposes steric demands that offset the otherwis e strong preference of the C-C backbone bonds for gauche states. This reversal in preference from gauche to trans causes the abrupt increase in CR since now the almost exclusively all-trans backbone of PDEO lac ks the distinctive conformational randomness of POM(3) and PDMO. In te rms of E(sigma), = E(t) - E(g) for the focal C-C bond in these polymer s, comparison of the MM2, MM3, and Discover (plus a variant Discover') force fields reveals a wide disparity of values for E(sigma) and more so for the constituent energy components E(stretch), E(bend), E(torsi on), E(vdW), and E(electrostatics). The preference of the focal C-C bo nd in PDEO for trans over gauche involves a surprising degree of inter play among these energy components; hence the conformational analysis of PDEO would represent a stringent test of any existing or prospectiv e force field.