Conformational Characteristics and Unperturbed Chain Dimensions of the Oxygen-Containing Polymers [−OCH2CR2CH2−]n with R = H, CH3, and CH2CH3

Abstract

MM3 molecular mechanics calculations reveal that poly(3,3‘-diethyloxetane) (PDEO) differs sharply from poly(3,3‘-dimethyloxetane) (PDMO) and poly(trimethylene oxide) (POM3) in terms of conformational preferences, notwithstanding the similarity of these three polymers as successive members of the homologous series [OCH2CR2CH2]n where R = CH2CH3, CH3, and H, respectively. Within the four-bond repeat unit OCH2CR2CH2, POM3 and PDMO both prefer ttgg while PDEO prefers tttt. These conformational differences are reflected in the predicted values of the characteristic ratio CR = 〈r2〉0/nl2 for the unperturbed chain dimensions (after allowance for the oxygen gauche effect) obtained from rotational-isomeric-state (RIS) calculations: 3.9 for POM3, 4.7 for PDMO, and 23 for PDEO. These CR values for POM3 and PDMO are consistent with experiment (3.9 and 4.3, respectively) and with previous theoretical treatments. The larger R substituent of PDEO compared with PDMO and POM3 imposes steric demands that offset the otherwise 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 lacks the distinctive conformational randomness of POM3 and PDMO. In terms of Eσ = Et − Eg for the focal C−C bond in these polymers, comparison of the MM2, MM3, and Discover (plus a variant Discover‘) force fields reveals a wide disparity of values for Eσ and more so for the constituent energy components Estretch, Ebend, Etorsion, EvdW, and Eelectrostatics. The preference of the focal C−C bond in PDEO for trans over gauche involves a surprising degree of interplay among these energy components; hence the conformational analysis of PDEO would represent a stringent test of any existing or prospective force field.