Conformational statistics of a polybenzoxazine

Abstract

A Rotational Isomeric State (RIS) model has been constructed for the polybenzoxazine with a repeating unit of [−C6H2(OH)(CH3)–CH2–N(CH3)–CH2−], with the four nonhydrogen substituents bonded to the aromatic ring arranged with the OH ortho to both CH2 and para to the single CH3. The model was derived from the conformational analysis of a dimer, N−(5-methyl-2-hydroxybenzyl)–N-(2–hydroxy-5-methyl benzyl)methylamine. This conformational analysis incorporates the two states that arise from inversion of the configuration at the tertiary amine. The analysis can be applied to any sequence of these two states, including the case where they have the same probability of occurrence, and are randomly distributed along the chain in accord with Bernoullian statistics. As expected, the calculated characteristic ratio, C∞, for the physically relevant random distribution of equally probable states is smaller than the result obtained for hypothetical chains that have a preference for one state over the other. The analysis also incorporates the two preferred states for each of the two C(sp)–O bonds from the aromatic ring to the hydroxyl groups. The computed values of C∞ are in the range 2.0–4.3, depending on the correlation between the states selected by neighboring C(sp)–O bonds. The most likely correlation of these states specifies a C∞ of about 2.4. The small value of C∞ signifies a rather compact average conformation for the chain. This conformation arises from compact states accessible to the spacer between consecutive aromatic rings and the participation of the hydroxyl groups in attractive intrachain interactions. These compact states produce a tendency for local order that is apparent in the scattering function calculated from the ensemble specified by the RIS model.