Effect of Chain Length on the Dipole Moment of Polyisobutylene Succinate Anhydride

Dipole moments for long-chain molecules are difficult to measure experimentally and also challenging to compute, given the very large number of degrees of freedom. We have computed dipole moments of oligomers of polyisobutylene succinate anhydride (PIBSA) as a function of chain length up to nine repeat units. We used a combination of classical potential calculations for generating conformations and density functional theory for optimizing structures and computing dipole moments to generate conformation-dependent dipole moments. We found that the conformational degrees of freedom significantly impact the PIBSA oligomer dipole moments, giving a range of about 3.2–5.6 D for the longest oligomer studied. We computed average dipole moments by using a Boltzmann weighting approach. We found that the dipole moment decreases slightly with increasing chain length, ranging from 4.8 ± 0.4 to 3.9 ± 0.6 D, due to a larger proportion of conformations having smaller dipole moments. We propose that the dipole moment of long-chain PIBSA polymers can be estimated from short-chain oligomers.