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.