Estimation of the molecular weights of noncovalent columnar polymers based on β-cyclodextrin by the measurement of the aggregation rate of their polymer inclusion complexes with polypropylene glycol

Noncovalent columnar polymers (NCPs) based on cyclodextrins (CD) are polymeric assemblies of molecules that have continuous hollow channels, the width of which is determined by the diameter of the cavity of the initial CDs. The repeating fragment in an NCP is the CD molecule. For NCPs that were obtained by the exclusion of polymer backbone macromolecule from the corresponding inclusion complexes (ICs) based on β-cyclodextrin (NCPexcl), the polymer length, expressed as the number of macrocycles in a single chain (n) is determined by the size of the included ligand, polypropylene glycol (PPG), and is the PPG polymerization degree divided by two. The determination of the molecular weight of an NCP obtained by the precipitation method (NCPprec) is rather difficult, since they are present in the aggregated state rather than in the form of individual molecules in solution. To estimate the molecular weight of NCPprec, an indirect method is used, which is based on the determination of the aggregation rate of the ICs formed as a result of the interaction between an NCP and polypropylene glycol with a fixed molecular weight (MW), in this case PPG 1000. The comparison of the aggregation rates of the inclusion between NCPexcb (which were synthesized using PPGs with different molecular weights) and PPG 1000 with the aggregation rate of the inclusion complex on the basis of NCPprec provided the estimation for the MWs of single polymer chains. The fact that the samples of NCPprec contain ∼30% of the monomeric β-CD was taken into account when constructing the calibration curve. It was demonstrated that the MW of the polypropylene glycol corresponding to NCPprec is 1320 Da. Consequently, ∼11–12 molecules of β-CD are included in the single chains of NCPprec.