Experimental and Theoretical Study of the Acidic Degradation of Poly(2,2′-dioxy-1,1′-biphenyl)phosphazene

Poly (2,2'-dioxy-1,1'-biphenyl)phosphazene (I) is a weak base that can be reversibly protonated with HBF4 in acetone. Acidic degradation of poly(2,2'-dioxy-1,1'-biphenyl)phosphazene was experimentally performed using HBF4, HCl, and H2SO4 yielding the same polymer I with lower molecular weights. The reaction takes place through protonation of some of the skeletal bonds thus producing a protonated P+-NH bond, and posterior rupture of those bonds by reaction with water. The hydrolysis breaks only these protonated skeletal bonds, leaving unaffected the regular P-N bonds. The degree of hydrolysis is controlled by the amount of time during which the system was treated with the acid. Molecular dynamics simulations performed on systems containing protonated and unprotonated oligomers together with water and HCl molecules proved that Cl- ions and water molecules get close enough to protonated P+-NH bonds as to produce a hydrolytical rupture, however, they do not get into such a proximity to the regular P-N bonds. The products of hydrolysis were characterized by (SEC-MALS) (i.e., size exclusion chromatography coupled with multiangle light scattering) and fluorescence. The results indicated that the parental polymer and all the degraded samples have the same chemical microstructure despite their very different molecular weight, i.e., they exhibit the same molecular calibration curve, unperturbed dimension, scaling law parameters, fluorescence emission, lack of cross-linking etc. It seems that the acidic degradation can be controlled to produce homogeneous samples in a much easier way than the thermal degradation used before.