Dielectric relaxation is used to verify the bidirectional growth of arms in poly(glycidyl phenyl ether) (PGPE) synthesized by ring-opening polymerization with ethylene glycol and a phosphazene base, t-BuP4, in the presence and absence of triisobutylaluminum (i-Bu3Al). The dielectric relaxation is highly sensitive to changes in the dipolar moment orientation along the chain contour, allowing us to precisely identify whether the polymer is composed by "one arm" or "two arms" as well as to verify the arm symmetry. By means of broadband dielectric spectroscopy (BDS), we evaluated the dipolar microstructure of synthesized polymers and observed that in the absence of i-Bu3Al the polymers are certainly composed by two symmetric arms, whereas those obtained in the presence of i-Bu3Al are contaminated with large amounts of "one-arm" byproducts, in agreement with the data obtained by MALDI-ToF MS. Furthermore, BDS allowed us to quantify the relative amounts of one-arm impurities in two-arm PGPE samples, an analysis that cannot be performed by either MALDI-ToF MS or NMR.