We report a high yielding AA-BB type step-growth synthesis of poly(in-phenylene oxides) (mPOs)by nucleophilic aromatic substitution (SNAr) reactions. Previously, poly(mPOs) have proven difficult to synthesize, especially by S(N)A(I), due to competing cyclooligomer (oxacalixarene) formation. We have found that the linear (polymeric) vs cyclic (oligomeric) product distribution is strongly dependent on the substitution pattern of the electrophilic monomer, and it model is described that predicts whether the reacting monomers will kinetically Favor formation of poly(mPOs) or oxacalixarenes. The polycondensation reaction tolerates a range of functional groups on both reactive monomers, and the synthesized poly(mPOs) have high thermal and aqueous stability but are readily depolymerized in polar aprotic media.