Tailored rigid-flexible block copolymers. 5. Synthesis and amphiphilic behavior of diblocks of poly(p-benzamide) and poly(ethylene glycol)

Tailored diblock copolymers of rigid poly(p-benzamide) (PBA) and flexible poly(ethylene glycol) (PEG) were prepared using two different polymerization schemes: (i) the one-pot two-step scheme of the Yamazaki reaction including benzoyl-terminated PBA and a PEG methyl ether modified by p-aminobenzoylation; (ii) the polymerization of p-aminobenzoic acid in the presence of the above NH₂-terminated PEG. The former method yields a copolymer coexisting with a large amount of PBA homopolymer. The latter method yields a copolymer with a reduced and controlled amount of unblocked PBA. The fraction of flexible segments in the copolymers varied between ca. 0.60 and ca. 0.40. In solutions of N,N-dimethylacetamide/LiCl, the solubility of the flexible homopolymer is larger at low LiCl content when the solubility of rigid PBA is, instead, negligeably small. However, large quantities of the latter are solubilized at low LiCl content, provided the copolymer is also present. The order of increasing solubility of the two components is reversed at high LiCl content. Micellar-type organizations of the amphiphilic copolymer with the rigid homopolymer are postulated. A lyotropic mesophase appears at a copolymer volume fraction v_pⁱ ca. 0.06 and it is fully formed at v_p^a ca. 0.08. The evolution of the structurization from diluted solutions to the mesophase and to the solid state is briefly considered.