Non-Covalent Interactions in Block Copolymers Synthesized via Living Polymerization Techniques

Abstract

Non-covalent interactions including nucleobase hydrogen bonding and ionic aggregation were studied in block and end-functional polymers synthesized via living polymerization techniques such as nitroxide mediated polymerization and anionic polymerization. The influence of non-covalent association on the structure/property relationships of these materials were studied in terms of physical properties (tensile, DMA, rheology) as well as morphological studies (AFM, SAXS).

Hydrogen bonding, a dynamic interaction with intermediate enthalpies (10-40 kJ/mol) was introduced through complementary heterocyclic DNA nucleobases such as adenine, thymine and uracil. Hydrogen bonding uracil end-functionalized polystyrenes and poly(alkyl acrylate)s were synthesized via nitroxide mediated polymerization from novel uracil-functionalized alkoxyamine unimolecular initiators. Terminal functionalization of poly(alkyl acrylate)s with hydrogen bonding groups increased the melt viscosity, and as expected, the viscosity approached that of nonfunctional analogs as temperature increased.

A novel difunctional alkoxyamine, DEPN2, was synthesized and utilized as an efficient initiator in nitroxide-mediated controlled radical polymerization of triblock copolymers. Complementary hydrogen bonding triblock copolymers containing adenine (A) and thymine (T) nucleobase-functionalized outer blocks were synthesized. Hydrogen bonding interactions were observed for blends of the complementary nucleobase-functionalized block copolymers in terms of increased specific viscosity as well as higher scaling exponents for viscosity with solution concentration. Multiple hydrogen bonding interactions were utilized to attach nucleobase-functional quaternary phosphonium ionic guests to complementary adenine-functionalized triblock copolymers.

Ionic interactions, which possess stronger interaction energies than hydrogen bonds (~150 kJ/mol) were studied in the context of sulfonated poly(styrene-b-ethylene-co-propylene-b-styrene) block copolymers. Strong ionic interactions resulted in the development of a microphase separated physical network and greater extents for the rubbery plateau in DMA analysis compared to sulfonic acid groups, which exhibit weak hydrogen bonnding interactions.

In contrast to the physical networks consisting of sulfonated or hydrogen bonding block copolymers, covalent networks were synthesized using carbon-Michael addition chemistry of acetoacetate functionalized telechelic oligomers to diacrylate Michael acceptors. The thermomechanical properties of the networks based on poly(propylene glycol) oligomers were analyzed with respect to the molecular weight between crosslink points (Mc) and the critical molecular weight for entanglement (Me).