Influence of random branching on multiple hydrogen bonding in poly(alkyl methacrylate)s

A series of linear and randomly branched poly(alkyl methacrylate)s with pendant 2-ureido4[1H]-pyrimidone (UPy) groups, which form quadruple hydrogen bonds, were synthesized, and the role of molecular topology on intermolecular hydrogen bonding was investigated. In solution rheological studies of poly(methyl methacrylate-co-UPy methacrylate) (PMMA-co-UPyMA) copolymers, a branched copolymer in the nonassociated state displayed a larger entanglement concentration (C _e) than a linear copolymer of equal molar mass, However, C _e of the branched copolymer approached the C _e of the linear analogue as the degree of hydrogen bond associations increased in solution, which suggested the reduced chain dimensions of the branched structure were overcome upon intermolecular association of the UPy groups. A series of linear and branched poly(2-ethylhexyl methacrylate-co-UPy methacrylate) (PEHMA-co-UPyMA) copolymers with 0-10 mol % UPy were utilized for melt rheological studies. A decrease in zero shear viscosity (η ₀) and relaxation time suggested that branching reduced entanglement couplings for the unfunctionalized PEHMA homopolymer, and the η ₀ and relaxation time of the branched and linear polymers approached each another as the UPy content was increased from 0 to 10 mol%. Furthermore, as the UPy content was increased in the copolymer, the plateau modulus (G _N ⁰) systematically increased, and the plateau region systematically broadened independent of the chain architecture. Thus, reversible hydrogen-bonding associations between UPy groups dominated the rheological behavior of linear and branched chains in both solution and the melt phase.