Degree of branching in hyperbranched polymers. 2. Enhancement of the db: Scope and limitations

Abstract

The degree of branching (DB) as well as the ANB (average number of branches) of polymers obtained from polycondensation of AB₂ and AB_m-type monomers in general is analyzed in detail on the basis of kinetic considerations. Various possibilities for the enhancement of the DB of hyperbranched polymers beyond the limit for a random AB_m polycondensation (0.5 for AB₂, 0.44 for AB₃) are considered, i.e., (i) enhanced reactivity of linear vs. terminal units; (ii) polymerization of prefabricated, perfect dendritic units and (iii) the slow addition technique. For AB₂ monomers, the formation of terminal (T), linear (L) and perfect dendritic (D) units in the course of the random one-pot polymerization is calculated. Furthermore, the evolution of the DB and ANB with conversion is calculated for AB_m monomers. It is shown that the definition of the DB for AB₂ systems (DB = 2D/(2D+L)) derived in the first paper on the DB (Acta Polym. 1997, 48, 30) in the case of a random one-pot polycondensation exhibits the same conversion dependence as the branching parameter α = 1/2P_A, defined by Flory for AB₂ polycondensations. The dilution principle is introduced, which is based on slow addition of AB_m monomers to a core molecule B_f or an AB_m-type hyperbranched molecule with an arbitrary DB. It is shown that the maximum DB value obtainable from such a dilution/slow addition process is 0.67 for AB₂ systems. Thus, the DB is considerably higher than in the case of a random one-step polymerization (0.5). The DB for AB_m-type hyperbranched polymers prepared according to the dilution principle has been calculated to be m/(2m–1).