Synthesis of hyperbranched polymers with controlled degree of branching

Abstract

Hyperbranched polymers (HBPs) have attracted considerable attention during the past decade because of their intrinsic globular structures and unique properties, such as low viscosity, high solubility and a high degree of functionality, compared with linear analogs. As HBPs are generally prepared by one-pot polycondensation of an ABx monomer, where A and B represent two different functional groups, they are regarded as less-expensive alternatives to dendrimers. However, HBPs have randomly branched structures and their degree of branching (DB) is determined by statistics and only reaches ∼50%. Recently, several research groups have successfully synthesized a 100% DB without any defect mainly by condensation or addition reactions. These methods are based on a new concept in which the ratio of the rate constants, the first reaction rate constant (k1)/the second reaction rate constant (k2) of an AB2 monomer, can be controlled. Recent development in the synthesis of a linear polymer, that is, a 0% DB HBP, from an AB2 monomer could make it possible to further precisely control the DB from 0 to 100% by using the same AB2 monomer. In this review article, the backgrounds and methodologies for controlling the DB are described, especially focusing on recent advances in the synthesis of HBPs with a DB of 100, 0% and any percentage between 0 and 100%. This paper reviews the synthetic strategies for HBPs with controlled DB by specially designing monomer architectures. Several research groups including our group have been overcoming the statistical determination of DB in a random polymerization by tuning the rate constant of the first substitution reactions (k1) and that of the second reaction (k2) of an AB2 monomers. As a result, HBPs with 100% DB, linear polymers (0% DB) and HBPs with any percentage of DB have been successfully synthesized.