New polymer syntheses, 81 . Poly(3‐oxybenzoate) randomly branched with 3,5‐dihydroxybenzoic acid or 5‐hydroxyisophthalic acid

Randomly branched (hyperbranched) Poly(3‐hydroxybenzoate), poly(3‐Hybe), was prepared by polycondensation of silylated 3‐acetoxybenzoic acid with either silylated 3,5‐diacetoxybenzoic acid or bis(trimethylsilyl) 5‐acetoxyisophthalate. The number of branching points was varied by changing the feed ratio of difunctional and trifunctional monomers. 1H NMR and 13C NMR spectroscopy proved the nearly random incorporation of the trifunctional “branching units”. Cocondensations with small amounts of acetylated bisphenol‐P allowed one to control the degree of polymerization (DP) and to determine the DP by 1H NMR spectroscopy. However, analogous copolycondensations with silylated 2‐(4‐tert‐butylphenoxy)terephthalic acid failed. According to GPC measurements, weight‐average molecular weights above 105 were obtained. DSC measurements revealed that the glass transition temperatures (Tg's) vary largely with the degree of branching (DB) and with the nature of the end‐groups. In the case of phenolic OH and acetate end‐groups, the relationship Tg vs. number of branching points passes through a minimum.