Chemistry of Polyester Polyol Synthesis

The polyester polyols are obtained by the polycondensation reactions between dicarboxylic acids (or derivatives such as esters or anhydrides) and diols (or polyols), or by the ring opening polymerisation of cyclic esters (lactones, cyclic carbonates). 

The polycondensation reaction between dicarboxylic acids and glycols is an equilibrium reaction, the equilibrium being shifted to the formation of polyester polyols by continuous elimination of water from the reaction system (reaction 8.1) [1-8, 12-27]. 

In order to generate terminal hydroxyl groups an excess of glycol is currently used. The reaction takes place in uncatalysed reaction conditions (self catalysis by the acidic carboxyl groups) but the best perfomances (low reaction time, low final acidity) are obtained in the presence of specific catalysts, such as p-toluene sulfonic acid, tin compounds (stannous octoate), antimony, titanium (tetrabutyltitanate), zinc (zinc acetate), manganese (manganese acetate) or lead compounds and more recently enzymic catalysts (lipases) [1, 25]. 

The direct polyesterification reaction of diacids with glycols is the most important industrial synthetic route to polyester polyols. The second most important synthetic route is the transesterification reaction between dimethyl esters of dicarboxylic or dibasic acids (dimethyl adipate, dimethyl terephthalate, dimethyl carbonate or even polyethylene terephthalate) and glycols (reaction 8.2) [1, 3-8]. 

Aliphatic polycarbonates (polyesters of carbonic acid) are obtained by the transesterification reaction of dialkyl carbonates (for example dimethyl carbonate) with aliphatic diols. A typical example is the polycondensation of dimethyl carbonate with 1,6 hexanediol (reaction 8.3) [4-8]  

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