Preparation of Polyester Polyols
The desired product of polyester polyol synthesis is a polyester with high levels of terminal hydroxyl functionality. Achieving this goal is facilitated by applying the Carothers equation for determining the ratio of acidic and alcoholic monomers to get the desired molecular weight and hydroxyl functionality [49, 50]. In preparation of a simple linear polyester, this would take the form of
where n is determined by considering one unit of diol as a chain initiator and the monomer unit to be the condensation unit of (diacid+diol-2H20). Thus, the target Mn is calculated as
As an example, if the desired result is a 2000 molecular weight polyester polyol of polybutylene succinate, the MW is
r j j ? monomer
n=moles monomer/moles initiator=moles adipic acid/(moles BDO-moles adipic acid) and a molar ratio of diol to diacid of 1.09. In practice, this result should be used as a theoretical guide and not as strictly predictive. The final result will reflect details of the polymerization procedure and systematic errors resulting in loss or decomposition of monomer or polymer over time. Nuclear magnetic resonance and chromatography are commonly applied to determine molecular weight, and titration of acid end groups by well-known techniques is applied to determine the level of hydroxyl functionality.
FIGURE 2.23 Accepted mechanism for coordination-insertion ring opening polymerization of e-caprolactone to polycaprolactone.
Polyester polymerization can often be done without the need of a solvent since many of the monomers melt and phase mix prior to reaction. Polyester formation involves formation of a condensation product (for instance, water when using a diacid, methanol when employing a dimethyl ester) and the polyester products are in equilibrium with the reactants . For this reason, condensation water (for instance) must be removed as the ester is formed, or high-molecular-weight polymers cannot be formed. This is usually achieved by sweeping a nonreactive gas such as N2 over the polymer surface. As polymer forms, the molecular weight increases and the viscosity of the melt increases—linearly at first and to the 3.4 power above the entanglement molecular weight. The increase in melt viscosity can interfere with removal of the condensation product and slow the reaction down markedly. There is a temptation to increase the melt temperature to reduce viscosity, which in principle will facilitate removal of the water or alcohol. But increasing temperature may also facilitate the differential evaporation of the monomers. This will upset the calculation of monomer ratios required to obtain a specific molecular weight with terminal hydroxyl functionality and result in a stubborn lack of molecular weight advancement or sudden increases in molecular weight and the number of acid terminal groups far above that expected based on calculation. Such occurrences, as often occur in the laboratory in the absence of well-established procedures, will initiate the addition of diacid or diol in order to adjust molecular weight and/or reduce the prevalence of acid end groups.
The production of polyester polyols by ring-opening polymerization of lactones has also been explored extensively and many synthetic procedures and catalyst alternatives have been developed. One of the most common approaches employs a small polyol initiator (diol or higher) as an initiator and propagates chain extension through a coordination-insertion mechanism (Fig. 2.23) [52, 53]. The catalyst of choice for this approach is usually a tin or titanium alkoxide or other common catalyst used for polyester polymerization. The case where R is a polyol results in a polymer with three polymer arms and can be used to control the final molecular weight of the product.