Polyesters with terminal reactive groups are widely used in thermosetting coatings and articles.  To achieve high crosslinking density, low molecular weights are preferred for such polyesters because high molecular weights lead to insufficient reactive groups.  Low molecular polyesters are usually oily oligomers which might not possess desirable properties, e.g., glass transition temperatures, thermal stability, easiness of purification, etc.  These types of polyesters are not suitable for applications where both reasonably high molecular weights and high crosslinking density are required.  For example, when used in antireflective coatings in photolithography, polyesters are required to have relatively high molecular weights for high thermal stability.  In the meantime, high contents of reactive groups in the polyesters are needed to achieve high crosslinking density to prevent intermixing with photoresists. 
In general, for high crosslinking to occur in compositions containing polyesters, polyesters preferably have pendant reactive groups.  The number of pendant reactive groups in such a polyester can be easily adjusted to obtain desired crosslinking properties.  The number of pendant reactive groups is independent of the molecular weight of the polyester. 
Reactive groups in polyesters suitable for crosslinking chemistry are usually hydroxyl, carboxylic acid, etc.  However, polyesters are normally prepared by condensation polymerization using monomers bearing the same reactive groups.  This usually makes it very difficult to prepare high molecular weight polyesters with high contents of reactive groups.  For some applications, e.g., powder coating, both high molecular and high contents of reactive groups can be achieved by preparing branched polyesters which are partially crosslinked.  Those partially crosslinked polyesters usually have very limited solubility in organic solvents.  For applications where polyesters need to be dissolved in solvents, highly branched polyesters are usually not very useful. 
The present invention relates to a novel polyester preparation method that can lead to high molecular weight polyesters with high contents of reactive groups.  Polyesters prepared by this method can be linear and/or branched and/or partially crosslinked and have good solubility in common organic solvents used in the end product in which the polyester is used.