The Weld-On site has a great deal of information on the proper application of PVC primer and cement: http://www.ipscorp.com/wo_html/basic.html
While it does not get into the information Richard supplies, an email to them mught get the actual chemical reaction. Some useful info is:
“To make consistently good joints the following points should be clearly understood.
1. The joining surfaces must be softened and made semifluid.
2. Sufficient cement must be applied to fill gap between pipe and fitting.
3. Assembly of pipe and fittings must be made while the surfaces are still wet and cement is still fluid.
4. Joint strength develops as the cement dries. In the tight part of the joint, the surfaces will tend to fuse together, in the loose part, the cement will bond to both surfaces.
These areas must be softened and penetrated
Penetration and softening can be achieved by the cement itself, by using a suitable primer or by the use of both primer and cement. For certain materials and in certain situations, it is necessary to use a primer. A suitable primer will usually penetrate and soften the surfaces more quickly than cement alone. Additionally, the use of a primer can provide a safety factor for the installer, for he can know under various temperature conditions when he has achieved sufficient softening. For example, in cold weather more time and additional applications may be required.”