A polymer is made up of many monomers; chemists define monomers as molecules that can react together with other monomers to form a larger polymer chain or 3-dimensional network. A simple model of this process called polymerization is that the monomers are like spheres with little hands sticking out of them; this allows them to flow, like a liquid, but under the influence of an energy source the hands from one bind together to grasp the hands of the others in the neighborhood, forming a solid mass.
A photopolymer uses the energy of light to stick the monomers together. An application of this principle that may be familiar to the reader is the 21st century method of filling tooth cavities, using a liquid composed of monomers to fill in the gap, than zapping it with ultraviolet photons to coerce the monomers to connect to each other, making a solid mass that perfectly conforms to the toothy abcess.
This technology as applied to holography changes the light sensitive layer into areas of hardened polymers in the bright fringe zones, drawing monomers from the dim fringe areas into those regions also, so the holographic interference pattern is represented by areas of 2 different refractive indices, which shape the replay light into the recorded object's replay wavefront.
The other ingredients to a holographic photopolymer are of course the base that it is coated on, generally plastic, with the layer being composed of ligannds, which holds the monomers in place, and a photo-initiator, which absorbs the light and transfers its energy to the monomers to get them to do their hooking up thing.
Here is a collection of instruction sheets and published papers from the major manufacturers of photopolymeric holographic recording materials. A couple of them were major players in the photography industry, like DuPont and Polaroid; Bayer is a giant in the medicine industry, havng given us aspirin and heroin; and Polygrama/Lynx/Zilux, simply coat for the holographers!
Click on the following links for your favorite brand! (All will be active sooner or later!)
Polaroid
Polygrama/Lynx/Zilux
Here is a Slide Show survey of all sorts of holographic recording materials' packages, from Agfa to Ultimate!