What is a prepolymer adhesive?
From Gene Wengert, Sawing and Drying Forum technical advisor:
Hot melt PUR is a hot-melt adhesive. When first applied hot, it is comprised of short, slippery molecules. With moisture, it reacts to form long chains, or polymers, that become non-slippery when they cool. These new polymers involve the chemicals within the wood as well, so you get both physical bonding due to the shape of the wood surfaces (like a jigsaw puzzle pieces fitting together) and chemical bonding.
Incidentally, the theory is, in part, that the smaller molecules (pre-polymerized) can penetrate into the wood better than a larger molecule. Also, the PUR adhesives cure over a period of several days as they react with moisture in the air and wood to polymerize and attach to the wood chemically.