Polymer modified asphalts are actually done "upstream" in the industry, typically by the liquid asphalt supplier.  The process typically takes polymer pieces of certain molecular structures (i.e. helical) and blends them into an asphalt using a mill.  However, the polymer doesn't bind to the asphalt without a reactant, and the reactant specific to completing the bonding in this situation is sulphur.  There are two different *modern* processes for injecting sulphur into the system. First being using the polymer mill to push the product to a "reaction" tank, and using another "mixing" tank to wet/mix the sulphur.  Once the sulphur is blended in the mix tank, the product is pumped into the reaction tank, where the binding process can be completed 

The other process, is inline, and this is the most modern system, developed by an American company by the name of Dalworth. Basically same concept as the reaction tank and the polymer mill, except the mix tank is a small tank on the polymer skid system. Sulphur is loaded by an automated auger into a small wetting tank supplied by an automated asphalt pump and the sulphur/asphalt mix is blended through the polymer mill and into a reaction tank.

After this though, you're left with a product called "Sty". This isn't usable on any road yet.  This sty is then blended with up to 5 (can't tell you what unfortunately, as I would be giving away my companies means of money making) different products, the main being a performance graded asphalt and other additives.