Wouldn't it be easier, cleaner and more efficient just to burn it to make heat and electricity? Polypropylene is chains of C3H6. Polyethylene is chains of C2H4. Plastic already is fuel.
Ssshhh! You just wrecked the "research". As you say, they already are a good fuel as solids.
One big problem in using recycled plastics as fuels, though, is the lack of will or ability to separate out the types. One piece of a chlorinated like PVC could damaged equipment and really mess things up.
I had a neighbor who used to slyly burn his rubbish in his wood stove. The greenish-yellow grey smoke would come rolling down his roof when the PVC went in. I suppose the phosgene was taking care of mosquitos, but now that they moved, it's nicer around here.