Its a thermal Diels-Alder reaction between two molecules of styrene, that eventually leads to a styryl radical. it is nicely exemplified in "The chemistry of radical polymerization" - by Moad and Solomon, pg 107-109.
You can browse those pages through Google books (http://books.google.com.au/books?id=EPA0NcrQIOIC&printsec=frontcover&dq=The+chemistry+of+radical+polymerization++By+Graeme+Moad,+David+Henry+Solomon&hl=en&ei=PNa5TtKfEObFmAWb1fGBCA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CC8Q6AEwAA#v=onepage&q&f=false)