1). Apart the polymerization, cyclopentadiene also forms cyclopentadienol and cyclopentadienone by allylic oxidation in air, which are further dimerized, trimerized, polymerized, etc., in situ. Cyclopentadiene’s allylic oxidation is slow in air but quite enough to decrease the yield of your reaction.
So, you work in inert atmosphere (N2 or Ar) and you add BHT at 0.5-1% w/w, at least. Note that BHT is more effective antioxidant than the BHA that you have used.
2). Cycloaddition reactions are sensible to steric hindrance. So, don’t expect such high yields as in the cited publication that starts from secondary halide moieties because you have tertiary bromide moieties, as drawn in your post.