{HEAD ~ The polymerizations} are catalyzed by {TAIL ~ selected transition metal compounds}, and sometimes other co catalysts 
are catalyzed by
{ HEAD ~ The polymerizations } are catalyzed by { TAIL ~ selected transition metal compounds } , and sometimes other co catalysts
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH