A crop harvesting header has a cutter bed to cut crop material as the harvester advances through a field. Parasitic capacitances are inherently associated with each current input of the current-switching pairs. 5% by weight, based on 100% by weight of monomers, of at least one hydrocarbon HC selected from compounds having 6 to 20 carbon atoms which on abstraction of a hydrogen atom form a pentadienyl radical or a 1-phenylallyl radical and from \u03b1-methylstyrene dimer comprises supplying the monomers to be polymerized to the polymerization reaction over the course of 3 hours.