to ppv does in fact liberate acid as one of its by products in thin film copolymer samples converted by heating in vacuo the acid is too rapidly removed to be effective in driving the conversion of the precursor to 2 5 dimethoxy para phenylene vinylene to completion in thick film samples prepared by static solution casting however the extent of conversion of the methoxy precursors is significantly enhanced this is clearly evidenced in their colour they are unfortunately too thick for optical absorption measurements which unlike the uniformly yellow thin film samples becomes increasingly red as the content of the precursor to 2 5 dimethoxy para phenylene vinylene in the copolymers increases it is also evidenced by the decrease of the strength during conversion of the characteristic c o stretch vibration in the infrared spectra that is associated with the methoxy modifier group on the benzylic carbon of the methoxy precursors to 2 5 dimethoxy para phenylene vinylene and para phenylene vinylene this behaviour can be understood as being due to the lower rate of loss of acid from the bulk of thick films allowing greater interaction with the units of the methoxy precursors and consequently a greater extent of their conversion further evidence supporting these differences between the thin spin coated films.