It is preferred that the difference

It is preferred that the difference in electronegative value between the phosphorus atom in Part A of the catalyst and the M element is as large as possible within the preferred range, giving improved catalytic activity when this value is larger. The presence of the anionic Lewis acid based part in the catalyst improves the catalytic activity of the catalyst in the polymerisation reaction of organosiloxanes. It also tends to make the catalyst more stable in itself thus preventing it from cyclisation and from polymerisation with other phosphonitrile compounds, even at higher temperatures.