SYNTHESIS OF POLYCARBOSILANE FROM LIQUID POLYSILANE WITH BENZOYL PEROXIDE AS AN INITIATOR

Abstract

The intermediate product of the pyrolysis process of polydimethylsilane(PDMS),liquid polysilane(LPS),is a mixture of low molecular weight silanes with different structure and reaction activity.The diversity of the reaction activity among the components leads to the different reaction degree of rearrangement and polycondensation,and it results in a low synthesis yield of polycarbosilanes(PCSs).In this work,PCSs were synthesized from LPS,which was obtained by thermal decomposition of PDMS,with an reaction additive——benzoyl peroxide(BPO) as the initiator.By using this method the yield of PCS from LPS has an increase of 20%～25%.The reaction process was traced by FTIR and GPC.The structure,composition and properties of PCS were investigated by EA,()~1H-NMR,()~(13)C-NMR,TG-DTA and XRD.The reaction mechanism of LPS with BPO was studied in detail.The results showed that,with raising the mass ratio of BPO to LPS and the reaction time,the weight average molecular weight(M_w) and softening point of PCSs increased.The Si—H bond content of PCSs increased with temperature in the range below 440℃ and decreased over 440℃,while the Si—CH_2 bond content of PCSs increased with temperature rising.The conversion process included initially the formation of PCS by thermal decomposition of LPS and also then the increase in molecular weight by condensation among the PCS molecules.And BPO can effectively catalyze the thermal decomposition and rearrangement of Si—Si bonds in LPS when heated,and also the formation of Si—CH_2—Si from Si radical and Si—CH_2 radicals.The molecular chain of PCS grows up,and the yield of PCS increases accordingly.Some phenyl and benzoyloxy groups are introduced into PCS molecules;it results in a little increase of carbon and oxygen contents of PCS.However the excessive BPO leads to excess crosslinking of PCS and makes the reaction uncontrollable.The proper softening point of the achieved PCS can be tailored by controlling the mass ration of BPO to LPS about 0.02,the reacting temperature(about 450℃) and reacting time(3～4 h),and a high synthesis yield of about 67% can be obtained,which is much higher than that of PCS synthesized by existed method.In addition,the carbon content of PCS increases by about 4%,and the oxygen content of PCS increases by about 0.4%.However they have little effect on the PCS's ceramic yield and pyrolysis conversion properties.When the softening points of PCS is in the range of 210～220℃,the ceramic yield is over 70%.It is proved by the TG-DTA and XRD analyses of the pyrolsis products of PCS.