Molecular Weight Distribution Of Polysiloxane By EROP Of Cyclic Siloxane:Simulation And Experiments Verification

Polysiloxane is the most commonly used silicone polymer,which could be polymerized by cyclic siloxane with catalysts like acid and base.These catalysts initiate reactants to perform equilibrium reaction,and thus such process is named as equilibrium ring-opening polymerization(EROP).As the name indicates,EROP consists of not only the main reaction,ring opening reaction,but also other side-reaction such as interchange and cyclization.So practically polysiloxane products contain a large amount of cyclic siloxane.Unfortunately,cyclic siloxanes are going to be listed as persistent organic pollutants(POPs)by Stockholm Convention,so it is essential to minimize their contents in polysiloxane products.To solve that problem by experiments is both time-consuming and costly;as a result,simulation was chosen to study this project.According to published papers,current simulation methods are unable to cope with the EROP problem,so a new method established on statistics was built and was tested by experiments.Homopolymerization is the most fundamental reaction in EROP,and naturally becomes the most suitable start of this research.By introduction of statistics method,a model describing EROP of siloxane was established.And this model was used to study the EROP of octamethylcyclotetrasiloxane(D4)and hexamethyldisiloxane(MM)catalyzed by acid.A series of simulations of different feeding ratio of D4 and MM ranging from 1:1 to 100:1 were conducted,whose results in equilibrium were given and compared with the results of experiments conducted at the same reaction conditions.Characterization of samples in experiments was carried out by gas chromatograph(GC)and gas chromatograph-mass spectrum(GC-MS).The comparison illustrated that our simulation results coincided with experiments very well,which verified our model.Further study indicated that the weight percentage of cyclic siloxane increases with feeding ratio at first,but it remains steady after reaching a range of 10%to 15%.In the meantime,the distribution of cyclic siloxane reaches an equilibrium state,which confirms that degree of polymerization of active center has little influence to the cyclization process.In contrast,the distribution of linear polysiloxane changes dramatically with different feeding ratio.Though its average molecular weight grows steady,its distribution becomes wider and the gap between average value and the designed value increases as well.Two reasons are responsible for this:the existence of cyclic siloxane and the tendency to become shorter chains caused by interchange process.Copolymerization of EROP is much more complicated than the homopolymerization process;as a result,related model should be based on the former developed model.But in order to cope with the sequence problem caused by copolymerization,this model relaxed the assumption that ignores the existence of triple cyclic and adopted a statistical method to describe sequence.The same simulations as proceeded in the former method were conducted by the new developed method,dynamic results of the simulation process were given.When compared with the former model,this model apparently underestimates the reactivity of the terminator,but could depict the distribution of cyclic siloxane more accurately.Subsequently this model was used to simulate the copolymerization of D4 and tetramethyltetravinylcyclotetrasiloxane(D4Vi).The distribution of the copolymer with the change of degree of polymerization and the number of vinyl was then given in a three-dimensional figure.After extension of the studied objects into silicone rubber,significant improvement of the distribution of the vinyl group in silicone rubber was quantitatively proved with increasing amount of D4Vi in reactants.The curing properties involved were therefore explained together with the mechanical properties of silicone rubber.