Study On The Anionic Polymerization Of Acrylonitrile Initiated By Lithium Dialkyl Amides

Polyacrylonitrile(PAN)has been widely used not only in textile industry but also as an important precursor to produce carbon fibers.PAN-based carbon fibers are superior composite materials in view of their high strength and modulus,light weight as well as low cost.It is known that the properties of PAN-based carbon fibers are improved with the increase in the molecular weight of PAN.PAN with high molecular weight(higher than 5 x 105 g/mol)is essential to produce high performance carbon fibers.Anionic polymerization of acrylonitrile is an important method for preparing high molecular weight PAN.Anionic polymerization takes some advantages such as clean polymerization system,high yield,and getting monodisperse polymers over radical polymerization.Therefore,this study aimed to explore the application of lithium dialkyl amides,which have properties including large steric hindrance,low nucleophilicity and strong alkalinity,on anionic polymerization of acrylonitrile to prepare PAN with high molecular weight.Firstly,lithium diisopropylamide was used as an example to explore the effects of experimental conditions on polymerization.Under optimized experimental conditions(T=-60?,nAN/nInitiator=6080),PAN with weight-average molecular weight ranging from 1.02 x 106 to 1.23× 106 g/mol and molecular weight distribution(MWD)ranging from 1.92 to 2.18 could be prepared utilizing lithium dialkyl amides derived from diethylamine,hexamethyldisilazane,dicyclohexylamine,and 2,2,6,6-tetramethylpiperidine as initiators.Next,the effects of different kinds of ligands on the anionic polymerization of acrylonitrile initiated by lithium dialkyl amides has been studied.The experimental results showed that,after the addition of lithium salts,the polymerization rate was effectively slowed down and the polymerization could proceed at moderate temperatures.For example,with lithium perchlorate as a ligand,PAN with weight-average molecular weight as high as 6.21 x 105 g/mol(MWD=1.68)could be prepared at-20?,and PAN with weight-average molecular weight of 5.85 x 105 g/mol(MWD=1.79)could be prepared at-20?.A flow microreactor system based on simple T-type mixers has been applied to the anionic polymerization of acrylonitrile initiated by lithium dialkyl amides for the first time in this study.A better control over the MWD(1.49?1.93)of obtained PAN has achieved compared with that in conventional lab-scale reactors.PAN with weight-average molecular weight as high as 8.32 x 105 g/mol(MWD=1.68)could be prepared at-40?.Even at 0?,PAN with weight-average molecular weight as high as 6.14 x 105 g/mol(MWD=1.93)could be prepared.Furthermore,the kinetic study of polymerization was conducted in the flow microreactor system.The experimental results showed that the apparent reaction order with respect to monomer concentration was 1 and to initiator concentration was 2,the apparent activation energy of the polymerization was 18.3 kJ/mol.