Synthesis Of Star-Comb Hperbranched Polyisoprene

Star-branched polyisoprene and its copolymers are notable for their good aging and low permeability air,and cannot be substituted in the sealant and rubber industry because of their special structure.Moreover,star-branched isoprene polymers have been extensively studied for a long time due to their markedly lower solution,bulk viscosities and less shear sensitivity. At the same time,isoprene can only be polymerized with a low branching structure.Therefore, the combination of anionic polymerization and other methodology is a very important method to synthesize novel structure and performance polymers.At the first part of the article,star polyisoprene is synthesized with cyciohexane as solvent and n-butyllithium(n-BuLi) as initiator,then coupled with methyltrichlorosilane (CH₃Cl₃Si).The molecular weight,molecular weight distribution(polydispersity index) and microstructure are measured by SEC/TALLS,IR and NMR.The polymers polydispersity index(PDI) are lower than 1.3 with the 1,4 content above 93%.The star polymers are epoxidized by Formic acid-Hydrogen peroxide in-situ method with toluene as solvent,and epoxidation degree of their polymers are determined by ¹H-NMR.Effects of different epoxidation time,different epoxidation temperature and different H₂O₂ and HCOOH proportion on the reaction are investigated.The epoxidation degre of star-branched polyisoprene can be well controlled at the range of 20%with H₂O₂ and HCOOH to double bonds ratio of 0.5,reaction in 40 min and temperature at 40℃.The rules of coupling reactions for isoprene homopolymerization cyclohexane as solvent, n-BuLi as initiator,and epoxidized star isoprene polymers as coupling agent are discussed. The results show that:the amount of coupling agent have a great effect on the branching degree,and the coupling efficiency reach the maximum 64%when the mole ratio of epoxy group/living end is 1.Coupling reaction time and coupling reaction temperature can promote the coupling reaction,but coupling efficiency decrease for the higher coupling reaction temperature.Coupling reaction efficiency is optimum when coupling reaction temperature is 60℃,and coupling reaction time is 24h.Different epoxidation degrees of coupling agents have a limited effect on the coupling reaction.A star polyisoprene substrate with a high(95%) 1,4-content is first epoxidized to introduce grafting sites randomly along the chain.Coupling of the epoxidized polyisoprene with polyisoprenyllithium yielded a comb branched or generation G1 star comb polyisoprene. Further epoxidation and grafting of the G1 polyisoprene lead to star-comb hyperbranched polyisoprene of G2,G3 and G4.The synthesis of G1-G4 arborescent polyisoprenes is using the optimized reaction conditions.The graft polymers are puried by precipitation fractionation and characterized by size exclusion chromatography(SEC),light scattering,and NMR spectroscopy.The molecular weight distributions of products are at the range of 1.20-1.50, and display geometric increases in molecular weight and branching functionality for successive generations.The absolute weight-average molecular weight values determined from light scattering measurement are much higher than the apparent GPC values because of the highly branched structure of the molecules.T_g of the products has a little increased for G1-G4 polymers.