Study on the Polymerization of Isobutylene by Half-Sandwich Scandium Complexes

Author:Yang Zuo

Supervisor:shen kai hua li yang


Degree Year:2019





In recent decades,high molecular weight polyisobutylene has served as an important synthetic rubber in biomedical materials,car tires and military,because of its excellent impermeability,high hysteresis,biocompatibility,chemical resistance,thermal and oxidative stability.However,for synthesis of high molecular weight polyisobutylene,there always are many serious issues,such as low polymerization temperature(Ca.-100℃),toxic solution(Ca.chloromethane),and few functional comonomers,which severely restrict the material performance of polyisobutylene.Therefore,the study on synthesis of high molecular weight polyisobutylene under mild conditions has significant scientific research value for optimizing isobutylene polymerization process and controlling the chemical composition and microstructure of polymer chains.Against this research background,in this paper half-sandwich rare earth metalmetal complexes were first used as an initiator system of isobutylene(IB)polymerization to synthesize high molecular weight(Mn)polyisobutylene products under mild conditions,including homopolyisobutylene(PIB),isobutylene/isoprene copolymers(IB-IP copolymers),random copolymers of IB with α-methylstyrene(α-MeSt)or α-MeSt derivatives and comb copolymers of IB with syndiotactic polystyrene(IB-sPS copolymers).The main study results are summarized as following:(1)A series of half-sandwich rare earth metal complexes bearing different alkyl ligands,Lewis base ligands and rare earth metal atoms,were synthesized to study the effects of complex structure on IB polymerization.Under C5Me4SiMe3Sc(CH2SiMe3)2THF(Sc1)/[Ph3C][B(C6F5)4](B1)initiator system,high Mn PIB was synthesized in toluene at-35 ℃,and Sc1/B1 initiator activity was relatively high(103 kgpolymer molsc-1·h-1).With increasing IB/Sc ratio,Mn of PIB rose linearly(up to 22×104 g/mol).Besides,with increasing B/Sc ratio,Sc1/B1 initiator activity rose dramatically,while,Mn of PIB first rose then fell and reached maximum when B/Sc ratio was 20/20(μmol/μmol).Moreover,as polar solution adding,IB conversion rose dramatically,but Mn of PIB obviously decreased.Furthermore,as temperature warming,during IB polymerization undesirable side reactions drastically reduced Mn of PIB,and according to Arrhenius equation,apparent activation energy of IB polymerization by Sc1/B1 initiator system was calculated,-14 kJ/mol.In addition,with extending reaction time,both polymerization yield and Mn of PIB rose gradually,and based on the kinetic rate law for a living polymerization,the polymerization follow first-order kinetics with respect to monomer,and the apparent rate constant for propagation(kapp)was 0.02 min-1.Finally,a conceivable mechanism of isobutylene carbocationic polymerization was proposed under Sc1/B1 initiator system,including metal cations initiation,carbocation propagation and β-H elimination termination.(2)Under Sc1/B1 initiator system at-35 ℃,copolymerizations of IB and IP were straightforward carried out to obtain a series of IB-IP copolymers.During the copolymerization with IP monomer ratio increasing,IP structure unit contents in copolymers rose gradually(IP=0-5.8 mol%),and based on Fineman-Ross equation,reactivity ratio for IB and IP were calculated respectively,rIB 11.23 and rIP=0.19.Besides,under the same polymerization conditions,through adding a small amount of IB into in advance,the copolymerizations of IB and a-MeSt were successfully carried out to obtain a series of random IB-a-MeSt copolymers.Within the same polymerization process,copolymerizations of IB and two alkenyl functionalized a-MeSt derivatives respectively,4-(2’-methyl-2’propenyl)-a-methylstyrene(FStl)and 4-(2’-propenyl)-a-methylstyrene(FSt2)produced a new family of high Mn random copolymers with pendant alkenyl groups(Mn:12.8-14.6×104 g/mol,C=C:1-20 mol%).Furthermore,during the copolymerization of IB with FSt2,1,4-diisopropenylbenzene(FSt3=1.0 mol%)was added as cross-linked agents to synthesize high Mn branched IB-FSt2 copolymers with pendant alkenyl groups(Mn=31.6×104 g/mol)under mild conditions.These high Mn copolymers with pendant alkenyl groups have relatively low Tg(-73 ℃~-47 ℃),which could serve as novel butyl rubber.(3)Under C5Me4SiMe3SC(CH2C6H4NMe2-o)2(Sc3)/[Ph3C][B(C6F5)4](B1)initiator system at 25 ℃,through the syndioselective copolymerizations of styrene(St)with five alkenyl functionalized styrenic monomers,a new family of versatile syndiotactic polystyrene materials with pendant alkenyl groups were obtained(C=C:5-20 mol%,Tm:220-250 ℃).These functional styrenic comonomers were divinylbenzene(FSt4),4-isopropenylstyrene(FSt5),4-(1’-isopropylvinyl)styrene(FSt6),4-(2’-propenyl)styrene(FSt7)and 4-(2’-methyl-2’-propenyl)styrene(FSt8).And under thiol-ene click chemistry and epoxidation,novel polar group functionalized sPS were synthesized efficiently,demonstrating that syndiotactic copolymers with pendant alkenyl groups can serve as reactive intermediates to produce various functional materials.Besides,under Scl/Bl initiator system at-35 ℃,the copolymerizations of IB and four alkenyl functionalized sPS copolymers as macromonomers were carried out to synthesize a series of crystalline comb IB-sPS copolymers.Especially,pendant isopropenyl groups in the functional sPS can completely react with IB during the copolymerization to form the comb IB-sPS copolymers.In comb copolymers IB unit composition is up to 37 mol%.Because of both crystalline sPS block and rubbery PIB block in the copolymer chains,these comb copolymers have high Tm(270 ℃)and low Tg(-67 ℃),meanwhile,stress and strain of copolymers can reach 8 MPa and 65%,respectively.Therefore,obtained crystalline IB-sPS comb copolymers could be novel thermoplastic materials in the future.