DCC/AlCl_3体系引发异丁烯正离子聚合

研究以对-二枯基氯(DCC)/AlCl3体系引发异丁烯在CH2Cl2/Hex(40/60,V/V)混合溶剂中进行正离子聚合,探讨了DCC用量、给电子试剂,如三苯胺(TPA)、2,6-二甲基吡啶(DMPy)对异丁烯正离子聚合转化率、产物分子量及其分布的影响.结果表明,在无给电子试剂存在时,DCC和体系中微量水均可与AlCl3产生络合竞争引起相继的引发竞争,聚合产物GPC谱图呈双峰分布,分子量分布宽,需要大量的引发剂DCC(DCC/H2O=5.3)来减少体系中微量水的不可控引发;在少量上述给电子试剂存在下,可提高DCC的引发效率,减少向单体链转移反应,提高聚合产物的分子量和使分子量分布呈较窄的单峰分布,即使在较低DCC用量下也可基本抑制体系中微量水的不可控引发,达到DCC定量引发,并得到分子量分布相对较窄(Mw/Mn≈2.3)的聚异丁烯产物.

CATIONIC POLYMERIZATION OF ISOBUTYLENE WITH DCC/AlCl3 INITIATING SYSTEM

The cationic polymerization of isobutylene (IB) by utilizing difunctional initiator dicumyl chloride (DCC) in conjunction with coinitiator AlCl₃ was carried out in CH₂Cl₂/Hex (40/60,V/V) solvent mixtures in the absence or in the presence of 2,6-dimethylpyridine (DMPy) and triphenylamine (TPA) respectively.The effects of concentrations of DCC,electron donor and AlCl₃ on monomer conversion,molecular weight and molecular weight distribution (MWD) of the product polyisobutylene (PIB) were investigated systematically.The small amount of water in the polymerization system in conjunction with AlCl₃ induced a conventional uncontrolled rapid polymerization of IB and led to polyisobutylene with broad molecular weight distribution.It is found that the GPC trace of resultant polymer obtained by DCC/H2O/AlCl₃ system exhibit a bimodal distribution,which was attributed to the competition of initiation from two different active centers,i.e.,H⁺AlCl₃OH^- (A) and AlCl₄^-+C(CH₃)₂C₆H₄(CH₃)₂C^+-AlCl₄ (B).The initiation from active center (B) increased in the polymerization system with an increase in DCC concentration.A large dosage of DCC (DCC/H₂O=5.3) was needed to decrease the initiation from active center (A) effectively,and the polymer with low molecular weight (about 1.8×10⁴) was obtained.According to the characterization results of GPC,NMR and UV,both the initiation efficiency of DCC and molecular weight of polymer increased and MWD narrowed by addition of small amount of TPA or DMPy.The mechanistic role of TPA or DMPy was to decrease the uncontrolled initiation of proton from active center (A) and the chain transfer reaction to monomer.The monomer conversion decreased, but the initiation efficiently increased, and MWD narrowed with increasing of TPA or DMPy concentration.The initiation from active center (A) could be suppressed at high DCC concentration or even at low DCC concentration in the presence of TPA or DMPy.A desirable cationic polymerization of IB would be achieved with DCC/AlCl₃ initiation system in the presence of adequate amount of TPA or DMPy, and polyisobutylene with relatively high molecular weight and relatively narrow unimodal molecular weight distribution could be obtained.