Kinetic and Mechanistic Studies of the Polymerization of Isobutylene Catalyzed by EtAlCl₂/Bis(2-chloroethyl) Ether Complex in Hexanes

Abstract

The kinetics and mechanism of the polymerization of isobutylene (IB) initiated by tert-butyl chloride and catalyzed by ethylaluminum dichloride (EADC)•bis(2-chloroethyl) ether (CEE) complex was studied in hexanes at 0°C. The polymerization is first order in [IB] and in dry hexanes the slopes of the first order plots are independent of the [CEE]/ [EADC] ratio in the range 1−1.5. From the slopes the steady state concentration of propagating cations [M n + ] = 1.8 × 10 −11 M was calculated, which suggests an equilibrium between dormant and active species. The olefin distribution is independent of IB concentration and conversion. At low conversion, the number average molecular weight (M n ) is proportional to the starting IB concentration. In hexanes saturated with water, the polymerization rate is 5 times higher at [CEE]/[EADC] = 1 compared to that in dry hexanes. However, at [CEE]/[EADC] = 1.5, the rates in dry or wet hexanes are identical. To account for the low concentration of active centers determined from the first order plots, an equilibrium between oxonium (dormant) and carbenium (active) ions is proposed. The existence of oxonium ions was confirmed by 1 H NMR studies and by polymerizations initiated from preformed oxonium ions. While identical rates and olefin distributions were obtained using EADC solutions in hexanes or toluene to form the EADC·CEE complex at short reaction times, at long reaction times, the exoolefin content decreased when EADC solution in toluene was used. This was attributed to a slow tert-butylation of toluene yielding 95% para isomer and the concomitant formation of AlCl 3 .

Catalytic chain transfer polymerization of isobutylene: The role of nucleophilic impurities

“…Some major drawbacks for the new HR PIB process systems include low process temperatures and/or use of polar chlorinated solvents that present serious limitations for industrial application . Recently, we reported an ethylaluminum dichloride (EADC) · bis(2‐chloroethyl) ether (CEE) complex as a fully hydrocarbon soluble catalyst that in conjunction with tert ‐butyl chloride ( t ‐BuCl) initiator in hexanes at 0–20 °C gave HR PIB with quantitative yields and up to 92% exo‐olefin end‐functionality – . In these studies, polymerizations were carried out under high purity conditions, which may be difficult to maintain in industry.…”

“…As per our previous report, the polymerization obtained in wet hexanes at [CEE]/[EADC] = 1.5 is identical with that determined for dry hexanes . Here, we demonstrated that water is not a necessary element in the case of polymerization without impurities under the specific conditions described in the experimental part.…”

“…) for one nucleophilic impurity (acetone) in the presence of EADC · CEE (1:1) complex and water at 0 °C. Upon addition of acetone to the complex, an upfield shift of the CEE protons (compared with EADC · CEE) and a downfield shift of the acetone protons are observed. This is due to formation of a complex between acetone and EADC.…”

“…Water content of hexanes before use in the polymerization of IB was measured using a Mettler Toledo KF coulometer DL39 . To adjust the water content in the polymerization, dry hexanes (0.3 mM water) was added to saturated hexanes (3.0 mM water) in proper amount and used in the polymerization system.…”

Micromixing enhanced synthesis of HRPIBs catalyzed by EADC/bis(2-chloroethyl)ether complex

“…More recently, some groups employed the soluble complex of ethylaluminum dichloride (EADC) and bis(2chloroethyl)ether (CEE) as catalyst, tert-butyl chloride (t-BuCl) as initiator, to overcome the limitations in the previous methods. [30][31][32][33][34] The HRPIBs with high content of exo-olen end groups (>80%), adjustable molecular weight and almost 100% conversion within 20 minutes could be attained in hexanes. However, the products exhibit broad molecular weight distribution (PDI) due to the poor performances of batch reactor in transfer and dynamic control.…”