Elsevier

Polymer

Volume 51, Issue 25, 26 November 2010, Pages 5960-5969
Polymer

A cost-effective process for highly reactive polyisobutylenes via cationic polymerization coinitiated by AlCl3

https://doi.org/10.1016/j.polymer.2010.10.012Get rights and content

Abstract

The initiating system consisting of AlCl3 with dialkyl ether such as di-n-butyl ether or diisopropyl ether has been successfully developed for providing a cost-effective process of synthesis of highly reactive polyisobutylenes (HRPIBs) with large proportion of exo-olefin end groups up to 93 mol% at temperatures ranging from −20 to +20 °C. The above dialkyl ethers played very important roles in promoting the directly rapid β-proton elimination from –CH3 of the growing chain ends to create exo-olefin end groups and decreasing or even suppressing the carbenium ion rearrangements to form the double bond isomers. Very importantly, the highly reactive PIBs with 80–92 mol% of exo-olefin end groups, having low Mns of 1300–2300 g mol−1 and monomodal molecular weight distribution (Mw/Mn = 1.7–2.0) could be achieved at 0–20 °C. These results are comparable to those of commercial HRPIBs produced industrially by the best BF3-based initiating system at far below 0 °C.

Introduction

Polyisobutylenes (PIBs) are important materials with excellent properties, such as low gas permeability and high chemical stability, and have been applied in many fields. PIBs are commercially produced by two processes with aluminum trichloride (AlCl3) and boron trifluoride (BF3) coinitiating systems respectively. The conventional PIBs with low content (<10 mol%) of exo-olefin end groups are industrially produced using H2O as initiator and AlCl3 as coinitiator, which having low reactivity for further functionalization reactions [1], [2], [3], [4]. The PIBs with large proportion of exo-olefin end groups more than 60 mol%, preferably more than 75 mol%, are normally referred to as highly reactive PIBs (HRPIBs). HRPIBs with number-average molecular weight (Mn) of 500–5000 g mol−1 are of great interest due to their applications as intermediates for production of the additives to lubricants and fuels, such as surfactants or dispersants [5], [6], [7], [7](a), [7](b), [7](c). Large proportion of exo-olefin end groups is favored to produce the additives since they have high reactivity in chemically modification with maleic anhydride. Commercial low molecular weight HRPIBs (Mns = 800–2500 g mol−1; Mw/Mn ∼2.0) with more than 80 mol% of exo-olefin end groups can be only produced by a single-step process via cationic polymerization of isobutylene initiated by BF3-based system at temperatures far below 0 °C [7]. Recently, the initiating systems consisting of solvent-ligated [M(NCMe)6]2+ (MII = Mn, Cu) complexes with the bulky, non-coordinating counter ions such as [C3H3N2{B(C6F5)3}2] have been reported for preparing HRPIBs with Mns of 1400–7000 g mol−1 at ambient temperatures by a single-step process and the polymerization process induced by these kinds of initiating systems sometimes required long time to get high monomer conversions [8], [8](a), [8](b), [9]. However, these solvent-ligated manganese(II) and Copper(II) complexes are not commercially available and need to be synthesized by several reactions [8], [9]. And these complexes are kept at room temperature for short periods of time and need to be stored at −35 °C under argon atmosphere to prevent decomposition and oxidation over long storage periods [8], [9]. Another initiating system consisting of alkyl zinc chloride (e.g. EtZnCl) and alkyl halide (e.g. t-BuCl) was also reported to produce medium molecular weight HRPIBs with Mns of 10000–29000 g mol−1 and Mw/Mn of 1.9–2.5 at room temperature [10]. On the other hand, HRPIBs could also be prepared by specific termination reaction after completion of cationic polymerization or by modification from tert-chloro terminated functional PIB chains [11, [12], [13], [14].

This research thinking about the potentially practical utilization of our initiating systems in industry in the future has led us to explore the AlCl3-based system for providing a cost-effective and simple process of synthesis of highly reactive PIBs since AlCl3, of greatest economic significance, is commonly applied in industrial production of conventional PIBs. To the best of our knowledge, no polymerization reaction to prepare highly reactive PIBs with such a high content of exo-olefinic groups (∼90 mol%) by using AlCl3 as coinitiator at 0 °c or above has been described so far. In this paper, we will report our investigation on the cationic polymerization of isobutylene (IB) by the initiating system consisting of H2O, AlCl3 and dialkyl ethers such as dibutyl ether (OB2) or diisopropyl ether (OP2) to prepare highly reactive PIBs containing large content of exo-olefin end groups even up to 93 mol% by a single-step process. The effects of polymerization conditions on monomer conversion, number-average molecular weight (Mn), molecular weight distribution (MWD, Mw/Mn) and end group structures of the resulting PIBs were investigated.

Section snippets

Materials

Dichloromethane (CH2Cl2, analytical reagent, Beijing Yili Fine Chemical Co.) was purified as described previously [15], [15](a), [15](b), [15](c). Di-n-butyl ether (99%, OB2, Tianjin Fuchen Chemical Co.) and diisopropyl ether (99%, OP2, Tianjin Guangfu Chemical Co.) were distilled from CaH2 under nitrogen before use. Anhydrous aluminum trichloride (AlCl3, 99%, Acros; packaging under nitrogen), isobutylene (99.9%, IB, Beijing Yanshan Petroleum Chemical Co.) and ethanol (analytical reagent,

Results and discussion

The most important initiating system from a scientific as well as a practical point of view is the cationogen/Friedel–Crafts acid system. The need for a cationogen to bring about cationic polymerization was clarified by the discovery of cocatalysis, i.e., that most Friedel–Craft acids, particularly some halides of boron, titanium and tin, require an additional cation source to initiate polymerization (coinitiation mechanism). Some authors claimed that cationic polymerization and particularly

Conclusions

This initiating system of H2O/AlCl3/dialklyl ether was successfully developed for the cationic polymerization of isobutylene to prepare low molecular weight polyisobutylenes (Mn = 1300–2200 g mol−1; Mw/Mn = 1.7–2.0) with exo-olefin end groups (structure A) of more than 80 mol%, even up to 94 mol% at 0 °C or higher temperatures. These obtained highly reactive polyisobutylenes can be comparable to those produced industrially by the best BF3-based initiating system. The more weakly coordinating

Acknowledgments

The financial supports from the National Natural Science Foundation of China (Grant No. 20774008 and 20934001) are greatly appreciated.

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