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Recent advances in catalytic chain transfer polymerization of isobutylene: a review

Tota Rajasekhar *, Gurmeet Singh, Gurpreet Singh Kapur* and S. S. V. Ramakumar
R&D Center, Indian Oil Corporation Limited, Sector-13, Faridabad-, Haryana, India. : ;

First published on 12th May

Abstract

This review presents the development of highly reactive polyisobutylene (HRPIB), a major commercial intermediate toward fuel and lubricant additives. Recent years have witnessed very substantial advances in the catalytic chain transfer polymerization (CCTP) of isobutylene/industrial Raffinate-1 (C4 Raffinate) to produce HRPIB, particularly in nonpolar solvents at elevated temperatures. The main subjects of this review are cationic polymerization of isobutylene, progress in HRPIB research and existing challenges, and recent advances of CCTP. New initiating/catalyst systems based on ionic liquids with Lewis acids are detailed, and this approach may open new views in the synthesis of HRPIB. Some current developments in CCTP of industrial Raffinate-1 and mechanistic studies are also described. This review strongly supports that the hydrocarbon soluble Lewis acid·ether (LA·ether) complex catalyzed CCTP will become the most popular technique for preparing HRPIB and could replace the traditional BF3 catalyzed industrial method.

Introduction

Dispersants are amphiphilic polymers possessing nonpolar, hydrocarbon chain and a polar moiety/chain at the chain end. Different types of dispersants are being typically used by the oil additive industry; for instance polyalphaolefins; polyisobutylene (PIB) and polypropylene, and acrylate-based polymers. PIB exhibits low ceiling temperature, 175 °C, which is an important characteristic of PIB that makes it well appropriate for the use of as ashless dispersants. Moreover, cationic polymerization is used to synthesize PIB component of dispersants. Polyisobutylene succinimide (PIBSI) dispersants are of the most common ashless dispersants used in engine oil, which have been initially developed by Le Suer and Stuart.3,4 The PIBSI dispersants are prepared in two steps. In the first step, PIB chain terminated with succinic anhydride (PIBSA) is generated mostly via Alder-ene reaction. Then, PIBSA is reacted with a polyamine to produce PIBSI dispersants (Scheme 1). In comparison to internal olefin ends (tri and tetra substituted) containing PIBs (conventional PIBs), PIB with terminal vinylidene functionality (exo group) is highly reactive toward maleic anhydride5–9 to give PIBSA. Because of the higher reactivity for post-polymerization reaction, exo group containing PIB is termed as highly reactive polyisobutylene (HRPIB).

Conventional PIBs have low reactivity for further post-polymerization functionalization due to steric crowding of the internal double bond.10–13 Earlier, the conventional PIBs required to chlorination followed by dehydrochlorination to react with maleic anhydride to give PIBSA, which was further reacted with oligo-alkylenimines to form PIBSI dispersants (Scheme 2).14–17 The main disadvantage of this method was that resulting dispersants contain residual chlorine as organic chlorides and also produces large quantities of chlorine containing waste water, which is the major environmental concern. Thus, HRPIB is the most preferred precursor for the preparation of ashless dispersants.

More than 750 000 metric tons of HRPIB, PIBs with terminal double bonds (exo bonds) are produced each year. Exxon, Amoco, BP, and BASF are globally major manufacturers of HRPIB. Dispersant properties are significantly affected by the molecular weight of polymer. Based on the molecular weight, PIB market size is divided into low molecular weight PIB (Mn < g mol−1), medium molecular weight PIB (Mn = 10 000–100 000 g mol−1) and high molecular weight PIB (Mn > 100 000 g mol−1). Viscosity is an important physical parameter to differentiate low, medium and high molecular weight HRPIBs. Viscosity of the low molecular weight HRPIB for Mn = g mol−1 is approximately 200 centistokes (cSt) at 100 °C, which is significantly lower than the viscosity of medium and high molecular weight HRPIBs. Amongst them, low molecular weight HRPIB is the most important class because of their demanding as lubricant and fuel additives. Low molecular weight HRPIB, – g mol−1 with polydispersity index of 2–3 are used as the additives.18,19

In the past 10 years, substantial advances in synthesis of commercially valued, HRPIB via different cationic polymerization approaches have been achieved.20–50 In particular, the catalytic chain transfer polymerization (CCTP) in nonpolar solvents using Lewis acid·ether complexes (LA·ether), has recently recognized much interest in the view of their industrial relevance.27–50 Because the recent CCTP embodies attractive features compared to the traditional BF3 catalyzed polymerization method,51,52 such as the simple experimental operation and cost, tolerance of ambient reaction conditions, and applicability of various kinds of reactor systems.19,53,54 To date, low molecular weight HRPIB is the most important industrial precursor material in the preparation of motor oil and fuel auxiliaries.

The endeavour of this perspective is clearly illustrated in Fig. 1 and Scheme 1, which presents importance and motivation on behind in the development of HRPIB. We will first briefly discuss the existing synthetic methods of engine oil/lubricant dispersant precursor and challenges. Subsequently, we will also review the basics of isobutylene cationic polymerization, and conventional & living polymerization of isobutylene approaches towards in development of HRPIB. This review finally covers the recent progress in CCTP of isobutylene/industrial Raffinate-1 feed for HRPIB synthesis and shows advantages of this method in comparison to conventional polymerization method. Furthermore, fundamental mechanistic aspects of chain transfer agent involved cationic polymerization will also be focused. Development of new initiating system and cost-effective processes for the production of HRPIB is currently an active area of research in academics and industry. An interested reader can consult previous reports on various developments in cationic polymerization of isobutylene.55–60

Synthesis of engine oil/lubricant dispersant precursor and challenges

The internal double bonds containing PIB preparation with low molecular weight has been well known as conventional process/Exxon process. In this process, isobutylene polymerization in the presence of cationogen and Lewis acid, AlCl3 or EtAlCl2 as coinitiator produces the low molecular weight conventional PIB.61–64 Importantly, the reaction mechanism of conventional PIB preparation is associated with a complex set of isomerisation and chain scission reactions. Faust et al., clearly demonstrated that the formation of ‘‘conventional’’ PIBs with internal tri- and tetra-substituted olefinic groups (Schemes 3 and 4) through complex mechanism of the carbocationic rearrangements.65 Here, the internal olefins formation in the polymerization of IB has been understood by ionization of PIB-Cl to PIB+ using EtAlCl2 in the presence of a proton trap.

A possible mechanism has been proposed by Faust et al., to explain the olefin structures. The mechanism involving a sterically hindered cation arising via hydride and methyde shifts from the chain growing PIB+. Tri-substituted olefins form through a distant hydride shift by backbiting followed by a methyde shift and chain scission of the PIB+. Moreover, the tri-substituted olefins possess of irregular carbon numbers, which clearly supports the chain scission pathway in the mechanism (Scheme 3). But the exo- and endo olefins are formed by simple β-proton elimination from the PIB+.

The formation of exo-, endo-, and tetra-substituted olefins was found accidentally, which form under milder conditions. The ionization of PIB-Cl at room temperature in the absence of a proton trap resulted to the formation of these three olefins. As shown in Scheme 4, PIB+ undergoes to hydride and methyde shifts followed by proton elimination, which results to tetra-substituted olefins.65

The external double bond containing PIBs (HRPIB) readily react with maleic anhydride,5 which clearly indicates that chlorination followed by dehydrochlorination is not essential to prepare PIBSA from HRPIB. Importantly, HRPIB route is environmental friendly one for the manufacture of ashless dispersants. Cationic polymerization of IB initiated by BF3 complex with either alcohol or ethers as catalyst system is the traditional process, to prepare commercial low molecular weight HRPIBs with more than 80 mol% of exo-olefin content. The process was first developed by BASF and is more commonly known as BASF process.66,67 Major drawback of this process is the critical low polymerization temperature, because of the requirement of low temperatures and highly purified feed making the process more expensive. Another disadvantage of this process is that usage of BF3 catalyst system produces fluorine based byproducts (Scheme 5). These fluorine based products, under the thermal stress, get converted to highly corrosive hydrogen fluoride. Moreover, in this process the fluorine content in HRPIB is noticed up to 200 ppm.54 Furthermore, researchers have been trying different ways to produce HRPIB with the aim to replace BF3 catalyst used in the traditional process.

In the recent years, significant contributions have been made by industries and academic-industrial collaborations for developing new catalyst systems and improvising process for producing HRPIB.20–54 Before going to discuss about the insights in development of HRPIB research, a summary is given below on historical developments in cationic polymerization of isobutylene. These past findings have been actually shown the route map to commercial valued HRPIB developments.

Cationic polymerization of isobutylene

Interestingly, overall activation energy of the isobutylene polymerization is negative, and increase in temperature causes decrease in the rate of propagation of polymerization.34,74 The activation energy can be expressed by the following equation.

The above equation is a fundamental statement for activation energy of polymerization. In the case of chain transfer polymerization, activation energy of termination could be replaced by activation energy of chain transfer.71 In the above equation, the sum of activation energies of initiation and propagation is always smaller than activation energy of chain transfer. Increase in polymerization temperature leads to shorter polymer chains, which is due to overcoming the energy barrier for the chain transfer reaction.34,76–81 This fundamental aspect helps in controlling the molecular weight of oil and lubricant additives during their synthesis. Moreover, chain transfer step plays a significant role in HRPIB synthesis,20–50 which could be discussed on detail in the following sections.

Cationic polymerization of isobutylene has a long story, the first report of the isobutylene polymerization by acid initiation at room temperature was revealed in , where the polymerization was restricted to formation of low molecular weight PIB oligomers. Since then, polymerization reactions of various conditions were examined,82–84 for which several types of catalyst systems have been developed to prepare high-molecular weight PIB.85,86 The discovery of butyl rubber/synthetic rubber in early s brings cationic polymerization into light. In the initial reports recognized that low temperatures and AlCl3 as catalyst have been required to obtain high molecular weight PIBs. Even though achieving of the molecular weight is more than 100 000 g mol−1, the polymer was not used for practical end use application. Later, the issue was overcome by copolymerization of isobutylene with isoprene. Here, the unsaturation unit of isoprene could be used for vulcanization of copolymer, butyl rubber in the formulation process. The discovery of butyl rubber led to in detail understanding of cationic polymerization.87 Control in cationic polymerization was lagging because of thermodynamically favoured chain transfer reaction. It was well known that chain transfer could be suppressed by lower temperatures, which favours higher molecular weight polymer. Based on these Kennedy et al., the conditions for the “quasiliving” polymerization of isobutylene have been realized.88 Moreover, the polymerizations having controlled initiation and the absence of termination or chain transfer during the propagation. These understandings gave direction for the controlled synthesis of low molecular weight PIBs having distinct end groups with predictable molecular weight. PIB of low molecular weights are valuable viscosity modifiers, fuel and lubricating oil additives.89–91 Relevant for this review are the currently practiced “conventional” and “living” cationic polymerizations of isobutylene as well as end group functionalities derived there from. The present review focused on chain end olefinic functionality of low molecular weight (< g mol−1) isobutylene-based polymers.

If you want to learn more, please visit our website PIBSI Dispersant.

Development of HRPIB synthesis

Recent developments in CCTP of isobutylene for HRPIB

Conclusion

Conflicts of interest

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