Cationic polymerization of isobutylene at room temperature


  • Sergei V. Kostjuk,

    1. Research Institute for Physical Chemical Problems of the Belarusian State University, 14 Leningradskaya St., 220030 Minsk, Belarus
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  • Hui Yee Yeong,

    1. Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany
    2. Organic Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden, Germany
    Current affiliation:
    1. BASF Coatings GmbH, Muenster, Germany
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  • Brigitte Voit

    Corresponding author
    1. Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany
    2. Organic Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden, Germany
    • Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany
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This review highlights recent approaches toward polyisobutylene (PIB) by an energy efficient room temperature cationic polymerization. Special focus is laid on our own work using modified Lewis acids and nitrile-ligated metal complexes associated with weakly coordinating anions. In both cases, suitable conditions have been found for efficient production of PIB characterized by medium to low molar masses and a high content of exo double bonds as end groups—the typical features of highly reactive PIB, an important commercial intermediate toward oil and gasoline additives. These and other approaches demonstrate that the cationic polymerization of isobutylene is still not fully explored, and new innovative catalyst systems can lead to surprising results of high commercial interest. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013


Cationic polymerization is an important technique to produce (co)polymers with predictable molecular weight and monomer sequence. The most important monomers polymerizing through a cationic mechanism are isobutylene (IB), styrene and its derivatives, vinyl ethers, cyclopentadiene, and N-vinylcarbazole.1, 2 Among these, IB is the most studied one as it polymerizes only by cationic mechanism3 and its polymer is the only organic polymer (in comparison with silicones or polyphosphazenes) to exhibit such unique properties as low permeability, good thermal and oxidative stability, chemical resistance, high hysteresis (mechanical dampening), and tack.4, 5 A very important commercial application arising from IB cationic polymerization is the manufacture of high- (Mn > 120,000 g mol−1), medium- (Mn = 40,000–100,000 g mol−1) and, especially, low-molecular-weight (Mn < 5,000 g mol−1) polyisobutylenes (PIB).5 Another important industrial application of cationic polymerization is the synthesis of high-molecular-weight IB-based elastomers such as butyl and halobutyl rubbers (worldwide production capacity is 760 thousand tons/year).5 A clear drawback of these polymerization processes is the necessity to work at cryogenic temperatures (down to −100 °C) to obtain desired high-molecular-weight polymers.

In the middle of 1980s, the controlled/living cationic polymerization technique was developed as a new promising tool for the synthesis of well-defined functionalized polymers, block copolymers (thermoplastic elastomers), star-like and dendritic polymers, and so on.2, 6, 7 Again, a great attention was paid to the polymerization of IB and its copolymerization with styrene derivatives to synthesize a family of brand-new elastomers (for example, poly(styrene)-poly(isobutylene)-poly(styrene) block copolymers)7 and biomaterials.8 Despite intensive academic investigations especially in the Akron group highlighted by Kennedy,9,10 there are only few examples of the commercialization of controlled cationic polymerization technique: (i) poly(vinyl ethers) production for use in liquid-crystal technology; (ii) poly(styrene)-poly(isobutylene)-poly(styrene) block copolymers (trade name SIBS) and functionalized PIB, which are marketed by Kaneka Inc. of Japan.5 One strong proof of the difficulty of carrying out such chemistry is the fact that these thermoplastic elastomers are produced in a much smaller scale compared to their analogues produced by anionic techniques e.g. poly(styrene)-b-poly(butadiene)-b-poly(styrene).

Clearly, the main drawbacks of the conventional/controlled cationic polymerization techniques are the necessity to use (i) low temperatures and (ii) strictly anhydrous conditions. Besides, for cationic polymerization of vinyl monomers the widely used reaction media are chlorinated solvents (methyl chloride and dichloromethane) or their mixtures with n-hexane because of their polarity, low price, and easy removal from the products.1 The biggest drawbacks of these chlorinated and volatile materials are their serious health and environment damaging effects. Last but not least, the current cationic polymerization processes typically utilities a large amount of Lewis acid coinitiators that leads to substantial quantities of waste.

In recent years, considerable efforts have been made on the development of alternative, mild and environmentally friendly polymerization conditions for the cationic polymerization of vinyl monomers including IB. Aqueous cationic polymerization has recently emerged as new and attractive method for conducting the polymerization reactions using mild experimental conditions (room temperature, water as nontoxic reaction media). This subject was recently reviewed11 and, therefore, is outside the scope of this highlight. Besides, to date only limited work is available regarding the IB cationic polymerization in aqueous media, and the polymerization was performed under stringent experimental conditions (H2SO4 or LiCl/NaCl/H2O as solvent, low temperature).12 Another intensively studied approach to conduct cationic (co)polymerization of IB under mild conditions is the use of special catalysts bearing weakly coordinating counteranions (WCAs) such as, for example, metallocenes (Cp*TiMe3, Cp2AlMe) activated by B(C6F5)3,13, 14 adducts of B(C6F5)3 with long chain carboxylic acids,15 Et3Si+B(C6F5)4−14,16 etc. It was shown that WCAs can play an important role in cationic polymerizations because of their extremely low proton affinities that enables cationic polymerization processes to be conducted at elevated temperatures without loss in molecular weight of synthesized (co)polymers.13–16 The application of such catalysts for the synthesis of high-molecular-weight PIBs and its copolymers with isoprene (butyl rubber) at relatively high operating temperatures (−30 °C) was already well reviewed.13

Low-molecular-weight poly(isobutylene) is the most important industrial class of IB polymers representing 75–80% of the total PIB market because of their intensive use for manufacturing of lubricants and fuel additives. Among them, PIBs containing a high proportion of vinylidene end groups (exo-olefin end groups), so-called highly reactive poly(isobutylene)s (HR PIBs), are favored because of their higher reactivity in postpolymerization functionalization reactions.17 Market prognoses clearly indicate that the production of HR PIB will increase in the coming years and higher quality products [higher content of exo-olefin terminal groups, narrower molecular weight distribution (MWD), precisely controlled Mn], on the one hand, and new cost-effective and environmentally friendly polymerization processes, on the other hand, will be needed.

In this highlight article, the development of new catalysts for the synthesis of mainly (but not only) HR PIB at room temperature is reported with a focus on two main approaches developed in our groups: (i) the utilization of modified conventional Lewis acids and (ii) the use of solvent-ligated metal complexes associated with WACs.


Modified Conventional Lewis Acids

The cationic polymerization of IB in the presence of AlCl3 (EtAlCl2) as coinitiators has been known for many years and is the base of the Exxon process for the production of low-molecular-weight PIB (conventional PIB) containing internal double bonds, which are characterized by low reactivity toward further functionalization.18 It was shown recently that a complex set of isomerization and chain scission reactions is responsible for the formation of “conventional” PIBs with internal tri- and tetra-substituted olefinic end groups19, 20 (Scheme 1).

Scheme 1.

Mechanism of formation of (a) tri- and (b) tetra-substituted olefinic end groups in the EtAlCl2(AlCl3)-co-initiated cationic polymerization of isobutylene proposed by Faust and coworkers.20

Until recently, only BF3-based initiating systems, i.e., complexes of BF3 with alcohols or ethers in hexanes, are known to produce HR PIB containing 75–85% of exo-olefin end groups (BASF process) at elevated temperatures (from −30 to 0 °C).21, 22 Very recently, we discovered that the use of AlCl3 in a form of complex with ether allowed to synthesize HR PIB containing 85–95% of exo-olefin end groups in contrast to conventional PIB typically obtained using neat AlCl3.23, 24 The similar initiating systems were later proposed by Wu and coworkers.25, 26 These initiating systems originated from our systematic investigations of the controlled cationic polymerization of styrene using of AlCl3 complex with dibutyl ether as an coinitiator with the aim to find an alternative to TiCl4-based initiating system in the synthesis of tri-block copolymers of IB with styrene.27 However, the cationic polymerization of IB with CumOH/AlCl3OBu2 initiating system led to oil-like product instead of the anticipated high-molecular-weight polymers, fortunately, with almost quantitative content of exo-olefin terminal groups (exo ∼ 95%). This was the beginning of our continuous interest to the investigation of room temperature cationic polymerization of IB.

Initial study on the cationic polymerization of IB was performed with CumOH/AlCl3OBu2 initiating system in a mixture of CH2Cl2/n-hexane 80/20 v/v as solvent.23 This initiating system induced quick polymerization of IB (50–80% of monomer conversion in 3 min) and afforded PIB with desired low molecular weight (Mn = 1000–3000 g mol−1), which can be controlled by the reaction temperature or initiator concentration, and relatively narrow monomodal MWD (Mw/Mn ≤ 1.8). 1H NMR spectroscopy revealed a high proportion of exo-olefin end groups (two signals at 4.64 ppm and 4.85 ppm) and small fraction of endo-olefin terminated PIB (5.15 ppm) (Fig. 1). The shoulder (4.82 ppm) to the downfield exo-olefin peak at 4.85 ppm (Fig. 1) was attributed either to the product of coupling of two PIB chains28 or rearrangements of growing cations.25

Figure 1.

A representative 1H NMR spectrum of polyisobutylene synthesized with CumOH/AlCl3OBu2 initiating system in CH2Cl2/n-hexane. (Reproduced from ref.23, with permission from American Chemical Society).

The advantages of CumOH/AlCl3OBu2 initiating system is the high reaction rate and the possibility to synthesize well-defined PIBs with a cumyl group at the α-end of polymer chain (Fn(Cum) = 75–90%) and an exo-olefin group at the ω-end.23, 29 This approach opens an access to difunctional exo-olefin terminated PIB.30 However, because of the partial decomposition of initiator, this initiating system could not be used at high reaction temperatures (T > 0 °C).23, 31 Therefore, the simplest initiating system, i.e., adventitious H2O/AlCl3OR2 (R = Bu2O, iPr2O), was then investigated to conduct the cationic polymerization of IB at high reaction temperatures.23, 25, 31

The effect of temperature on the cationic polymerization of IB induced by H2O/AlCl3OBu2 in CH2Cl2 was studied by Wu and coworkers.25 It was shown that the increase of the temperature from −20 to 20 °C led to decrease of molecular weight from 3800 g mol−1 to 1500 g mol−1 but almost did not influence the exo-olefin terminal groups content (88% at −20 °C and 84% at 10 °C), MWD (Mw/Mn = 1.8−2.2) or monomer conversion (∼45% in 20 min).25 The decrease of coinitiator concentration from [AlCl3OBu2] = 0.04 to 0.005 M allowed to increase the content of exo-olefin terminal groups up to about 90% but at the expense of decreasing the reaction rate (23% of monomer conversion in 20 min).25 Similar low-molecular-weight PIBs (Mn = 800−2200 g mol−1) with quite good functionality (82−91% of exo-olefin terminal groups) were also obtained during the cationic polymerization of IB with a H2O/FeCl3/R2O initiating system (R2O = Et2O, Bu2O, iPr2O) in CH2Cl2 in a temperature range between 0 and 20 °C.32 One of the interesting observations from this work is the much higher concentration of PIB chains than that of initiator (H2O) indicating intensive chain transfer under investigated conditions.

One significant drawback of the aforementioned initiating systems is the conducting of the polymerization in toxic chlorinated solvent. To replace CH2Cl2, the polymerization of IB with H2O/AlCl3OBu2 initiating system was studied in different solvents (toluene, α,α,α-trifluorotoluene, and n-hexane).29, 31, 33 In toluene, the H2O/AlCl3OBu2 initiating system afforded low-molecular-weight HR PIB (Mn = 1000−2100 g mol−1) with relatively narrow MWD (Mw/Mn = 1.4−2.6) in a good yield (70−90% in 10 min) and even with a better functionality (90−94% of exo-olefin end groups) compared to CH2Cl2 at T > 0 °C (Table 1). Besides, the increase of the monomer concentration from [IB] = 0.9 M up to [IB] = 5.2 M almost did not influence the functionality of the obtained polymers (exo = 86−90%) (Table 1).31

Table 1. Effect of Temperature on the Isobutylene Polymerization Using H2O/AlCl3OBu2 Initiating System in Toluenea b
inline image

In n-hexane, however, the H2O/AlCl3OBu2 initiating system in a temperature range from −20 to 20 °C led to a slower polymerization of IB than in toluene, and the reaction was typically terminated at incomplete monomer conversion (15−60%).29, 31, 33 This behavior of the H2O/AlCl3OBu2 initiating system was attributed to the very low concentration of initiator in the system ([H2O]<4 × 10−4 M).29, 33 The synthesized PIBs had Mn in between 2000 and 10,000 g mol−1, and a quite broad MWD (Mw/Mn = 3−7) that is consistent with the heterogeneous nature of the polymerization. In addition, the content of exo-olefin end groups in PIB synthesized in n-hexane is considerable lower when compared with CH2Cl2 or toluene due to the growing cation rearrangements/chain scission leading to the formation of tri- and tetra-substituted olefins (Scheme 1) along with exo-olefin-terminated PIB (compare Fig. 1 and Fig. 2). It was shown that the functionality could be improved by using an excess of ether toward AlCl3, but at the expense of decreasing the monomer conversion.29, 33

Figure 2.

A representative 1H NMR spectrum of polyisobutylene synthesized with H2O/AlCl3OBu2 initiating system in n-hexane at 10 °C.33

From these intriguing results, an important question arose regarding the difference in the polymerization mechanism in the presence of AlCl3OR2 in comparison with neat AlCl3. Initially, a mechanism depicted in Scheme 2 was proposed to explain the behavior of AlCl3OR2-based initiating systems in the polymerization of IB. According to Scheme 2, free Lewis acid, which is generated by dissociation of AlCl3OR2 complex (step A), participates in the initiating and propagating steps of the polymerization. The propagation occurs until selective β-H abstraction by free Bu2O (step B), competitive reinitiation of polymerization by H+ formed after β-H abstraction (step C), or ion pair (H+…AlCl3OH) collapse leading to the inactive AlCl2OH (step D) take place.23, 31 A similar mechanism was also proposed for the cationic polymerization with H2O/FeCl3/R2O initiating system.32

Scheme 2.

Mechanism for isobutylene polymerization using CumOH/AlCl3OBu2 initiating system proposed by Kostjuk and coworkers.31

It was unambiguously shown that basicity and concentration of electron donor play a key role in the synthesis of PIBs with high content of exo-olefin end groups.23, 25, 26, 31, 32 Indeed, the use of complexes of AlCl3 with weaker than Bu2O (pKa = −5.4) electron donors such as ethyl acetate (EtOAc; pKa = −6.5) or diphenyl ether (Ph2O; pKa = −6.54) led to the total loss of the control over the selectivity of β-H abstraction.23

However, the use of stronger bases such as diethyl ether (pKa = −3.59) and pyridine (pKa = 5.25) resulted in a considerable decrease in the monomer conversion and, in some cases, in the content of exo-olefin end groups.23, 31, 32 More sterically hindered ethers (iPr2O, iBu2O) gave higher IB conversion, while the influence of ether structure on the functionality is more complex.29, 32, 33

Recently, an alternative mechanism for the IB polymerization by AlCl3/ether complexes was proposed by Faust and coworkers. (Scheme 3).29

Scheme 3.

Mechanism for isobutylene polymerization by AlCl3/ether complexes proposed by Faust and coworkers.29

The main difference of this mechanism from the one depicted in Scheme 2 is that AlCl3OR2, not free uncomplexed AlCl3, is the true coinitiator. This conclusion has been made based on the observed inactivity in the presence of 2,6-di-tert-butylpyridine as proton trap of usually effective initiators like 2-chloro-2,4,4-trimethylpentane or tert-butyl alcohol in the polymerization of IB. According to the mechanism depicted in Scheme 3, the reaction of water with AlCl3×ether complex yields H+AlCl3OH with the simultaneous release of the ether. Protonation of the monomer and propagation take place until ether assisted chain transfer to monomer yields a PIB exo-olefin and a new protonated monomer. Termination by ion collapse yields PIB–Cl, which cannot be reionized.29

To summarize, the complexes of AlCl3 with ether show a great potential in the synthesis of low-molecular-weight PIBs (Mn = 1000−10,000 g mol−1) with perfect functionality (exo-olefin end groups content 70−95%) at room or close to room temperatures. Evidently, these complexes could be a good alternative to BF3-based initiating systems, which are currently used in industry for the production of HR PIB. However, further fundamental research has to be done before transferring this promising catalyst to industry. First of all, the problem of low activity and poor solubility of the catalytic complex in n-hexane should be solved. Then, we anticipate further intensive fundamental investigations devoted to the improvement the initiating system in terms of selectivity of β-H abstraction (especially in n-hexane) by using both new Lewis acids and new electron donors. Finally, in very preliminary experiments we showed that complexes of AlCl3 with appropriate electron donors could also generate relatively high molecular weight PIBs (Mn up to 30,000 g mol−1) at elevated temperatures (from −20 to 0 °C).

Initiating Systems Involving WCAs

As already indicated, in the recent years new polymerization strategies have appeared for the preparation of PIB, which are based on initiator or metal complex systems associated with weakly coordinating anions (WCAs).34–47

Various types of weakly coordination counteranions or WCAs based on borates, carbonates, alkoxy, and aryloxy metallates have been referred to in the literature. WCAs will preferentially coordinate (despite weakly) with the most electrophilic and sterically accessible moiety in their environment. Hence, the coordinating ability of each anion is limited by its most basic site.48–50

It is well known that in cationic polymerization, the nature of the counteranions exerts a significant role in the polymerization reactions particularly on the rates of chain transfer reactions.1, 18 Over the years, there has been an increase of interest in the synthesis and application of complexes incorporating WCAs for the homopolymerization or copolymerization of IB which covered both the optimization of the complexes and related mechanistic studies. A comprehensive review of the different categories of the complexes according to their properties and its applications has been published by Kühn and coworkers.51

Early on prominent and interesting results achieved by the application of complexes associated with WCAs in IB polymerization led to a strong interest to further explore the potential of these complex systems. Thus, compared to the conventional AlCl3, an initiating system based on the in situ formation of [(CH3)3Si]+[B(C6F5)4] via SiMe3Cl and Li[B(C6F5)4] led to unusually high-molecular-weight copolymers of IB and isoprene (butyl rubber) at relatively high temperature up to −8 °C and under close to neat conditions, as described by Kennedy and coworkers.16

Metallocene-like initiators such as [(η5-C5Me5)TiMe2{μ-Me}B(C6F5)3] and certain similar compounds were applied by Baird et al. in the polymerization of IB and copolymerization of IB and isoprene.13, 52–54 The 10 electron, cationic species [Cp*TiMe2]+ formed via the dissociation of the borate anion from the methyl-bridged compound [(η5-C5Me5)TiMe2{μ-Me}B(C6F5)3]13 was demonstrated to be a very effective initiator for the homopolymerization of IB at −40 °C achieving molecular weights in the range of 106 g mol−1.53

Shaffer et al. also elaborated on the role of WCAs in carbocationic polymerization with a series of metallocene [[Cp*TiMe3] and [Cpmath imageMMe2] (CpR = Cp, Cp* or Me2Si(TSI), TSI = tetrahydroindenyl; M = Zr or Hf)] and cationogenic (Ph3C+, R3C+, H+, Li+, and R3Si+) complexes incorporating [B(C6F5)4] and [RB(C6H5)3] as WCAs. They noted that these complexes showed numerous advantages over other catalysts due to their non nucleophilic nature while permitting carbocationic initiation. Using those WCA complexes, high-molecular-weight PIB was achieved at relatively high temperature (−20 °C) allowing the conclusion that the WCAs did not catalyze chain transfer and did not contribute to termination.14

Nitrile-Ligated Metal Complexes Associated with WCAs

Nitrile-ligated metal complexes involving WCAs48 have already attracted quite some interest due to the labile coordination mode of the ligands which can be easily replaced.55, 56 Kühn et al. have investigated the application of the monometallic complexes with the structure [MII(NCCH3)6][A]2 (MII = Cr, Mn, Fe, Co, Ni, Cu, Zn and A = BFmath image or [B(C6H3(CF3)2)4]) for the cationic cyclopentadiene polymerization in a homogeneous phase, demonstrating high activity for the metal complexes bearing Cr (II), Mn (II), Fe (II), and Zn (II) while on the contrary, the analogous V (II) and Ni (II) metal complexes appeared to be nearly or completely inactive.48, 57–59 It was further demonstrated that the WCA complex [Mn(CH3CN)6][B(C6H3(CF3)2)4]2 resulted in a significantly higher activity for cyclopentadiene polymerization when compared with [Mn(CH3CN)4][BF4]2.48, 57

The application of more weakly coordinating anions such as [B(C6F5)4], [B{C6H3(m-CF3)2}4], or [(C6F5)3B-C3H3N2-B(C6F5)3] (Fig. 3) in lieu of the BFmath image anions, allowed the polymerization and copolymerization of challenging but industrially more interesting monomers such as IB or isoprene.34, 36, 48, 51 The initial application of the nitrile-ligated metal complexes associated with the perfluoroborate WCAs for a room temperature synthesis (temperature range from 20 to 60 °C) of PIB was first published by Nuyken and coworkers in 2003.34–37 Interestingly, the resulting products had the features of the highly reactive polyisobutylene (HR PIB) meaning relatively low molar mass combined with a high content of exo double bonds. Since then, a series of nitrile-ligated metal complexes of the structure [M(RCN)6]2+ (M = Cr(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II); R = CH3, C6H5) and [MoCl(NCCH3)5]2+ incorporating the perfluoroborate WCAs, mainly (1) [B(C6F5)4], (2) [B{C6H3(m-CF3)2}4] and (3) [(C6F5)3B-C3H3N2-B(C6F5)3] (Fig. 3) were investigated by us for the polymerization of IB at room temperature.38–45

Figure 3.

Structures of the nitrile-ligated metal cations and perfluoroborate (1–3) and perfluoroalkoxyaluminates (4) WCAs applied for the polymerization of IB. (R = CH3 or C6H5 and M = corresponding metal center).34–47

It was found quickly that the synthesis of HR PIB at room temperature using WCA-associated metal complexes provides an alternative to the conventional cationic polymerization of IB which is generally carried out in cryogenic temperature (−10 to −100 °C) to suppress the chain transfer reactions.38–46 Since the initial report by Nuyken and coworkers, significant improvements in the polymerization features have been achieved by variation of the central metal in the metal complexes, such as shorter reaction times and the possibility to use nonchlorinated solvents in the polymerization process.38–42

The first series of nitrile-ligated complexes associated with the perfluoroborate WCAs reported were based on manganese (II) as central metal. These complexes were successfully applied for the polymerization of IB (and copolymerization with isoprene) at room temperature leading to HR PIB with molecular weights in the range of 500–13,000 g mol−1 with an exo double bond content over 90%.36 Both the acetonitrile and benzonitrile-ligated manganese (II) complexes were shown to be active for the polymerization of IB at room temperature.38 Subsequently, acetonitrile and benzonitrile copper (II) complexes associated with the perfluoroborate WCAs39, 40 showed considerable faster reaction time (from 20 h with the manganese (II) complexes down to 15 min) and allowed us to use nonhalogenated solvents like toluene and n-hexane rich binary solvent mixtures (80/20 vol/vol % n-hexane/dichloromethane). PIB with molecular weights of 1500 to 5500 g mol−1 and with an exo double bond content within 85 to 95% was achieved in high yields but only under optimized conditions as there are low complex concentration (0.5 × 10−4 mol L−1), very short reaction times (15 min), and use of nonpolar solvents.39, 40 More recently, we could show that molybdenum (III) complexes associated with the perfluoroborate WCAs series with the chemical structure of (1) [Mo(NCCH3)5Cl][B(C6F5)4]2 (2) [Mo(NCCH3)5Cl][B{C6H3(m-CF3)2}4]2 and (3) [Mo(NCCH3)5Cl][(C6F5)3B-C3H3N2-B(C6F5)3]2 can also be successfully used for the polymerization of IB at 30 °C, and these molybdenum (III) complexes exhibit similar activity also in pure toluene as demonstrated by the copper complexes. E.g., with the [Mo(NCCH3)5Cl][(C6F5)3B-C3H3N2-B(C6F5)3]2 complex under optimal conditions ([complex] = 0.5 × 10−4 mol L−1; 2 h) an IB conversion of 67% was achieved, and the product had a Mn of 600 g mol−1, PDI of 1.5, and an exo double bond content of 90%.41, 42 Overall, the central metal of the complexes has been varied widely, and particularly, the first row transition metal complexes ligated with acetonitrile and associated with the [B(C6F5)4] have so far been examined with regard to the homopolymerization of IB at 30 °C.43 In addition, very recently, also the WCAs have been further varied by applying aluminate structures (Fig. 3, WCA 4) associated with nitrile-ligated Cu (II) and Zn (II) allowing also the preparation of HR PIB at RT in high yields within 2 h of reaction.47

The nitrile-ligated metal complexes associated with the perfluoroborate or aluminate WCAs exhibit certain similar trends and traits in the room temperature IB polymerization reaction. First, it was noted that the metal center, its ligands (acetonitrile or benzonitrile), and the associated WCAs can influence the RT polymerization of IB significantly, similarly as solvent polarity and complex concentration; thus, for each complex system optimized conditions had to be found. Low-molecular-weight PIB (regardless of the complex concentration) with high exo double bond content corresponding to HR PIB was obtained under optimized conditions (reaction time adjusted for sufficient yield and high exo double bond content, low catalyst concentration) in all cases when the complex showed any activity.34–43 Yet at the same time, each of the metal complexes exhibits also certain associated features. For example, the manganese (II) complexes exhibited a very slow polymerization reaction, whereas the copper (II) and molybdenum (III) complexes led to a much faster reaction. The manganese (II) complexes had been less effective in increasing nonpolar media, whereas the copper (II) and molybdenum complexes (III) showed optimal results in nonhalogenated solvents such as toluene.36, 38, 39, 40, 42

The low-molecular-weight PIBs obtained with the various WCA metal complexes are similar in structure,36, 38, 40, 42 and in accordance with the typical cationic polymerization showing a t-butyl head group and unsaturated groups. Apart from the exo and endo double bonds, various unsaturated structural units are found due to isomerization of the end groups. The isomerization tendency strongly depends on the metal cation used; the most active complexes for polymerization like the Cu (II) complexes demonstrate also the highest isomerization tendency. In general, the isomerization tendency increases significantly with prolonged reaction time and is more favored at higher catalyst concentration (optimized complex concentration was found to be 0.5 × 10−4 mol L−1 irrespective of the metal center). The different unsaturated structural units detected can be attributed to the 1,3-methide shift and concerted 1,2-hydride and 1,2-methide shift (see also Scheme 1 and discussion therein).17, 38, 40, 60 Figure 4 shows the 1H NMR spectra obtained with the manganese (II) complexes.

Figure 4.

1H NMR analysis of the olefinic region for the low-molecular-weight PIB obtained with the manganese (II) complexes with (a) indicating a PIB with nearly exclusively endo and exo end groups (prepared at low catalyst concentration of 0.5 × 10−4 mol L−1) and (b) indicating a PIB with a significant amount of internal double bond (marked by *) prepared at 10 times higher catalyst concentration. (Results taken from Ref.38)

Generally, the isomerization reactions observed for the room temperature polymerization process applying this series of complexes associated with WCAs are similar to those reported for the conventional cationic polymerization of the IB.25, 32, 38, 40, 60 We postulated that the exo double bonds, which are kinetically favored, are dominant in the early stage of the polymerization. Prolonged reaction time and higher concentration of the complexes applied favor isomerization reactions mainly due to reprotonation of the polymer chain. Online 1H NMR investigations were very helpful for an in-depth study of the polymerization reaction.38, 42 Figure 5(a) shows the time-dependent monomer conversion and the corresponding development of exo double bonds obtained with [Mn(NCCH3)6][(C6F5)3B-C3H3N2-B(C6F5)3]2 in dichloromethane.38 In comparison, the results obtained with the ([[Mo(NCCH3)5Cl][(C6F5)3B-C3H3N2-B(C6F5)3]2] complex in toluene, as shown in Figure 5(b) demonstrate a much faster polymerization reaction but also a more prominent isomerization in the first hour of the polymerization.

Figure 5.

Polymerization of isobutylene at 30 °C followed online by 1H NMR spectroscopy regarding monomer conversion and exo double bonds content using two different complexes in different solvents. ([IB] = 1.78 mol L−1, [complex] = 0.5 × 10−4 mol L−1) (Results taken from Refs.38 and42).

First postulates for a mechanism for this room temperature polymerization process using the WCA metal complexes were published by Nuyken and coworkers on 200636, 37 considering the following facts: (1) the possibility of the polymerization of IB at elevated temperature (30 °C), (2) the high content of exo double bonds, and (3) the relative stability of the manganese (II) complex towards water. Thus, two mechanism were considered: (a) initiation via metal–monomer interaction and (b) initiation via protonation (Scheme 4)36, 37, 51

Scheme 4.

(a) Initiation mechanism via the metal–monomer interaction and b) protonation mechanism involving the interaction of the water molecules with the metal cation complex, both as proposed by Nuyken et al.37

Based on the early work by Vierle et al., particularly the low polymerization rates obtained with the manganese (II) complexes and their relatively low sensitively toward water, it was postulated that the initiation via protonation is not viable. The inactivity of the aqua complexes bearing the water ligands, [Mn(H2O)2(CH3CN)]2+ toward the polymerization of IB further supported this postulate. Nevertheless, the release of a ligand, leaving a vacant coordination site for subsequent coordination by a substrate was presumed to be the first step for the polymerization reaction.36, 37 It is possible that once the active species is formed, cationic polymerization continues to proceed which is then most likely terminated by proton transfer. Nuyken and coworkers, however, favored a mechanism based on an M-H intermediates (from the initiation mechanism postulated via metal-monomer interaction) which could explain the high exo double bond content plus the presence of the tert-butyl head group and the continuation of the polymerization to similar molecular weight products after addition of a fresh batch of monomer.36, 37

Alternatively, in accordance with results from our further investigation, a more likely “acid complex” polymerization reaction was proposed based on initiation via protonation and the occurrences of transfer reactions.61 Online 1H NMR investigation of the manganese (II) complexes demonstrated that the integral value of all the unsaturated end groups was equivalent to the tertiary butyl head group. The investigation also indicated that no fragments of the metal complexes were incorporated in the polymer chains as head or end groups, and the tert-butyl head group detected was in agreement with the conventional cationic polymerization. We reasoned that the manganese (II) complexes might, through certain specific intermediates, assist in the proton transfer as indicated by the slow polymerization rate and the slow isomerization of the end groups. It was also concluded that proton abstraction resulted in the termination of a propagating chain, probably under the involvement of the manganese (II) complexes.38

In the subsequent investigations with the molybdenum (II) complexes, the real initiating species was assumed to be actually a proton. Such protons resulted most likely from the “generation” of protons assisted by the metal complexes. At the same time, we also presumed that no excess “free” protons were present in the polymerization system.42, 61 Therefore, it was suggested that initiation proceeds via an “acid complex” formed with anions or with other bases. We also postulated that the nitrile ligands might assist in stabilizing the propagating chain which suppresses transfer reaction and hence allowing the room temperature reaction to proceed.61

Recently, it was also demonstrated that the copper (II) and zinc (II) complexes associated with the perfluoroalkoxyaluminates, in particular the [Al(OC(CF3)3)4] counteranion as shown in Figure 3(WCA 4), can also be applied for the polymerization of IB at 30 °C.47 Similar results were obtained with the perfluoroalkoxyaluminates as with the perfluoroborate WCAs. For example, the molecular weights of the PIB obtained were rather low and seemly independent of the concentration of the complexes used and under favorable conditions relatively high contents of exo double bonds were obtained whereas isomerization of the end groups was observed with increasing reaction time and concentration of the complexes used.47

Another crucial aspect of the polymerization of IB examined was the effect of the addition of water and its influence on the results obtained. It was reported for the manganese (II) and copper (II) complexes associated with the perfluoroborate WCAs that the addition of more than 10-fold excess of water leads to a significant decrease in the polymer yields which was attributed to the formation of the aqua complex.36, 37, 40 In contrary, the copper (II) and zinc (II) complexes associated with the perfluoroalkoxyaluminate WCA showed a significant higher tolerance toward water. No significant decrease in polymer yield was detected with these complexes despite the addition of ∼100 fold excess of water. However, a decrease in molecular weight and PDI, and interestingly, an increase of isomerization was detected which we attributed to an increase of “protonic activity” (Fig. 6). Based on these observations, it was presumed that the increase of “protonic activity” was due to the generation of protons (in accordance with the increasing amount of water added) which subsequently lead to the generation of new polymer chains and the isomerization of the end groups. Hence, these recent results again indicate that the initiation via protonation (due to the interaction of the adventitious water with the ligated metal cations) as shown in Scheme 4(b) is most likely while propagation proceeds cationically and β proton abstractions contribute to the high exo double bold contents with dominant transfer reaction.47

Figure 6.

1H NMR spectra showing the dominant exo end groups obtained by the [Zn(NCC6H5)6][Al(OC(CF3)3)4]2 complex in (a) predried dichloromethane without the addition of water and (b) the increase in isomerization detected with the addition of 100 fold excess of water. [[complex] = 0.5 × 10−4 mol L−1, [IB] = 1.78 mol L−1, reaction time = 2 h. (Results taken from Ref.47).

Temperature dependent online 1H NMR with the copper (II) complex associated with the perfluoroalkoxyaluminates WCA showed that no polymerization was detected between the temperature range of −10 to 10 °C. Traces of polymerization can be seen above 10 °C and polymerization becomes rapid at 30 °C. This again indicated that the polymerization process is truly dependent on temperature and only possible at elevated temperatures above 0 which is consistent with earlier observation reported with the complexes associated with the perfluoroborate WCAs47 and fully in contrast to conventional free proton initiated cationic polymerization.

Certainly, the WCAs have to play a crucial role in the observed room temperature polymerization of IB. Shaffer and Ashbaugh14 and also Bochmann and coworkers62 pointed out already advantages associated with the application of WCA-based complexes in the IB polymerization. The very low nucleophilicity of the WCAs disfavors the deprotonation of the propagating carbocation, leading to a reduction in chain transfer and therefore allows high molecular weight products at relatively high temperature.62 Furthermore, clearly significant variations in reactivity were observed for different metal complexes associated with the different WCAs34–43

But why are those complexes not active below 0 °C? Previous investigations into the metal-ligated complexes indicated that their effectiveness or activity depends upon the ease of losing one of the nitrile ligands, and hence creating a free coordination sites for subsequent substrate coordination.43 An increasing amount of acetonitrile as the second solvent resulted in a significantly decreased polymer yield which was attributed to the competition of coordination sites with possible substrates.36 At the same time, also postulated by Vierle et al., the released nitrile ligands might exert the role as a stabilizing agent stabilizing the end group of the growing polymer chain.36, 63 Meanwhile, our ligand exchange studies showed that the ligands of the complexes were readily replaced by stronger coordinating substrates or nucleophiles such as water, methanol, or free benzonitrile and temperature dependent online 1H NMR investigation demonstrated that the nitrile ligand attached to the metal centre was released at temperatures over 0 °C. This observation indicates that the release of ligand which resulted in a vacant coordination site is the prime and rate determining step for the room temperature polymerization reaction.61

In summary, we can state that nitrile-ligated metal complexes associated with WCAs also pose a highly interesting room temperature alternative to conventional cationic initiating systems used to produce HR PIB. Much insight was gained in the different aspects of these complex systems, and the role of the various components on the success of the room temperature polymerization of IB. However, a full clarification of the mechanism has not been achieved so far which will be highly needed to control the polymerization and to allow technical use of these systems. Further drawbacks, like the increase in isomerization with increased reaction time and aspects of catalyst recovery need also to be addressed.

Other Catalytic Systems

During the last decade, a number of other homogeneous and heterogeneous initiating systems for the cationic polymerization of IB or its copolymerization with isoprene at room temperature have been reported in the literature.

RZnCl (R = Et or Oct)-Based Initiating Systems

The very simple and interesting initiating system, tert-butyl chloride (tBuCl)/EtZnCl, was shown to be effective in the synthesis of medium-molecular-weight PIBs (Mn = 10,000−29,000 g mol−1) with relatively narrow MWD (Mw/Mn < 2.5) and high content of exo-olefin terminal groups (60−92%) in CH2Cl2 at room temperature.64, 65 Importantly, the method of introducing the zinc component into the reaction is crucial for the success of the polymerization: the injection of EtZnCl into a IB solution in CH2Cl2 containing tBuCl led to rapid polymerization, whereas premixing of EtZnCl and tBuCl gave instantly a white precipitate (probably ZnCl2), which was inactive in the polymerization of IB. In addition, ZnCl2 did not induce the polymerization due to its insolubility in reaction media, whereas ZnEt2 was not acidic enough to coinitiate the polymerization. To clarify the polymerization mechanism, the reaction of tBuCl and EtZnCl in CD2Cl2 was monitored by 1H NMR spectroscopy. Although the presence of tert-butyl cations was not detected, the rapid formation of precipitate (ZnCl2) and IB oligomers occurred. Therefore, it was assumed that the polymerization proceeds via cationic mechanism and that the mode of initiation involves not only [CMe3]+[ZnCl2Et] but also in situ formation of “molecular” ZnCl2, which is sufficiently active to generate [CMe3]+[ZnCl3] (see Scheme 5). The last assumption, however, is questionable since the use of OctZnCl instead of EtZnCl led to polymers with considerably higher Mn indicating a distinct counteranion effect.65

Scheme 5.

Reactions of EtZnCl with tBuCl.64

The molecular weight of PIB synthesized with the tBuCl/EtZnCl initiating system was effectively controlled by the tBuCl concentration that is consistent with the cationic mechanism of the polymerization, where tBuCl acts as an initiator and EtZnCl as coinitiator or activator. The monomer conversion was gradually increased with increasing of tBuCl concentration indicating that irreversible termination is a main chain breaking process under investigated conditions.

In summary, the cationic polymerization of IB with the tBuCl/EtZnCl initiating system represents a rare example of synthesizing relatively high-molecular-weight PIBs (Mn up to 30,000 g mol−1) at room temperature. The clear advantage of this initiating system is also the possibility to synthesize copolymers of IB with isoprene (up to 4% of isoprene incorporation) at room temperature without any significant decrease in polymer yield or molecular weight when compared with homopolymerization of IB. These results were explained by the formation of WCA ([RZnCl2], R = Et or Oct).64, 65 Unfortunately, although a relevant mechanism explaining the behavior of tBuCl/EtZnCl initiating system in the polymerization of IB was proposed, no explanations for the predominant formation of exo-olefin end groups were given. The inability of tBuCl/EtZnCl to polymerize IB in nonpolar solvents such as, for example, toluene could be considered as a main disadvantage of this initiating system.64, 65

Methylaluminoxane-Mediated Cationic Polymerization

Methylaluminoxane (MAO) is widely used as an activator for the early transition metallocenes in the polymerization of α-olefins.66 On the other hand, it was expected that due to its unique structure MAO would generate so-called WCA and, therefore, could be used for the conducting the cationic polymerization at elevated temperatures. Indeed, relatively high-molecular-weight polymers (Mn from 2,000 and up to 60,000 g mol−1) were obtained using MAO as catalyst in the room temperature cationic polymerization of styrenes,67, 68 isobutyl vinyl ethers,67, 68 or cyclopentadiene.69 Unfortunately, only one work is related to the investigation the cationic polymerization of IB mediated by MAO.70 It was shown that MAO induced the cationic polymerization of IB at 25 °C in CH2Cl2 (monomer/solvent ∼1:1 v/v) to afford polymers with viscosity-average molecular weight (Mv) between 8,000 and 190,000 g mol−1 depending on the MAO concentration. Despite the high Mv values observed at low MAO concentrations (Mv = 61,000−190,000 g mol−1), all obtained polymers were colorless viscous sticky liquids, which indicates that relatively low-molecular-weight polymers were in fact formed. Unfortunately, any data about the number average molecular weight and polydispersity of synthesized PIBs are absent in this article.70 The chain end analysis of obtained PIBs by means of 1H NMR and 13C NMR spectroscopy revealed the tert-butyl head group and exo-olefin end group, respectively.

It should be also noted that MAO showed good activity in the cationic polymerization of IB only in polar CH2Cl2, whereas only traces of polymer were formed in toluene. Finally, MAO also initiated the cationic copolymerization of IB with isoprene, although relatively low-molecular-weight copolymers in comparison with homopolymers of IB were obtained at ambient temperature (Mv = 4,000−12,000 g mol−1 at 0 °C).70

Conventional Lewis Acids

Conventional Lewis acids, such as SnCl4 or TiCl4, could be also used for the cationic polymerization of IB to synthesize well-defined polymers at high temperatures (from 0 to 60 °C) under specific reaction conditions.71–73 Toman et al.71 investigated the polymerization of IB in neat monomer or in n-hexane ([IB] = 7.2 M) using SnCl4 as coinitiator at different temperatures. At low temperature (from −20 to 35 °C) polymerization did not proceed, whereas above 40 °C the reaction was completed in 1−3 h depending on the reaction conditions (Fig. 7). The obtained polymers had low molecular weights (Mn = 600–2,000 g mol−1) and broad but monomodal MWD (Mw/Mn = 3–5). According 1H and 13C NMR spectroscopy, all synthesized polymers contained only unsaturated terminal groups (the absence of tert-chloride end groups was verified by elemental analysis) prevailingly of the useful exo-type (up to 80%).71

Figure 7.

Effect of reaction temperature on the attainable conversion in the polymerization of isobutylene: [SnCl4] = 0.18 M. Reaction conditions: (1) n-hexane, 1 h; (2) bulk, 1 h; (3) n-hexane, 3 h; (4) bulk, 3 h. (Reproduced from Ref.71, with permission from Wiley).

Based on the obtained results71 and earlier investigations,74 it was proposed that the Lewis acid formed charge transfer complexes with IB under the investigated conditions (neat monomer or nonpolar n-hexane as solvent). At high temperatures (T > 35 °C), the energy is sufficiently high for fast excitation of the complex to generate radical-cations, which quickly recombined with the formed dication and propagation proceeded via cationic mechanism (Scheme 6).71 There are two observations in favor of the proposed mechanism: (i) the clear inhibition effect of oxygen; (ii) the higher molecular weights than those obtained with a 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 initiating system at the same temperature although in supercritical carbon dioxide as solvent.

Scheme 6.

Proposed mechanism for the isobutylene polymerization with SnCl4×isobutylene charge transfer complex (CT).71

The first cationic polymerization of IB in supercritical carbon dioxide at 32.5 °C and pressure as high as 120 bar using TMPCl/LA (LA = TiCl4, SnCl4 or = TiCl4/BCl3 mixture) initiating systems has been accomplished by Kennedy and coworkers.73, 75 These initiating systems induced slow polymerization of IB (∼45% of monomer conversion in ca. 1 day) to afford polymers with Mn ∼ 2000 g mol−1 and Mw/Mn ∼ 2.0, in comparison with Mn ∼ 200 g mol−1, which was predicted based on Arrhenius plot for PIB obtained with H2O/TiCl4 initiating system in a “conventional” liquid-phase polymerization. The microstructure of PIB obtained with various Lewis acids was studied by 1H NMR spectroscopy. Basically, PIBs synthesized with TMPCl/TiCl4 and TMPCl/SnCl4 contained a variety of ill-defined olefinic end groups. On the contrary, the use of TMPCl in conjunction with a mixture of TiCl4/BCl3 led to well-defined PIBs (Mn ∼ 1800 g mol−1; Mw/Mn ∼ 1.4) containing only tert-chloride end groups indicating the virtual absence of chain transfer reactions even at such a high temperature as 32.5 °C. This unique behavior of mixed coinitiator was tentatively attributed by the authors to the low stability (high nucleophilicity) of the BClmath image counterion that led to rapid irreversible termination with the formation of PIB–Cl and BCl3.75 It was shown also that polymerization of IB in supercritical CO2 with TMPCl/TiCl4/BCl3 initiating system was very sensitive to temperature: even an insignificant increase of reaction temperature (from 32.5 to 42.5 °C) led to decrease both the molecular weight (from Mn = 2000 g mol−1 to 875 g mol−1) and monomer conversion (from 40% to 7%).75

An interesting example of conducting the cationic polymerization of IB at ambient temperatures (−20 to 0 °C) with TMPCl/TiCl4/N,N,N′,N′-tetramethylethylenediamine (TMEDA) using α,α,α-trifluorotoluene (BTF) as “environmentally benign” solvent was communicated by Ivan et al.72 Compared to CH2Cl2, BTF has a similar dielectric constant (9.04 and 9.18, respectively), but a higher dipole moment (1.89 D and 2.86 D, respectively).76 The molecular weight of PIB prepared in BTF exactly matched the target value (Mn = 2000 g mol−1) and MWD was quite narrow (Mw/Mn < 1.2) even at 0 °C, whereas PIB synthesized in CH2Cl2 exhibited bimodal MWD, and the Mn was twice of the target value indicating that chain coupling took place in CH2Cl2. Interestingly, the polymers synthesized in CH2Cl2 possessed only olefinic (both exo and endo) terminal groups, whereas 1H NMR spectra of PIB synthesized in BTF under the same conditions exhibited mainly tert-chloride end groups indicating that side reactions are suppressed in BTF.72 These very preliminary results showed that a controlled cationic polymerization could be achieved at ambient temperatures using the initiating system, which was shown to be efficient only at cryogenic temperatures,2 via rational selection of the polymerization solvent.

Heteropolyacid Salts

Researches at Lubrizol proposed salts of phosphotungustic acid (M0.5.H0..5PW12O40, where M = Cs, NH4) as a new class of heterogeneous catalysts for the cationic polymerization of IB at ambient temperature (−5 °C).77 The selection of these salts was based on their high inherent acidity, thermal stability, commercial availability, and recyclability. It was pointed that the polymerization proceeded in inert hydrocarbon solvent for 15–30 min, but the monomer conversion was rather low (20–40%). The obtained PIBs had low molecular weight (Mn = 1000–3500 g mol−1) and very broad MWD (Mw/Mn = 5–25). Heteropolyacid salts were characterized by high selectivity toward end group formation: chain end analysis of synthesized PIBs by means of 1H NMR spectroscopy revealed predominantly exo-olefin terminal groups (70–85%), endo-olefin end groups (10–20%), and tetra-substituted olefinic groups (2–5%) were found in minor amounts. The observed predominant formation of exo-olefin end groups was explained by the formation of weakly coordinating heteropolyanion in contrast to, for example, strongly coordinating, basic [AlCl3OH] generated through the conventional AlCl3-coinitiated IB polymerization leading mainly to isomerized tri-substituted double bonds at the chain ends.77 Thus, heteropolyacids or their salts represent an interesting and promising type of catalysts for the cationic polymerization at room temperature. Apart from IB, these catalysts were used recently for the cationic polymerization of styrene78 and β-pinene79 at ambient temperatures (−10 to 20 °C).

Heterogeneous Polymerization

The heterogeneous cationic polymerization of IB was described by Boisson and coworkers.80 Solid catalyst (YCl3 supported on dehydroxylated SO2) was shown to induce the cationic polymerization of IB in n-heptane as solvent at 20 °C to afford low-molecular-weight PIBs (Mn = 2900 g mol−1) with reasonable MWD (Mw/Mn = 2.6). Unsaturated chain ends were quantified by 1H NMR spectroscopy. It was shown that the obtained PIBs contained high percentage of tri-substituted end groups formed due to isomerization and chain scission reactions (see Scheme 1 for details). Belbachir and coworkers81 used proton exchanged montmorillonite clay as catalyst for heterogeneous cationic polymerization of IB in bulk, CH2Cl2 or n-hexane at −7 °C. PIBs with Mn ∼ 3500 g mol−1 and very narrow MWD (Mw/Mn = 1.16) were obtained in CH2Cl2 and n-hexane, while only oligomers (Mn = 500–700 g mol−1) with broad MWD (Mw/Mn = 4–5) were synthesized in a neat IB. Unfortunately, only the 1H NMR spectrum of PIB sample synthesized in bulk was discussed in the article and, basically, ill-defined oligomers containing exo-, endo- and also significant fraction of tri-substituted end groups were formed under these conditions.

The selective oligomerization of IB, which represents an important domain of cationic polymerization of IB at elevated temperatures (40–100 °C), was already adequately reviewed82 and, therefore, will be not considered here.


Is it possible to draw any general conclusions after reviewing the various approaches developed recently to allow room temperature cationic polymerization of IB? One fact should certainly be highlighted and that is that the cationic polymerization of IB is still full of surprises. It still cannot be considered fully explored or even “mature” as often said for polymerizations leading to long standing and important commercial products. The special features which dominate that polymerization, especially the high tendency for transfer reactions at elevated temperatures, certainly limits the production of high molar mass PIB above 0 °C, and this will also not change with the new systems explored recently. However, these special characteristics of IB polymerization offer chances for new catalytic and initiating systems when one aims for lower molar mass products and especially when a high exo double bond content in the product is desired (HR-PIB). However, to exploit these new possibilities it requires a deep understanding of the underlying mechanisms and processes to design suitable systems which allow controlled chain transfer but avoid further isomerization.

The two major systems outlined above—modified Lewis acids and WCA complexes—have a high potential for commercial exploitation and both have as common features that the ß-proton abstraction is regulated whereas the protic activity is reduced. Hence, with these systems the cationic polymerization toward HR-PIB is shown to be fully feasible and efficient at room temperature, and under certain circumstances even possible only at higher temperatures as demonstrated via the nitrile-ligated metal complexes associated with WCAs. At the same time, it has to be emphasized that the understanding and control of the various aspects of the studied systems such as the protonation mechanism, the regulation of the transfer reactions, the optimizing of the β proton abstraction, and the properties of the counteranions (weakly coordinating) and how those are influenced by the initiator and complex components are vital for the success of the cationic polymerization of IB at room temperature. Not all aspects are fully understood presently, yet we are very optimistic that with the current developments and further progress made with regard to mechanistic insights and development of tailor-made systems, the room temperature cationic polymerization of IB will finally emerge as an also technically feasible alternative toward existing low temperature processes. One can also expect a renaissance in research activities on the cationic polymerization not only of IB but in very general aspects which may further allow to overcoming current boundaries and limitations.


The authors thank F. Kühn and O. Nuyken for their excellent cooperation in the cationic polymerization of IB using nitrile-ligated metal complexes. Furthermore, they thank BASF SE for partial financial support of this work and fruitful discussions. All coworkers who contributed to the work are gratefully acknowledged.

Biographical Information

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Sergei Kostjuk has received his Ph.D. degree in polymer chemistry in 2002 from Belarusian State University. In 2002, he joined Research Institute for Physical Chemical Problems and since 2008 he is heading the laboratory of catalysis of polymerization processes in the same Institute. From 2005, he has also spent several periods as invited scientist in the Institute Charles Gerhardt (France). His research interests lie in the fields of polymer design and synthesis (cationic and anionic polymerization).

Biographical Information

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Hui Yee Yeong, after graduating with a Master in Industrial Chemistry from Technische Universität München (TUM) and National University Singapore (NUS), completed her doctorate research work under the supervision of Prof. Brigitte Voit at Leibniz-Institut für Polymerforschung Dresden e. V. (IPF) and Technische Universität Dresden (TUD), Germany. Her research was focused on the room temperature polymerization of isobutylene in collaboration with the group of Prof. F. Kühn from TUM and BASF SE. Recently she joint BASF Coatings GmbH.

Biographical Information

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Brigitte Voit received her PhD in Macromolecular Chemistry in 1990 from University Bayreuth, Germany. After postdoctoral work in 1991/1992 at Eastman Kodak in Rochester, USA, and habilitation at TUM, in1997 Brigitte Voit was jointly appointed professor for (Organic Chemistry of Polymers at TUD and Head of the Institute of Macromolecular Chemistry at IPF Dresden which she is heading since 2002 also as Scientific Director. Her major research interest is in the synthesis of new functional polymer architectures.