Univalent Gallium Salts of Weakly Coordinating Anions: Effective

Oct 31, 2013 - Addressing the Chemical Sorcery of “GaI”: Benefits of Solid-State Analysis Aiding in the Synthesis of P→Ga Coordination Compounds...
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Univalent Gallium Salts of Weakly Coordinating Anions: Effective Initiators/Catalysts for the Synthesis of Highly Reactive Polyisobutylene Martin R. Lichtenthaler,† Alexander Higelin,† Anne Kraft,† Sarah Hughes,‡ Alberto Steffani,§ Dietmar A. Plattner,§ John M. Slattery,∥ and Ingo Krossing*,† †

Institut für Anorganische und Analytische Chemie, Freiburger Materialforschungszentrum (FMF) and Freiburg Institute for Advanced Studies (FRIAS) Section Soft Matter Science, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany ‡ Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada § Institut für Organische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany ∥ Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K. S Supporting Information *

ABSTRACT: The scope of the univalent gallium salts [Ga(C6H5F)2]+[Al(ORF)4]− and the new completely characterized [Ga(1,3,5-Me3C6H3)2]+[Al(ORF)4]− (RF = C(CF3)3) was investigated in terms of initiating or catalyzing the synthesis of highly reactive poly(2-methylpropylene)highly reactive polyisobutylene (HR-PIB)in several solvents. A series of polymerization reactions proved the high efficiency and quality of the univalent gallium salts for the polymerization of isobutylene. The best results were obtained using very low concentrations of [Ga(C6H5F)2]+[Al(ORF)4]− (down to 0.007 mol%) while working at reaction temperatures of up to ±0 °C and in the noncarcinogenic and non-water hazardous solvent toluene. Under these conditions, HR-PIB with an α-content of terminal olefinic double bonds up to 91 mol% and a molecular weight of 1000−2000 was obtained in good yields. Upon changing [Ga(C6H5F)2]+[Al(ORF)4]− for the electron richer [Ga(1,3,5-Me3C6H3)2]+[Al(ORF)4]−, polymerization temperatures could be increased to +10 °C. The reactivity of the gallium(I) cations therefore seems to be tunable through ligand exchange reactions. Experimental results, density functional theory calculations, and mass spectrometric investigations point toward a coordinative polymerization mechanism.

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INTRODUCTION

1). Being sterically exposed, the terminal olefinic double bonds are more reactive than their internal analogues and therefore can be easily functionalized.4 These functionalized derivatives as well as highly reactive polyisobutylene (HR-PIB) itself are essential additives for lube oils and fuels.5 In addition to butyl rubber (a copolymer of PIB with small amounts of isoprene),

The classical commercial process of polymerizing isobutylene (IB) dates back to the year 1931.1 Using the strong Lewis acid boron trifluoride (BF3) in combination with a proton-donating species, high-molecular-weight polyisobutylene (PIB) was prepared.1,2 Over the past 80 years, this approach has been augmented by several proton-, carbocation-, and metal-based systems, allowing for the syntheses of low- to high-molecularweight PIB. Depending on its molecular weight (Mn), PIB can be divided into three major groups. High-molecular-weight PIB (Mn > 200000), a rubberlike and physically harmless compound, has gradually substituted natural chicle as the main chewing gum base.3 Medium-molecular-weight PIB (Mn = 40000−85000 g mol−1), a highly viscous liquid with an excellent barrier against moisture and various gases, is used as an adhesive or in sealants.3 The main application of lowmolecular-weight PIB (Mn = 1000−2300 g mol−1), a honeylike and colorless liquid, relies on the PIB being highly reactive, requiring that the polymer features more than 60 mol% of terminal olefinic double bonds (α-content >60 mol%) (Figure © XXXX American Chemical Society

Figure 1. Terminal olefinic double bonds contributing to the αcontent of the PIB and internal olefinic double bonds to the β-content. Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Received: June 14, 2013

A

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Table 1. Compilation of Systems That Are Capable of Initiating/Catalyzing the Synthesis of HR-PIBa system AlCl3OBu2·CumOH FeCl3·H2O/i-Pr2O BF3·C6H11OH EtZnCl/t-BuCl 2-chloro-2,4,4-trimethylpentane/TiCl4; S(tBu)2/MeOH [H(OEt2)2]+[Al(ORF)4]− [H(Oi-Pr2)2]+[Al(ORF)4]− H+[Al(ORF)4]− [Mn(NCMe)6]2+[N2C3H3(B(C6F5)3)2]−2 [Mo(NCMe)5Cl]2+[N2C3H3(B(C6F5)3)2]−2 [Mo(NCMe)5Cl]2+[B(C6F5)4]−2 [Fe(NCMe)6]2+[B(C6F5)4]−2 [Cu(NCPh)6]2+[B(C6F5)4]−2 [Zn(NCMe)6]2+[Al(ORF)4]−2

conversn (%)

Mn (g mol−1)

α-contentc (mol%)

3 10 10 30 >30

62 65 (22) (7) −d

1560 2200 1600 (26900) 2200

91 90 92 92 100

9 10 11 12 17

45 −d 30 (1200) 120 15 (300) 15 60

−d 76 55 83 67 (16) 88 73 87

2390 (11993) (6800) 2400 (600) 800 900 (600) 1500

91 >95 92 75 90 88 62 76 72

19−21 22 19,21 28 29 27 30 31 32

cb (mol%)

solvent

T (°C)

t (min)

(2.418) (0.275) (0.476) 0.059 (0.270)

n-Hex/(CH2Cl2) (CH2Cl2) n-Hex/(CH2Cl2) (CH2Cl2) n-Hex/CH3Cl

(−40) −10 −20 +20 (−60)

0.020 0.038 0.023 0.028 0.003 0.003 0.003 0.003 0.003

n-Hex/(CH2Cl2) (CH2Cl2) toluene (CH2Cl2) toluene toluene (CH2Cl2) toluene (CH2Cl2)

(−40) (−50) (−30) +30 +30 +30 +30 +30 +30

ref

a

Disadvantages of each system are given in parentheses. bConcentrations of the used initiators/catalysts refer to the amount of the IB used in each synthesis. If the initiator is composed of a Lewis acid and a cocatalyst, the concentration of the Lewis acid was used for calculation. cThe content of terminal olefinic double bonds of HR-PIB is referred to as α-content and can be calculated using 1H NMR spectroscopy. dNo value stated.

obtained primary products. The initiators, however, have to enable a living or quasi-living polymerization of IB, as only these can be terminated in a controlled manner. Hence, Kennedy, Iván, and more recently Storey have developed several two- or three-step processes yielding HR-PIB.16 For example, Storey et al. quenched living polymerizations of IB with organic sulfides SR2 (R = t-Bu, i-Pr (CH(CH3)2), phenyl, n-alkyl) and subsequently treated the obtained PIB−sulfonium ions with bases, such as amines or alcohols, thus obtaining HRPIB with an α-content of up to 100 mol%.17 However, the temperatures for the polymerization and the quenching process are as low as −60 °C. New Initiator/Catalyst Systems for the Polymerization of IB. To our knowledge, new systems are based on protons or metals, but not on carbocations. Thus, highly active heterogeneous initiators that contain both Lewis and Brønsted acid sites were developed for the polymerization of IB by supporting yttrium or lanthanide chloride salts on silica.18 Though the systems yield PIB in excellent conversions and at polymerization temperatures of +20 °C, the α-content of the obtained PIB was only 15 mol%. Our group together with BASF19−23 showed that cationic Brønsted acids, such as [H(OR2)2]+[Al(ORF)4]− (R = Et, i-Pr; RF = C(CF3)3), initiate the polymerization of IB with high reactivity, which is again due to the weakly coordinating nature of the anion used. In comparison to the related and similarly performing [B(C6F5)4]− WCAs, the implemented [Al(ORF)4]− WCAs feature the advantage that their synthesis is relatively simple and circumvents the need for handling explosive LiC6F5.24 With the very good [Al(ORF)4]− anion, provided that reaction temperatures remain low (around −50 °C), polymerizations yield HRPIB with a reproducible α-content of 90−95 mol%.21,23 Turning to metal-based initiators, the latter often are cationic metallocenium alkyl complexes, such as [Cpx2MR]+ (M = Ti, Zr, Hf, Y, Al; Cpx = C5H5, C5Me5, C5H4SiMe3; R = alkyl), and the corresponding anions are usually WCAs. These systems were developed and improved by many groups, and Bochmann gives a good overview.25 However, the metallocenium alkyl complexes lead to PIBs with Mn values too high to be attributed as highly reactive. True catalysts for the polymerization of IB have been developed by only very few groups. In 2003, Nuyken

HR-PIB is therefore one of the most important classes of IB polymers.6 Industrially, HR-PIB is still produced via the classical approach: using Lewis acids combined with matching cocatalysts.7 However, working with combinations of BF3 and isopropyl alcohol or methanol (MeOH), low polymerization temperatures down to −60 °C are essential.8 Without sufficient cooling, sudden temperature changes can occur, which pose a safety risk and potentially lead to PIB with a broad spectrum of olefinic double bonds rather than to HR-PIB. Overall, there is a need to improve the established production methods of HRPIB. Improving the Classical Approach. Among others, Kostjuk, Wu, and Bochmann introduced a number of promising 1:1 adducts of Lewis acids and cocatalysts, initiating the protic or carbocationic polymerization of IB with high reactivity and leading to HR-PIB with an α-content higher than 90 mol%.9−12 For example, Bochmann’s “EtZnCl/t-BuCl” system (Et = C2H5, t-Bu = C(CH3)3) enables polymerization temperatures of up to +20 °C but relies on carcinogenic and water-hazardous CH2Cl2 as a solvent and delivers mediuminstead of low-molecular-weight HR-PIB with low yield.12 Applying multifunctional cocatalysts is a recent supplement, through which Baird et al. established a number of 2:1 adducts of B(C6F5)3 and different carboxylic acids CH3(CH2)nCO2H (n = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20) that initiate the polymerization of IB with very high reactivity.13 Structurally, the remarkable reactivity of these 2:1 adducts, e.g. H+[nC17H35CO2{B(C6F5)3}2]−, results from the weakly coordinating properties of the corresponding anion. Thus, weakly coordinating anions (WCAs)voluminous and singly charged anions with poly- or perfluorinated surfacesare capable of stabilizing the highly reactive cationic polymerization intermediates and are therefore the key to further progress.14 Low reaction temperatures, down to −50 °C, are indispensable, however, and the obtained IB polymers feature high Mn values.13 In a similar approach Piers and Collins applied chelating diboranes (e.g. o-C6F4[B(C6F5)2]2 or o-C6F4(9BC12F8)2) in combination with cumyl chloride, cumyl methyl ether, or MeOH.15 Another approach combines cationic initiated IB polymerizations with well-defined termination methods and, if necessary, with subsequent treatment of the B

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to compound 1 (Scheme 1). Upon removal of the solvent C6H5F, 2 resulted as a slight grayish solid which was stored at room temperature under an argon atmosphere.

et al. introduced different transition-metal complexes such as [Mn(NCMe)6]2+[A]−2 (A = boron-based WCAs such as [N2C3H3{B(C6F5)3}2]−, [B(C6F5)4]−) that catalyze the polymerization of IB with high activities, yielding HR-PIB with αcontents higher than 95 mol%.26 Since then, especially Kühn et al. have refined the architecture of the transition-metal complexes, thus lowering initial reaction times of 20 h to 15 min.27 The key advantage of the catalytic systems is that they favor a less exothermic polymerization mechanism in comparison to their initiating congeners, thus permitting polymerization temperatures as high as +30 °C. Some drawbacks, however, are the use of CH2Cl2 as solvent, as well as partially moderate yields and/or moderate α-contents of the obtained HR-PIB. In summary, most syntheses of HR-PIB can be improved upon in at least one point: e.g., low reaction temperatures, CH2Cl2 as solvent, long reaction times, or relatively high concentrations of the initiating/catalyzing species. Table 1 shows a compilation of the discussed systems that are capable of yielding PIB in its highly reactive form. Disadvantages of each system are given in parentheses. In this work, the univalent group 13 cation salt [Ga(C6H5F)2]+[Al(ORF)4]− was tested for initiating or catalyzing the synthesis of HR-PIB in several solvents. During this course, a novel modification of the [Ga(C6H5F)2]+[Al(ORF)4]− salt was obtained. In addition, the new [Ga(1,3,5-Me3C6H3)2]+[Al(ORF)4]− salt was prepared, completely characterized, and tested for the polymerization of IB. Both univalent gallium salts initiate/catalyze the polymerization of IB with significant reactivity and yield HR-PIB in excellent conversions. The polymerization mechanism was investigated by experimental and computational methods.

Scheme 1. Synthesis of 2 via a Ligand Exchange Reactiona

a

The reaction enthalpy (ΔrH°) and the Gibbs free energy (ΔrG°) of the reaction were calculated at the RI-BP86/SV(P) level, in the gas phase and at 298.15 K. They are clearly exothermic and exergonic.

A direct synthesis of 2 by applying Ag+[Al(ORF)4]− and elemental gallium in 1,3,5-Me3C6H3 was not successful. The initial presence of C6H5F is therefore crucial to stabilize gallium in its univalent oxidation state. Crystal Structure Determination of the Gallium(I) Salts. During the synthesis of 1, a novel modification of the C6H5F stabilized gallium(I) salt was obtained (Figure 2).

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RESULTS AND DISCUSSION Some Background to Gallium(I) Chemistry. As a group 13 element, the oxidation state +1 of gallium is thermodynamically only metastable.33 However, many univalent gallium compounds have been stabilized in the past decades: “GaI”,34 which in fact contains mixed-valence Ga 2 I 3 3 5 or (Ga+)2[Ga2I6]2− 36 and therefore is of limited use as a gallium(I) starting material; Ga+[GaCl4]−,37 which gave rise to the first gallium(I) arene complexes38 but regarding further gallium(I) chemistry is limited by the chemically reactive and coordinating halogallate anions [GaX4]− (X = halide anion);38 solid and dissolved GaCl,39 which is metastable and therefore disproportionates above temperatures of ±0 °C.40 Turning to the materials used for this article, our group developed a novel simple route to stable, univalent gallium salts of WCAs by oxidation of elemental gallium with the silver(I) salt Ag+[Al(ORF)4]− (RF = C(CF3)3) in fluorobenzene (C6H5F) and with ultrasonic activation.41 In contrast to the aforementioned Ga+[GaX4]− compounds, the [Al(ORF)4]− anions hardly interact with the obtained gallium(I) cations and therefore exclude con- and disproportionation pathways of the latter.41,42 The weak π-base C6H5F makes [Ga(C6H5F)2]+[Al(ORF)4]− an excellent precursor for further gallium(I) chemistry, e.g. homoleptic phosphine41,43 or carbene complexes,44 as well as a crown ether complex of univalent gallium.45 Synthesis and Characterization of the Gallium(I) Salts for This Work. [Ga(C6H5F)2]+[Al(ORF)4]− (1) was prepared on a multigram scale as a colorless solid and was stored at room temperature under an argon atmosphere. [Ga(1,3,5Me3C6H3)2]+[Al(ORF)4]− (2) was prepared by a ligand exchange reaction using 2 equiv of 1,3,5-Me3C6H3 with respect

Figure 2. Molecular structure of 1. The thermal displacement ellipsoid of the gallium(I) cation is shown at 50% probability. The complete asymmetric unit of the crystal structure is shown in the Supporting Information.

The asymmetric unit of 1 contains only one [Ga(C6H5F)2]+ cation and therefore differs from the crystal structure obtained by Slattery et al. (JS), which contains both a [Ga(C6H5F)2]+ as well as a [Ga(C6H5F)3]+ cation.41 In addition, the structural parameters of the [Ga(C6H5F)2]+ cations in 1 and JS deviate so that the angle between the centroids of the C6H5F ligands and the gallium(I) cation in 1 is much larger than in JS, a consequence most likely due to packing effects. In comparison to JS, the interactions between the gallium(I) cation and its C6H5F ligands in 1 are more distinct, whereas the interactions with the corresponding [Al(ORF)4]− anion are attenuated. Hence, the gallium(I) cation in 1 only shares one Ga−F contact, with its corresponding [Al(ORF)4]− anion being shorter than the sum of the van der Waals radii (Table 2). Compound 2 was crystallized from highly concentrated solutions of the reactants in C6H5F; its single-crystal X-ray structure was determined (Figure 3) and compared to the [Ga(arene)2]+ cations of JS and 1 (Table 2). Multinuclear Magnetic Resonance Spectroscopic Analysis of the Gallium(I) Salts. Compounds 1 and 2 C

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Table 2. Comparison of Key Structural Parameters for [Ga(arene)2]+ in 1 and 2 in Comparison to the [Ga(C6H5F)2]+ Cation of Slattery et al. (JS) min Ga−C (Å) max Ga−C (Å) av Ga−centa (Å) cent−Ga−centb (deg) closest Ga−F contact (Å) no. of Ga−F contactsc

JS

1

2

2.913 3.084 2.669 141.7 3.054 2

2.865 3.027 2.622 170.3 3.243 1

2.857 3.024 2.595 140.9 3.480 0

a

Average distance from the gallium(I) cation to the centroids of the C6 rings. bAngle formed by the two centroids of the C6 ring and the gallium(I) cation. cNumber of Ga−F contacts shorter than the sum of the van der Waals radii (3.34 Å).

Figure 4. 71Ga NMR spectra of 1 (20 mg, 16 μmol; red trace) and of 2 (20 mg, 16 μmol; blue trace) recorded in o-C6H4F2 (700 μL) at +25 °C. The δ(71Ga) signal of 1 occurred at −758 ppm and featured a fwhm of 250 Hz; the δ(71Ga) signal of 2 occurred at −739 ppm while featuring a fwhm of 300 Hz.

shielded. This explanation is in agreement with the known crystal structure of JS.41 A solution of 2 in o-C6H4F2 appears to still feature the isolated [Ga(1,3,5-Me3C6H3)2]+ cations, as in the solid state. Apparently, the larger 1,3,5-Me3C6H3 is too bulky to allow for the formation of a mixed [Ga(1,3,5Me3C6H3)2(o-C6H4F2)]+ cation (calculations at the RI-BP86/ SV(P) level did not converge for this process). Therefore, the 71 Ga resonances of 2 occurred at a lower field. This hypothesis is supported by investigations of Schmidbaur et al., who found that exchange reactions at gallium(I) centers have very low activation barriers, depending on the nature of the ligand.38 Further indications supporting the occurrence of [Ga(C 6 H 5 F) 2 (o-C 6 H 4 F 2 )] + cations for 1 and [Ga(1,3,5Me3C6H3)2]+ cations for 2 in o-C6H4F2 were the full line widths at half-maximum (fwhm) of the respective 71Ga resonances. That is, the fwhm for 1 (250 Hz) was smaller than that for 2 (300 Hz). With a higher symmetry, the trisarene cations in 1 featured longer relaxation times and therefore a sharper 71Ga resonance in comparison to the lower symmetry bis-arene cations in 2 (Figure 4).38,49,51 The 27Al and 19F NMR spectra in o-C6H4F2 both revealed clean reactions with intact [Al(ORF)4]− anions (δ(27Al) 33.8 ppm and δ(19F) −74.9 ppm for 1 and 2; see also the Supporting Information).52,53 In addition, the 19F NMR spectrum of 1 featured a resonance at −111.8 ppm (triplet of triplets (3J(H,F) = 8.9 Hz, 4J(H,F) = 5.7 Hz)) clearly attributable to the C6H5F ligands of the gallium(I) complex of 1 (Figure 2 in the Supporting Information).52 The 1,3,5-Me3C6H3 ligands in 2 featured a characteristic singlet at δ(1H) 2.37 ppm induced by its methyl groups (Figure 5 in the Supporting Information).52 Initial Solvent Screening for the Polymerization of IB. Relatively low initiator/catalyst loads of 0.01−0.007 mol% (about 20 mg) of 1 were applied in all reactions. The solvent of choice should be non-carcinogenic, non-water hazardous, inexpensive, and volatile. In n-pentane, due to its low polarity and insolubility of 1, no polymerization of IB was observed (Table 3, entry 1). A heterogeneous initiating or catalytic effect of 1 on the polymerization of IB therefore seems to be unlikely. Also in the donor solvents diethyl ether (Et2O) and acetonitrile (NCMe) no polymerization was observed (Table 3, entries 2 and 3), as both solvents inhibit a conceivable cationic initiated or a coordinative polymerization mechanism.54,55 CH2Cl2 is, despite its unfavorable properties, a typical solvent for the

Figure 3. Molecular structure of the [Ga(1,3,5-Me3C6H3)2]+ cation in 2. The thermal displacement ellipsoid of the gallium(I) cation is shown at 50% probability. The asymmetric unit also contains a cocrystallized 1,3,5-Me3C6H3 molecule (see also the Supporting Information).

were dissolved in o-difluorobenzene (o-C6H4F2) and characterized by NMR spectroscopy. Beyond the crystal structure determination, the 71Ga NMR spectra are the most significant indicators for the successful stabilization of the +1 oxidation state of gallium. Hence, the 71Ga nuclei (abundance 39.9%, spin 3 /2, quadrupole moment 10.7 fm2) in 1 and 2 featured characteristic sharp signals at chemical shifts (δ(71Ga)) of −758 ppm for 1 and of −739 ppm for 2.46,47 The pronounced highfield shifts are attributable to a reduction of the deshielding paramagnetic contribution (σpara) to δ(71Ga).38,48−50 As σpara inversely correlates with the gap (ΔE) between the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the [Ga(arene)2]+ complexes, stronger interactions between the aromatic ligands and the gallium(I) cations lead to higher ΔE values, smaller σpara values, and conclusively to shifts toward higher field.50,51 Unexpectedly, the 71 Ga resonance of 1 occurred at a higher field than that of 2 (Figure 4). Thus, one would initially anticipate a stronger interaction between the gallium(I) cation and the electron richer ligand 1,3,5-Me3C6H3 in 2 than with the electron poorer ligand C6H5F in 1, as supported by the analysis of the molecular structures of both [Ga(arene)2]+ complexes in Table 2. However, the situation in solution differs in that 1 may bind a third aromatic ligand, e.g. o-C6H4F2 (cf. ΔrH° = −39.2 kJ mol−1, ΔrG° = +1.13 kJ mol−1; calculated at the RI-BP86/ SV(P) level), causing the gallium(I) cations in 1 to be better D

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Table 3. Solvent Effects on the Polymerization of IB using 1a entry

solvent

c(1) (mol%)

T (°C)

t (min)

conversn (%)

1 2 3 4 5

n-pentane Et2O NCMe CH2Cl2 toluene

0.007 0.01 0.007 0.007 0.007

−20 −20 −20 −20 −10c

70 70 60 1b 120

−d −d −d 100 71

observed in proton- or carbocation-initiated polymerizations, the latter are a first hint pointing toward a cationic initiating property of 1 in CH2Cl2. This assumption was supported by the inverse relationship between T and Mn (Table 4, columns 2 and 6). Hence, elimination reactions in cation-initiated polymerizations are associated with higher activation barriers than propagation reactions and preferably occur at higher T values.55 The obtained IB polymers featured broad molecular weight distributions, and a clear tendency among their polydispersity indices (PDIs) was not observed (Table 4, column 7). The polymerizations were therefore unlikely to be of a living nature and potentially featured slow initiation and/or transfer reactions.55 Conversions of IB directly correlated with T (Table 4, column 4). All in all, using 1 in CH2Cl2, PIBs with high α-contents and relatively low Mn values were obtained with T values as low as −40 °C. However, the Mn values were still too high to classify the IB polymers as highly reactive and the conversions of IB at such T values were only moderate. Polymerizations of IB in Toluene. The behavior of 1 in toluene significantly differed from its behavior in CH2Cl2 (Table 5). Using the same concentrations of 1 (0.007 mol%) in toluene, T values had to be as high as −10 °C to achieve any conversion of IB at all, and reasonable conversions of IB were only obtained with reaction times (t) as long as 2 h: i.e., the kinetics in toluene and CH2Cl2 notably differed. Regarding the extent of the kinetic changes, we believe that the latter not only are attributable to the known solvent dependency of cationic chain growth polymerizations55 but also suggest that 1 in toluene promotes a different, i.e. a coordinative, polymerization mechanism for IB. This hypothesis is supported by room temperature model polymerizations conducted in flame-sealed NMR tubes that showed much faster reaction rates in CH2Cl2 than in toluene (Figures 9 and 10 in the Supporting Information). The proposed mechanism will be discussed in detail in Quantum-Chemical Investigations, and here we only want to point out that toluene, due to its increased π-donating properties and excess quantity, presumably substitutes C6H5F as a ligand in 1 (cf. ΔrH° = −38.9 kJ mol−1, ΔrG° = −32.5 kJ mol −1 ; calculated at the RI-BP86/SV(P) level). [Ga(toluene)2]+ cations were therefore likely to initiate/catalyze the polymerization of IB. Varying the concentration of 1 in toluene had only a small effect. However, the temperature T and its correlations with the α-content, Mn, and the conversion of IB were crucial to successfully synthesize PIB with the desired criteria. Though partially attributable to the chain transfer propagating properties of toluene,27 the reproducible differing characteristics of the obtained PIB also point toward a changed polymerization mechanism (Figure 11 and 12 in the Supporting Information). Hence, PIB with high α-contents and low Mn values was synthesized. To our knowledge this is the first successful synthesis of HR-PIB by solely applying a maingroup-metal, since the reported aluminum-based initiators did not yield PIB in its highly reactive form.58 In comparison to the IB polymerizations in CH2Cl2, the obtained HR-PIB in toluene featured only small olefinic impurities (Figure 5) and the reproduction of preliminary polymerization results was straightforward. Because of the promising results, the gallium(I) salt 2 was also tested in toluene for its initiating/catalyzing properties on the polymerization of IB (Table 6). Though the behavior of 2 in toluene resembled that of 1, some important differences were observed. After applying 2 at identical T and t values, it

a

Condensed IB (20 mL) was added to a solution of 1 (20 mg, 0.007 mol% referring to the amount of IB used) in the respective solvent (50 mL) at the temperature (T) indicated. The reaction mixture was stirred for the given time and quenched by adding i-PrOH (5 mL). bA sudden temperature jump (ΔT) of 62 °C occurred upon addition of IB. The polymerization was quenched immediately by adding i-PrOH (5 mL). cBelow −10 °C no conversion of IB was observed. dNo value stated.

polymerization of IB.56 Using 1 in CH2Cl2, high yields of PIB were obtained in less than 1 min (Table 3, entry 4). As a more economical and ecological solvent replacement, toluene was also tested giving a 71% conversion (conversn) of PIB within 2 h (Table 3, entry 5). From Table 3, CH2Cl2 and toluene emerged as the most promising solvents for polymerizing IB and were investigated in more detail in the next sections. Polymerizations of IB in CH2Cl2. Working in CH2Cl2, manageable concentrations of very reactive 1 to work with were as low as 0.007 mol% (20 mg). Varying the concentration of 1 had only a small effect on the polymerization of IB, and higher concentrations led to a slight decrease of the Mn value of the obtained PIB.57 A blank run in CH2Cl2 was futile. The polymerization temperature (T), on the other hand, was the key element to successfully synthesize PIB, and distinct correlations among T, the α-content, Mn, and the conversion (conversn) of IB were observed (Table 4). While low T values Table 4. Polymerization of IB using 1 in CH2Cl2a entry

T (°C)

t (min)

conversn (%)

α-contentb (mol%)

Mn (g mol−1)c

PDId (mol%)

1 2 3 4

−50 −40 −30 −20

60 64 1e 1e

24 45 100 100

94 81 45 43

12130 3520 2740 2670

2.9 2.3 4.0 5.7

a

Condensed IB (20 mL) was added to a solution of 1 (20 mg, 0.007 mol%, referring to the amount of IB used) in CH2Cl2 (50 mL) at the given T. The reaction mixture was stirred for the given time and quenched by adding i-PrOH (5 mL). bBoth α- and β-contents were calculated using 1H NMR spectroscopy. The terminal olefinic double bonds featured characteristic signals at δ(1H) 4.64, 4.66, and 4.85 ppm, whereas the internal olefinic double bonds featured one signal at δ(1H) 5.15 ppm. cThe Mn value of the obtained PIB was calculated using 1H NMR spectroscopy and additionally validated by gel permeation chromatography (GPC) measurements. As the values calculated by 1H NMR spectroscopy are more precise, they are shown here. dThe polydispersity indices (PDI) of the obtained PIBs were measured using GPC. eA ΔT value of 63 °C occurred upon addition of IB. The polymerization was quenched immediately by adding i-PrOH (5 mL).

led to high α-contents, high T values promoted the formation of internal olefinic double bonds and unsaturated side products (Table 4, columns 2 and 5). From −30 °C onward, sudden temperature jumps (ΔT) and concomitant fast reaction rates occurred once IB was added to a solution of 1 in CH2Cl2 (Table 4, entries 3 and 4). As such ΔT values are usually E

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Table 5. Polymerization of IB using 1 in Toluenea entry

T (°C)

conversn (%)

α-contentb (mol%)

Mn (g mol−1)c

PDId (mol%)

1 2 3e

−10 −5 ±0

71 78 81

93 75 72

1980 1840 1140

2.7 2.2 2.5

a

Condensed IB (20 mL) was added to a solution of 1 (0.007 mol%, referring to the amount of IB used) in toluene (50 mL) at the given T. The reaction mixture was stirred for 120 min and quenched by adding i-PrOH (5 mL). bBoth α- and β-contents were calculated using 1H NMR spectroscopy. The terminal olefinic double bonds featured characteristic signals at δ(1H) 4.64 and 4.85 ppm, whereas the internal olefinic double bonds featured one signal at δ(1H) 5.15 ppm. cThe Mn values of the obtained PIB were calculated using 1H NMR spectroscopy and additionally validated by GPC measurements. As the values calculated by 1H NMR spectroscopy are more precise, they are shown here. dThe PDI values of the obtained PIBs were measured using GPC. eA ΔT of 50 °C occurred upon addition of IB. The polymerization was quenched immediately by adding iPrOH (5 mL).

relationship between T and the α-content of the obtained IB polymers, which was direct and not inverse (Table 6, columns 2 and 4). The relations among T, the α-content, and Mn enabled us to synthesize PIB at relatively high T values (up to +10 °C before a ΔT occurred) with α-contents of 79 mol% and conversions of 71% but with Mn values too low to be classified as HR-PIB. Overall, the reactivity of the univalent gallium salts seems to be tunable by varying the aromatic ligands. Using that fact, we expect that one can adjust the α-content and the Mn of the obtained PIB in future work. Quantum-Chemical Investigations. To further elucidate the polymerization mechanism of 1 and 2 in toluene, we performed orienting density functional theory (DFT) calculations at the RI-BP86/SV(P) level.60 Because of computing capacity, the [Al(ORF)4]− anions were excluded from the calculations. However, it should be noted that the [Al(ORF)4]− anions play a crucial role in the proposed polymerization mechanism: they hardly interact with the cationic intermediates and therefore not only have an accelerating effect on the polymerization but also exclude possible decomposition pathways. In the following, we investigated the primary interactions between the [Ga(arene)2]+ cations (arene = C6H5F, toluene, 1,3,5-Me3C6H3) and one IB unit as well as the initial sequence, the propagation and the termination of the polymerization of IB. The primary interactions between the [Ga(arene)2]+ cations and one IB unit are independent of the nature of the polymerization mechanism and attributable to the interaction between the frontier molecular orbitals (FMO) of the reactants (Figure 6). As the [Ga(arene)2]+ cations feature both vacant 4p orbitals and a lone pair of electrons, they are ambiphilic and can act as a Lewis acid or as a Lewis base.61 However, with the ΔE values between the LUMOs of the [Ga(arene)2]+ cations and the HOMO of an IB unit being distinctly smaller than vice versa, the gallium(I) cations are likely to act as Lewis acids (see

Figure 5. Olefinic region of a 1H NMR spectrum of HR-PIB (20 mg) in CDCl3 (700 μL) recorded at +25 °C. The IB polymer was synthesized using 1 (0.007 mol% referring to the used amount of IB) in toluene at T = −10 °C. The terminal olefinic double bonds featured characteristic resonances at δ(1H) = 4.65 and 4.86 ppm and added up to an α-content of 93 mol%. The internal olefinic double bonds (βcontent) featured one signal at δ1H = 5.16 ppm. In addition, some minor olefinic impurities induced signals at δ(1H) 4.81, 4.98, and 5.12 ppm. The impurities were derived from different olefinic double bonds along the polymeric backbone and can be attributed to the following substructures: −C(CH3)2C(CH2)C(CH3)2− and −C(CH3) CHC(CH3)2−.28,59

was found that conversions of IB were smaller than for 1 (Table 6, column 3). For better conversions of IB one had to further increase T. Hence, electron-rich and sterically demanding ligands seem to have an intrinsic mitigating effect on the reactivity of the gallium(I) salts. We note that the electron-rich 1,3,5-Me3C6H3 ligands, are in contrast to C6H5F, unlikely to be substituted by toluene (cf. ΔrH° = +25.8 kJ mol−1, ΔrG° = +16.3 kJ mol−1; calculated at the RI-BP86/SV(P) level). Using 2 in toluene, the active species initiating the polymerization of IB should therefore be [Ga(1,3,5-Me3C6H3)2]+ cations. Another important difference between 1 and 2 was the Table 6. Polymerization of IB using 2 in Toluenea entry

T (°C)

conversn (%)

α-contentb (mol%)

Mn (g mol−1)c

PDId (mol%)

1 2 3e

−10 ±0 +10

16 60 71

59 69 79

1070 630 290

2.0 2.0 2.0

a

Condensed IB (20 mL) was added to a solution of 2 (0.007 mol%, referring to the amount of IB used) in toluene (50 mL) at the given T. The reaction mixture was stirred for 120 min and quenched by adding i-PrOH (5 mL). bBoth α- and β-contents were calculated using 1H NMR spectroscopy. The terminal olefinic double bonds featured characteristic signals at δ(1H) 4.65 and 4.86 ppm, whereas the internal olefinic double bonds featured one signal at δ(1H) 5.16 ppm. cThe Mn value of the obtained PIB was calculated using 1H NMR spectroscopy and additionally validated by GPC measurements. As the values calculated by 1H NMR spectroscopy are more precise, they are shown here. dThe PDI values of the obtained PIBs were measured using GPC. eA ΔT of 54 °C occurred upon addition of IB. The polymerization was quenched immediately by adding iPrOH (5 mL). F

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Figure 7. Schematic DFT investigation of the initial sequence of the polymerization of IB starting from [Ga(arene)2]+ cations. The shown reaction path is the energetically most favored. The intermediate formation of the mixed [Ga(arene)(IB)]+ cations are feasible (e.g., ΔrH° = +3.27 kJ mol−1 for [Ga(C6H5F)(IB)]+) but, for better visualization, are not shown here. Upon addition of the third IB unit, oxidative additions with concomitant cyclization become increasingly important. The short Ga−C bond distances in the cyclic substructures indicate that the oxidation state of gallium changes from +1 to +3. All structures were calculated at the RI-BP86/SV(P) level in the gas phase. The given standard reaction enthalpies (ΔrH°) are zero point energy corrected and correspond to 298.15 K and 1.013 bar.

Figure 6. Molecular orbital diagram for the interaction between the HOMO of an IB unit and the LUMO of the different [Ga(arene)2]+ cations (RI-BP86/SV(P) level, cutoff 0.08 au). The FMO energies were additionally calculated in toluene (εr = 2.38 F m−1) by COSMO at the RI-BP86/SV(P) level. The negligible lowering of the LUMO energy of the IB unit from −5.871 to −5.891 eV is not shown.

the Supporting Information). With the HOMO of an IB unit lying at −5.871 eV, respective interactions with the LUMO of a [Ga(C6H5F)2]+ (−5.991 eV) or a [Ga(toluene)2]+ cation (−5.516 eV) are better than the interaction with the LUMO of a [Ga(1,3,5-Me3C6H3)2]+ cation (−5.095 eV). Therefore, the [Ga(C6H5F)2]+ and [Ga(toluene)2]+ cations are more reactive intitiators/catalysts in comparison to the [Ga(1,3,5Me3C6H3)2]+ cation. This fact is in good agreement with the experimental results. To further simulate the interactions of the reactive species with toluene (relative permittivity εr = 2.38 F m−1) conductor-like screening model calculations (COSMO) were conducted at the RI-BP86/SV(P) level.62 The solvent interactions had a pronounced effect on the energy levels of the LUMOs of the [Ga(arene)2]+ cations: e.g., being raised by 1.76 eV in the case of the [Ga(C6H5F)2]+ cation. The solvation effect on the HOMO of the IB unit on the other hand was very small, with the HOMO of the IB unit only being lowered by 0.02 to −5.891 eV (not shown in Figure 6 for a better visualization). Taking these solvation effects into account, however, the [Ga(C6H5F)2]+ cation (−4.231 eV) and the [Ga(toluene)2]+ (−3.858 eV) cation are still more reactive than the [Ga(1,3,5-Me3C6H3)2]+ cation (−3.596 eV), as their energy gaps to the corresponding IB unit (−5.891 eV) are the smallest. The investigations of the FMOs are therefore in good agreement with the experimental results. Regarding the initial sequence of the IB polymerization, possible interactions between a gallium(I) cation and a number of IB units were investigated at the RI-BP86/SV(P) level (Figure 7).60 With the εr value of toluene being substantially smaller than the εr value of CH2Cl2 (8.93 F m−1), we believe that the results in the gas phase resemble a toluene rather than a CH2Cl2 environment. As a large excess of IB was applied, different [Ga(IB)x]+ adducts (x = 1, 2, 3, ...) are likely to form. The interactions between the gallium(I) cation and the first IB unit are primarily coordinative (d(sp2-C−Ga(I)) = 240 pm) and get weaker upon addition of the second IB unit (average d(sp2-C−Ga(I)) = 251 pm). However, on addition of a third IB unit, coordinative interactions and the formation of a fivemembered cyclic gallium(III) structure ([C8H16Ga]+) compete.

We believe that this [C8H16Ga]+ moiety is formed by an oxidative addition of a gallium(I) cation and two IB units, thus resulting in a cyclogalla(III)pentanium cation. As the cyclogalla(III)pentanium cation is covalently interacting with the third IB unit, the positive charge is mainly left on a C8H16GaCH2C+(CH3)2 moiety. Upon addition of a fourth IB unit, the formation of the cyclogalla(III) cations clearly becomes energetically favored, and thus a seven-membered ring (cyclogalla(III) heptanium cation) forms by insertion of the coordinated IB unit into the cyclogalla(III) pentanium ring. The fourth IB unit again covalently interacts with the enclosed gallium(III) cation (d(sp2-C−Ga(III)) = 229 pm). Figure 7 also includes the energetics of the formation of the [Ga(IB)x]+ adducts from the [Ga(arene)2]+ cations. The calculations show the intrinsic mitigating effect of the aromatic ligands and are in good agreement with the experimental results. We assume that the proposed cyclogalla(III) cations play an important role during the initiation of the polymerization of IB in toluene. Direct evidence for this hypothesis comes from investigations performed by Jordan et al., who established low-coordinate cyclic cationic aluminum(III) and gallium(III) amidinate complexes as initiators for various olefin polymerizations.63 Such gallium(III) species are extremely reactive and difficult to handle. Hence, the gallium(I) salts provide a temperature-stable and easily applicable storage form of the apparently initiating/ catalyzing gallium(III) species which are only formed during the polymerization of IB: a behavior more reminiscent of transition-metal complexes. However, how do the cyclogalla(III) cations contribute to the propagation of the polymerization of IB? To answer this question, we simulated different reaction paths at the RI-BP86/SV(P) level. The energetically most favored calculated route is shown in Figure 8 and resembles a transition-metal-catalyzed coordinative and not a cationic initiated chain growth polymerization mechanism, as one might have anticipated from the carbocationic structure of G

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[Ga(IB)x]+ complexes, the cations produced from electrospray were reacted with IB (p(IB) = up to 100 mTorr) in O1. In doing so, two new species with m/z 125 and 181 were observed. The characterization of the two cations by CID (p(Ar) = 0.08 mTorr in O2) resulted in loss of the IB molecules, thus suggesting a structural assignment of [Ga(IB)]+ for m/z 125 and of [Ga(IB)2]+ for m/z 181. As the loss of the IB molecules was stepwise and as the energy used for the CID was very low (ECM = 0.97 eV; see also the Supporting Information), it is obvious that both IB molecules coordinate as distinct ligands to the gallium(I) cation and that the formation of a cyclogalla(III) pentanium cation has not taken place yet. These findings are in perfect accordance with the energy diagram obtained from the DFT calculations (Figure 7). Finally, the reactivity of the [Ga(C6H5F)]+ cation was confirmed by a second gas-phase reaction. Hence, the selection of [Ga(C6H5F)]+ in Q1 and its reaction with IB (p(IB) = 0.22 mTorr) in O2 resulted in replacement of the C6H5F ligand by one and two IB molecules, respectively (Figure 9). At this

Figure 8. Proposed coordinative propagation mechanism. The shown reaction path is the energetically most favored. All structures were calculated at the RI-BP86/SV(P) level in the gas phase. The given standard reaction enthalpies (ΔrH°) are zero point energy corrected and correspond to 298.15 K and 1.013 bar.

the cyclogalla(III) pentanium cation. While the reactive species may form by β-hydride elimination and subsequent hydrogallation, thus accounting for the high α-content of the HRPIB, the actual propagating step consists of a migratory insertion. The slightly exothermic downhill trend of the entire calculated mechanism is in good agreement with the high T values at which polymerizations have to be conducted in toluene, thus supporting the validity of our calculations. In terms of a possible termination for the polymerization of IB, we currently can only speculate. Hence, the gallium(III) cations could be reductively eliminated from the polymeric chain and the originating gallium(I) cations trapped by toluene or 1,3,5Me3C6H3, thus regenerating the original initiating/catalyzing gallium(I) species, [Ga(toluene)2]+ or [Ga(1,3,5-Me3C6H3)2]+. While the calculated thermodynamics speak in favor of such a termination (ΔrH° = −102 kJ mol−1, ΔrG° = −58.4 kJ mol−1 for [Ga(toluene)2]+ and ΔrH° = −127 kJ mol−1, ΔrG° = −74.7 kJ mol−1 for [Ga(1,3,5-Me3C6H3)2]+, calculated at the RIBP86/SV(P) level for these processes), thus explaining the lower Mn values of PIB obtained using 2 in toluene, experimental verifications are still in progress: i.e., as the obtained IB polymers should feature distinct stereoirregularities at the polymeric chain end, we are currently investigating the latter in the course of a primary end group analysis (Figure 11, in the Supporting Information).

Figure 9. CID of the [Ga(C6H5F)]+ cation (m/z 165) in O2. The collision with IB yielded [Ga(IB)]+ (m/z 125) and [Ga(IB)2]+ (m/z 181) as the sole products. The formation of a mixed [Ga(C6H5F)(IB)]+ cation was not observed, as the latter is a weakly endothermic process (Figure 7) and thus unlikely to survive a collisional activation. Experimental conditions: ESI voltage 4.6 kV, Tcapillary = 100 °C, ECM = 0.97 eV, p(IB) in O2 = 0.22 mTorr.

point, the question arises as to why the uptake of more than two IB molecules cannot be observed. The answer lies within the formation of the [Ga(IB)x]+ adducts being a multicollision process, on the one hand, and the [Ga(IB)3]+ adduct resting in a shallow minimum, on the other hand (Figure 7). Thus, the [Ga(IB)3]+ adduct is unlikely to survive the collisional activation incurred by the multiple collision in O1 or O2. Further experiments to quantify the elementary reaction steps are in progress.

Electrospray Ionization Mass Spectrometry Experiments. In order to corroborate the importance of the [Ga(IB)x]+ complexes (x = 1, 2, ...), electrospray ionization mass spectrometry (ESI-MS) experiments were conducted. ESI-MS has proven to be a valuable tool in identifying reactive intermediates in organometallic cycles64 and in particular has been used to study the Ziegler−Natta oligomerization in the gas phase.65 A solution of 1 in C6H5F (10−3 mol L−1) was transferred from a sealed pressurized flask66 to the electrospray source. The configuration of the mass spectrometer was octopole−quadrupole−octopole−quadrupole (O1/Q1/O2/ Q2) as described earlier. 67 In Q2 scanning mode [Ga,C6,H5,F]+ was detected as the sole gallium-containing species at a mass to charge ratio (m/z) of 165 and 167a characteristic of the 69Ga/71Ga isotope ratio. As the collisioninduced dissociation (CID) of m/z 165 (p(Ar) = 0.06 mTorr) in O2 yielded only Ga+ (m/z 69) as a detectable fragment, it is safe to assume that m/z 165 corresponds to [Ga(C6H5F)]+, the first observed gallium(I) monoarene complex. To generate

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CONCLUSION The novel univalent gallium salts are promising species for initiating/catalyzing the synthesis of HR-PIB. According to the experimental results and the DFT calculations, the initiating or catalyzing nature of the gallium(I) salts and therefore the nature of the polymerization mechanism rely on the solvent used: i.e., we suggest a cationic initiated chain growth polymerization in CH2Cl2 and a coordinative polymerization in toluene for IB. This hypothesis is very well reflected in the solvent dependence of the polymerization kinetics or the properties of the obtained IB polymers and additionally H

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90.813(2)°, γ = 90°, V = 3857.1(4) Å3, Z = 4, ρcalcd = 2.116 Mg m−3, F(000) = 2384, λ = 0.71073 Å, T = 100(2) K, absorption coefficient 0.959 mm−1, absorption correction multiscan, Tmin = 0.6583, Tmax = 0.7464, GOF = 1.176, R1 = 6.88 and wR2 = 19.65 for reflections I > 2σ(I), R1 = 7.25 and wR2 = 19.81 for all reflections. Crystal data for 2 (CCDC 936462): C34H24O4F36AlGa, Mw = 1277.23, monoclinic, space group P21/c, a = 10.5086(3) Å, b = 20.9035(5) Å, c = 20.5072(5) Å, α = 90°, β = 94.1300(10)°, γ = 90°, V = 4493.0(2) Å3, Z = 4, ρcalcd = 1.888 Mg m−3, F(000) = 2512, λ = 0.71073 Å, T = 99.97 K, absorption coefficient 0.821 mm−1, absorption correction multiscan, Tmin = 0.613, Tmax = 0.7457, GOF = 1.012, R1 = 3.94 and wR2 = 8.56 for reflections I > 2σ(I), R1 = 7.52 and wR2 = 9.92 for all reflections. Preparation of 1. Ag+[Al(ORF)4]− (5.2 g, 4.8 mmol, 1.0 equiv) and gallium (0.7 g, 9.7 mmol, 2.0 equiv) were allowed to react in C6H5F (10 mL) under ultrasonic activation with concomitant heating to about +50 °C for a minimum of 24 h. The reaction mixture was cooled to +6 °C, decanted, and filtered through a G4 filter. The solvent was removed from the filtrate by vacuum distillation, and pure 1 (4.9 g, 4.0 mmol, 83%) was obtained as a colorless powder. From highly concentrated solutions (163 mmol L−1) of 1 in C6H5F colorless platelet-shaped crystals were obtained. The latter were suitable for single-crystal X-ray analysis. 1H NMR (400 MHz, o-C6H4F2, calibrated to o-C6H4F2 7.12 ppm,70 298 K): δ 6.83−7.42 (m, Ar−H). 19F NMR (377 MHz, o-C6H4F2, calibrated to o-C6H4F2 −139 ppm,52 298 K): δ −112 (tt, C6H5F, 3J(19F,1H) = 8.9 Hz, 4J(19F,1H) = 5.7 Hz), −74.9 (s, CF3). 27Al NMR (104 MHz, o-C6H4F2, calibrated to [Al(ORF)4]− 33.8 ppm,53 298 K): δ 33.8 (s). 71Ga NMR (122 MHz, o-C6H4F2, 298 K): δ −758 (s). Preparation of 2. 1 (2.5 g, 2.1 mmol, 1.0 equiv) was reacted with a bimolar amount of 1,3,5-Me3C6H3 (0.6 mL, 4.2 mmol, 2.0 equiv) in C6H5F (3 mL) under ultrasonic activation with concomitant heating to about +50 °C for 19 h. The reaction mixture was cooled to +6 °C, decanted, and filtered through a G4 filter. The solvent was removed from the filtrate by vacuum distillation, and pure 2 (2.4 g, 89%) was obtained as a slightly grayish powder. Using highly concentrated solutions (163 mmol L−1) of 1 in C6H5F as well as an equimolar amount of 1,3,5-Me3C6H3 (40.7 μL, 291 μmol), colorless plate-shaped crystals suitable for single-crystal X-ray analysis were obtained. 1H NMR (400 MHz, o-C6H4F2, calibrated to o-C6H4F2 7.12 ppm,70 298 K): δ 2.37 (9H, s, CH3), 6.83−7.36 (m, Ar−H). 19F NMR (377 MHz, o-C6H4F2, calibrated to o-C6H4F2 −139 ppm,52 298 K): δ −74.9 (s, CF3); 27Al NMR (104 MHz, o-C6H4F2, calibrated to [Al(ORF)4]− 33.8 ppm,53 298 K): δ 33.8 (s). 71Ga NMR (122 MHz, o-C6H4F2, 298 K): δ −739 (s). Polymerization of IB: General Instructions. 1 or 2 (16.3 μmol) was weighed into a two-necked Schlenk flask. After the chosen solvent was added (50 mL), the reaction mixture was cooled to the desired starting temperature. With T being monitored by a Voltcraft K101 digital thermometer, condensed IB (20 mL, 472 mmol) was added to the reaction mixture. In case of a ΔT, the polymerization was terminated by adding i-PrOH (5 mL) as soon as T reached its maximum. In cases where there was no ΔT, the reaction mixture was stirred for the time (t) indicated in the tables before adding i-PrOH (5 mL). In the following, water (50 mL) was added to the reaction mixture and the obtained emulsion was stirred for at least 60 min. In some cases evolution of a colorless gas was observed. The latter was attributable to unconverted IB. After addition of CH2Cl2 (50 mL), the combined organic phases were washed with water (2 × 50 mL) and subsequently isolated. The organic solvents were removed under reduced pressure. Thus, PIB was obtained as a medium or highly viscous colorless gel. The latter was characterized using 1H NMR spectroscopy and GPC. Though the obtained IB polymers were dried under high vacuum (10−2 mbar), they obtained traces of the solvent used. 1H NMR (400 MHz, CDCl3, calibrated to CHCl3 7.26 ppm,71 298 K): δ 1.00 (s, CH3, PIB), 1.04 (s, PIB), 1.10 (s, CH3, PIB), 1.12 (s, CH3, PIB | repeating unit), 1.27 (m, CH2CH3, ethyl acetate), 1.34 (s, PIB), 1.38 (s, CH2, PIB), 1.42 (s, CH2, PIB | repeating unit), 1.79 (s, PIB), 2.01 (s, CH2, PIB), 2.36 (s, CH3, toluene), 4.08 (m, CH, iPrOH), 4.65 (m, CH2, α-content, PIB), 4.81−4.83 (s, −C(CH3)2C(

confirmed by model polymerizations conducted in flame-sealed NMR tubes. Working in toluene, the gallium(I) salts can be seen as a relatively stable precursor for cyclogalla(III) cations which, being much more electrophilic than gallium in its oxidation state +1, initiate/catalyze the polymerization of IB with high reactivity: a behavior more reminiscent of transitionmetal complexes. The best results were obtained using very low concentrations (3.1 × 10−4 mol L−1) of 1 in toluene with T values as high as −10 °C. The α-content of the obtained IB polymer was very high (93 mol%), and the Mn value (1980 g mol−1) was within the range to classify the latter as highly reactive. In comparison to other initiators/catalysts for the polymerization of IB, the gallium(I) salts do not rely on expensive transition metals and are, to our knowledge, the first main-group-metal compounds able to initiate/catalyze the synthesis of HR-PIB. Furthermore, the gallium(I) salts enable IB polymerizations in non-chlorinated solvents at relatively high T values, thus providing a more economical and ecological approach. Finally, it seems that one may fine tune the reactivity of the gallium(I) salts by ligand exchange reactions, providing an excellent opportunity to directly control the α-content and Mn of the obtained PIB and thus to meet the needs of potential end users. Further investigations to isolate potential oxidative addition products of the gallium(I) salts and IB, to prove the relationship between the nature of the ligands and the properties of the gallium(I) cations, and to verify the proposed polymerization mechanism are currently underway.

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EXPERIMENTAL SECTION

General Procedures. Apart from isolation of the freshly synthesized PIB, all manipulations were performed using Schlenk or glovebox techniques with an argon atmosphere (H2O and O2