Highly Branched Polyisobutylene by Radical Polymerization under Li

Article pubs.acs.org/Macromolecules

Highly Branched Polyisobutylene by Radical Polymerization under Li[CB11(CH3)12] Catalysis Victoria Volkis,†,§ Richard K. Shoemaker,† and Josef Michl*,†,‡ †

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Boulder, Colorado 80309-0215, United States Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám 2, 16110 Prague 6, Czech Republic

‡

S Supporting Information *

ABSTRACT: In the presence of a nonoxidizing radical initiator, azo-tert-butane, and a high concentration of LiCB11(CH3)12, isobutylene undergoes thermal or light-induced radical polymerization to b-PIB, a highly branched polymer of modest molecular weight (mostly a few thousand and up to ∼25 000 g/mol based on GPC with polystyrene standards). The structure of b-PIB was elucidated by NMR spectroscopy of a low-molecular-weight fraction. The polymer is branched on every carbon atom of the main chain; one chain end carries an isobutenyl group, and the other carries a tert-butyl group originating in the initiator. The branches are short segments of l-PIB (linear polyisobutylene), on the average composed of five IB units. A mechanism of formation if this dendrimer-like structure is proposed.

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∼500. Presently, we provide full details of the formation and structural proof of b-PIB formed from IB under these conditions. Other terminal alkenes such as 1-hexene and 1octene have been oligomerized under the same conditions.9

INTRODUCTION The polymerization of α-olefins by a radical mechanism has been a challenge for several decades. Generally, radical polymerization of an alkene is difficult due to a high activation barrier for the radical attack on a double bond of alkene relative to abstraction of its allylic hydrogens, which forms a stable allylic radical.1 Among terminal alkenes, isobutylene (IB) is the least likely candidate for radical polymerization. IB is normally polymerized by cationic polymerization, initiated with a proton or carbocation derived from water, an alcohol, or an alkyl halide in the presence of a co-initiator, mostly of the Lewis acid type. Haloalkanes or aromatic hydrocarbons are usually used as reaction medium. Butyl rubber, i.e., a copolymer of IB and isoprene, is manufactured at low temperature in liquid ethylene.2 It was discovered accidentally3,4 that poorly solvated Li+ cations catalyze radical-initiated oligomerization and polymerization of unactivated alkenes in solvents of low donicity, and LiCB11(CH3)12 (1)5,6 was rapidly established as a representative catalyst of this type. More recently, it became clear that the process is strongly affected by the presence of small amounts of additives such as sulfolane and by the nature of the initiator used. Thus, the polymerization of IB initiated by peroxides or air proceeds by concurrent radical and cationic mechanism, the former producing a highly branched polymer that had not been described previously (b-PIB) and the latter producing linear polyisobutylene with a carborate anion as one chain end, l-PIB.7 In a brief communication,8 it was reported that polymerization of IB catalyzed by 1 but initiated under nonoxidizing conditions (azo-tert-butane, ATB) proceeds by the radical mechanism only and produces b-PIB with a degree of polymerization up to © XXXX American Chemical Society

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EXPERIMENTAL PART

General. All manipulations with air-sensitive materials were carried out with rigorous exclusion of oxygen and moisture in a dual manifold Schlenk line equipped with Airfree vacuum pump ((1−3) × 10−4 Torr). Argon was purified by passage through a MnO oxygen-removal column and a Davison 4 Å molecular sieve column. All chemicals were reagent grade (Aldrich) and were used as purchased. All solvents were anhydrous grade. CD2CMe2, 13CH2 CMe2, and CH2C(CD3)2 were purchased from CDN Isotopes, Inc. Me3NHCB11H12 was purchased from Katchem Ltd. (E. Krásnohorské 6, 11000 Prague1, Czech Republic) or synthesized by a published procedure.10 Sulfolane and acetone-d6 were dried with activated Davison 4 Å molecular sieves. 1,2-Dichloroethane (DCE), dichloromethane, and 1,1,2,2-tetrachloroethane were dried with CaH2 and then distilled under argon. Tetrahydrofuran (THF), ether, benzene, and toluene were distilled from Na/benzophenone. Catalyst Synthesis. The preparation of Li[CB11Me12] (1) and its regeneration after use generally followed published procedures8,11,12 but is described in more detail in the Supporting Information since the course of the polymerization is sensitive to the presence of minor additives. Polymerization of IB. All solvents are dried and freshly distilled immediately before use. The catalyst 1 is placed into a 50 mL stainless steel reactor and dried for 24 h under reduced pressure. The solvent Received: September 3, 2012 Revised: October 31, 2012

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dx.doi.org/10.1021/ma301850c | Macromolecules XXXX, XXX, XXX−XXX

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dimensional NMR spectra (gCOSY, gHSQC, gHSQC with boron decoupling) were measured with a Varian INOVA-500 spectrometer equipped with a Nalorac IDTXG-500-5 indirect triple resonance probe tuned for 1H observation (500.37 MHz), with 13C and 11B decoupling (125 and 160.4 MHz, respectively). 2H NMR experiments (76.7 MHz) were performed using a Nalorac broadband observe probe, with the 2H channel modified to yield 2H 90° pulses of 29.5 μs. 13C, DEPT, and DOSY spectra were measured with a Varian INOVA-400 spectrometer (400.16 MHz for 1H NMR), equipped with a broadband gradient probe capable of pulsed field gradients of 66 G/cm. 800 MHz NMR spectra were acquired using a Varian VNMRS-800 NMR spectrometer at 799.36 MHz for 1H observation and 201.01 MHz for 13 C observation. The instrument is equipped with a salt-tolerant, triple-resonance (H, C, N) cold probe, optimized for 1H observation as well as direct 13C detection. Because of the long ring-down time for 13 C observation on the cold-probe, digital signal processing with reverse linear prediction to correct the initial 21 data points in the FID was applied to achieve a flat baseline in the 13C NMR spectra. ESI-MS spectra were recorded with a Hewlett-Packard 5989 ESI mass spectrometer. MALDI-TOF was recorded with a Voyager DE STR TM spectrometer.

and the ATB initiator are added. The reactor is connected to a vacuum manifold equipped with a differential manometer and a 1 L glass flask. The solution in the reactor is frozen; the system is evacuated and then filled with dry isobutylene until the pressure reaches 1 atm. IB is refilled as necessary to keep 1 atm pressure. Reactor temperature is controlled with an oil bath. Unless stated otherwise, it was 80 °C and the polymerization time was 24 h. In photoinitiated polymerization experiments, the reaction mixture is placed in a 3 mL quartz reactor, equipped with circulating water bath for keeping constant temperature. The reactor is placed into a UV merry-go-round apparatus and irradiated at 254 nm (Rayonet Reactor model RPR-100, The Southern New England UV Co., Branford, CT). In all experiments the polymerization reaction is quenched with methanol, and all liquids are evaporated from the reactor under reduced pressure. The residue is extracted with 3 × 30 mL of hexane at 45−50 °C. Hexane is evaporated, and the polymeric powder product is dried in a vacuum oven at 100 °C for 24 h. The NMR spectrum is recorded at room temperature in chloroform-d3. The catalyst 1 is recovered essentially quantitatively as described below. Its HPLC and ESI-MS analysis showed no indications of decomposition. All samples were examined by GPC using a Waters instrument equipped with a refractive index (RI) detector and standard set of Styragel columns (THF, 35 °C). Relative calibration was based on the use of polystyrene standards (Aldrich). All data in tables refer to these conditions. Catalyst Regeneration. After a polymerization reaction the catalyst is precipitated with hexane, filtered, and dried under reduced pressure. The regeneration is best done after at least 500 mg of the used catalyst has been collected. It is dissolved in ether (100 mL), the solution is filtered and extracted with 20% aqueous solution of CsCl (3 × 20 mL), and ether is evaporated under reduced pressure. The residual solid is dissolved in CH2Cl2 and loaded onto a normal phase silica gel column packed in CH2Cl2 and first rinsed CH2Cl2 (100 mL) to remove low-molecular-weight organic impurities. A mixture of acetonitrile and CH2Cl2 (30−70 vol %) is used as a mobile phase. The column is washed with pure acetonitrile (50 mL), solvents are evaporated under reduced pressure, and the regenerated Cs[CB11Me12] is recrystallized from water at 70 °C and dried under reduced pressure overnight at 120 °C. The solution of this Cs salt in ether (100 mL) is filtered and extracted with a 20% aqueous solution of LiCl (3 × 20 mL), and ether is evaporated under reduced pressure. The regenerated catalyst is dried in a Kugelrohr under reduced pressure for 8 h at 80 °C and then for 24 h at 180 °C. Sulfolane is added if necessary to reach the optimal composition, monitored by 1H NMR. Triple Detection GPC. Selected polymer samples were also analyzed using triple detection GPC. The GPC system (Viscotek) includes an isocratic pump, vacuum-membrane degasser, nonthermostated autosampler for up to 200 samples, a set of two thermostated columns (G4000 and G6000 from Viscotek), and a triple detection system including a refractive index (RI) detector, a lightscattering (LS) detector with low (7°) angle and right (90°) angle detection, and a viscosity (IV) detector. THF (HPLC quality stabilized by Organox, 200 ppm) was used as the mobile phase. Columns and detectors were at 35 °C, and the working flow was 1 mL/min. The calibration and calculations were done using the Universal method for triple detection.13 For a narrow polystyrene standard (Viscotek) with Mw = 98 000 in THF solution, dn/dc was 0.185 mL/g (obtained from exact concentration and exact dilution). Concentrations were in a range 2.5−6 mg/mL. Solutions of samples were prepared 24 h prior to measurement in order to get full dissolution, cluster breakup, and thus the real size of polymer particles in solution under conditions of full solvation. DSC and TGA measurements were performed using TA Instruments Q600 serial instruments with nitrogen flow and the temperature ramp rate of 5 °C/min. 5−10 mg samples were used. The index of refraction was measured at 632.8 nm with a Rudolf AutoEL ellipsometer. Spectroscopy. 11B NMR spectra were measured with a Varian VXRS-300 spectrometer at 96.2 MHz. 1H and indirectly observed two-

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RESULTS Polymerization of IB. In control runs, it was ascertained that no polymer was formed under the standard conditions of 50 mg of 1, 50 mg of ATB, 80 °C, 1 atm of IB, and 24 h reaction time in DCE, when either 1 or ATB was left out or when 50 mg of either TEMPO or 4-crown-12 was added. No polymer was formed when dichloroethane (DCE) was replaced with THF. Table 1 illustrates the increase in polymerization Table 1. Influence of the Amounts of Catalyst 1, Radical Initiator ATB, and Solvent on Polymerization Activitya run

solvent, 0.5 mL

ATB/ mg

1/mg

b-PIB/ mg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

DCE DCE DCE DCE toluene toluene toluene toluene DCE DCE DCE DCE toluene toluene toluene toluene

0 10 50 100 0 10 50 100 50 50 50 50 50 50 50 50

50 50 50 50 50 50 50 50 0 10 50 100 0 10 50 100

0 11 24 58 0 55 130 169 0 21 31 47 0 69 119 160

Mw/ g mol−1 b

Db

660 1800 2400

1.09 1.11 1.19

1900 4900 6800

1.08 1.21 1.24

1500 1650 1880

1.09 1.10 1.16

2100 3900 5900

1.22 1.26 1.25

24 h; 80 °C; 1 atm of IB. bApparent weight-average molecular weight Mw and dispersity D obtained by GPC with PS calibration. a

activity in DCE and toluene as the amounts of catalyst 1 and radical initiator ATB grow. The activity is ultimately limited by the solubility of 1, which is about 30% in DCE and less in toluene and benzene. In benzene, the activity is low and little polymer forms. Although 1 dissolves well in this solvent at first, some of it then precipitates and its concentration in the hot reaction mixture is uncertain. Table 2 provides polymer yield as a function of the amount of catalyst and initiator present in 0.5 mL of solvent (1 atm of IB, 24 h). Three main factors influence the polymerization B


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Table 2. b-PIB by UV-Initiated Polymerization of IBa run

solvent

additive

PIB/mg

1 2 3 4

toluene DCE toluene DCE

none none naphthalene naphthalene

210 20 240 60

Mw/g mol 2200 680 1900 1200

−1

Table 3. Influence of Sulfolane on the Polymerization Reaction in DCEa,b

D 1.8 1.2 1.6 1.4

a

10% 1 by weight, 10% ATB by weight, in 0.5 mL of solvent, 1 atm of IB, 24 h, RT.

activity: the amount of the catalyst 1, the amount of the radical initiator (ATB), and the nature of the solvent. Under all conditions the activity is higher in toluene than in DCE; 1 is fully soluble in both, even after the addition of IB. Table 2 illustrates data from polymerization experiments at room temperature under UV irradiation. In the absence of 1, no polymer was formed. Solution Properties of b-PIB from Triple-Detection GPC. Triple-detection GPC results in THF at 35 °C for an ATB/1/IB sample thermally polymerized in DCE (run 3 in Table 1) are shown in Figure S1 (Supporting Information). The intrinsic viscosity is 0.0165 dL/g, and the hydrodynamic radius is 5.08 nm. The Mark−Houwink plot is linear, with a slope of 0.126 (Figure 1; for additional details, see Supporting

run

sulfolane/1 mol:mol

M PIB, mg

Mw

D

1 2 3 4 5 6 7 8 9 10

0 0.025 0.05 0.075 0.1 0.15 0.175 0.2 0.3 1.5

0 6 31 34 40 90 98 46 23 9

1300 6500 7000 28000 9800 2300 1600 1300

1.3 1.54 1.9 2.24 2.2 1.9 1.2 1.3

a

Sample for run 1 was prepared by passing the methanol solution of LiCB11Me12 through ion-exchange resin followed by treatment with LiOH solution. Samples for runs 2−10 were prepared by addition of sulfolane to DCE solution of 1 immediately before starting the polymerization. b1 (50 mg); ATB (50 mg); DCE (0.5 mL); IB (1 atm); 80 °C; 24 h.

NMR was measured (Figure 2) and differentiated peaks associated with a large diffusion coefficient and low Mw

Figure 1. Mark−Houwink plot for b-PIB sample from thermally induced polymerization in DCE at 80 °C.

Information). The Mn and Mw values from universal calibration are 13 000 and 55 000 g mol−1, respectively (dispersity D = 4.25). For the same sample, Mw based on polystyrene standards is 25 000 (D = 2.4). All molecular weights reported elsewhere in this paper are based on polystyrene standards, and the few comparisons that we have performed indicate that they are generally 2−3 times lower than the values obtained from triple detection GPC with universal calibration. The Need for a Polar Additive: Sulfolane. Meticulously purified 1 containing no residual sulfolane has no catalytic effect. Table 3 shows the effect of small amounts of sulfolane on the catalytic activity. The optimal molar ratio sulfolane/Li+ for the polymerization of IB initiated with ATB in DCE is 0.15/1. Excess sulfolane suppresses all catalysis. In toluene, the optimal sulfolane/Li+ ratio is similar. Macroscopic Properties of Neat b-PIB. The oligomer/ polymer is liquid below Mw = 600 g mol−1 and semisolid above this molecular weight. The index of refraction for b-PIB with molecular weight Mw = 1600 g mol−1 was found to be 1.475, compared to 1.490 for commercial l-PIB of similar Mw. DSC measurements showed Tg = 14−16 °C for b-PIB and ∼−40 °C for commercial l-PIB. A TGA measurement suggests that b-PIB is less stable thermally than commercial l-PIB, 240 vs 300 °C. Structure of b-PIB by NMR. For the structural analysis of b-PIB by NMR low-Mw samples with narrow molecular weight distribution (D up to 1.2) were prepared. First, 1H DOSY

Figure 2. 1H-DOSY NMR of l-PIB (Mw, 500−600) in CDCl3.

(∼120 g/mol) compared to the polymer (Mw = ∼650 g/ mol) were attributed to traces of solvents and other organic impurities. These were excluded from further analysis and are marked by crosses in figures. 1H NMR integration of the chainend signals as a fraction of the total suggested an approximate Mn value of 450, in rough agreement with the value deduced from GPC using polystyrene standards. 1 H (Figure 3A) and 13C NMR (Figure 3B) spectra of low-Mw b-PIB are extremely complex, indicating that the structure of this new polymer is highly branched and very complicated, precluding an exact and unambiguous structural determination. Through careful analysis of many one-dimensional and twodimensional of NMR spectra, it is possible to present a generalized structure that reflects the structural moieties that are present throughout the polymer sample. This structure does not reflect the heterogeneity of the polymer with respect to molecular weight distribution and with respect to the defects present. We present herein the experimental NMR data and describe how they were interpreted to arrive at the global C


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Figure 3. b-PIB structure elucidation: (A) 1H NMR of b-PIB in CDCl3; (B) 13C NMR of b-PIB in CDCl3. For enlarged key segments of the spectra, see Figures S4 and S5 in the Supporting Information.

The Isobutenyl Chain End. The recognition of peaks associated with the unsaturated chain end was facilitated by the realization that they are identical with those in l-PIB.7,14,15 Figure 5 illustrates the identification of the isobutenyl chain end using gCOSY and gHMBC methods, and Figure 6 illustrates

structure presented in Figure 4. We use color coding to facilitate the understanding of the relation of the NMR signals to the structural features proposed.

Figure 6. Gradient HMBC NMR of b-PIB with m = 0. Connection between the isobutenyl chain end and the polymer chain.

how this isobutenyl group is connected to the main chain. Polymer structure pictured in Figure 6 has no short linear fragment between the isobutenyl chain end and the first branch. This seems to be the dominant structure. However, spectra of 2 D- and 13C-labeled samples of l-PIB suggest the additional presence of minor constituents in which a short l-PIB chain

Figure 4. Proposed structure of b-PIB with carbon atom numbering.

Figure 5. b-PIB in CDCl3: (A) gradient 1HCOSY NMR; (B) gradient HMBC NMR. Isobutenyl chain end. D


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Figure 7. b-PIB in CDCl3: (A) gradient COSY NMR branch terminating isobutyl groups; (B) gradient HMBC. Linear part of branches.

Figure 8. b-PIB in CDCl3: (A) DEPT determination of quaternary carbons and CH groups (isobutenyl chain end is not shown); (B) gradient HSQC (blue: CH2; red: CH and CH3).

separates the isobutenyl end from the first branch, m > 0 (see below). The Branches. Peaks clearly attributable to terminal isopropyl groups have high intensity, which shows that they terminate the branches. Figure 7A illustrates a correlation between these i-Pr groups and the neighboring CH2 groups, demonstrating that the branch terminating groups actually are isobutyl moieties. One can discriminate here between two sets of signals. One set is attributed to the bulk of the branches located on the main chain, whereas the second set is for the branch located the closest to the isobutenyl chain end. Because of the low Mw of the sample, the intensities in both sets are nearly equal in the spectra shown, but this is no longer the case when Mw is high. Then, the second set has much lower intensity and sometimes cannot be detected at all. Figure 7B shows the gHMBC NMR zoomed to the region of branches. Here we can clearly discriminate between the fragment CH2− C(Me)2 connected to backbone, the same fragment attached to the terminal i-Pr, and a group of fragments located between the two. On the basis of the average of 1H NMR integration ratios, we have estimated the average branch chain length as five IB fragments, where the first carbon atom of the first IB unit is contained in the backbone. At this point, a check of consistency can be obtained by identifying (i) all quaternary carbons by DEPT NMR (Figure 8A), (ii) all CH groups in natural abundance b-PIB and in bPIB obtained from 13CH2CH(CH3)2 by gHMBC NMR

(Figures S2 and S3 in Supporting Information), and (iii) correlations between groups identified using 1H-based and those found by 13C-based NMR, using gHSQC NMR (Figure 8B). As a result, most of the signals in 1H and 13C NMR spectra are now assigned. The tert-Butyl Chain End and a General Cross-Check. This part of the analysis depended on the use of polymers prepared from CD2C(CH3)2 and 13CH2C(CH3)2. Figure 9 illustrates the signals observed in 2H NMR of b-PIB from CD2C(Me)2. All the expected deuterium signals were found, but the resolution is insufficient to allow conclusion about the length of the branches. Figure 10 illustrates 13C and DEPT-135 spectra of b-PIB obtained from 13CH2CMe2 collected during

Figure 9. 2H NMR of b-PIB synthesized from CD2CMe2. E


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coiled backbone and accommodates all the branches in an alternating manner. The cone angle between neighboring branches is ∼67°. We have also modeled the backbiting by the propagating radical end that is required during the growth of the highly branched b-PIB and found that the smallest distance between the radical and H atom of the chain is reached after six IB units have been added (Figure 12). After hydrogen transfer, this leaves a five IB-long branch. The favored analogous conformation of the chain formed from seven IB units is coiled, as shown in Figure 12.

Figure 10. NMR of l-PIB prepared from 15 13CH2CMe2: (A) 13C NMR; (B) DEPT-135 (top) and 13C NMR (bottom) CH region.

a short time such that only 13C-labeled groups give intense NMR peaks. gHSQC and gHMBC spectra of the same sample were used to assign 13C NMR signals, and they pinpointed the expected t-Bu chain end of the polymer (Figure 10). Using this labeled sample, we have also found a linear fragment connecting the isobutenyl chain end with the main branched chain, present as a minor constituent not detectable in unlabeled samples. In addition, gHMBC spectrum with no decoupling was used to measure JC−H coupling constants for 13 C labeled groups (Figures S2 and S3 in Supporting Information). In the NMR spectra of both unlabeled b-PIB and b-PIB prepared from CH2C(CD3)2, methyl groups of the terminal isobutyls represent ∼20% of the total integrated intensity of all methyl groups; hence, the maximum average branch length is 5 IB units. MM2 Calculations for b-PIB. In order to check whether the very highly branched structure proposed for b-PIB in Figure 4 is realistic, MM2 calculations were done for a chain with 32 adjacent branches, close to the sample with the highest Mw obtained. The calculated structure (Figure 11) has a weakly

Figure 12. MM2 modeling of backbiting during the formation of bPIB chain of five (A), six (B), and seven (C) IB units, leading to branch lengths of four, five, and six units, respectively.

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DISCUSSION Radical Polymerization of IB. Although by all normal standards the feasibility of radical polymerization of IB appears very remote, there is no doubt that in the simultaneous presence of high concentrations of both the catalyst 1 and the radical initiator ATB such polymerization process takes place, albeit to molecular weights that are only in the thousands or tens of thousands at best. A radical process is compatible with the observed high dispersity. The proposed active participation of “naked” Li+ in a radical polymerization is supported by experiments with a radical trapping additive and with Li+ complexing additives or solvent. The observation that direct or sensitized photochemical decomposition of ATB at room temperature works about as well as its thermal decomposition at 80 °C demonstrates that it is not just the decomposition of the initiator that is catalyzed by 1, but also the propagation step itself. At a constant concentration of 1, polymerization activity is higher in toluene than in DCE, where the solubility of 1 is highest (Table 1). A fair comparison with benzene, where the activity is the lowest, is difficult because the catalyst precipitates and its concentration in solution is low and uncertain. Clearly, the form in which Li+ is present is important, but its exact nature is not known. Qualitative observation of light scattering by the solutions of 1 suggests that they are colloidal, and the nature of the aggregates could be a sensitive function of the choice of solvent and the presence of additives such as sulfolane. It is not clear whether the catalyzed polymerization occurs on the surface of the colloidal aggregates or in their interior or whether it is due to some small fraction of 1 that is present as a true solution. The detailed structure of the Li+carrying species present in the catalytic solutions needs to be investigated next. The Structure of b-PIB. 1H and 13C NMR results support the general structure shown in Figure 4. The backbone of b-PIB consists of adjacent −CHR units, where R is a branch of five IB monomeric fragments on average, terminated with an isobutyl

Figure 11. MM2 calculated structure for b-PIB with Mw of ∼8600 g mol−1 (30 branches). F


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group. One of the polymer chains is terminated by an isobutenyl moiety, and the other reflects the structure of the radical initiator. In our case, this was ATB and the terminal group is tert-butyl. For low-molecular-weight samples we were able to discriminate in the NMR spectra between the branch R located next to the isobutenyl chain end and the other branches R. However, this difference cannot be discerned in highmolecular-weight samples. The highly branched nature of b-PIB makes this onedimensional polymer unique and almost dendrimer-like, and it is not surprising that its physical properties are quite distinct from those of industrially available linear l-PIB. These properties are compatible with the proposed structure. The intrinsic viscosity is relatively low, indicating a high-density polymer. The dependence of log Rh on log Mw is almost linear (Figure 1), with a Mark−Houwink slope of 0.118, characteristic of spherical molecules in solution, with branches shorter than the backbone. Formation Mechanism of b-PIB. The likely feasibility of radical polymerization of simple alkenes complexed to Li+ was first implied by early ab initio calculations16 that demonstrated that in the gas phase the activation barrier for the addition of the methyl radical to ethylene is reduced by a factor of ∼3 when the double bond is complexed to a naked Li+ cation. This result suggests that the polymerization will proceed by an attack of a chain radical terminus on an IB molecule complexed with a Li+ containing species, perhaps an Li+/carborane anion pair or perhaps a more complicated aggregate. In order to account for the presence of a branch on every carbon of the backbone, we propose backbiting (Scheme 1). As

Scheme 2. Proposed Overall Mechanism for the Formation of b-PIB under Nonoxidizing Conditions (Orange, Initiation; Blue, Propagation; Purple, Backbiting; Red, Termination by Chain Transfer)

needed before the encumbered chain can reach back comfortably. The backbiting translocates the radical center and results in the formation of a first branch. The propagation then continues and after the next six IB insertions a second backbiting takes place and forms a second branch. This sequence of propagation and backbiting continues until the chain transfer process terminates the formation of a molecule of b-PIB. The proposed mechanism is in accord with the observed effects of reaction conditions on the polymerization activity, and the difficulty in reaching high molecular weights probably stems from the highly sterically encumbered environment of the radical-carrying chain end.

Scheme 1. Backbiting as a Route to the Formation of Branches in b-PIB

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CONCLUSIONS In the presence of high concentrations of 1 and a nonoxidizing radical initiator (ATB), isobutylene can be polymerized at atmospheric pressure and ambient temperature by the radical mechanism to a b-PIB polymer of moderate molecular weight that carries a branch on every carbon of the backbone. One of the polymer chains is terminated by an isobutenyl functional group, and the other reflects the structure of the radical initiator. The branches are segments of linear PIB containing five IB subunits on the average. This result is different from that obtained with oxidizing initiators, which produce a mixture of b-PIB and l-PIB by concurrent radical and cationic mechanisms.

written, this process is uphill energetically, since a tertiary radical is being converted to a secondary one, and in an equilibrium the former should prevail. The reactivity of the much less hindered secondary radical toward further IB addition would then have to be sufficiently higher than that of the tertiary radical to compensate for this handicap. It is also possible that the situation is more complicated than indicated in Scheme 1, for instance by complexation of Li+ to the radicals, which could affect both their relative energies and their reactivities. A possible overall mechanism is proposed in Scheme 2. The polymerization starts by the activation of the IB double bond by an unknown form of the “naked” Li+ cation provided by 1. The complexed double bound then is attacked by a t-Bu radical formed as the result of thermal or UV initiated decomposition of the radical initiator ATB. This step is repeated, and IB insertions continue until the chain is long enough for the backbiting. From our modeling studies, six IB insertions are

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ASSOCIATED CONTENT

S Supporting Information *

List of abbreviations, descriptions of catalyst preparation, Figures S1−S3, and detailed results from triple detection GPC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. G


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Present Address §

Department of Natural Sciences, School of Agricultural and Natural Sciences, University of Maryland Eastern Shore, 1 Backbone Road, Princess Anne, MD 21853. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by, or in part by, the U.S. Army Research Laboratory and the U.S. Army Research Office under Contract/Grant W911NF-11-1-0345. We are grateful to Mr. Michael Hoffmann for help with some of the experiments and to Drs. Petr Vlček and Jan Merna for useful discussions.

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REFERENCES

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Highly Branched Polyisobutylene by Radical Polymerization under Li

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