Bulk and Solution Polymerization of Isobutene Using Tris

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Bulk and Solution Polymerization of Isobutene using Tris(pentafluorophenyl)gallium and Aluminum Nathan T. Hand, Robert T. Mathers, Krishnan Damodaran, and Stewart P Lewis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4038483 • Publication Date (Web): 28 Jan 2014 Downloaded from http://pubs.acs.org on February 1, 2014

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Industrial & Engineering Chemistry Research

Bulk and Solution Polymerization of Isobutene using Tris(pentafluorophenyl)gallium and Aluminum Nathan Hand,1 Robert T. Mathers,1 Krishnan Damodaran,2 and Stewart P. Lewis.3* 1. Department of Chemistry, The Pennsylvania State University, New Kensington PA 15068. USA 2. Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260. USA 3. Innovative Science Corp., Roanoke, VA 24019. USA KEYWORDS Cationic, Polyisobutene, Polymerization, Tris(pentafluorophenyl)aluminum, Tris(pentafluorophenyl)gallium. ABSTRACT From the standpoint of green chemistry, tris(pentafluorophenyl)gallium (7) and aluminum (8) outperform all previously explored perfluoroarylated Lewis acids (PFLAs) for the polymerization of isobutene. In comparison to other PFLA based systems, 7 and 8 do not require ultra-high purity monomer, toxic chlorinated solvents, or expensive and highly sensitive initiators (e.g. metallocenes). Moreover, compared to other PFLA based initiator systems they provide much larger yields of high to medium molecular weight polyisobutene even at polymerization temperatures up to ambient. Stopping experiments indicate strong Brønsted acids formed in situ by reaction with adventitious moisture (9 and 10, respectively) induce cationic polymerization protonically. Unlike other PFLA compounds which require expensive and/or energetically unstable precursors, 7 and 8 are readily synthesized from their corresponding Group 13 halides in conjunction with bis(pentafluorophenyl)zinc (6) thereby lending themselves to commercial implementation. 1 ACS Paragon Plus Environment


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INTRODUCTION The cationic polymerization1-3 of olefins is of industrial significance and serves as the sole methodology for the synthesis of a number of commodity polymers including those derived from terpenes4 and isobutene (IB).5 With regards to green polymerization methods,6 this technique presents a unique set of challenges including the inverse relationship of molecular weight (MW) on reaction temperature (T),7,8 the hydrolytic instability of useful Lewis acid (LA) coinitiators,9 and the need for polar solvents to promote polymerization by ionic species.1-3,10,11 Despite their long entrenched use, industrial processes based on this technique still require increased energy efficiency and the elimination of both toxic solvents and waste streams. A great deal of research has been directed at achieving these goals and has yielded significant improvements in terms of MW-T profile and/or the elimination of halogenated solvents resulting from judicious design of initiator systems.5 One new class of initiator systems that has attracted a great deal of attention is based on perfluoroarylated Lewis acids (PFLAs).12-57 Due to the weakly coordinating nature of the anions contained in these initiator systems, chain transfer (CT) events are reduced allowing for the preparation of polymers with MWs approximating those yielded by γ-radiation58-62 while in many cases operating effectively in nonpolar media. Despite their obvious benefits, most PFLA based systems are not conducive to industrial use as they require the purposeful addition of exotic initiator components (e.g. metallocenes) and are highly sensitive to adventitious moisture requiring ultra-high purity monomer and solvent. The added cost of PFLAs due to difficulty in their manufacture also further detracts from the utility of such systems. Another disadvantage of most PFLA based systems is they are reported to only operate at temperatures below -20 °C and thus energy intensive cooling is still required for polymerization.5 Although a great deal of research has been conducted in the area of IB polymerization using PFLAs, reports on systems where adventitious moisture functions as the initiator for the polymerization of IB (Chart 1) has been limited to: tris(pentafluorophenyl)boron (1)36-39 and its salts lithium tetrakis(pentafluorophenyl)borate (2) and trityl tetrakis(pentafluorophenyl)borate (3) (all in chlorinated solvents),38-40 the chelating diborane (1,2-C6F4[B(C6F5)2]2, 4) and diborole (1,2-C6F4[9-BC12F8]2, 5),41-46 and bis(pentafluorophenyl)zinc (6),47,48 the latter being the least efficient in terms of productivity. Systems based on nitrile ligated transition metal tetrakis(pentafluorophenyl)borate salts30-35 are likely to initiate polymerization protonically5 although no experimentation has been conducted to support this assumption and these systems have limited utility as they are incapable of operating in aliphatic solvents and only give rise to low MW polymers. Our research group therefore had particular interest in the behavior of other Group 13 PFLAs as coinitiators for protonic initiation of IB polymerization. Specifically, since other Group 13 PFLA congeners {i.e. tris(pentafluorophenyl)gallium (7) and aluminum (8)} have been shown to exhibit much higher activity for protonic initiation of IB polymerization in aqueous media56 compared to their boron and zinc counterparts would they also possess enhanced activity for polymerization of IB under bulk conditions or in aliphatic solvents?

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In order to gain insight into this question a number of polymerizations using 7 and 8 for the bulk and solution polymerization of IB were conducted. The results contained in this article show that 7 and 8 are indeed superior as coinitiators for polymerization of IB under these conditions when compared to their boron and zinc analogs. Moreover, 7 and 8 incorporate a greater number of the twelve basic principles of green chemistry63 when compared to other systems based on PFLAs that have been developed for the polymerization of IB. EXPERIMENTAL Safety Disclaimer: The polymerization of isobutene is exothermic and presents a potential explosion hazard. Therefore, necessary safety measures should be taken while conducting such polymerizations in the absence of cryogenic cooling. Tris(pentafluorophenyl)aluminum has been reported to be energetic,64 thus its synthesis and handling should be undertaken only by trained professionals. As a safety precaution, unused 8 and its residues should be hydrolyzed with isopropanol. During our experimentation we have not experienced any problems using this compound or the procedures described herein. Solvents: Hexanes (> 60 % n-hexane, BDH) and toluene (99.5 %, BDH) were distilled from Na, subjected to three sequential freeze/pump/thaw cycles and stored over activated 3 Å molecular sieves. Monomer: Isobutene (Chemically Pure, 99 %, Matheson) was used as received. Coinitiators and Proton Trap: All fluorinated precursor compounds employed in the synthesis of PFLA coinitiators were sourced from Matrix Scientific and used without further modification. Tris(pentafluorophenyl)gallium (99 %) and tris(pentafluorophenyl)aluminum were donated by Innovative Science Corporation and prepared according to existing procedures.65 AlBr3 (98%, Alfa Aesar) was freshly sublimed under vacuum prior to use. 2,6-di-tert-butylpyridine (DTBP) was used as received (97 %, Acros). Polymerization Reactors: Specific details concerning the construction of polymerization reactors used in IB polymerizations have been detailed in previous publications.56,66 Polymerization Procedure for IB: The polymerization reactor (containing a magnetic stir bar) was assembled inside of a glove box followed by cooling via immersion in a -78 °C isopropanol/CO2(s) bath for 20 minutes. A desired quantity of IB was then condensed inside. Next, if hexanes were used they were injected into the reactor via syringe. For stopping experiments, a stock solution of 2,6-di-tert-butyl pyridine (in hexanes) was then injected. The reactor was then equilibrated to the desired polymerization temperature by immersion into an appropriate dry ice/solvent, ice water, or water bath with stirring for a period of 20 min. Polymerization was then initiated by injection of the coinitiator (in toluene) and allowed to proceed at the desired temperature for one hour. In most instances refluxing of monomer occurred almost immediately upon injection of the coinitiator accompanied by a concomitant rise in pressure as the polymerization charge solidified or thickened. Following polymerization, all PIBs were dissolved in hexanes (if not already in solution), pipetted into tarred jars, and concentrated by evaporation of volatiles. In the instance of stopping experiments, DTBP was removed by first washing the resultant polymer solution with one 10.00 mL portion of 10.00 wt % HCl(aq) and then three 10.00 mL aliquots of deionized water followed 3 ACS Paragon Plus Environment


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by drying over activated alumina. The resultant solution was filtered through a plug of cotton and concentrated by evaporation of volatiles before finally precipitating the polymer with ethanol. Polymer yields were determined gravimetrically. Instrumentation: Polymer molecular weights were determined by gel permeation chromatography (GPC). The GPC system was equipped with a three-angle Wyatt Mini Dawn light scattering detector, a Wyatt ViscoStar viscometer and a Wyatt OptiLabReX differential refractometer. Assuming 100 % mass recovery of the injected polymers, the specific refractive increments (dn/dc) were calculated with Wyatt Technology’s Astra V software. The GPC samples (≈ 10 mg·mL-1) were injected onto two columns (PL gel, 5 µm MIXED-C linear, 300 mm x 7.5 mm) and eluted with tetrahydrofuran (1.0 mL·mL-1) at 35 °C. NMR measurements were done using a Bruker Avance III 400 MHz NMR spectrometer and a 5 mm smart probe with 16 scans being collected. Results and Discussion Introductory experimentation focused on the bulk polymerization of IB as a function of temperature using toluene stock solutions of 7 (Table 1). In contradistinction to its boron analog (1) and its derivative salts (2 and 3), 7 was highly active for polymerization of neat IB in conjunction with adventitious moisture. Polymerization at low temperatures (Table 1, entry 1) proceeded smoothly up to a limiting conversion which appears to be imposed by entrapment of monomer within a solid charge of polyisobutene (PIB). The negative effect that charge solidification has on yield appears to be compounded as polymerization temperature is decreased due to lower polymer solubility (in neat monomer or solvents) and a reduction in CT which in turn lowers the total number of polymer chains generated. The MW of the resultant polymer is high and its polydispersity index (PDI) relatively small. The high MW and small PDI are likely the result of reduced CT from the low nucleophilicity of [HOGa(C6F5)3]-. This behavior is in line with previous research involving the aqueous polymerization of IB using 7 which also gave rise to PIBs with high MWs and unusually low PDI values.56 In both instances, polymerization is believed to be initiated by the hypothetical Brønsted acid (BA) 9, generated from reaction of 7 with adventitious moisture present in the monomer (Scheme 1). Evidence for the existence of species similar to BA 9 {e.g. its Al analog (BA 10)} have been gathered by previous researchers and have been shown to be relatively stable species, especially at low temperatures.67,68 As the reaction temperature was increased polymerization became distinctly more violent in nature with a small reflux and concomitant pressure buildup occurring on injection of 7 (Table 1, entry 2) at -42 °C. Although, the MW of the polymer was still relatively high it was dramatically decreased in comparison to that produced at lower temperature and its PDI value was slightly increased. These results are most likely due to increased CT from the exothermic nature of polymerization as witnessed by refluxing IB (b.p. = -6.9 °C)69 which is difficult to control for polymerization occurring under bulk conditions. As the reaction temperature was progressively increased the severity of polymerization became more pronounced to the point that at room temperature the pressure buildup was high enough to rupture the syringe needle used to introduce 7 as it was being withdrawn while simultaneously tripping the overpressure safety valve (≈ 100 psi) resulting in loss of some monomer to the atmosphere. 4 ACS Paragon Plus Environment

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Despite the uncontrolled nature of the majority of these polymerizations, and thus the fact that the MW values reported here are most likely artificially depressed compared to what is theoretically possible (due to the temperature of the polymerization charge exceeding that of the cooling bath), they in fact closely mirror or even exceed the performance of previously reported PFLA systems16,41,42,44 that operate under low polarity conditions (Figure 1). For example, the MW-T profile for 7 in neat IB strongly mirrors that reported for [(C5H4SiMe3)2ZrH2]2/[Ph3C][B(C6F5)4] in neat IB containing small amounts (ca. 5 vol %) of dichloromethane.16 It should be noted that of the PFLA systems reported to date, those based on zirconocene hydrides produce PIBs with some of the highest MWs. The superior performance of 7 is further demonstrated by the fact that the results obtained for [(C5H4SiMe3)2ZrH2]2/[Ph3C][B(C6F5)4] involved polymerizations where temperature control was stated to be ± 3 °C in highly purified IB. Systems based on metallocenes are overly sensitive to trace moisture and in the case of the aforementioned zirconocene hydrides monomer containing less than 1 µmol water per 1 mol IB was required for replicable results, large yields, and high MWs. Finally, 7 is unique in that it is capable of efficiently polymerizing IB at ambient temperature whereas PFLA/transition metal complex systems have only be reported to operate at temperatures below -20 °C. It may be possible that systems based on boron containing PFLAs do not operate satisfactorily above this temperature as the B-C6F5 bond is unstable towards cleavage by C+,70 a problem which might be potentially negated by substitution of other Group 13 elements. Even in the absence of solvent and using unpurified IB, the MW-T profile for polymerization of IB using 7 is superior to that for solution polymerization of IB as induced by the strongly acidic chelating diborane 4 (or diborole 5) in hexane.41,42,44 In the latter systems both monomer and solvent were purified by vacuum transfer from tri-n-octylaluminum where the concentration of water was shown to be on the order of ~ 50 µM.41 The main detraction to using 7 is that Ga is not a perfect “green” metal due to its scarcity. Encouraged by these results attention was turned to the use of the Al congener 8 under very similar reaction conditions. Initial experiments involving the use of 8 centered on bulk polymerization of IB (Table 2) where the concentration of coinitiator was reduced to a factor of 10 less than that employed for experiments involving 7 (for safety) as previous studies on aqueous polymerization had proven the Al PFLA was much more active than its Ga analog. Indeed, even at low reaction temperatures and reduced concentration of coinitiator, polymerization under bulk conditions occurred in an uncontrolled manner with immediate reflux of monomer and concomitant solidification of the polymerization charge (Table 2, entry 1). The resultant PIB had a much lower MW and broader PDI than that yielded under essentially identical conditions using 7 (Table 1, entry 1) and it was speculated that much of this was the result of insufficient heat dissipation for such a rapid polymerization. A number of control experiments were conducted to further prove that 8 was solely responsible for polymerization as had been demonstrated to be the case involving its use in analogous aqueous polymerization.56 Although the main contaminate for 8 has been previously shown to be pentafluorobenzene56 this PFLA could in theory contain trace amounts of AlBr3 used in its synthesis, a LA which is known to readily polymerize IB in aliphatic solvents.71-75 It was therefore decided that a comparison run using only AlBr3 should be made to determine whether enieidic76,77 (i.e. more than one chain carrier) polymerization was occurring. In this instance the 5 ACS Paragon Plus Environment


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use of the word “enieidic” is meant to include not only propagating species that differ by their degree of iconicity (e.g. ion pairs vs. fee ions) but also due to a difference in the chemical composition of their counteranions. Since AlBr3 can induce polymerization protonically in conjunction with adventitious moisture or HBr impurities and by self-ionization (Scheme 2) the presence of trace amounts of this LA could in theory give rise to a minimum of three chemically distinct counteranions in addition to those derived from 8. Polymerization of bulk IB using AlBr3 proceeded very slowly to give a reduced yield of PIB with greater MW and larger PDI compared to that yielded by 8 (Table 2, entry 2). It was therefore concluded that the amount of polymer that could be attributed to this potential impurity was negligible in this study. These results further support the assumption that insufficient heat dissipation (and not anion nucleophilicity) is most likely responsible for the depressed MWs of PIBs obtained using 8. Studies involving polymerization of IB with supported methylaluminoxane in conjunction with silyl or organo chlorides are illustrative of this effect.5,66 For example, when easily ionizable 3° silyl halides (e.g. chlorotrimethylsilane {TMSCl}) are used as initiators at ambient temperature polymerization is exceedingly violent giving rise to quantitative yields of low-medium MW (ca. 20 kg·mol-1) PIBs within 1-2 minutes. Under identical conditions, less ionizable initiators (e.g. CHCl3) produce close to 100 % yields of medium MW (ca. 50 kg·mol-1) PIBs, albeit full polymerization requires much more time (ca. 4 h). In both systems the counteranion is (in theory) the same as is the total heat of polymerization but in the former (i.e. TMSCl) this heat is liberated so rapidly that it cannot be adequately dissipated by the cooling bath (internal T >> ambient) whereas in the latter (i.e. CHCl3) heat is released so gradually that the charge does not deviate from ambient. The end result is depression of MW for polymerizations that reach high yields in short time frames (e.g. 8 vs. 7 or AlBr3) compared to polymerizations that gradually produce polymer due to increased CT caused by heat buildup. Regardless, it was realized that for 8, polymerization needed to be effected in solution if it were to proceed in a somewhat controllable fashion. When polymerization was conducted in hexanes (Table 2, entries 3 and 5-9), behavior similar to that seen for 7 (as used in bulk IB) was witnessed. That is, at low reaction temperatures (i.e. -78 °C) polymerization proceeded in a steady manner to yield a solidified charge of high MW PIB containing entrained monomer; whereas, at higher reaction temperatures refluxing of monomer with concomitant thickening of the charge was observed. In all instances, polymerizations induced by 8 proceeded with a much higher velocity than those using 7 despite the lower concentration of the former PFLA and the use of solvent. PIBs yielded by the former PFLA had depressed MWs and higher PDI values compared to those produced by the latter PFLA at identical reaction temperatures (Tables 1 vs. 2 and Figure 1). The primary cause of this disparity (in MW and PDI values) is believed to be due to increased CT caused by reduced heat dissipation experienced in experiments involving 8 compared to those containing 7 as polymerizations with the Al congener were always much more violent. Interestingly, solution polymerizations conducted using AlBr3 (Table 2, entries 4 and 10) again proceeded slowly to afford reduced yields of PIBs with higher MWs compared to those made by 8 reinforcing the suggestion that insufficient heat removal exacerbates CT for latter system. In spite of the poor heat dissipation, the MW-T profile for polymerization of IB in hexanes using 8 is still favorable to that yielded by 4 or 5 under similar conditions. Again, it should be noted that


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these improved results were obtained despite the fact that a significant portion of the polymerization charge consisted of monomer of moderate purity. It was speculated that polymerizations involving 7 and 8 were initiated protonically by adventitious moisture (Scheme 1). In order to test this assumption, stopping experiments involving the use of the sterically hindered pyridine (SHP), 2,6-di-tert-butyl pyridine (DTBP) were conducted, the presence of which should in theory prevent polymerization processes involving proton mediated initiation (Scheme 3, Table 3). Although a large excess of DTBP to 8 was used, polymerization is so facile in the presence of this PFLA that it was impossible to prevent the formation of small amounts of PIB; however, inclusion of the DTBP dramatically reduced polymer yields indicating that water was indeed the initiator. Compared to polymerizations run in its absence (Table 2, entries 3 and 5), inclusion of DTBP (Table 3) resulted in polymers with higher MWs and lower PDI values. This may indicate that exothermicity of polymerization coupled with CT (in absence of DTBP) are most likely responsible for the reduced MWs and larger PDIs of PIBs made by 8 versus those yielded using 7. It should be noted, that SHPs are not diagnostic tools for PFLA based initiator systems containing carbocation synthons or transition metal complexes capable of generating metal cations as numerous studies have shown they are capable of reacting with electrophiles other than H+ when paired with anions derived from PFLAs.24,41,49 Indeed, sterically hindered bases have even been shown to be capable of abstracting β-H- from living PIB bearing counteranions derived from traditional Lewis acids (e.g. TiCl4).78-81 Since bulk and solution polymerization of IB with 7 and 8 yielded PIBs with modest MWs at 22 and 0 °C, respectively (Table 1, entry 4 and Table 2, entry 9), it was decided that 1H NMR (Figure 2) could be used to elucidate the chain-ends. Such analysis82-84 showed that these polymers were slightly enriched in endo-olefinic functionalized chain-ends (ca. 57 % in both cases) which is in contrast to PIBs produced using the same initiators in aqueous media56 which contained essentially equimolar amounts of exo and endo-olefinic functionalized chain-ends. The exo content of PIBs afforded in solution using 7 or 8 are substandard when compared to polymers produced by other PFLAs30-35 and those made using ethereal adducts of AlCl3,85-88 which have long been recognized for their ability to polymerize IB.89-95 CONCLUSIONS From a green chemistry standpoint, tris(pentafluorophenyl)gallium (7) and aluminum (8) outperform all known PFLAs for polymerization of IB. Such polymerizations conform to as many as 6 of the 12 basic principles of green chemistry:63 • • • • •

Reduced Toxicity: Neither 7 nor 8 require toxic chlorinated solvents (e.g. CH3Cl) that are detrimental to the environment. Reduction of Auxiliary Substances: Using 7, polymerization can be effected in neat monomer and neither 7 nor 8 require the use of exotic initiators. High Yields: Quantitative yields of PIBs can be made. Reduced Energy Consumption: Medium to high MW PIBs can be synthesized at temperatures more closely approximating ambient thereby reducing energy consumption. Reduction of Synthetic Procedures: 7 and 8 do not require ultra-high purity monomer. 7 ACS Paragon Plus Environment


•

Safety and Cost: Compared to other PFLAs these coinitiators lend themselves to commercial implementation as they are readily made from low cost, non-energetic precursors.

Future publications will detail the application of 7 and 8 to the polymerization of renewable monomers (e.g. β-pinene) as well as use of supported analogs57 for the production of PIB. FIGURES Figure 1. MW-T profiles for 7 (■) and 8 (◊) versus [(C5H4SiMe3)2ZrH2]2/[Ph3C][B(C6F5)4] (●)16 and 1,2-C6F4[B(C6F5)2]2 (∆),41,42,44 respectively. Solid markers and lines = bulk polymerizations, hollow markers and dashed lines = solution polymerization in hexane.

6.5

6

5.5 Log Mw

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Ga 5

Al diborane

4.5

[Cp'2ZrH2]2/PhC3X

4

3.5 3

3.5

4

4.5 K-1

5

5.5

X 1000


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Figure 2. 1H NMR spectra for polyisobutene made using Al(C6F5)3 (top spectrum; Table 2, entry 9) and Ga(C6F5)3 (bottom spectrum: Table 1, entry 4).

SCHEMES Scheme 1. Initiation of polymerization of IB by tris(pentafluorophenyl)gallium and aluminum in conjunction with adventitious moisture.

CH 3 M(C 6 F5) 3 + H 2O 7 or 8

H2O M(C 6 F5) 3

H +[HOM(C 6F5)3] 9 or 10

[HOM(C 6F5) 3] -

H 3C C CH 3

7 and 9, M = Ga; 8 and 10, M = Al



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Scheme 2. Enieidic polymerization of IB coinitiated by AlBr3.

Scheme 3. Stopping experiments using 2,6-di-tert-butylpyridine.

CHARTS Chart 1. Perfluoroarylated Lewis acid based initiator systems known to induce polymerization of IB protonically.


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TABLES. Table 1. Polymerization of neat IB using Ga(C6F5)3.a Entry Temp. Yield

Mw

(°C)

(%)

(kg·mol-1)

1

-78

22.7

1450

1.13

2

-42

100

383

1.28

3

-23

100

126

1.88

4b

22

79.7

38.6

1.42

a

Mw / Mn

IB = 194 mmol, time = 1 hr, Ga(C6F5)3 = 0.200 mmol dissolved in toluene. 11 ACS Paragon Plus Environment


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b

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Loss of monomer through safety valve.

Table 2. Bulk and solution polymerization of IB using Al(C6F5)3 and AlBr3.a Entry

1

Initiator

Hexanes Temp. Yield

Mw

Mw / Mn

[µmol]

[mL]

(°C)

(%)

(kg·mol-1)

Al(C6F5)3

―

-78

40.5

85.4

2.62

―

-78

20.6

208

4.50

15.42

-78

66.6

268

1.62

15.42

-78

30.0

721

1.60

23.11

-78

77.1

300

2.17

23.11

-42

100

127

3.13

―

-23

47.7

13.0

1.53

23.11

-23

25.9

47.0

1.88

23.11

0

12.8

9.86

1.93

15.42

0

3.94

21.3

2.17

20.0 2

AlBr3 7.9

3

Al(C6F5)3 20.0

4

AlBr3 7.9

5

Al(C6F5)3 20.0

6

Al(C6F5)3 20.0

7

Al(C6F5)3 20.0

8

Al(C6F5)3 20.0

9

Al(C6F5)3 15.0b

10

AlBr3 7.9

a

IB = 194 mmol; time = 1 hr. All coinitiators dissolved in toluene, DTBP dissolved in hexanes.

b

Made in two separate 7.5 µmol injections spaced 30 minutes apart. 12 ACS Paragon Plus Environment

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Table 3. DTBP stopping experiments for solution polymerization of IB using Al(C6F5)3. Entry

DTBP

Hexanes

Temp.

Yield

Mw

[mmol]

[mL]

(°C)

(%)

(kg·mol-1)

1

0.200

15.17

-78

2.09

416

1.16

2

0.800

14.42

-78

0.59

339

1.37

Mw / Mn

a

IB = 194 mmol; time = 1 hr. Al(C6F5)3 = 0.0200 mmol dissolved in toluene. DTBP dissolved in hexanes.

GRAPHICAL ABSTRACT AND TOC GRAPHIC Compared to other initiator systems based on perfluoroarylated Lewis acids, tris(pentafluorophenyl)gallium and aluminum offer improved sustainability for polymerizing isobutene. They operate in conjunction with adventitious moisture both in unpurified monomer and nonpolar solvents to produce large yields of high molecular weight polymers at elevated temperatures. Since they are readily synthesized from cheap, non-energetic precursors and do not require expensive initiator components (e.g. metallocenes) they are more ideally suited to commercial implementation.

AUTHOR INFORMATION Corresponding Author 13 ACS Paragon Plus Environment


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*Correspondence to: Stewart P. Lewis (E-mail: [email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT SPL would like to acknowledge H.J. Lewis MD for a donation used to fund this work and Mr. B. Hickory for assistance in manuscript preparation. ABBREVIATIONS BA, Brønsted acid; CT, Chain transfer; DTBP, Di-tert-butylpyridine; LA, Lewis acid; MW, Molecular weight; PDI, Polydispersity index; PFLA, Perfluoroarylated Lewis acid; PIB, Polyisobutene; SHP, Sterically hindered pyridine; T, Temperature REFERENCES (1) The Chemistry of Cationic Polymerisation.; Plesch, P., Ed.; Pergamon Press: Oxford, 1963. (2) Gandini, A.; Cheradame, H. Cationic Polymerisation: Initiation Processes with Alkenyl Monomers. Adv. Polym. Sci. 1980, 34-35, 1-284. (3) Cationic Polymerizations.; Marcel Dekker, Inc.: New York, 1996. (4) Mathers, R. T.; Lewis, S. P. Monoterpenes as Polymerization Solvents and Monomers in Polymer Chemistry.; In Green Polymerization Methods: Renewable Starting Materials, Catalysis and Waste Reduction; Mathers, R. T., Meier, M. A. R., Eds.; Wiley-VCH: New York, 2011; pp 91-128. (5) Lewis, S. P.; Mathers, R. T. Advances in Acid Mediated Polymerizations.; In Renewable Polymers, Synthesis, Technology and Processing; Vikas, M., Ed.; Wiley-VCH: New York, 2011; pp 69-173. (6) Mathers, R. T. How well can renewable resources mimic commodity monomers and polymers? J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1-15. (7) Flory, P. J. Principles of Polymer Chemistry.; Cornell University Press: Ithaca, 1971; pp 217224. (8) Thomas, R. M.; Sparks, W. J.; Frolich, P. K.; Otto, M.; Müeller-Cunradi, M. Preparation and Structure of High Molecular Weight Polybutenes. J. Am. Chem. Soc. 1940, 62, 276-280. (9) Olah, G. A. Friedel-Crafts Chemistry.; John Wiley and Sons: New York, 1973.


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Bulk and Solution Polymerization of Isobutene Using Tris

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