Isobutene Polymerization Using a Chelating Diborane Co-Initiator

Article abstract of DOI:10.1021/ja037725y

Lewis acidic diborane 1 (J. Am. Chem. Soc. 1999, 121, 3244?3245) is highly effective for both proton- and cationogen-initiated isobutene polymerization in hydrocarbon media at low temperature. Reactions of diborane 1 with cumyl chloride and cumyl methyl ether were studied by variable-temperature ¹H and ¹⁹F NMR spectroscopy. At low temperatures stable ion pairs 2a and 2b are formed; at higher temperatures these ion-pairs form phenyl-1,3,3-trimethylindan (3) with concomitant release of HCl to form 1 in the case of 2a or degradation of the anion (2b). Reaction between Ph₃C-Cl and diborane 1 resulted in the generation of an ion-pair 4 consisting of the Ph₃C cation very weakly associated with the chelated, μ-Cl counteranion as revealed by X-ray crystallography. Copyright

Full text of DOI:10.1021/ja037725y

Published on Web 11/07/2003
Isobutene Polymerization Using a Chelating Diborane Co-Initiator
Stewart P. Lewis,† Nicholas J. Taylor,‡ Warren E. Piers,§ and Scott Collins*,†
Department of Polymer Science, UniVersity of Akron, Akron, Ohio 44325-3909, Department of Chemistry,
UniVersity of Waterloo, Waterloo, Ontario, Canada N2L 3G1, Department of Chemistry, UniVersity of Calgary,
Calgary, Alberta, Canada T2V 2R3
Received August 3, 2003; E-mail: collins@polymer.uakron.edu
There is considerable interest in counteranion design and
synthesis in the context of olefin polymerization using single-site
metallocene catalysts.1 The counteranion influences catalyst activity/
stability, polymer MW, and tacticity in a manner that is reasonably
well understood. Less attention has been devoted to systematic
modification of the counteranion, despite its importance in chain
transfer processes,2 in cationic polymerization of isobutene (IB).
Previous work, which involved the use of weakly coordinating
counteranions (WCA)3 to hinder chain transfer in IB homo-/
copolymerization, has focused on various transition metal and main
group initiators paired with a WCA.4 The use of chelating diborane
1 (see Scheme 1) or related compounds5 as activators in olefin
polymerization has been explored,6 and chelated WCA derived from
such compounds {e.g., [Ph3C][1-(µ-X)] (X ) OMe, OC6F5 etc.)}
present advantages in ethylene polymerization over trityl activators
partnered with mononuclear WCA.6a We expected chelating di-
borane 1 might be effective in cationic polymerization of IB, and
this communication presents preliminary results to this effect.
Figure 1. (a) 1H NMR spectrum (400 MHz, CD2Cl2, -40 °C) and (b) 19
F
Scheme 1
NMR spectrum (376 MHz, CD2Cl2, -20 °C) of a mixture of diborane 1
(2.0 equiv) and CumCl (1.0 equiv). (c) 19F NMR spectrum (282 MHz,
CD2Cl2, 25 °C) of ion-pair 4. Signals due to ion-pair 2a are highlighted in
dark gray, while those due to 1 and indan 3 are in light gray. The peaks
marked with an asterisk are due to 2,3,5,6-tetrafluoro-p-xylene, n-hexane,
and toluene.
Scheme 2
We investigated the reaction of diborane 1 with cumyl chloride
(CumCl) and cumyl methyl ether (CumOMe) by variable-temper-
ature 1H and 19F NMR spectroscopy. As shown in Scheme 1,
ionization is observed at low T with either of these initiators and
diborane 1 to form ion-pairs 2a-b in which the cumyl cation7
(Figure 1a) is partnered with symmetrical, counteranions as is
evident from the 19F NMR spectra (Figure 1b).
In the case of ion-pair 2b, the known [1(µ-OMe)] counteranion
is formed (see Supporting Information).6a By analogy, reaction with
CumCl forms ion pair 2a, featuring the unknown [1(µ-Cl)] anion
partnered with the cumyl cation. On the basis of the extent of
ionization, the Lewis acidity of 1 toward CumCl exceeds that of
BCl3 and is comparable to that of SbF5 where ionization is
essentially irreversible.8
other major byproducts of this decomposition (Scheme 1). A similar
process also ensues for 2b although competitive degradation of the
counteranion is observed as will be described in detail elsewhere.
As a model for ion-pair 2a we investigated the reaction of Ph3C-
Cl with diborane 1; the expected ion-pair 4 is quantitatively formed
in either benzene-d6 or CD2Cl2 solution at room temperature
(Scheme 2, 19F NMR spectrum in Figure 1c).9
The X-ray structure of this compound appears in Scheme 2.10
Notable features include a puckered, five-membered ring for the
chelated counteranion with marginally dissimilar B(1)-Cl(1) and
B(2)-Cl(1) bond lengths of 2.0329(16) and 2.0399(15) Å, respec-
tively. The B(1)-Cl(1)-B(2) angle is acute at 94.34(6)° so as to
accommodate the bridging Cl between the two B atoms. Even with
this feature and concomitant puckering of the ring, there is evidence
Ion-pair 2a is stable at -80 °C, but on warming, decomposition
to the known indan 38 occurs while diborane 1 and HCl are the
† University of Akron.
‡ University of Waterloo.
§ University of Calgary.
9
14686
J. AM. CHEM. SOC. 2003, 125, 14686-14687
10.1021/ja037725y CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Polymerization of IB Initiated Using Diborane 1a
for the controlled polymerization of IB and other susceptible
monomers.
[CumCl]
(mM)
[DTBMP]
(mM)
[IB]
(M)
conv.
(%)
Mhw
(K)
f
entry
PDI
(%)b
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada and the University of Akron
for financial support of this work. We also acknowledge the
assistance of Mr. J. Page in conducting GPC analyses of PIB
samples and Professor W. Clegg (U. Newcastle) for also solving
the X-ray structure of compound 4.
1c
2
3
-
0
0
2.76
2.76
2.76
2.76
2.76
2.76
2.76
2.35
2.76
2.35
2.76
100
100
5.8
1.4
0.47
0.10
172
285
302
552
753
98
361
255
146
779
-
2.60
2.40
2.48
3.60
4.60
1.80
1.72
1.92
1.75
2.11
-
-
-
-
2.0
5.0
15.0
0
20.0
20.0
20.0
2.0
20.0
-
4
-
-
5
-
-
6d
7
-
-
0.20
0.30
0.55
0.20
0.20
42
156
68
134
166
-
8
9
21
40
93
0
Supporting Information Available: Experimental procedures,
selected NMR spectra and tables of crystallographic and refinement
data, atomic coordinates and isotropic thermal parameters, bond lengths
and angles, anisotropic thermal parameters, H-atom coordinates, and
isotropic thermal parameters for compound 4 (PDF). This material is
available free of charge on the Internet at http://pubs.acs.org.
10
11d
--
a
Diborane 1 dissolved in toluene (0.3 mL) was added to a solution of
IB in hexane (total volume 24.5 mL) at -78 °C containing DTBMP, and
then a solution of CumCl in CH2Cl2 (0.2 mL) was added. Final concentration
of [1] ) 2.0 mM. Initiator efficiency f ) 100(Xn /Xn) where Xno ) ([IB]o/
b
o
c
[CumCl]o)(conv./100). Diborane 1 in hexane solution added to monomer
References
d
in hexane at -78 °C. Final [1] ) 0.5 mM. B(C6F5)3 (4.0 mM) was
substituted for diborane 1.
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of angle strain as exemplified in the acute C(1)-B(1)-Cl(1) and
C(2)-B(2)-Cl(1) angles of 98.03(9) and 98.98(9)°, respectively.
Evidently, the “bite” angle of diborane 1 is not optimal for chloride
binding as judged by comparison to the structure of [PPN][1,8-
(Cl2B)2C10H6(µ-Cl)] in which both the B-Cl bond lengths are
shorter [av 1.92(1) Å] and the angles within the six-membered ring
less distorted from tetrahedral values.11
The triphenylmethyl cation has a propeller arrangement of the
phenyl rings about the central carbon which features sp2 hybridiza-
tion [Σ C(7) ) 359.99(13)°]. While this atom is oriented toward
the bridging Cl of the counteranion, this interaction may be
electrostatic in nature; the C(7)-Cl(1) distance of 3.899(15) Å is
significantly longer than the sum of the van der Waals radii of Cl
and sp2-hybridized C (3.65 Å).
Diborane 1 is an effective initiator for IB polymerization even
in apolar media such as hexane (Table 1, entries 1 and 2). Despite
the use of a vacuum line, silanized glassware, and final purification
of both monomer and solvent by vacuum transfer from tri-
noctylaluminum, it proved impossible to prevent protic initiation
by diborane 1 except through the addition of 2,6-di-tert-butyl-4-
methylpyridine (DTBMP). As shown in the Table 1, entries 3-5,
about a 5-10-fold excess of DTBMP with respect to 1 was required
to inhibit protic initiation. Evidently under these conditions (1.5-
2.0 ppm H2O!) protic initiation of IB polymerization is highly
effective. The MW in the absence of DTBMP appears limited by
these low levels of H2O as seen from the increase in MW with
[DTBMP] (Table 1, entries 3-5).12
Controlled polymerization of IB, initiated by CumCl and excess
diborane 1, was possible in the presence of a large excess of
DTBMP in hexane solution (Table 1, entries 7-9) where MW
correlates with CumCl concentration. Under more typical conditions
(entry 10 vs 7),13 polyIB of high-molecular weight is obtained at
high conversion. Calculated initiator efficiencies (f) routinely
approach or exceed 100%, consistent with effective initiation and
chain transfer. The initiator efficiency of diborane 1 in concert with
CumCl is superior compared to that in some other initiator systems
which feature WCA under similar conditions.4i-k
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Sci. Eng. 1999, 80, 495. (e) Jacob, S.; Kennedy, J. P. Ionic Polymerizations
and Related Processes. NATO Sci. Ser., Ser. E 1999, 359, 1-12. (f) Barsan,
F.; Karam, A. R.; Parent, M. A.; Baird, M. C. Macromolecules 1998, 31,
8439-8447. (g) Carr, A. G.; Dawson, D. M.; Bochmann, M. Macromol.
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(9) Compound 4: 1H NMR (CDCl3, 300 MHz): δ 8.23 (tt, 3H, p-C6H5),
7.83 (m, 6 H, m-C6H5), 7.61 (dd, 6 H, o-C6H5). 19F NMR (CDCl3, 282
MHz) δ -132.0 (d, 8F, o-C6F5), -136.1 (d, 2F, o-C6F4), -158.6 (t, 4F,
p-C6F5), -162.1 (d, 2F, m-C6F4), -165.6 (m, 8F, m-C6F5). 13C{1H}(CDCl3,
1
75 MHz): δ 210.8 (s, CPh3), 147.9 (d, JC-F ) 245 Hz, o-C6F5), 146.9
(d, 1JC-F ) 242 Hz, o-C6F41), 143.7 (s, p-C6H5), 142.4 (s, o- C6H5), 139.8
1
(s, ipso- C6H5), 139.8 (d, JC-F ) 251 Hz, p-C6F5), 138.4 (d, JC-F
)
1
240 Hz, m-C6F4), 136.7 (d, JC-F ) 249 Hz, m-C6F5), 133.7 (br s, ipso-
C6F4), 130.6 (s, m-C6H5), 118.1 (br s, ipso-C6F5). Anal. Calcd for
C49H15B2F24Cl: C, 52.70; H, 1.35. Found C, 52.36, H, 1.40.
(10) Molecular structure of compound 4 with 30% thermal ellipsoids
depicted: Triclinic, P1h, a ) 11.2135(4) Å, b ) 11.7384(5) Å, c )
19.3086(8) Å, R ) 72.610(1)°, â ) 89.354(1)°, γ ) 62.433(1)°, V )
2125.66(15) Å3, Z ) 2, R ) 0.0468, wR ) 0.0744 based on 12, 469
independent reflections with I > 2σ(I).
(11) Katz, H. E. Organometallics 1987, 6, 1134-36.
That the chelated nature of the counteranion is important for
effective initiation was revealed by comparative experiments using
B(C6F5)3. As shown in Table 1, this borane was ineffective for IB
polymerization using either CumCl (entry 11) or protic initiation
(entry 6) even at equivalent concentrations with respect to boron.14
Future work will explore the utility of diborane 1 as co-initiator
(12) Garratt, S.; Carr, A. G.; Langstein, G.; Bochmann, M. Macromolecules
2003, 36, 4276-87 and references therein.
(13) Storey, R. F.; Curry, C. L.; Hendry, L. K. Macromolecules 2001, 34,
5416-32.
(14) For use of [(Cp′2Zr)2H3][(C6F5)3B-(µ-X)-B(C6F5)3] (X ) CN, NH2) in
IB polymerization see ref 12.
JA037725Y
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J. AM. CHEM. SOC. VOL. 125, NO. 48, 2003 14687