Aqueous suspension polymerization of isobutene initiated by 1,2-C 6F4[B(C6F5)2]2

Article abstract of DOI:10.1021/ja0445387

Lewis acidic, chelating diborane 1 forms stable oxonium acids 2 in the presence of excess MeOH or water. Diborane 1 is shown to be an effective co-initiator for the suspension polymerization of isobutene in aqueous media at sufficiently low temperatures.

Full text of DOI:10.1021/ja0445387

Published on Web 12/16/2004
Aqueous Suspension Polymerization of Isobutene Initiated by
1,2-C6F4[B(C6F5)2]2
Stewart P. Lewis,† Lee D. Henderson,‡ Brett D. Chandler,‡ Masood Parvez,‡ Warren E. Piers,*,‡ and
Scott Collins*,†
Department of Polymer Science, UniVersity of Akron, Akron, Ohio 44325-3909, and
Department of Chemistry, UniVersity of Calgary, Calgary, AB Canada
Received September 9, 2004; E-mail: wpiers@ucalgary.ca; collins@uakron.edu
Aqueous suspension or emulsion polymerization processes are
of practical utility, combining high conversion with control over
latex morphology.1 In recent years, the aqueous suspension or
emulsion polymerization of common alkenes such as ethylene, using
transition-metal catalysts,2 and styrene, using cationic initiators,3
has been reported. However, early attempts at the polymerization
of isobutene (IB) in aqueous suspension yielded only dimers and
trimers,4 thus leading to the general belief that cationic polymer-
ization of this monomer in the presence of significant quantities of
water is unfeasible.5 We report here the first polymerization of IB
and synthesis of butyl rubber (IIR) in aqueous suspension using
diborane 1,2-C6F4[B(C6F5)2]2 (1).
Figure 1. Variable-temperature 19F NMR spectra of a solution of (a)
diborane 1 and MeOH (0.5 equiv) and (b) diborane 1 and a 10-fold excess
MeOH in CD2Cl2 solution. Signals due to 1 are highlighted in pink, 2 in
blue, 3 in green, and 4 in yellow.
Recently, we reported on the use of diborane 1 (Scheme 1) as
an initiator of IB polymerization, in combination with either cumyl
chloride or adventitious moisture, in hydrocarbon media.6 Further
work has revealed that diborane 1 is effective for protic initiation
at [1] ≈ 10-5 M in hexane.7 In view of the heightened reactivity
of 1 in the presence of water, the reaction of 1 with MeOH was
investigated by NMR spectroscopy to identify the products formed.
The addition of 0.5 equiv of MeOH to 1 in CD2Cl2 solution at
-80 °C leads to formation of an ion pair 2 featuring the [1(µ-
OMe)] counteranion,8 partnered with an oxonium ion, which, based
while the O2-O3 and O3-O4 separations of 2.465(5) and 2.495-
(5) Å differ slightly within experimental error. There are six
H-bonds or short contacts between the remaining two, ordered
MeOH molecules, and F-atoms of different counterions7 that
stabilize this monomeric, oxonium ion within the lattice.
Because the basicity of water and MeOH are similar,9 we
expected that aqueous suspension polymerization of IB should be
possible at sufficiently low T. Experiments involving the addition
of a hexane solution of 1 to IB suspended in a 68:32 MeOH/H2O
solution at -60 °C were encouraging, but only a low yield (<10%)
of PIB (Mw ) 102 K, PDI ) 3.3) formed under these conditions.
In contrast, the use of aqueous solutions of strong electrolytes
such as a eutectic salt mixture consisting of LiCl, NaCl, and H2O,14
38% aqueous H2SO4 or 48% aqueous HBF4 proved more promising,
giving moderate to high yields of PIB of moderate to high MW
over the T range -60 to -80 °C (Table 1). The addition of a polar
cosolvent such as CH2Cl2 led to a pronounced increase in MW
accompanied by a slight decrease in conversion (Table 1, entry 2
vs 1). We interpret these positive results as arising from partial
“drying” of the organic phase through physical contact with these
electrolyte solutions.15
Polymerization in the presence of the surfactants sodium dodecyl
sulfate (SDS) or dodecyltrimethylammonium bromide (DTBr),
triflate (DTOTf), and tetrafluoroborate (DTFB) led to a decrease
in yields, and this was especially true for those polymerization
reactions using DTBr (Table 1, entry 3 vs 6). In contrast, when
partnered with a weakly nucleophilic counteranion, as in DTOTf
or DTFB, the yields of PIB were comparable to those obtained in
the presence of SDS (Table 1, entry 9 vs 10, 13 vs 14).
1
on the H NMR spectrum,7 corresponds to [(MeOH)2H] (Scheme
1). On warming above -60 °C this ion pair decomposes, and all
of the added MeOH is consumed, forming equimolar amounts of
borinic ester 3 (independently prepared from (C6F5)2BCl and
1
MeOH)7 and borane 4, which was identified from its H and 19F
NMR spectra (Figure 1a).7 Evidently, 2 is susceptible to chemose-
lective protonolysis of the B-C bonds of the o-C6F4 moiety.
In contrast, ion pair 2 is stable in the presence of excess MeOH,
even at 25 °C (Figure 1b), a feature that we attribute to the reduced
acidity of the oxonium acid, as the solvation of the proton by MeOH
is increased.9 Single crystals of the ion pair [(MeOH)3H][1(µ-OMe)]
could be grown from a solution of 1 in toluene and MeOH, and
the X-ray structure appears in Scheme 1.10
The anion of this ion pair is analogous to that found in [(Et2O)2H]-
[o-C6F4{B(C6F5)2}2(µ-OMe)],11 but features a µ-OMe group that
is planar at O1 with essentially equivalent B-O bond lengths of
1.558(3) and 1.566(3) Å, respectively. The endocyclic C-B-O
angles at the two B atoms are reduced from the tetrahedral value
to 97.8(2) and 97.6(2)°, reflecting angular strain within the five-
membered ring, while the B1-O1-B2 angle is 118.0(2)°.
The [(MeOH)3H] cation has not been observed in oxonium acids
+
derived from methanol,12 but is similar to the H7O3 ion found in
a number of structurally characterized salts.13 The central MeOH2
moiety of this oxonium acid is disordered with the one electron
(and two protons) occupying sites between O2-O3 and O3-O4,7
Copolymerizations of IB were carried out with 8 mol % isoprene
(IP) in the feed in a manner similar to homopolymerizations (entries
4 and 8). Although IP normally acts as a chain transfer agent in
traditional copolymerizations,5 this comonomer led to the production
of higher MW polymer (albeit in lower yield) in suspension. We
† University of Akron.
‡ University of Calgary.
9
46
J. AM. CHEM. SOC. 2005, 127, 46-47
10.1021/ja0445387 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Table 1. Polymerization of IB in Aqueous Suspensiona
Supporting Information Available: Experimental procedures and
selected NMR spectra, crystallographic, refinement, and metrical data
for ion pair 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
entry
mediumb
[1] (mM)
T (
°
C)
yield (%)
Mw (K)
PDI
1
2
Ac
0.43
0.43
0.63
0.57
0.63
0.63
0.63
0.57
0.63
0.63
0.43
0.63
0.63
0.63
-60
-60
-60
-60
-60
-60
-60
-60
-60
-60
-80
-80
-80
-80
48
32
44
24
32
5
29
19
18
14
85
58
42
52
66.1
121
2.55
1.96
2.30
2.89
2.37
2.74
2.05
2.97
2.69
2.82
2.16
2.36
2.35
2.12
Ac,d
3
A
A
19.8
86.2
25.4
57.2
38.4
57.1
61.0
55.7
138
References
4e
5
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A/DTBr
B
6
7
8e
9
B
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Å, R ) 71.61(1)°, â ) 86.79(2)°, γ ) 83.47(2)°; V ) 1798(1) Å3; Z )
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B/SDS
B/DTOTf
Cc
10
11
12
13
14
C
50.8
36.3
39.9
C/SDS
C/DTFB
a Suspension polymerizations were conducted by the addition of a toluene
solution of diborane 1 over a period of 10 s (Caution: strongly exothermic)
to a vigorously stirred suspension of IB in aqueous media (ca. 50:50 v:v)
at the indicated T for 1 h. b A ) 7.2 M LiCl, 0.22 M NaCl in water; B )
38 wt % aqueous H2SO4; C ) 48 wt % aqueous HBF4 with 0.1 g of
surfactant where applicable. c A hexane solution of diborane 1 was added
over 5 min. d The organic phase was 50:50 v:v IB/CH2Cl2. e 8.0 mol % IP
was added to the initial feed.
tentatively attribute this unusual observation to stabilization of the
allylic chain ends toward termination by water.
1H NMR spectroscopic analysis of the copolymers produced with
8 mol % IP feed indicated an average of 5 mol % trans-1,4 IP
units of which an average of 16% served as branch points.5,16
Other water-resistant co-initiators of IB polymerization, including
16
[Li(Et2O)n][B(C6F5)4],16 [Ph3C][B(C6F5)4],16 and B(C6F5)3 were
ineffective, even in the presence of a polar solvent such as CH2-
Cl2. Even the strongly Lewis acidic, but nonchelating, dibora-
17
anthracene 9,10-(C6F5B)2C12F8 failed to yield PIB. Finally, a
strong Brønsted acid such as [(Et2O)2H][B(C6F5)4] (5)18 (pKa )
-5.19b) did not lead to polymerization.
We suspect the features that allow use of 1, either in very dilute
solution in hydrocarbon media or in aqueous suspension, relate to
its ability to chelate water or related donors, thus transiently
generating the acid [o-C6F4{B(C6F5)2}2(µ-OH2)]. Because both 5
and 2 fail to initiate IB polymerization under these conditions,7
the pKa of such an acid is no higher than -5. Future work will
concentrate on the generality and applications of this novel process.
(14) Akopov, E. K. Zh. Prikl. Khim. 1963, 36, 1916-1919.
(15) Control experiments in the absence of diborane 1 using these mineral
acids failed to consume monomer.
(16) Shaffer, T. D.; Ashbaugh, J. R. J. Polym. Sci., Part A: Polym. Chem.
1997, 35, 329-344.
Acknowledgment. We thank the University of Akron and
NSERC of Canada for funding this work. We also thank Prof.
Joseph P. Kennedy for helpful insight into the polymerization of
IB under these conditions and Mr. Jon Page for GPC analyses of
polymer samples.
(17) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks, T. J.
Angew. Chem., Int. Ed. 2000, 39, 1312.
(18) Jutzi, P.; Mu¨ller, C.; Stammler, A.; Stammler, H.-G. Organometallics 2000,
19, 1442-1444.
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