Influence of chain length on the electrophilic reactivity of carbocations

Article abstract of DOI:10.1021/ma0120299

The electrophilic reactivities of the bis(p-methylphenyl)carbenium ion 1b⁺ and its macromolecular analogue 1a⁺ have been compared in slow reactions with allylsilanes and in fast reactions with silyl enol ethers. Treatment of 1b-Cl wi

Full text of DOI:10.1021/ma0120299

Macromolecules 2002, 35, 4611-4615
4611
Influence of Chain Length on the Electrophilic Reactivity of
Carbocations
Her ber t Ma yr * a n d Holger Sch im m el
Department Chemie, Ludwig-Maximilians-Universita¨t, Butenandtstr. 5-13,
D-81377 Mu¨nchen, Germany
Sh in jir o Koba ya sh i,* Ma sa sh i Kota n i, a n d T. R. P r a ba k a r a n
IFOC, Kyushu University, Higashi-ku, Fukuoka 812 J apan
La szlo Sip os a n d Ru d olf F a u st*
Polymer Science Program, Department of Chemistry, University of Massachusetts Lowell,
One University Avenue, Lowell, Massachusetts 01854
Received November 19, 2001; Revised Manuscript Received March 25, 2002
ABSTRACT: The electrophilic reactivities of the bis(p-methylphenyl)carbenium ion 1b+ and its macro-
molecular analogue 1a + have been compared in slow reactions with allylsilanes and in fast reactions
with silyl enol ethers. Treatment of 1b-Cl with TiCl4 or GaCl3 and of 1a -Cl with GaCl3 in CH2Cl2 at -70
°C gave solutions of the carbocations 1b+ and 1a +, the concentration of which was measured
photometrically. Addition of allylsilanes led to the exponential decay of the benzhydryl cation concentra-
tions, from which the second-order rate constants have been derived. The rate constants determined for
the reactions of 1a + or 1b+ with allyltriphenylsilane and with allylchlorodimethylsilane, respectively,
agree within experimental error. Laser flash photolyses of 1a -Cl and 1b-Cl in acetonitrile/dichloromethane
mixtures yielded the corresponding benzhydryl cations, which showed pseudo-first-order decay in the
presence of variable concentrations of 1-trimethylsiloxycyclohexene or 1-trimethylsiloxycyclopentene. The
second-order rate constants ranged from 1 × 108 to 5 × 108 L mol-1 s-1 but differed for 1a + and 1b+ by
less than 30%. It is concluded, therefore, that macromolecular carbocations and their low molecular weight
analogues have the same reactivity.
In tr od u ction
The conclusion drawn from these experiments may
be disputed, however, since the second-order rate con-
stants in Scheme 1 are smaller than 102 L mol-1 s-1
whereas the retarding effect of a macromolecular back-
bone might only become effective in fast reactions, when
diffusion processes become rate controlling. It was the
goal of this work to compare the electrophilic reactivities
of macromolecular carbocations (1a +) with those of their
low molecular weight analogues (1b+), that can be
photometrically detected, in fast reactions.
The rate constants of carbocationic polymerizations
are still the topic of substantial controversy.1 For the
propagation step of the cationic polymerization of isobu-
tylene, values from 6 × 103 to 7 × 108 L mol-1 s-1 have
been reported (Table 1).2 For the carbocationic polym-
erization of styrene the published rate constants are
∼103 L mol-1 s-1 4
. There is evidence, however, that
these numbers may underestimate the real kp values
by as much as 6 orders of magnitude.2e Since solvent
polarity has a large influence on ionization and dis-
sociation equilibria5 but not on the rate of attack of
carbocations on olefins or other neutral nucleophiles,6,7
and temperature has only a small effect on the rates of
fast bimolecular reactions,8 the huge differences in the
reported kp cannot be due to the different reaction
conditions employed.
Plesch has recently attempted to explain the discrep-
ancies in propagation rate constants by monomer sol-
vation of the propagating carbocation at high monomer
concentration (>4 M).1b The diffusion clock method,9
however, yielded virtually identical propagation rate
constants with that recently reported by one of us for
[IB] ) 1-2.5 M2e at higher IB concentrations up to 6 M
in hexanes/methyl chloride 60/40 (v/v) at -80 °C.10
Alternatively, one might assume that macromolecular
carbocations possess lower reactivity than low molecular
weight model compounds. Two of us have shown,
however, that the reactivity of 1,1-diphenylethylene-
capped polyisobutylene carbocations toward various
nucleophiles does not differ from that of the 3,3,5,5-
tetramethyl-1,1-diphenyl-1-hexyl cation (Scheme 1).11
Exp er im en ta l Section
Ma ter ia ls. Methyl chloride and isobutylene were dried in
the gaseous state by passing them through in-line gas-purifier
columns packed with BaO/Drierite. They were condensed in
the cold bath of a glovebox prior to polymerization. 2,6-Di-tert-
butylpyridine (DTBP, Aldrich, 97%), titanium tetrachloride
10.1021/ma0120299 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/09/2002

4612 Mayr et al.
Macromolecules, Vol. 35, No. 12, 2002
Sch em e 1. Ra te Con sta n ts (L m ol-1 s-1) of Ad d ition of Sila n es to Dip h en ylca r ben iu m Ion s in CH2Cl2 a t -70 °C
(Cou n ter ion : Ti2Cl9-)
Ta ble 1. Rep r esen ta tive P r op a ga tion Ra te Con sta n ts in
th e Ca tion ic P olym er iza tion of Isobu tylen e
Sch em e 2. Syn th esis of 1a -Cl
no.
solvent
T/°C
kp/L mol-1 s-1
ref
1
2
3
4
5
6
CH2Cl2
CH3Cl
heptane
bulk
-78°
-48
-14
-78
-78
9.1 × 103
1 × 104a
6 × 103b
1.5 × 108
6 × 108
2a
2b
2c
2d
2e
2f
CH2Cl2
hexanes/CH3Cl 60/40 (v/v) -80
7 × 108
a
+ b
According to ref 1 this is probably kp
.
According to ref 1 this
( 3
is probably kp
.
(Aldrich, 99.9%), silver triflate (Aldrich, 99%), lithium alumi-
num hydride (Aldrich, 95%), and p-toluoyl chloride (Aldrich,
98%) were used as received. Dichloromethane (Doe & Ingalls)
was distilled, then refluxed over CaH2, and distilled before use.
HCl was generated from sodium chloride with concentrated
sulfuric acid and passed through a column filled with drierite.
Hexanes (Hex, mixture of hexane isomers plus methylcyclo-
pentane, Aldrich, 99%) was refluxed for 24 h with concentrated
sulfuric acid, washed with 5% NaOH solution and then with
distilled water until neutral, dried on anhydrous Na2SO4, and
distilled from CaH2 under nitrogen before use. Cumyl chloride
was made by hydrochlorination of R-methylstyrene.
P olym er iza t ion of IB a n d Su b seq u en t Met h yla t ion .
The polymerization of IB was carried out under a dry nitrogen
atmosphere ([H2O] < 1 ppm) in an MBraun 150 M stainless
steel glovebox in hexanes/CH3Cl 60/40 (v/v) solvent at -80 °C
using the following concentrations: [cumyl chloride] ) 0.015
M, [IB] ) 0.493 M, [TiCl4] ) 0.036 M, [DTBP] ) 0.006 M. The
total volume of the polymerization mixture was 1 L. After 1.5
h polymerization time (CH3)2Zn ([CH3)2Zn] ) 0.144 M) was
added, and the mixture was left to react overnight. The
resulting polymer I (Scheme 2) exhibited Mn ) 2160 and Mw/
Mn ) 1.12 determined by GPC.
Acyla tion of I w ith a Mixed An h yd r id e of p-Tolu ic
Acid a n d Tr iflu or om eth a n esu lfon ic Acid . Into a 500 mL
one-neck flask 8.48 g (33.0 mmol) of silver triflate was placed
in an inert atmosphere glovebox. The material was suspended
in 10 mL of CH2Cl2, and 4.32 mL (32.7 mmol) of p-toluoyl
chloride was added slowly under vigorous shaking. The blue
suspension was mixed with 30 mL of CH2Cl2 solution of 13.84
g of polyisobutylene I (Scheme 2). The flask was closed and
taken out of the glovebox. After stirring for 20 h at room
temperature, the mixture was cooled in an ice bath, and ∼5
mL of saturated aqueous NaCl solution was added dropwise.
The suspension was filtered and taken up in 200 mL of
hexanes. The organic phase was washed with 5% aqueous
NaOH solution and then with water until neutral. The solution
was dried on anhydrous Na2SO4, and the solvent was evapo-
rated to obtain 15.1 g of crude product. The product was
purified on silica gel (100 g), by bonding it to the stationary
phase, washing with 300 mL of hexanes, and eluting with 600
mL of hexanes/CH2Cl2 50:50 v/v mixture. (On silica gel TLC
plate the retention factor for starting polyisobutylene I and
acylated polymer II were 0.92 and 0.05, respectively, with
hexanes as eluent.) Yield: 13.79 g (93.7%). Mn ) 2490 (GPC),
Mn ) 2250 (NMR).
with ∼0.5 g of LiAlH4 at room temperature for 3 h. Then the
excess LiAlH4 was destroyed at 0 °C by the dropwise addition
of ethyl acetate. The solution was mixed with 200 mL of
hexanes and washed with aqueous HCl solution (0.5 M) and
then with water. The organic phase was dried on anhydrous
Na2SO4, and the solvent was evaporated to yield 12.0 g of crude
III. The product was purified on silica gel (130 g) with hexanes/
CH2Cl2 50:50 v/v mixture. Yield: 10.8 g (92.5%). Mn ) 2570
(GPC), Mn ) 2410 (1H NMR).
Hyd r och lor in a tion of III. 1a -Cl was finally obtained by
the hydrochlorination of III with HCl gas in CH2Cl2 at
0 °C.
Ch ar acter ization . Molecular weights and molecular weight
distributions were measured at room temperature, using a
Waters HPLC system equipped with a model 510 pump, a
model 486 tunable UV/vis detector, a model 250 dual refrac-
tometer/viscometer detector (Viscotek), a model 712 sample
processor, and five Ultrastyragel GPC columns connected in
the following series: 500, 103, 104, 105, and 100 Å. The flow
rate of THF, which was used as an eluent, was 1.0 mL/min.
NMR spectra were recorded on Bruker 250 MHz spectrometer
with CDCl3 as a solvent (Cambridge Isotope Laboratories,
Inc.).
Syn th esis of 1b-Cl. Bis(p-tolyl)methyl chloride, 1b-Cl, was
synthesized analogously from bis(p-tolyl)methanol,5b which
was either purchased or prepared from p-methylbenzophenone
and p-methylphenylmagnesium bromide.
Red u ction of II w ith LiAlH4. 11.7 g of II (Scheme 2) in
200 mL of dry THF, freshly distilled from LiAlH4, was reacted

Macromolecules, Vol. 35, No. 12, 2002
Electrophilic Reactivity of Carbocations 4613
F igu r e 1. 1H NMR spectrum of 1a -Cl.
Kin etic In vestiga tion of th e Slow Rea ction s. The
kinetics of the reactions of 1a + and 1b+ with allyltriphenyl-
silane and with allylchlorodimethylsilane were determined in
CH2Cl2 solution by means of the working station described
previously.6a Details of the individual kinetic experiments
(concentrations, rates at variable temperature) are given in
Tables S1-S4 of the Supporting Information.
Kin etic In vestiga tion of th e F a st Rea ction s. For the
laser experiments, the solutions (optical densities/cm were
0.5-1.5) were photolyzed by 6 ns pulses (30 mJ ) of 266 nm
from a Continuum Powerlite 9010 Nd:YAG laser with fourth
harmonic generator. The signals of the photomultiplier were
digitized by a Tektronix TDS744A oscilloscope. The rate
constants were determined by plotting the observed rates (at
five different concentration of 1-trimethylsiloxycyclohexene or
1-trimethylsiloxycyclopentene) as a function of the concentra-
tion of the nucleophile. These plots were all linear with R >
0.99.
reflux temperature and in CH2Cl2 at -25 °C (which was
the lowest temperature at which both the polymer and
the AlCl3-p-toluoyl chloride complex dissolved com-
pletely) for 20 h. In both cases 1H NMR spectroscopy
indicated incomplete acylation. Furthermore, degrada-
tion was a severe side reaction according to a compari-
son of the GPC traces of the starting material and the
products. Acylation was also attempted at -80 °C in a
hexanes/CH2Cl2 50/50 (v/v) solvent mixture for 20 h. In
this system the polymer dissolved, but the AlCl3-toluoyl
chloride complex precipitated. It was not surprising,
therefore, that neither acylation nor degradation oc-
curred under these conditions.
Complete acylation was finally achieved with the
mixed anhydride of p-toluic acid and trifluoromethane-
sulfonic acid, synthesized in situ from silver triflate and
p-toluoyl chloride, at room temperature in CH2Cl2
solvent.16 Acylation is clearly evident from the 1H
(Figure S1 in Supporting Information) and 13C (Figure
S2) NMR spectra of the acylated PIB, which showed the
resonance signals for CH3-Ph- protons at δ ) 2.47 and
the carbonyl 13C signal at δ ) 196.6. By comparing the
integrals for the methyl protons in the repeating unit
of polyisobutylene and the methyl protons in the CH3-
Resu lts a n d Discu ssion
Ch oice of System s. Laser flash photolytic genera-
tion of carbocations has been used for investigating fast
reactions of carbocations with nucleophiles.12 While
photolyses of benzhydryl halides and p-cyanophenolates
have been reported to proceed with homolytic and
heterolytic cleavage of the C-X bond to give benzhydryl
radicals and cations, respectively, photolysis of benzhy-
dryl acetate and diphenylmethanol was found to give
homolytic cleavage exclusively.13 Since the carbocations
obtained by capping of polyisobutylene with 1,1-diphen-
ylethylene do not form stable adducts with halides or
phenolates,14 we could not synthesize suitable precur-
sors for the photolytic generation of the carbocations
shown in Scheme 1. For that reason we have decided
to investigate benzhydryl cations linked to a polyisobu-
tylene chain. The synthetic strategy for the precursor
benzhydryl chloride linked to polyisobutylene is shown
in Scheme 2.
1
Ph- headgroup in the H NMR spectrum, Mn ) 2250
was calculated. This value is in good agreement with
Mn ) 2490 measured by GPC and virtually identical to
the Mn ) 2170 determined for the precursor polymer I
1
by H NMR spectroscopy.
The reduction of the acylated polyisobutylene was
carried out with LiAlH4 in dry THF. The doublet at
1
δ ) 5.82 in the H NMR spectrum of the product can be
attributed to the CH group formed in the reduction, and
the doublet at δ ) 2.12 corresponds to the OH group
(Figure S3). In the 13C NMR spectrum the carbonyl 13
C
signal absent and a new resonance signal appeared at
δ ) 76.3 (Figure S4).
Hydrochlorination of the alcohol yielded the final
First, isobutylene was polymerized by the cumyl
chloride/TiCl4 initiating system in a hexanes/CH3Cl 60/
40 (v/v) solvent mixture at -80 °C. After complete
conversion (CH3)2Zn was added to methylate quantita-
tively the ω-end.15 Friedel-Crafts acylation of the
aromatic headgroup of the methylated polymer was
attempted with toluoyl chloride and AlCl3 in CS2 at
product 1a -Cl. The peak assigned to the methine proton
1
moved downfield from δ ) 5.83 to 6.13 in the H NMR
spectrum (Figure 1), and the carbon resonance shifted
from δ ) 76.3 to 64.66 in the 13C NMR spectrum (Figure
2). Importantly, the molecular weight and molecular

4614 Mayr et al.
Macromolecules, Vol. 35, No. 12, 2002
F igu r e 2. 13C NMR spectrum of 1a -Cl.
Ta ble 2. Ch a r a cter istics of th e P olym er s a t Differ en t
Step s of th e Syn th esis
compd
yielda (%)
Mn (GPC)
Mw/Mn
Mn (NMR)
I
II
III
1a -Cl
94.6
93.7
92.5
2360
2490
2570
2540
1.12
1.09
1.07
1.07
2170
2250
2410
2430
100.0
a
Purified product.
weight distribution remained constant during the four
steps of the synthesis (Table 2).
Kin etic In vestiga tion s w ith Lon g-Lived Ca r -
boca tion s. Treatment of 1b-Cl with TiCl4 or GaCl3 in
CH2Cl2 gave solutions of the carbocation 1b+, the
concentration of which was detected photometrically
using fiber optics and the working station described
previously.11 While the ionization of 1a -Cl with TiCl4
caused problems, the reaction with GaCl3 led to the
quantitative formation of 1a +. Addition of 5-120 equiv
of the allylsilanes led to the exponential decay of the
benzhydryl cations, from which the second-order rate
constants have been derived.
F igu r e 3. Electronic spectra of benzhydryl cations 1a +,b+ in
acetonitrile/CH2Cl2 (3/1).
Table 3 shows that the rate constants (-70 °C)
determined for 1a + and 1b+ with both allylsilanes agree
within experimental error, indicating that replacement
of a p-methyl group of 1b+ by a polyisobutylenyl group
does not change the electrophilicities of the benzhydryl
cations. Similar rate constants obtained with different
the C-Cl bond, while in acetonitrile homolytic and
heterolytic cleavage occurred with simultaneous forma-
tion of benzhydryl radicals and cations, respectively.12
Since 1a -Cl turned out to be insoluble in acetonitrile,
the laser flash photolyses were carried out in acetoni-
trile/CH2Cl2 mixtures (v/v ) 1/3). As shown in Figure
3, photolysis of 1b-Cl gave a mixture of the radical 1b•
(λmax ) 338 nm) and the cation 1b+ (λmax ) 469 nm) in
accord with previous observations.13 From the molar
absorption coefficients reported in ref 13 and the
spectrum shown in Figure 3, one can derive a cation/
radical ratio of [1b+]/[1b•] ) 0.4 compared to 0.67 in
pure acetonitrile.13 While the absorption maxima of the
radical 1a • (λmax ) 342 nm) and the carbocation 1a +
(λmax ) 482 nm) experience a slight bathochromic shift
relative to 1b• and 1b+, respectively, Figure 3 indicates
the formation of the same cation/radical ratio from both
precursors 1a -Cl and 1b-Cl, when the two cations as
-
-
counterions GaCl4 (or Ga2Cl7-) and Ti2Cl9 are in
accord with rate-determining formation of the carbon-
carbon bond followed by a rapid desilylation step (eq
1).
Kin etic In vestiga tion s of La ser F la sh P h otolyti-
ca lly Gen er a ted Ca r boca tion s. Previous work has
shown that photolysis of benzhydryl chlorides in dichlo-
romethane resulted in exclusive homolytic cleavage of

Macromolecules, Vol. 35, No. 12, 2002
Electrophilic Reactivity of Carbocations 4615
Ta ble 3. Ra te Con sta n ts a n d Activa tion P a r a m eter s for th e Rea ction s of th e Ben zh yd r yl Ca tion s 1a +,b+ w ith Allylsila n es
in CH2Cl2
Ar2CH+X-
nucleophile
k/(-70 C°) L mol-1 s-1
∆H‡/kJ mol-1
∆S‡/J mol-1 K-1
-
1a GaCl4-
H2CdCH-CH2-SiPh3
H2CdCH-CH2-SiPh3
H2CdCH-CH2-SiClMe2
H2CdCH-CH2-SiClMe2
H2CdCH-CH2-SiClMe2
391 ( 23
385 ( 19
24.6 ( 2.3
25.6 ( 1.3
19.9 ( 0.7
20.26 ( 1.39
18.68 ( 0.89
16.83 ( 1.38
19.98 ( 0.57
19.76 ( 0.61
-92.4 ( 6.7
-100.3 ( 4.2
-132.3 ( 6.4
-116.4 ( 2.6
-119.6 ( 2.7
1b GaCl4-
1a GaCl4-
1b GaCl4-
1b Ti2Cl9
Ta ble 4. Ra te Con sta n ts (L m ol-1 s-1) for th e Rea ction s of
th e Ben zh yd r yl Ca tion s 1a +,b+ w ith Silyl En ol Eth er s in
Aceton itr ile a n d Aceton itr ile/CH2Cl2 (1/3 v/v)a
than 30%, even when the rate constants under consid-
eration are greater than 108 L mol-1 s-1. We can
conclude, therefore, that macromolecular carbocations
with Mn ∼ 2400 and low molecular weight analogues
do not differ in reactivity with the consequence that the
high kp values reported for the polymerization of iso-
butylene are real.
1a
1b
TMSOCP TMSOCH TMSOCP TMSOCH
CH3CN
4.23 × 108 1.15 × 108
CH3CN/CH2Cl2 3.56 × 108 1.26 × 108 4.62 × 108 1.60 × 108
(R)
(0.997)
(0.9996)
(0.994)
(0.9996)
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C NMR
spectra of II and III and tables of rate constants for reactions
of 1a + and 1b + with allylchlorodimethylsilane and allyltri-
phenylsilane in CH2Cl2. This material is available free of
charge via the Internet at http://pubs.acs.org.
a
TMSOCP ) 1-trimethylsilyloxycyclopentene; TMSOCH
)
1-trimethylsilyloxycyclohexene.
Refer en ces a n d Notes
(1) (a) Plesch, P. H. Prog. React. Kinet. 1993, 18, 30. (b) Plesch,
P. H. Macromolecules 2001, 34, 1143.
(2) (a) Plesch, P. H. Philos. Trans. R. Soc. London, Ser. A. 1993,
342, 469. (b) Magagnini, P. L.; Cesca, S.; Giusti, P.; Priola,
A.; DiManna, M. Makromol. Chem. 1977, 178, 2235. (c)
Marek, M.; Chmelir, M. J . Polym. Sci. 1968, 177, 3225. (d)
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Macromolecules 2000, 33, 8225.
(3) This specification was added at the request of one of the
reviewers. In our view, however, there is hardly a difference
between kp( and kp+. See: Mayr, H. Cationic Polymerizations;
Matyjaszewski, K., Ed.; Marcel Dekker: New York, 1996; pp
87-91.
(4) (a) Vairon, J . P.; Rives, A.; Bunel, C. Macromol. Chem.,
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(5) (a) Reinhardt, C. Solvents and Solvent Effects in Organic
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(b) Schneider, R.; Mayr, H.; Plesch, P. H. Ber. Bunsen-Ges.
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(6) (a) Schneider, R.; Mayr, H.; Schade, C.; Bartl, J .; Bederke,
R. J . Am. Chem. Soc. 1990, 112, 4446. (b) Hagen, G.; Mayr,
H. J . Am. Chem. Soc. 1991, 113, 4954. (c) Mayr, H.; Basso,
N.; Hagen, G. J . Am. Chem. Soc. 1992, 114, 3060.
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938.
F igu r e 4. A typical exponential decay of the benzhydryl
cations in a reaction with silyl enol ethers (1-trimethylsilyl-
oxycyclohexene ([TMSOCH] ) 7.451 × 10-3 M).
well as the two radicals are assumed to possess identical
molar absorption coefficients, respectively.
(8) (a) Schneider, R.; Mayr, H.; Grabis, U. J . Am. Chem. Soc.
1990, 112, 4460. (b) Patz, M.; Mayr, H.; Bartl, J .; Steenken,
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The pseudo-first-order decay of the benzhydryl cation
absorption in the presence of different concentration of
1-trimethylsiloxycyclohexene and 1-trimethylsiloxy-
cyclopentene was observed; a typical decay plot is shown
in Figure 4. These plots were fitted with an exponential
function to determine the rate of reaction. The second-
order rate constants listed in Table 4 were determined
by plotting the observed rates (at five different concen-
tration of 1-trimethylsiloxycyclohexene or 1-trimethyl-
siloxycyclopentene) as a function of the concentration
of the nucleophile. In accordance with the results of
previous investigations, the change from acetonitrile to
acetonitrile/dichloromethane solvent had only little
influence on the reactivity of the carbocation.5 More
importantly, however, the rate constants determined for
the ditolylcarbenium ion 1b+ differ from those of the
analogous polymer-bound carbenium ion 1a + by less
(9) Roth, M.; Mayr, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
2250.
(10) Sipos, L.; Faust, R., to be submitted to Macromolecules.
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MA0120299