Living carbocationic polymerization of p-methoxystyrene using p-methoxystyrene hydrochloride/SnBr4 initiating system: Determination of the absolute rate constant of propagation for ion pairs

Article abstract of DOI:10.1021/ma0490451

The carbocationic polymerization of p-methoxystyrene (p-MeOSt)/SnBr ₄ using p-Methoxystyrene hydrochloride was studied in CH ₂Cl₂ at ?30°C. The living polymerization was observed to be the first order in SnBr₄ w

Full text of DOI:10.1021/ma0490451

7930
Macromolecules 2004, 37, 7930-7937
Living Carbocationic Polymerization of p-Methoxystyrene Using
p-Methoxystyrene Hydrochloride/SnBr4 Initiating System:
Determination of the Absolute Rate Constant of Propagation
for Ion Pairs
P r iya d a r si De 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 May 14, 2004; Revised Manuscript Received August 6, 2004
ABSTRACT: The carbocationic polymerization of p-methoxystyrene (p-MeOSt) was studied in CH2Cl2
at -30 °C using 1-chloro-1-(p-methoxyphenyl)ethane (p-MeOStCl) as initiator in conjunction with different
Lewis acids, BF3‚O(C2H5)2, BCl3, ZnCl2, TiCl4, SnCl4, and SnBr4. The best results, moderate rates,
theoretical Mns, and low polydispersities, were obtained in conjunction of SnBr4. The living nature of the
polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear Mn vs conversion plots
in the temperature range of -60 to -20 °C. The number-average molecular weight of the polymers
increased in direct proportion to monomer conversion up to Mn ) 120 000 and agreed with the calculated
molecular weight, assuming that one polymer chain forms per molecule of p-MeOStCl. Kinetic studies
suggest that the polymerization is first order in [SnBr4], and the rate, Mn, and polydispersity index are
not affected by excess proton trap, 2,6-di-tert-butylpyridine. The living nature of the polymerization was
further demonstrated by chain extension experiment. The stability of the propagating chain end of poly-
(p-MeOSt) and monomeric chain end of p-MeOSt+ was studied, and a slow decomposition of the active
chain end was observed under monomer starved conditions. Employing the model compound p-MeOStCl
in conjunction with SnBr4 in CH2Cl2 UV-vis spectroscopy was used to determine the equilibrium constant
of ionization (Ki) at -30 and -60 °C. From Ki values and the apparent rate constant of propagation
(kapp), the absolute rate constant of propagation for ion pairs, kp( ) 1.07 × 105 L mol-1 s-1 for -30 °C and
3.83 × 104 L mol-1 s-1 for -60 °C, was calculated. To determine kp( separately, competition experiments
were carried out in the presence of a nucleophile, phenylsilane. NMR spectroscopy, gel permeation
chromatography, and MALDI-TOF MS analysis suggested complete capping of the polymeric cation and
the absence of side reactions. From the limiting conversion and limiting number-average degree of
(
(
polymerization kp was calculated using the known rate constant of capping. The kp values obtained
from the competition experiments agreed well with those determined from the UV-vis spectroscopy.
In tr od u ction
StCl)/Yb(OTf)3 initiating system in the presence of 2,6-
di-tert-butyl-4-methylpyridine.9 The authors also claimed
the controlled cationic polymerization of p-MeOSt with
very slow polymerization in aqueous media using the
p-MeOStCl/Yb(OTf)3 initiating system. Relatively nar-
row PDI (∼1.4) were observed, and the molecular
weights increased in proportion to the monomer conver-
sion. More recently, surfactants,10 sulfonic acid-based
initiators,11 and various phosphonic acid initiators12
were used for the cationic polymerization of p-MeOSt
in aqueous medium. Weak Lewis acids such as SnBr4
have not been used as co-initiators to study the living
cationic polymerization of p-MeOSt.
The absolute rate constant of propagation for ion pairs
(kp() in the cationic homopolymerization of isobutylene
(IB)1 and styrene (St)2 was recently determined by the
(
diffusion clock method. For both IB and St, the kp
values were in good agreement with those predicted by
the linear free energy relationship (LFER) by Mayr;3
however, they were 4-6 orders of magnitude higher
than previously accepted values.4 We are extending our
work to St derivatives and have recently reported
preliminary results for 2,4,6-trimethylstyrene (TMeSt),
where both UV spectroscopy and competition experi-
ments at -70 °C yielded similar kp ∼ (1-3) × 104 L
(
mol-1 s-1.5 The present study focuses on p-methoxysty-
rene (p-MeOSt). The living cationic polymerization of
p-MeOSt was first reported with the HI/ZnI2 initiating
system in toluene at -15 to 25 °C.6 Living polymeriza-
tions were also attained in the more polar solvent, CH2-
Cl2, with the HI/I2 and HI/ZnI2 initiating systems in the
presence of tetra-n-butylammonium salts (nBu4NX; X
) Cl, Br, I).7 Comparable but less controlled polymer-
ization of p-MeOSt has been reported using iodine as
an initiator in carbon tetrachloride.8 This system gives
rise to long-lived but not truly living polymerization. A
recent report indicated the controlled cationic poly-
merization of p-MeOSt with controlled molecular weights
and relatively narrow molecular weight distribution
(PDI ) 1.4) using the p-MeOSt-HCl adduct (p-MeO-
For the cationic polymerization of p-MeOSt using both
calorimetry and spectrophotometry, Cotrel et al. re-
ported kp ) 7.4 × 104 and 1.4 × 104 L mol-1 s-1 at -15
and 10 °C, respectively.13 Stopped-flow/rapid-scanning
spectroscopy was also applied to study the polymeriza-
tion of p-MeOSt in CH2Cl2 and in mixed solvents by
numerous initiators, and considering the absorption of
the propagating cation at 380 nm, kp values ranging
from 4 × 103 to 1.3 × 105 L mol-1 s-1 have been
reported.14 As pointed out by Plesch,4 however, assign-
ment of the 380 nm peak for the p-MeOSt cation is
erroneous. Recently, using laser flash photolysis, Mc-
Clelland et al.15 have shown that λmax for the p-MeOSt
cation is at 340 nm, which was earlier given by De Sorgo
et al.16
10.1021/ma0490451 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/23/2004

Macromolecules, Vol. 37, No. 21, 2004
Polymerization of p-Methoxystyrene 7931
Ta ble 1. Resu lts for th e Ca tion ic P olym er iza tion of
p-MeOSt Usin g Differ en t Lew is Acid /p-MeOStCl
In itia tin g System in CH2Cl2 a t -30 °Ca
In this study, the polymerization of p-MeOSt initiated
by p-MeOStCl, a model propagating chain end, was
studied in CH2Cl2 at -20 to -60 °C using SnBr4, a weak
Lewis acid. UV-vis spectroscopy and competition ex-
[Lewis acid] time conv
(mol L-1
)
(min) (%)
Mn
PDI
(
expt
Lewis acid
periments have been carried out to determine kp
separately.
1
2
3
4
5
6
7
8
BF3.O(C2H5)2
BF3.O(C2H5)2
BCl3
BCl3
BCl3
ZnCl2
ZnCl2
ZnCl2
TiCl4
TiCl4
SnCl4
SnCl4
SnBr4
SnBr4
SnBr4
0.02
0.07
42
10
5
5
5
30
30
30
1
1
0.5
66.7 267 300 1.8
98.2 221 000 2.1
0.003
0.006
0.012
0.003
0.009
0.016
0.0035
0.006
0.0035
0.01
98.2
100
100
17.2
19 000 1.7
20 600 1.6
16 500 2.1
5 400 2.7
Exp er im en ta l Section
Ma ter ia ls. The p-methoxystyrene (p-MeOSt, Aldrich, 97%)
was distilled from calcium hydride (CaH2) under reduced
pressure and stored under nitrogen at -20 °C, and just before
use it was again distilled from CaH2 under reduced pressure.
The 1-chloro-1-(p-methoxyphenyl)ethane (p-MeOStCl) was
synthesized just before use by bubbling dry HCl gas into
p-MeOSt in CH2Cl2 (1:10 p-MeOSt/CH2Cl2 (v/v)) under stirring
at -78 °C, and then dry nitrogen was bubbled to remove excess
56.2
94.5
12 100 1.1
19 100 1.3
21 400 1.7
18 400 1.9
21 000 1.4
22 400 1.7
14 300 1.05
15 600 1.04
22 600 1.05
9
100
100
97.6
100
68.9
72.5
100
10
11
12
13
14
15
1
5
3
5
0.004
0.008
0.025
1
HCl. Finally, CH2Cl2 was removed using a vacuum pump. H
NMR spectrum: p-MeOStCl; 7.41 (d), 6.94 (d), 5.1-5.2 (q), 3.86
(s), and 1.89 ppm (d). The CH2Cl2 was shaken with 5% NaOH
and then repeatedly with water. Then it was predried with
anhydrous Na2SO4 and distilled from CaH2 under nitrogen.
This distilled CH2Cl2 was refluxed under nitrogen overnight
with P2O5 and distilled to a round-bottom flask over P2O5. It
was refluxed under nitrogen overnight and distilled just before
use. Phenylsilane (97%, Aldrich), SnBr4 (99%, Aldrich), and
2,6-di-tert-butylpyridine (DTBP, 97%, Aldrich) were used as
received. All other chemicals and solvents were purified as
described previously or used as received.1
P olym er iza tion . Polymerizations were carried out under
a dry nitrogen atmosphere ([H2O] < 0.5 ppm) in an MBraun
150-M glovebox (Innovative Technology Inc., Newburyport,
MA). Large (75 mL) culture tubes were used as polymerization
reactors. The total volume of the reaction mixture was 15 mL.
After predetermined time polymerization was terminated by
the addition of 0.5 mL of prechilled methanol. The polymer
was recovered and purified two times by reprecipitation from
CH2Cl2/methanol. Monomer conversions were determined by
gravimetric analysis.
[DTBP] ) 0.004 mol L-1, [p-MeOSt] ) 0.15 mol L-1, and
a
[p-MeOStCl] ) 0.001 mol L-1
.
spectrometer using CDCl3 as a solvent (Cambridge Isotope
Lab., Inc.). 1H NMR spectra of solutions in CDCl3 were
calibrated to tetramethylsilane as internal standard (δH 0.00).
MALDI-TOF MS. The MALDI-MS and post source decay
(PSD) MALDI-MS/MS measurements were performed with a
Bruker BIFLEX III mass spectrometer equipped with a time-
of-flight (TOF) analyzer. In all cases 19 kV acceleration voltage
was used with pulsed ion extraction (PIE). The positive ions
were detected both in the reflectron mode (20 kV) and in the
linear mode. A nitrogen laser (337 nm, 3 ns pulse width, 106-
107 W/cm2) operating at 4 Hz was used to produce laser
desorption.
The samples (5 mg/mL) in THF were prepared with a
dithranol matrix (20 mg/mL in THF). To produce silver
cationized peaks, silver trifluoroacetate (AgTFA) dissolved in
THF at a concentration of 1 mg/mL was added to the matrix/
analyte solution. The solutions were mixed in 10:2:1 v:v ratio
(matrix:analyte:AgTFA). A volume of 0.5 µL of these solutions
was deposited onto the sample plate (stainless steel) and
allowed to air-dry.
UV-Vis Sp ectr oscop y. UV-vis spectroscopic measure-
ments were carried out under a dry nitrogen atmosphere in
the glovebox. A quartz immersion probe (661.300-QX, Hellma,
optical path: 0.5 or 0.02 cm) connected to a fiber-optic visible
(tungsten light source, Ocean Optics) and UV (AIS model UV-
2, Analytical Instrument Systems, Inc.) light source and a
Zeiss MMS 256 photodiode array detector was used. The latter
was connected to a personal computer via a TEC5 interface,
and the spectra were recorded using the “Aspect Plus” software
(Zeiss).
Competition experiments were carried out in the presence
of phenylsilane as a capping agent. In a typical competition
experiment the polymerization was carried out in CH2Cl2 at
-30 °C using the following concentration: [p-MeOStCl] )
0.001 mol L-1, [DTBP] ) 0.006 mol L-1, [p-MeOSt] ) 0.15 mol
L-1, [TiCl4] ) 0.003 mol L-1, [phenylsilane] ) 0.75 mol L-1
.
Into a 75 mL culture tube at -30 °C 12.1 mL of CH2Cl2 at
room temperature, 0.45 mL of DTBP stock solution in CH2Cl2
(0.2 mol L-1) at -30 °C, 0.31 mL of p-MeOStCl stock solution
in CH2Cl2 (0.045 mol L-1) at -30 °C, 1.0 mL of p-MeOSt stock
solution in CH2Cl2 (2.25 mol L-1) at -30 °C, and 1.4 mL of
phenylsilane at room temperature were added and mixed
thoroughly. The polymerization was started by the addition
of 0.47 mL of TiCl4 solution (0.09 mol L-1, in CH2Cl2) at -30
°C. After a predetermined time (15 min) 0.5 mL of prechilled
methanol was added to the reaction mixture to quench the
polymerization. The polymer was precipitated twice from CH2-
Cl2/methanol to obtain 107 mg of polymer (conversion ) 35.3%,
Mn,GPC ) 7400, PDI ) 2.7).
Resu lts a n d Discu ssion
Livin g Ca t ion ic P olym er iza t ion of p -MeOSt .
Sea r ch for Lew is Acid Coin itia tor . The cationic
polymerization of p-MeOSt was carried out using p-
MeOStCl as initiator (0.001 mol L-1) in the presence of
DTBP (0.004 mol L-1) as proton trap in CH2Cl2 at -30
°C using BF3.O(C2H5)2, BCl3, ZnCl2, TiCl4, SnCl4, and
SnBr4 as Lewis acids. The results are presented in Table
1. The cationic polymerization of p-MeOSt using BF3‚
O(C2H5)2 as coinitiator is uncontrolled and yields poly-
mers with Mns much higher than the theoretical value
and broad molecular weight distribution. Using BCl3,
theoretical molecular weights but high polydispersities
were obtained since the polymerization was too fast.
Polymerization with ZnCl2 (in diethyl ether) in the
concentration range 0.01-0.02 mol L-1 gives poly(p-
MeOSt) with theoretical molecular weight. The molec-
ular weight distribution decreases to 1.1 at 60-80%
Ch a r a cter iza tion . Molecular weights were measured using
a Waters HPLC system equipped with a model 510 HPLC
pump, a model 250 dual refractometer/viscometer detector
(Viscotek), a model 486 UV/vis detector, a model 712 sample
processor, and five ultra-Styragel GPC columns connected in
the following series: 500, 103, 104, 105, and 100 Å. Tetrahy-
drofuran (THF) was used as eluent at a flow rate of 1.0 mL/
min at room temperature. The measurements were carried out
at room temperature. The number-average molecular weights
(Mn) and polydispersities (PDI) were determined using the
conventional calibration curve and Viscotek TriSEC GPC
software. Molecular weights were also calculated by the
universal calibration obtained by polystyrene standards; how-
ever, the Mn values were very similar to that obtained from
1
conventional calibration using polystyrene standards. The H
NMR spectroscopy was carried out on a Bruker 250 MHz

7932 De and Faust
Macromolecules, Vol. 37, No. 21, 2004
F igu r e 1. Polymerization of p-MeOSt initiated with p-
F igu r e 2. First-order plot of ln([M]0/[M]) vs time for the
cationic polymerization of p-MeOSt initiated by p-MeOStCl/
SnBr4 in CH2Cl2; [p-MeOStCl] ) 0.001 mol L-1, [DTBP] )
0.006 mol L-1, [p-MeOSt] ) 0.15 mol L-1, [SnBr4] ) 0.004 mol
MeOStCl/ZnCl2 in the presence of increasing concentration of
DTBP at -30 °C in CH2Cl2; [p-MeOSt] ) 0.15 mol L-1
[p-MeOStCl] ) 0.001 mol L-1, and [ZnCl2] ) 0.016 mol L-1
,
.
L-1
.
conversion and then increases to 1.2-1.3 at 100%
conversion. Sequential monomer addition experiments
(not shown), however, proved the nonliving nature of
the polymerization. As shown in Figure 1, the poly-
merization rate decreases with increasing DTBP con-
centration, suggesting a complex nature of the poly-
merization with possible complexation between ZnCl2
and DTBP. Cationic polymerization of p-MeOSt using
the p-MeOStCl/TiCl4 initiating system gives poly(p-
MeOSt) with theoretical molecular weight, but because
of the extremely rapid polymerization, the molecular
weight distribution is broad. The theoretical molecular
weight was also obtained with PDI ) 1.4-1.7 using
SnCl4, but again the polymerization is extremely rapid.
Importantly, the polymerization induced by SnBr4
proceeds with a moderate rate, and the polymers
exhibited narrow molecular weight distribution (PDI <
1.1). On the basis of these results, we have selected the
p-MeOStCl/SnBr4 initiating system in CH2Cl2 at -30
°C to study the living cationic polymerization of p-
MeOSt in detail.
F igu r e 3. Variation of Mn and PDI with conversion. Reaction
conditions are the same as in Figure 2.
Effect of Tem p er a tu r e. The cationic polymerization
of p-MeOSt with SnBr4 was carried out at different
temperatures. At all temperatures, the first-order plots
of ln([M]0/[M]) vs time were linear starting at the origin
(Figure 2), and the slope of this plot was calculated to
determine the apparent rate constant of propagation
(kapp). The linear first-order plots obtained at all tem-
peratures indicate the absence of irreversible termina-
tion during the polymerization. The number-average
molecular weight (Mn) and the polydispersity index (PDI
) Mw/Mn) vs conversion plots are shown in Figure 3.
The molecular weights of the poly(p-MeOSt) are directly
proportional to the conversion and follow the theoretical
Mn-conversion line at all temperatures, indicating the
absence of chain transfer during the polymerization. For
polymerizations at all temperatures the PDI decreases
with increasing conversion and reaches well below 1.1
after ∼60% conversion. The linear Mn-conversion plots
together with the linear first-order plots indicate that
the polymerization of p-MeOSt is living in the studied
temperature range.
F igu r e 4. Arrhenius plot of ln(kapp) vs 1/T to determine
apparent energy of activation, Ea, for the cationic polymeri-
zation of p-MeOSt initiated by p-MeOStCl/SnBr4 in CH2Cl2;
[p-MeOStCl] ) 0.001 mol L-1, [DTBP] ) 0.006 mol L-1
,
[p-MeOSt] ) 0.15 mol L-1, [SnBr4] ) 0.004 mol L-1
.
Ea has already been reported for the cationic polymer-
ization of various monomers such as St,2 IB,17 R-meth-
ylstyrene,18 TMeSt,5 and indene.19 The negative Ea has
been attributed to the decrease of the extent of chain
end ionization with increasing temperature.
The apparent rate constant of propagation, kapp
,
increases with decreasing temperature. The apparent
activation energy of polymerization, Ea ) -3.5 kJ mol-1
was calculated from the Arrhenius plot of ln(kapp) vs 1/T
(Figure 4). It is noteworthy to mention that negative
,

Macromolecules, Vol. 37, No. 21, 2004
Polymerization of p-Methoxystyrene 7933
F igu r e 6. Mn and PDI of the polymers obtained from the
polymerization of p-MeOSt initiated with p-MeOStCl/SnBr4
in the presence of increasing concentration of DTBP at -30
°C in CH2Cl2; [p-MeOSt] ) 0.15 mol L-1, [p-MeOStCl] ) 0.001
mol L-1, and [SnBr4] ) 0.004 mol L-1: (solid symbols) Mn;
(open symbols) PDI; straight line indicates the Mn assuming
that one polymer chain forms per molecule of p-MeOStCl.
F igu r e 5. Polymerization of p-MeOSt initiated with p-
MeOStCl/SnBr4 in the presence of increasing concentration of
DTBP at -30 °C in CH2Cl2; [p-MeOSt] ) 0.15 mol L-1
[p-MeOStCl] ) 0.001 mol L-1, and [SnBr4] ) 0.004 mol L-1
,
.
Control experiments (direct initiation) were also car-
ried out in the absence of initiator. Under the same
conditions, 5.5%, 11.3%, 15.1%, and 23.1% monomer
conversions were obtained after 5, 11, 17, and 30 min,
respectively. This confirms that direct initiation is
operational during the cationic polymerization of p-
MeOSt initiated by p-MeOStCl/SnBr4, but it is much
slower than in the presence of initiator, p-MeOStCl, and
the kinetics of polymerization is unaffected.
Effect of DTBP . The DTBP, a hindered pyridine, is
generally used in the carbocationic living polymerization
to prevent protic initiation.20 Thus, we have studied the
polymerization of p-MeOSt in the presence and absence
of DTBP with the p-MeOStCl/SnBr4 initiating system
in CH2Cl2 at -30 °C. In absence of DTBP 100%
conversion was reached in less than 1 min. The molec-
ular weight was higher than the expected theoretical
molecular weight (Mn ) 25 400, PDI ) 1.821 vs theoreti-
cal Mn, Mn,theo ) 20 100). When the polymerization
experiments were carried out in the presence of DTBP,
the difference from the theoretical molecular weight
decreased with increasing DTBP concentration. The
polymerization rates also decreased as [DTBP] in-
creased from 0.0005 to 0.001 mol L-1; however, above
this value the polymerization rate remained unchanged
as shown in Figure 5. At [DTBP] ) 0.001 mol L-1, close
to theoretical molecular weights and very narrow mo-
lecular weight distributions are observed (Figure 6), as
apparently all protic impurities have been scavenged.
Above [DTBP] ) 0.001 mol L-1 the polymerization rates,
Mns, and PDIs are independent of DTBP concentration;
thus, the polymerization is unaffected by excess DTBP.
So in all further experiments [DTBP] ) 0.006 mol L-1
was used in the polymerization.
Dep en d en ce of P olym er iza tion Ra te on [Sn Br 4].
The reaction order in [SnBr4] was investigated for the
polymerization of p-MeOSt in CH2Cl2 at -30 °C. First-
order plots were constructed for the polymerization of
p-MeOSt using the p-MeOStCl/SnBr4 initiating system
in CH2Cl2 at -30 °C at four different concentrations of
SnBr4 (0.004-0.008 mol L-1). As expected, the rate of
polymerization increases with increasing concentration
of SnBr4. The first-order plots were linear at all SnBr4
concentrations. The Mn of the polymers increased in
direct proportion to the monomer conversion at all
SnBr4 concentrations. The apparent rate constant of
F igu r e 7. Plot of log(kapp) vs log([SnBr4]) in the polymerization
of p-MeOSt in CH2Cl2 solvent at -30 °C; [p-MeOSt] ) 0.15
mol L-1, [p-MeOStCl] ) 0.001 mol L-1, and [DTBP] ) 0.006
mol L-1 at [SnBr4] ) 0.004-0.008 mol L-1
.
polymerization (kapp) was obtained from the slope of the
first-order plots. The plot of log(kapp) vs log([SnBr4])
(Figure 7) gives a slope nearly equal to 1, suggesting a
first-order dependence on SnBr4 and the presence of
SnBr4Cl- counterions. For the cationic polymerization
of St22 in CH2Cl2, a first-order dependence of the
polymerization rate on [SnCl4] was already reported in
the literature, and one can expect the first-order de-
pendence on [SnBr4] in the present polymerization
system.
Con tr ol of P olym er Molecu la r Weigh t. So far, the
living nature of the cationic polymerization of p-MeOSt
was studied in the molecular range up to Mn ≈ 20 000.
To confirm the absence of chain transfer and termina-
tion, a series of experiments were carried out by
increasing the initial molar ratio of monomer to initiator
([M]0/[I]0), keeping all other conditions constant. The
([M]0/[I]0) ratio was varied from 180 to 900. As shown
in Figure 8, the polymer molecular weights at 100%
conversion are directly proportional to the ratio of
p-MeOSt to p-MeOStCl. The Mn values are in acceptable
agreement with those calculated with the assumption
that each molecule of p-MeOStCl initiates one living

7934 De and Faust
Macromolecules, Vol. 37, No. 21, 2004
F igu r e 8. (a) GPC RI traces and (b) Mn and PDI vs [M]0/[I]0
of poly(p-MeOSt) in the polymerization of p-MeOSt initiated
with p-MeOStCl/SnBr4 at -30 °C in CH2Cl2; [p-MeOSt]0/[p-
MeOStCl] ) 180-900 and [SnBr4] ) 0.004 mol L-1, [DTBP]
) 0.006 mol L-1, [p-MeOStCl] ) 0.001 mol L-1, polymerization
time was 24 min. In (b) 9: Mn; 0: PDI; and straight line
indicates the Mn based on 100% conversion.
F igu r e 10. GPC RI traces of poly(p-MeOSt); [p-MeOSt]0
)
[p-MeOSt]increment ) 0.15 mol L-1, [p-MeOStCl] ) 0.001 mol
L-1, [DTBP] ) 0.006 mol L-1, and [SnBr4] ) 0.004 mol L-1 at
-30 °C in CH2Cl2.
F igu r e 11. 1H NMR spectrum of poly(p-MeOSt) obtained with
p-MeOStCl/SnBr4 initiating system in CH2Cl2 at -30 °C.
[p-MeOSt] ) 0.15 mol L-1, [p-MeOStCl] ) 0.007 mol L-1
,
[DTBP] ) 0.006 mol L-1, and [SnBr4] ) 0.004 mol L-1 at 100%
conversion; Mn,GPC ) 3300 and PDI ) 1.15.
F igu r e 9. Monomer increment experiments in the cationic
polymerization of p-MeOSt initiated with p-MeOStCl/SnBr4
at -30 °C in CH2Cl2; [p-MeOSt]0 ) [p-MeOSt]increment ) 0.15
mol L-1, [p-MeOStCl] ) 0.001 mol L-1, [DTBP] ) 0.006 mol
L-1, and [SnBr4] ) 0.004 mol L-1. (a) 9: Mn; 0: PDI; and
straight line indicates the Mn based on conversion. (b) GPC
RI traces of poly(p-MeOSt) at different conversions.
addition technique. The unquenched polymerization
reaction mixture was left at -30 °C under monomer-
starved conditions for different times, and then after a
certain time p-MeOSt in CH2Cl2 at -30 °C was added
to polymerize for another 20 min. The GPC RI chro-
matograms (Figure 10) obtained after the polymeriza-
tion of the second monomer increment become bimodal
with increasing aging time. Thus, under monomer-
starved conditions the polymeric chain ends are stable
up to ∼30 min in CH2Cl2 at -30 °C; however, they
slowly decompose at longer time.
polymer chain, and over the whole range of the [M]0/
[I]0 ratio, the PDI of the polymers remained very narrow
(PDI < 1.1). Thus, the present initiating system, p-
MeOStCl/SnBr4, efficiently generates high molecular
weight poly(p-MeOSt) with very narrow PDI.
In another experiment DTBP, SnBr4, and p-MeOStCl
were mixed in CH2Cl2 at -30 °C, and polymerization
was started by the addition p-MeOSt after 0, 15, 30, 60,
and 120 min and polymerized for 25 min. The Mns of
the polymers were 19 800, 21 500, 24 400, 26 700, and
28 400 respectively when p-MeOSt was added after 0,
15, 30, 60, and 120 min to the premixed DTBP, SnBr4,
and p-MeOStCl in CH2Cl2 at -30 °C. The PDI, however,
remained unchanged (1.05-1.07). This indicates that
monomeric chain end also decomposes (most likely
eliminates H+ which is trapped by DTBP) with increas-
ing aging time in the presence of SnBr4, and the initiator
efficiency decreases from ∼100% to ∼71% in 120 min.
Ch a in Exten sion Exp er im en t. The living nature
of the p-MeOSt polymerization with the p-MeOStCl/
SnBr4 initiating system was further studied by chain
extension; i.e., a new feed of p-MeOSt was added to the
polymerization mixture before the initial charge of the
monomer had been completely consumed. The ad-
ditional p-MeOSt was smoothly polymerized at nearly
the same rate as in the first stage. As shown in Figure
9, the Mn of poly(p-MeOSt) increased in direct propor-
tion to monomer conversion and was very close to the
theoretical value for living polymers. The molecular
weight distribution remained very narrow (<1.1), and
no tailing appeared in the low molecular weight region.
Thus, the polymerization of p-MeOSt by p-MeOStCl/
SnBr4 initiating system in CH2Cl2 at -30 °C is living,
as chain transfer and termination are undetectable.
1
An a lysis of th e En d Gr ou p . The H NMR spectro-
scopy was used to study the end group structure of the
1
polymers. Figure 11 depicts the H NMR spectrum of a
typical poly(p-MeOSt) sample obtained with the p-
MeOStCl/SnBr4 initiating system in CH2Cl2 at -30 °C
at 100% conversion; Mn,GPC ) 3300 and PDI ) 1.15. The
Lifetim e of th e Livin g En d s. The lifetime of living
poly(p-MeOSt) was studied by the incremental monomer

Macromolecules, Vol. 37, No. 21, 2004
Polymerization of p-Methoxystyrene 7935
F igu r e 12. Plots of absorption at different time for the
ionization of p-MeOStCl with SnBr4; [p-MeOStCl] ) 0.03 mol
L-1, [DTBP] ) 0.006 mol L-1, and [SnBr4] ) 0.08 mol L-1 in
CH2Cl2 solvent at -60 °C.
F igu r e 13. Ionization of p-MeOStCl by SnBr4 in CH2Cl2 at
-30 and -60 °C; [p-MeOStCl]0 ) 0.04 mol L-1 (-30 °C), 0.03
mol L-1 (-60 °C) and [DTBP] ) 0.006 mol L-1
.
value of the intercept was subtracted from the original
absorbance, and percent ionization was calculated using
the corrected value of absorbance. The molar absorption
coefficient ꢀmax ) 27 300 L mol-1 cm-1 was determined
from the absorption spectrum of p-MeOStCl at 345 nm
with TiCl4 at -30 °C. A further increment of TiCl4
confirmed the quantitative ionization of p-MeOStCl to
the p-MeOSt+ cation. This ꢀmax value is in excellent
agreement with the value reported in the literature for
p-MeOSt cation14c,23 (ꢀmax ) 28 000 L mol-1 cm-1 at room
temperature) and p-methoxy-R-methylstyrene cation24
(ꢀmax ) 28 500 L mol-1 cm-1 at -72 °C). From the results
presented in Figure 13 the average equilibrium constant
of ionization, Ki ) 9.16 × 10-3 L mol-1 and Ki ) 3.11 ×
10-2 L mol-1, was calculated for -30 and -60 °C,
respectively.
signals at 6.25-7.05 and 3.6-4.0 ppm are due to the
aromatic protons and -OCH3 protons attached to the
aromatic ring, respectively. The signals at 1.2-2.4 ppm
are due to the -CH2 and -CH protons in the main chain
of the polymer. The peak at 0.93-1.1 ppm corresponds
to the H3C- protons from the initiator fragment (p-
MeOStCl). The peak at 2.92-3.08 ppm is due to meth-
oxy terminal group resulting from a reaction with
methanol. MALDI-TOF MS measurements also con-
firmed the methoxy end group,; the peaks were consis-
tent with CH3-CH(C6H4-OCH3)-(-CH2-CH(C6H4-
OCH3))n-OCH3. The methoxy end group, however, is
not the result of in situ termination of cations since the
cation concentration is very low (see later). More likely
fast methanolysis of the chloro end yields the methoxy
end groups. The number-average degree of polymeri-
1
During the polymerization of p-MeOSt, ion pairs are
considered as active species since in the presence of
DTBP, ion-pair dissociation is suppressed1,25 by common
anions of the pyridinium salt formed from existing
traces of moisture in the presence of SnBr4. The rate of
polymerization (Rp) can be expressed as
zation for poly(p-MeOSt) determined from the H NMR
spectrum as Mn,NMR ) 3500 (from the peak intensity
ratio of the -OCH3 protons attached to the aromatic
ring at 3.6-4.0 ppm to the H3C- terminal group at
0.93-1.1 ppm) is in reasonable agreement with the
calculated value based on the assumption that one living
chain is formed from one molecule of initiator, p-
MeOStCl. On the basis of the above discussion and in
agreement with the molecular weight analysis, the
general formula of poly(p-MeOSt) can be written as
shown in Figure 11.
Deter m in a tion of th e Absolu te Ra te Con sta n t of
P r op a ga tion (kp () of th e Ca tion ic P olym er iza tion
of p-MeOSt. a . UV-Vis Sp ectr oscop y. To determine
the equilibrium constant of ionization (Ki), the ionization
of the model compound p-MeOStCl with SnBr4 was
studied by UV-vis spectroscopy in CH2Cl2 at -30 and
-60 °C. Mixing p-MeOStCl with the solution of DTBP
and SnBr4 in CH2Cl2 at -60 °C under stirring, the UV-
vis spectrum exhibited a maximum absorption at 344-
348 nm. Also, as shown in Figure 12, two other peaks
centered at 427 and 550 nm developed. The intensity
of these peaks is much higher at the higher temperature
(-30 °C) under the same conditions. As we discussed
earlier, the peak at 344-348 nm is due to the p-MeOSt
cation and was used to determine the Ki. The plot of
absorbance vs SnBr4 concentration showed a positive
intercept for both temperatures. This intercept is due
to the impurities (possibly p-methoxyacetophenone)
present in the commercially available p-MeOSt. A
similar observation was also observed during the ioniza-
tion of hydrochlorinated 2,4,6-trimethylstyrene. This
Rp ) -d[p-MeOSt]/dt ) kp([Pn+][ p-MeOSt] )
kapp[p-MeOSt] (1)
where [Pn+] is the concentration of the active center,
[p-MeOSt] is the concentration of the monomer (M), and
kapp is the first-order apparent rate constant of propaga-
tion obtained from the first-order plot of ln([M]0/[M]) vs
time. After integration, eq 1 can be expressed as
[M]0
ln
) kappt ) kp (Ki[SnBr4][Pn-Cl]0t
(2)
[M]t
where Ki is the equilibrium constant of ionization and
[Pn-Cl] is the concentration of total chain ends. At -30
°C, the plot of ln([M]0/[M]) vs time (Figure 2) gives the
slope ) kapp ) 3.9 × 10-3 s-1. Using Ki ) 9.16 × 10-3
L
mol-1, [SnBr4] ) 0.004 mol L-1, and [p-MeOStCl] )
0.001 mol L-1, [Pn+] ) 3.664 × 10-8 mol L-1 was
calculated at -30 °C. Using kapp, kp ) 1.07 × 105
L
(
mol-1 s-1 was obtained at -30 °C. By a similar calcula-
tion, kp( ) 3.83 × 104 L mol-1 s-1 was obtained for -60
°C.
b. Com p etition Exp er im en ts. Recently competition
experiments have been successfully used to determine

7936 De and Faust
Macromolecules, Vol. 37, No. 21, 2004
a
Ta ble 2. Com p etition Exp er im en t of p-MeOSt w ith P h en ylsila n e Ca p p in g Agen t in CH2Cl2 Usin g Sn Br 4
M
(
temp (°C)
kc( (L mol-1 s-1
)
conv (%)
Mn (GPC)
PDI
kp(/kc( (X
)
kp(/kc( (DPn,
)
kp × 10-4 (L mol-1 s-1
)
-30
-40
-50
-60
340.1
188.6
99.4
59.2
50.3
65.9
62.6
12 300
10 700
13 400
12 900
1.2
1.3
1.2
1.2
538
419
645
590
566
455
657
614
18.8
8.2
6.5
49.4
3.0
a
[p-MeOStCl] ) 0.001 mol L-1, [DTBP] ) 0.006 mol L-1, [p-MeOSt] ) 0.15 mol L-1, [SnBr4] ) 0.004 mol L-1, and [phenylsilane] ) 0.6
mol L-1
.
Ta ble 3. Com p etition Exp er im en t of p-MeOSt w ith
P h en ylsila n e Ca p p in g Agen t in CH2Cl2 Usin g TiCl4
Sch em e 1
a
kc
conv
(%)
Mn
(GPC)
kp × 10-4
(
(
temp (°C)
(L mol-1 s-1
)
PDI
(L mol-1 s-1
)
-30
-40
-50
-60
340.1
188.6
99.4
35.3
34.3
39.7
39.9
7400
7600
8700
8300
2.7
2.9
2.7
2.8
11.4
6.3
4.0
49.4
1.9
a
[p-MeOStCl] ) 0.001 mol L-1, [DTBP] ) 0.006 mol L-1
,
[p-MeOSt] ) 0.15 mol L-1, [TiCl4] ) 0.003 mol L-1, and [phenyl-
silane] ) 0.75 mol L-1
.
(
kp for the cationic polymerization of IB,1 St,2 and
TMeSt.5 In the competition experiments the polymer-
izations are carried out in the presence of a suitable
trapping/terminating agent at well-chosen concentra-
tions, so conversion (X∞M) and the number-average
degree of polymerization (DPn,∞) reach limiting values.
The reactivity ratio (kp(/kc() can be calculated from both
only ∼20% when SnBr4 and phenylsilane were premixed
and added last to initiate the polymerization under the
same conditions. Using SnCl4 or the mixture of (SnCl4
+ phenylsilane) in the competition experiment at -30
°C, 64.6% and 36.8% limiting conversions were obtained,
respectively. This suggests a complex nature of the
reaction, which may be due to hydride transfer from
phenylsilane to Sn halides. Hydride transfer from
ordinary silanes to weak Lewis acids such as BCl3 has
already been reported.27 It is also known that TiCl4 does
not react with ordinary silanes at low temperature.28
Therefore, TiCl4 was used in further competition experi-
ments, and indeed at -30 °C the limiting conversions
were almost the same when the polymerization experi-
ments were carried out by adding TiCl4 or the mixture
of TiCl4 and phenylsilane.
M
X∞ and DPn,∞ using eqs 3 and 4.
(
(
M
kp
kc
ln(1 - X∞
)
)
(3)
(4)
ln(1 - [p-MeOStCl]0/[PhSi]0)
(
(
kp
kc
ln(1 - DPn,∞[p-MeOStCl]0/[p-MeOSt]0)
ln(1 - [p-MeOStCl]0/[PhSi]0)
)
In eqs 3 and 4, [p-MeOStCl]0 is the initial concentration
of the initiator, which equals that of the chain ends,
[PhSi]0 is the initial concentration of the capping agent,
phenylsilane, and [p-MeOSt]0 is the initial concentration
of the monomer. Using eqs 3 and 4, kp( can be calculated
if kc( is known. The kc( values at different temperatures
have been determined for the reaction of p-MeOSt+
cation with phenylsilane in CH2Cl2,26 and the Eyring
Using [TiCl4] ) 0.003 mol L-1, competition experi-
ments were carried out at different temperatures.
Limiting conversions indicated in Table 3 were reached
(
in less than 5 min. The calculated kp values are also
1
listed in Table 3. The H NMR spectrum confirms the
absence of methoxy chain ends in the polymers obtained
in the competition experiments. Also, the MALDI-TOF
MS analysis revealed that the end group is -H for the
poly(p-MeOSt) obtained from the competition experi-
ment of p-MeOSt with phenylsilane capping agent in
CH2Cl2 using TiCl4. The peaks were consistent with the
chemical structure CH3-CH(C6H4-OCH3)-(-CH2-
CH(C6H4-OCH3))n-H.
(
parameters are reported in Scheme 1. The kc values
used for the calculation of kp( at different temperatures
are presented in Table 2.
Since the kc( is known for the reaction of phenylsilane
to the p-MeOSt cation in CH2Cl2 at different tempera-
tures, the polymerization of p-MeOSt was carried out
in the presence of phenylsilane with the p-MeOStCl/
SnBr4 initiating system in CH2Cl2 in the presence of
DTBP as proton trap. First, the time necessary to reach
limiting conversion was determined at -30 °C. Limiting
conversion was reached in less than 60 min, suggesting
complete capping of the polymer cation. Therefore,
competition experiments at other temperatures were
also carried out for 60 min. The limiting conversions,
M
The reactivity ratios were calculated from X∞ using
eq 3 and from the DPn,∞ using eq 4. These two methods
yielded similar kp(/kc( values. Using the kc( value at a
(
certain temperature, kp at the corresponding temper-
ature was calculated from the average of kp(/kc( values
obtained from eqs 3 and 4, respectively. Table 3 shows
(
that kp is greatly dependent on temperature and
increases considerably with increasing temperature.
Figure 14 exhibits the Arrhenius plot of the data as ln-
(kp() vs 1/T; from the slope, the activation energy of
(
limiting Mn’s, and the calculated kp values at all
temperature are shown in Table 2. Table 2 also shows
that the molecular weight distribution of the polymers
obtained in the competition experiments is much nar-
rower (PDI ∼ 1.2-1.3) than the expected most probable
distribution (PDI ∼ 2) for bimolecular termination.
While at -30 °C the limiting conversion was 59.2%
when SnBr4 was added to the mixture of initiator,
pMeOSt, and phenylsilane, the limiting conversion was
propagation, Eap, was calculated to be 24.9 kJ mol-1
.
(
According to Table 3 kp ) 1.1 × 105 L mol-1 s-1 at
-30 °C and decreases to 1.9 × 104 L mol-1 s-1 at -60
°C. These values are similar to that obtained with SnBr4
listed in Table 2; however, because of the unexpectedly
low polydispersities we cannot consider these values.
The different nature of the Lewis acids employed in the

Macromolecules, Vol. 37, No. 21, 2004
Polymerization of p-Methoxystyrene 7937
Refer en ces a n d Notes
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Vairon, J .-P. Macromolecules, submitted. (b) Preliminary
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activation energy of propagation, Ea
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classical method and competition does not prevent a
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(
propagation rate constant is independent of the nature
of Lewis acid.1,2 In this comparison we find that the two
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(21) While the molecular weight distribution was broad but
monomodal when the SnBr4 stock solution was prepared by
us using high-purity CH2Cl2, employing a SnBr4 solution in
CH2Cl2 from Aldrich resulted in bimodal molecular weight
distributions.
The living carbocationic polymerization of p-MeOSt
can be achieved using the 1-chloro-1-(p-methoxyphenyl)-
ethane (p-MeOStCl)/SnBr4 initiating system in CH2Cl2
at -60 to -20 °C. Polymerization rates, Mns, and PDIs
are unaffected by excess DTBP. The polymerization is
first order in [SnBr4], which suggests the presence of
SnBr4Cl- counterions. The propagating chain end of
poly(p-MeOSt) has very similar stability to that of
monomeric chain end of p-MeOSt+ cation.
Both UV-vis spectroscopy and competition experi-
ments can be used to determine the absolute rate
constant of propagation for ion pairs for the living
(
cationic polymerization of p-MeOSt. The kp value
increases (activation energy of propagation ) 24.9 kJ
mol-1) and the overall polymerization rate decreases
with increasing temperature (apparent activation en-
ergy of propagation ) -3.5 kJ mol-1). This is attributed
to a larger decrease in the active center concentration
with increasing temperature compared to the corre-
sponding increase in kp(. The kp( values obtained from
the two methods are similar, confirming the general
applicability of the competition experiments for the
determination of absolute rate constants of propagation
in carbocationic polymerization.
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Mu¨nchen, 2000.
Ack n ow led gm en t. Support by the National Science
Foundation (CHE-0131106) is gratefully acknowledged.
We also thank Prof. M. Zsuga and co-workers (Depart-
ment of Applied Chemistry, University of Debrecen,
Hungary) for the MALDI-TOF MS measurements.
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Su p p or tin g In for m a tion Ava ila ble: MALDI-TOF-MS
spectra of poly(p-MeOSt) and plots of absorption-time for
ionization of p-MeOStCl with SnBr4 and TiCl4. This material
is available free of charge via the Internet at http://pubs.
acs.org.
MA0490451