living carbocationic copolymerization of isobutylene with styrene

Living Carbocationic Copolymerizationof Isobutylene with Styrene

JUDIT E. PUSKAS,1 SAM W. P. CHAN,1,2 KIMBERLEY B. MCAULEY,2 GABOR KASZAS,3* SOHEL SHAIKH4

1Department of Polymer Science, University of Akron, Akron, Ohio 44325-3909

2Department of Chemical Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6

3LANXESS, Incorporated, 1265 Vidal Street South, Sarnia, Ontario, Canada, N7T 7M2

4Department of Chemical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9

Received 27 September 2006; accepted 12 December 2006DOI: 10.1002/pola.21945Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The carbocationic copolymerization of isobutylene (IB) and styrene (St),initiated by 2-chloro-2,4,4-trimethylpentane/TiCl4 in 60/40 (v/v) methyl chloride/hex-ane at �90 8C, was investigated. At a low total concentration (0.5 mol/L), slow initia-tion and rapid monomer conversion were observed. At a high total comonomer con-centration (3 mol/L), living conditions (a linear semilogarithmic rate and Mn–conver-sion plots) were found, provided that the St concentration was above a critical value([St]0 � 0.6 mol/L). The breadth of the molecular weight distribution decreased withincreasing IB concentration in the feed, reaching Mw/Mn � 1.1. St homopolymeriza-tion was also living at a high total concentration, yielding polystyrene with Mn

¼ 82,000 g/mol, the highest molecular weight ever achieved in carbocationic St poly-merization. An analysis of this system by both the traditional gravimetric–NMRcopolymer composition method and FTIR demonstrated penultimate effects. IBenrichment was found in the copolymers at all feed compositions, with very little driftat a high total concentration and above the critical St concentration. VVC 2007 Wiley Peri-

odicals, Inc. J Polym Sci Part A: Polym Chem 45: 1778–1787, 2007

Keywords: isobutylene; living carbocationic copolymerization; penultimate effects;reactivity ratios; real-time FTIR; styrene

INTRODUCTION

Polyisobutylene (PIB)-based polymers exhibitunique properties such as impermeability andhigh damping. The most important PIB-basedpolymer is commercially known as butyl rubber,a copolymer of isobutylene (IB) and 1–3 mol %

isoprene (IP), so the material can be vulcanizedto obtain desired properties.1,2 Butyl rubber hasbeen used in the tire industry as inner liners andtubes and in applications such as dampeners forvibrating equipment. A new type of butyl rub-ber, an IB–para-methylstyrene (PMS) copolymercalled EXXPRO with improved resistance to heataging, is marketed by ExxonMobil.3 Experimen-tal terpolymers—brominated IB–IP–styrene (St)and IB–IP–PMS—have also been introduced.4

Some alternatives to vulcanized/crosslinkablebutyl rubbers are thermoplastic elastomers(TPEs) based on PIB–polystyrene (PS) block

*Present address: Goodyear Tire & Rubber Company,Akron, Ohio

Correspondence to: J. E. Puskas (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 1778–1787 (2007)VVC 2007 Wiley Periodicals, Inc.

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copolymers. Kennedy and coworkers5,6 patentedthe first synthesis of linear PS-b-PIB-b-PS tri-block copolymers with good physical strengthwithout solvent extraction. Since then, otherstructures have been reported, the latest beingTPEs with an arborescent PIB core.7 The route tothe synthesis of these TPEs is via controlled car-bocationic polymerization. Controlled/living car-bocationic polymerization processes are currentlybeing commercialized (e.g., SIBSTAR by KanekaCo. and IBS Oppanol by BASF). Boston ScientificCo. markets its drug-eluting coronary stent usingPS-b-PIB-b-PS under the trade name Transluteas the drug carrier.8 Thus, the copolymerizationof IB with St is of both academic and industrialinterest. This article investigates the carboca-tionic copolymerization of these two monomerswith real-time fiber-optic FTIR monitoring.

BACKGROUND

Carbocationic Homopolymerization of IB and St

The homopolymerization of IB initiated by 2-chloro-2,4,4-trimethylpentane (TMPCl)/titaniumtetrachloride (TiCl4) is the most investigated liv-ing carbocationic system, producing high-molecu-lar-weight PIBs with narrow molecular weightdistributions (MWDs).9 However, under condi-tions in which [TMPCl]0 was greater than [TiCl4]0,initiation and monomer consumption proceededsimultaneously.10 Aliquots of the formed polymerwere taken to measure the instantaneous frac-tional initiation efficiency, which was less thanunity but rose toward unity at complete conver-sion. Despite this, the semilogarithmic monomerconsumption plot was linear. A similar phenom-enon had earlier been reported by Kamigaitoet al.11 in living vinyl ether polymerization. Thereason for the apparent insensitivity of the mono-mer consumption rate to the changing polymerconcentration is that the quasi-steady-state(QSS) approximation of d[Pþ]/dt & 0 applied toboth systems, yielding the following rate ex-pression:

ln½M�0½M�

8>>:

9>>; ¼ kpKeq½I�0½LA�n0t ð1Þ

where M is the monomer, I is the initiator, LAis the Lewis acid, and Keq is the equilibriumconstant between active and dormant species.In both IB and St polymerizations initiatedby 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene

[blocked dicumyl chloride (bDCC)]/ TiCl4 (Fig.1), a period of rapid monomer consumption(RMC) that coincides with the process of initiatorconsumption at the beginning of a living poly-merization has been reported.12,13 The end of theRMC occurs when all initiators have undergoneat least one successful ionization event and havethereby been converted into a dormant chain endcharacteristic of the monomer. The apparent rateconstant for monomer consumption during RMC[kapp(RMC)] is typically larger than that observedafter RMC. Later, De et al.14 also found RMC inSt polymerization initiated by p-methyl-styryl-chloride/TiCl4. Our interpretation of this phe-nomenon is that during RMC, d[Pþ]/dt is greaterthan 0, so QSS does not apply. When the carboca-tions forming from the initiator are more stablethan the propagating active polymer, Pþ, tempo-rary accumulation of [Iþ] may occur. This maylead to initial RMC until the steady state isreached. In Figure 1, kapp(RMC) of St is about 10times that of IB, and this scales with their kp val-ues measured by Mayr and Faust. Interestingly,Figure 1 shows that after RMC, IB polymerizesfaster than St (the linear part of the plot).

Reports for St homopolymerization initiatedby TMPCl/TiCl4 are controversial; RMC has beenreported under some conditions,10 but not underothers.15–19 However, in all cases, slow initiationand broad MWDs have been consistently found.The TMPCl/TiCl4 system is particularly impor-tant because it mimics blocking PS from living

Figure 1. RMC reported for carbocationic IB and Stpolymerizations12,13 ([TiCl4] ¼ 0.0278 mol/L; [St]0¼ [IB]0 ¼ 0.5 mol/L; [bDCC]0 ¼ 0.0066 mol/L; [DtBP]¼ 0.004 mol/L; [n-Bu4NCl] ¼ 0.0005 mol/L; MCHex/MeCl ¼ 60/40 v/v; temperature ¼ �70 8C).

LIVING CARBOCATIONIC COPOLYMERIZATION 1779

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

PIB in PIB–PS block copolymer synthesis. It wasshown that when St was added to living PIB at ahigh residual IB concentration, blocking wasslow20 because of slow copolymerization. Kimand Faust21 demonstrated penultimate effects inIB polymerization initiated by 3-chloro-1,1,3-tri-methylbutyl)benzene. When the first St unitreacted with TMPþ in TMPCl/TiCl4-initiated Stpolymerization, the more stable TMP–Stþ cationscould accumulate, leading to a transitory periodin which QSS did not apply. Kinetic analysis ledto the conclusion that in this system the rateof initiation depended on the absolute monomerconcentration. Increasing [St]0 from 0.5 to 2 mol/L accelerated the rate of initiation with res-pect to the rate of monomer consumption. At[St]0 ¼ 2 mol/L, no RMC was found, even thoughinitiator and monomer consumption coincideduntil a relatively high monomer conversion.18,19

Evidently, under these conditions, QSS prevailed.The reported apparent rate constant of initiation,ki,app ¼ 1.4 L2/mol2 s, obtained from initiatorconsumption data,18,19 agreed well with ki,app¼ 1.3 L2/mol2 s obtained from RMC.13

Carbocationic Copolymerization of IB with St

The carbocationic copolymerization of IB with Sthas been studied extensively. Table 1 lists reac-tivity ratio data for carbocationic IB–St copoly-merizations, which were compiled by Kennedyand Marechal.22 Most data indicate that IB ismore reactive than St, but for a few systems, thereverse has been reported. The authors con-cluded that the data were inconsistent. Two sets(bolded in Table 1) were reevaluated with theKelen–Tudos (K–T) method,23 yielding the reac-tivity ratios rIB ¼ 1.5 and 2.7 and rSt ¼ 0.17 and0.7, respectively, and four systems (italicized inTable 1) indicated penultimate effects. Subse-quently, Kaszas and coworkers produced idealIB–St copolymers whose composition was identi-cal to the feed over the entire monomer concen-tration range, using the TMPCl/TiCl4 initiatingsystem24 and the concept of forced ideal copoly-merization.25 Recently, Hull and Kennedy26

reported rIB ¼ 3.41 and rSt ¼ 1.40 and azeotropiccopolymerization at 21 mol % (11 wt %) IB, usingthe cumyl chloride (CumCl)/TiCl4 � BCl3 initiat-ing system. The living copolymerization of IBwith St has not been reported yet.

Although there is still disagreement concern-ing rate constants in carbocationic polymeriza-tions,9 most data show kp,St > kp,IB. Mayr’s linear

free energy relationship (LFER)27 predicts kIB¼ 5.5 3 108 L mol�1 s�1 and kSt ¼ 5.8 3 109 Lmol�1 s�1, and this is supported by experimentalrate constants obtained from competition experi-ments.16,28 LFER also predicts high values forcross-propagation constants: we calculated k12¼ 7.35 3 107 and k21 ¼ 6.05 3 1010 L mol�1 s�1

from Mayr’s data, where 1 refers to IB and 2refers to St. On the basis of the LFER-predictedkinetic rate constants, IB–St reactivity ratios ofr1 ¼ k11/k12 ¼ 7.5 and r2 ¼ k22/k21 ¼ 0.10 would beexpected. Although these predictions should beviewed with caution because of the very high rateconstants, they have worked well in predictingreactivity ratios in IB–IP copolymerizations.29

Against this background, we investigated thecarbocationic copolymerization of IB with St,using both real-time FTIR and traditional gravi-metric methods.

EXPERIMENTAL

Materials

Methyl chloride (MeCl; 99.9%; Matheson) and IB(99%; Matheson) were dried by being passedthrough a packed bed consisting of CaCl2/BaO.Methylcyclohexane (MeCHx; 99%; Aldrich) wasdried over molecular sieves, refluxed over sodiumunder dry nitrogen, and distilled before use. St(polymerization-grade; Aldrich) was vacuum-dis-tilled before use. TMPCl was synthesized asreported24 and was purified by vacuum distilla-tion; its purity was checked by 1H NMR. TiCl4,2,6-di-tertiary-butylpyridine (DtBP), and N,N-dimethylacetamide (DMA; Aldrich) were used asreceived.

Polymerization and Real-Time Data Collection

Polymerizations were conducted in an MBraunglovebox under a dry nitrogen atmosphere. TheO2 (�5 ppm) and moisture levels (<5 ppm) weremonitored during the reaction. The temperatureof the bath was controlled at �90 8C with liquidnitrogen and a PID (Proportional-Integral-Differ-ential)/fuzzy logic controller. A typical experi-ment was conducted as follows. A three-necked,round-bottom flask was charged with a solventmixture of MeCl and MeCHx (60/40 v/v), followedby monomers IB and St, initiator TMPCl, DtBP,and DMA. The reaction mixture was cooled to�90 8C, and a chilled stock solution of TiCl4 in

1780 PUSKAS ET AL.


MeCHx (50–50 v/v) was then added to initiatethe reaction. Tables 2 and 3 list the conditions forexperiments with low (L) and high (H) overallmonomer concentrations, respectively. The num-ber represents the molar percentage of IB (L20 ¼20 mol % IB).

The progress of the polymerization was moni-tored with attenuated total reflectance (ATR)probes for high monomer concentrations andwith transmission (TR) probes for low monomerconcentrations. Each probe was immersed in thepolymerization medium and interfaced to a Bio-

Table 1. Reactivity Ratios Compiled for Carbocationic IB–St Copolymerizationsa

Reaction Conditionsand Temperature (8C)

Reactivity Ratio

Referencer1 r2

SnCl4/EtCl/0 1.60 0.17 Lyudvig et al. (1959)c-Ray/EtCl/�78 3.5 0.33 Abkin et al. (1961)AlCl3/MeCl/�92 9.02 6 0.77 1.99 6 0.24 Rehner et al. (1953)AlCl3/MeCl/�30 2.51 6 0.05 1.21 6 0.06 Rehner et al. (1953)AlCl3/MeCl/�30 2.36 6 0.06 0.76 6 0.13 Rehner et al. (1953)EtAlCl2/MeCl2/�100 2.8 6 0.2 1.4 6 0.2 Kennedy and Chou (1975)AlCl3/MeCl/�90 1.66 6 0.02 0.42 6 0.02 Rehner et al. (1953)AlCl3/MeCl/�90 1.79 6 0.02 0.24 6 0.02 Rehner et al. (1953)AlCl3/MeCl/�103 3 6 1 0.6 6 0.3 Tegge (1953)TiCl4/Tol/�78 1.78 6 0.10 1.20 6 0.10 Okamura et al. (1961) and

Imanishi et al. (1965)TiCl4/Hx/�78 0.37 6 0.07 2.41 6 0.12 Okamura et al. (1961) and

Imanishi et al. (1965)TiCl4/Hx/�20 0.54 6 0.24 1.20 6 0.11 Imanishi et al. (1965)TiCl4/Hx/MeCl2 (75:25)/�20 2.63 6 0.52 5.50 6 0.55 Imanishi et al. (1965)TiCl4/Hx/MeCl2 (50:50)/�20 3.25 6 0.25 2.75 6 0.25 Imanishi et al. (1965)TiCl4/Hx/MeCl2 (25:75)/�20 4.11 6 0.19 1.70 6 0.07 Imanishi et al. (1965)TiCl4/MeCl2/�20 4.48 6 0.28 1.08 6 0.07 Imanishi et al. (1965)SnCl4/Hx/MeCl2 (25:75)/�20 3.75 6 0.45 1.92 6 0.41 Imanishi et al. (1965)TiCl4/CS2/�20 1.51 6 0.15 2.44 6 0.15 Okamura et al. (1961)TiCl4/decalin/�20 1.53 6 0.03 2.61 6 0.4 Okamura et al. (1961)TiCl4/MeCHx/�20 0.75 6 0.15 1.30 6 0.15 Okamura et al. (1961)SnCl4/SO2/�78 3.1 1.1 Iino and Tokura (1964)SnCl4/SO2�NOEt/�78 2.2 1.1 Iino and Tokura (1964)SnCl4/SO2�EtCl2/�78 1.5 1.0 Iino and Tokura (1964)SnCl4/SO2�NOBz/�78 0.8 0.5 Iino and Tokura (1964)SnCl4/SO2�Bz/�78 1.9 0.6 Iino and Tokura (1964)

a The two bolded entries were reevaluated with the K–T method. The four italicized entries showed penultimate effects.

Table 2. Copolymerization Dataa

Sample[IB]0

(mol/L)[St]0

(mol/L)

Initial kapp (1/s) QSS kapp 1/s

IB St IB St Total

L100 0.50 0.00 — — 0.0123 — 0.0123L80 0.40 0.10 0.0044 — 0.0016 0.0001 0.0017L66 0.33 0.17 0.0077 — 0.0014 0.0003 0.0017L50 0.25 0.25 0.0062 — 0.0008 0.0002 0.0010L34 0.17 0.33 0.0034 0.0034 0.00010 0.0002 0.0012L20 0.10 0.40 0.0056 0.0020 0.0009 0.0004 0.0013L0 0.00 0.50 — 0.0088 — 0.0027 0.0027

a [IB]0 þ [St]0 ¼ 0.5 mol/L; [TiCl4]0 ¼ 0.04 mol/L; [TMPCl]0 ¼ 0.004 mol/L; [DtBP] ¼ 0.007 mol/L; DMA ¼ 0.002 mol/L.



Rad FTS 175C FTIR unit by fiber-optic cablesthrough a port in the dry box. A backgroundspectrum (300 transients) was taken after theinitiator and the solvents were added to establisha net zero baseline and minimize backgroundnoise. After initiation with TiCl4, a spectrum(each consisting of 32 transients) was collectedevery 14–18 s. The spectra then were collatedinto one multifile (50–250 spectra). Data collec-tion was accomplished online with Bio-Rad Win-IR software. In addition, in selected experiments,gravimetric samples were taken for NMR andsize exclusion chromatography (SEC) analysis.The reactions were terminated by the addition ofprechilled methanol, and the products were driedin vacuo at 40 8C. The FTIR data were evaluatedwith the GRAMS software with a Lorentziancurve fit. Calibration was used to establish pro-portionality between the IR peak areas and con-centrations in the simultaneous presence of thecomonomers.29

NMR spectra were recorded on a BrukerAvance 500 instrument in deuterated chloroformwith TMS as the internal reference.

The polymer molecular weights were deter-mined by SEC with a Viscotek model 300 tripledetector system equipped with a mixed-bed col-umn (Viscogel GMHHRH) thermostated at 35 8C,a four-capillary differential viscometer, a right-angle laser light scattering detector, and an RIdetector. THF was used as the mobile phase at aflow rate of 0.5 mL/min.

RESULTS AND DISCUSSION

The copolymerizations were carried out in a morepolar solvent mixture (60/40 MeCl/MeCHx) than

we normally use to achieve reasonable rates inboth low- and high-concentration experiments.

Low-Monomer-Concentration Experiments

Figure 2 shows a typical waterfall plot for [IB]0þ [St]0 ¼ 0.5 mol/L, monitoring the C¼¼C stretchsignals (1655 cm�1 for IB and 1630 cm�1 for St)by the TR probe.

On the basis of the linearity of the calibrationplots, plots of ln(A0(time zero)/At(time t)) ¼ ln([M]0/[M]t) versus the time were constructed for eachexperiment. Figure 3 shows plots for St homopo-lymerization (L0) and the copolymerization inFigure 2 (L66).

Similarly to Figure 3(b), fast initial IB poly-merization was observed in all five copolymeriza-tion experiments, reminiscent of the RMCreported by Storey and coworkers12,13 and Faustet al.16 with similar monomer and initiator con-centrations. RMC was also observed in St homo-polymerization [L0; Fig. 3(a)]. After the initialRMC burst, a steady consumption rate was dis-played by both IB and St. The IB homopolymeri-zation rate plot was linear (L100, not shown).The values of the apparent rate constant (kapp),calculated from both the initial and steady rateswith eqs 2 and 3, are summarized in Table 2:

ln½IB�0½IB�

8>>:

9>>; ¼ kappðIBÞ t ð2Þ

ln½St�0½St�

8>>:

9>>; ¼ kappðStÞ t ð3Þ

IB homopropagation was faster than St homopro-pagation—kp,app (1/s) ¼ 0.0123 versus kp,app (1/s)¼ 0.0088 (RMC) and kp,app (1/s) ¼ 0.0027 (steadyrate)—in contrast to the measured kp data. Asmentioned before, Storey and Thomas13 alsofound faster propagation of IB than St following

Table 3. Copolymerization Dataa

Sample [IB]0 (mol/L) [St]0 (mol/L) kapp (1/s)

H100 3.00 0.00 0.0143H95 2.85 0.15 0.0126H80 2.40 0.60 0.0036H66 2.00 1.00 0.0027H50 1.50 1.50 0.0040H34 1.00 2.00 0.0029H20 0.60 2.40 0.0042H5 0.15 2.85 0.0133H0 0.00 3.00 0.0320

a [IB]0 þ [St]0 ¼ 3 mol/L; [TiCl4]0 ¼ 0.128 mol/L; [TMPCl]0¼ 0.004 mol/L; [DtBP] ¼ 0.007 mol/L; [DMA] ¼ 0.002 mol/L.

Figure 2. Waterfall plot of real-time FTIR monitor-ing of an IB–St copolymerization (L66, TR probe).

1782 PUSKAS ET AL.


RMC (see Fig. 1). Both the IB and St consump-tion rates were suppressed in the presence of theother monomer, and IB polymerization proceededfaster than St polymerization at all monomerratios. The steady rates of propagation of IB andSt differed the most at [IB]0 ¼ 80 mol %, whereaskapp(IB) changed very little in the 20–50 mol %[IB]0 range.

As mentioned earlier, Puskas and coworkersshowed that the rate of initiation in St polymer-ization was directly proportional to both [TiCl4]0and [St]0,

18 and at [St]0 ¼ 2 mol/L, no RMCwas seen even when the initiator and monomerconsumption proceeded simultaneously.19 Thus,copolymerization experiments were carried outat high TiCl4 and monomer concentrations(Table 3).

High-Monomer-Concentration Experiments

In the high-concentration experiments ([M1]0þ [M2]0 ¼ 3 mol/L), rate plots were constructedfrom gravimetric conversion data, and the kappvalues are listed in Table 3. The kapp values forH100 and H95 do not differ significantly fromthat for L100. It is likely that at a high IB con-centration, a rate-determining reaction reachedits diffusion limit. In contrast to the low-concen-tration measurements, no RMC was observedand St homopolymerized faster than IB, withkp,app ¼ 0.032 versus 0.014 L/mol s, in agreementwith the measured kp data. Similarly to the low-

concentration experiments, the copolymerizationrates were very slow. The rate plot with [St]0¼ 0.6 mol/L (H80 in Table 3), shown in Figure 4,reveals RMC. However, in the experiments with[St]0 > 0.6 mol/L, no RMC was found. A represen-tative plot is shown in Figure 5. ATR–FTIR con-firmed the absence of RMC; a representative plotis shown in Figure 6. Unfortunately, at a highmonomer concentration, the FTIR data scatterwas generally high. Figure 6 shows that at 34%

Figure 3. Plots of ln([M]0/[M]) versus the time for (a) St homopolymerization (L0)and (b) IB–St copolymerization from Figure 2 (L66): (n) St and (u) IB.

Figure 4. Plot of ln([M]0/[M]) versus the time for[IB]0 ¼ 2.4 mol/L and [St]0 ¼ 0.6 mol/L (Table 3,H80).



IB, this comonomer was consumed much fasterthan St (kapp,IB ¼ 0.0021 s�1 and kapp,St ¼ 0.0013s�1). At 20% IB, however, the rates were muchcloser, with kapp,IB ¼ 0.0027 s�1 and kapp,St¼ 0.0022 s�1 (not shown), with no RMC for eithermonomer.

At [St]0 > 0.6 mol/L, living conditions wereachieved; Figure 7 demonstrates a representativelinear plot of Mn versus the conversion for H34.The measured final Mn values agreed within theexperimental error with the theoreticallyexpected Mn data, assuming 100% initiating effi-

ciency. Figure 8 shows that the breadth of theMWD decreased with increasing IB contentin the feed, as expected in polymerizations gov-erned by dormant–active equilibria. Beyondabout 50 mol % IB in the feed, relatively narrowMWDs (Mw/Mn � 1.2–1.1) were achieved.

In summary, the results presented here dem-onstrate that both the relative and absolute [St]0values have a profound influence on the polymer-izations rates, molecular weights, and MWDs.Living copolymerization was achieved at a hightotal comonomer concentration (3 mol/L). Belowthe critical concentration of [St]0 ¼ 0.6 mol/L,slow initiation was found, but at full conversion,

Figure 5. Plot of ln([M]0/[M]) versus the time for[IB]0 ¼ 1.0 mol/L and [St]0 ¼ 2.0 mol/L (Table 3,H34).

Figure 6. Plot of ln([M]0/[M]) versus the time forthe C¼¼C stretch of IB (1655 cm�1) and St (1630cm�1) monitored with an ATR probe (H34, Table 3):(n) St and (u) IB.

Figure 7. Plots of the molecular weight versus theconversion for [IB]0 ¼ 1.0 mol/L and [St]0 ¼ 2.0 mol/L(Table 3, H34).

Figure 8. Plot of MWD versus the IB content forthe high-concentration experiments (Table 3).

1784 PUSKAS ET AL.


copolymers with theoretical Mn values (withinthe experimental error) and Mw/Mn � 1.1 wereproduced. Above the critical St concentration, alinear semilogarithmic rate and Mn–conversionplots were achieved, indicating living conditions,but Mw/Mn increased with increasing St con-centration. St homopolymerization yielded Mn

¼ 82,000 g/mol, the highest ever produced by car-bocationic polymerization.

Copolymer Composition

Table 4 shows copolymer composition data by 1HNMR. We found IB enrichment in all copolymers.It can also be seen that below the critical St con-centration, there is copolymer composition driftwith increasing conversion in both the low- andhigh-concentration experiments. In contrast,above the critical value of [St]0 ¼ 0.6 mol/L, littleor no drift was found (H34, H20, and H5). H20

showed nearly isotropic conditions, and this wassimilar to what was reported by Hull and Ken-nedy.26

The K–T plot (extended to high conversions)constructed for the high-concentration experi-ments is displayed in Figure 9. The plot clearlyshows penultimate effects in the IB–St copoly-merization system. Reactivity ratios were esti-mated separately in the high and low IB concen-tration ranges (see the inset in Fig. 9). In thehigh [IB]0 range, rIB ¼ 7.24 and rSt ¼ 1.96 repre-sent blocky behavior. In the low [IB]0 (high St)range, with rIB ¼ 1.02 and rSt ¼ 0.37, the systemshifts to more random behavior. It is also inter-esting to note that rIB at the high [IB]0 range andrSt at the high [St]0 range (rIB ¼ 7.25 and rSt¼ 0.0.37) are close to the values predicted byMayr’s LFER.

From the M–L (Mayo-Lewis) equation, wederived the following:30

rIB ¼ kappðIBÞkappðStÞ ¼ rIBðinstÞ ð4Þ

rSt ¼ kappðStÞkappðIBÞ ¼ rStðinstÞ ð5Þ

The instantaneous reactivity ratios obtainedfrom FTIR data of low-monomer-concentrationexperiments with eqs 3 and 4 varied with thefeed composition in a wide range (rIB(inst) ¼ 38.43to �2.35 and rSt(inst) ¼ 0.07–0.81).30 From FTIRdata of high-concentration experiments (H34 and

Table 4. Copolymer Compositions by 1H NMR

Sample Conversion (%) Composition (mol % IB)

L34 27 44.139 42.157 36.970 36.489 32.4

L20 50 29.663 25.577 24.5

H95 58 99.083 98.795 98.4

H80 67 93.490 92.7

H66 47 82.571 79.474 72.185 73.2

H50 39.4 59.045.0 54.948.0 51.8

H34 37 42.754 39.963 42.772 40.983 40.4

H20 57 26.777 24.985 25.8

H5 52 12.376 13.281 12.1

Figure 9. Extended K–T plot for the high-concen-tration data listed in Table 5.



H20, Table 3), rIB ¼ 1.61 and 1.23 and rSt ¼ 0.6and 0.81 were obtained. Thus, at a high totalmonomer concentration and above the criticalvalue of [St]0 � 0.6 mol/L, the IB–St copolymer-ization tends to be random.

For comparison, Table 5 lists the reactivityratios reported for the copolymerization of IBwith IP. In this system, the terminal model wasfound to be valid, and reactivity ratios were cal-culated from FTIR data, each polymerizationexperiment being treated as a series of low-con-version experiments.29 These reactivity ratioswere found to agree well with rIB ¼ 1.32 and rIP¼ 0.74, which were calculated with Mayr’s nucle-ophilicity scale. We calculated reactivity ratiosfrom apparent rate constants measured by FTIRin these experiments and list these data in Table5. The reactivity ratios from apparent rate con-stants are independent of the feed composition inthe 90–50% IB range and agree well with the val-ues obtained by the other reported FTIR techni-ques.29

CONCLUSIONS

Real-time FTIR monitoring, together with tradi-tional gravimetric sampling, has given unprece-dented insight into IB–St copolymerization. At ahigh total comonomer concentration (3 mol/L),living conditions (a linear semilogarithmic rateand Mn–conversion plots) were found, providedthat the St concentration was above a criticalvalue ([St]0 � 0.6 mol/L). The breadth of theMWD decreased with increasing IB concentra-tion in the feed, reaching Mw/Mn � 1.1. St homo-polymerization was also living at a high mono-mer concentration, yielding PS with Mn ¼ 82,000g/mol, the highest molecular weight ever ach-ieved in carbocationic St polymerization. Ananalysis of this system by both the traditionalgravimetric–NMR copolymer composition method

and FTIR demonstrated penultimate effects. Wefound IB enrichment in the copolymers at allfeed compositions, with very little drift at a hightotal comonomer concentration and above thecritical St concentration.

Financial support by LANXESS, Inc. (Canada), andthe National Science Foundation (grant DMR-050968)is greatly appreciated.

REFERENCES AND NOTES

1. Puskas, J. E.; Wilson, G.; Duffy, J. Ullman’s En-cyclopedia of Industrial Chemistry, 7th ed.; Wiley:New York, 2004; Section 3.3.

2. Puskas, J. E.; Kaszas, G. Encyclopedia of PolymerScience and Technology; Wiley: New York, 2003;Vol. 5, p 382.

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Table 5. Reactivity Ratios of the IB–IP Copolymerization Systems

IB (mol %) Probe

r1 r2

K–T kapp(1)/kapp(2) K–T kapp(2)/kapp(1)

90.00 TR 1.15 1.05 0.87 0.9579.75 ATR 1.05 1.03 0.92 0.9862.50 TR 1.13 1.14 0.89 0.8850.20 ATR 1.23 1.07 0.81 0.93All data combined ATR/TR 1.17 1.07 0.89 0.94

1786 PUSKAS ET AL.


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living carbocationic copolymerization of isobutylene with styrene

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