Chapter 9
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Noncoordinating Anions in Carbocationic Polymerization Tris(pentafluorophenyl)boron as a Lewis Acid Catalyst Timothy D. Shaffer Baytown Polymers Center, Exxon Chemical Company, 5200 Bayway Drive, Baytown, TX 77522-5200 This chapter reports on several aspects of isobutylene homo- and copolymerization catalyzed by trispentafluorophenylboron, (B(pfp) ). The nature of initiation, termination and chain transfer are addressed by kinetic analysis. The first order chain transfer constant (k /k ) at -40°C is also reported. Comonomers addressed by this account include isoprene (2.1 mol%) and p-methylstyrene (2.0 mol%). The characteristics (i.e. M , M /M , comonomer incorporation, branching, BSB triad) of copolymers prepared by B(pfp) catalysis are compared with that known from other catalysts. 3
tr,M
n
w
p
n
3
The initiation of carbocationic polymerizations with metallocene initiators that contain the noncoordinating anions (NCA's), B(C6F4)4" and R(B(C6F5)3", has been recently demonstrated (7-7). Initiation is believed to occur by σ addition of the olefin to the metallocation.
However, protic initiation may also result as a consequence of the metallocation's reaction with adventitious moisture in the solvents (7). The source of initiation is dependent upon the relative concentrations of the metallocation and water. Protic initiation can be avoided with the addition of a proton trap, but the metallocation's concentration must exceed the concentration of adventitious moisture to ensure polymerization. Initiation from the metallocation is very inefficient. Since the metallocation largely participates only in initiation, the attributes of these polymerizations are generally due to the N C A (7). This realization opens opportunities for N C A catalysts that do not contain expensive metallocenes. Indeed, Ph3C , Li+, and R 3 S i salts of NCA's can initiate and catalyze carbocationic homo- and copolymerizations (7,7). These initiators provide many of the attributes observed in the metallocene based systems. In addition, some offer the advantages of stability to water and handling ease. These advantages are offset +
+
N O T E : This article is part two in a series.
96
© 1997 American Chemical Society
In Cationic Polymerization; Faust, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
9. SHAFFER
Noncoordinating Anions in Carbocationic Polymerization
by the multistep syntheses required for their preparation. The less synthetically challenging trispentafluorophenylboron (B(pfp)3) may provide similar polymerization attributes when used as a Lewis acid catalyst. If so, this represents a less complicated way to access the advantages offered by NCAs. The use of B(pfp)3 to catalyze isobutylene polymerization is known (5,7). In both cases, water borne protons are assumed to be the initiator. Evidence of protic initiation came from proton trapping experiments in which the presence of a proton trap successfully prevented polymerization.
Downloaded by STANFORD UNIV GREEN LIBR on August 27, 2012 | http://pubs.acs.org Publication Date: May 1, 1997 | doi: 10.1021/bk-1997-0665.ch009
ionization:
B(pfp)
3
+ H 0 2
Β(ρίρ) ·Η 0 3
2
+
H B(pfp) OH 3
cationation: The B(pfp)3/H20 system requires more polar solvents to support ionization as polymerizations do not occur in hydrocarbon solvents. As such, B(pfp)3 behaves more like B C I 3 than B F 3 in this regard. Complexation studies with B(pfp)3 rank it between, B F 3 than B C I 3 in Lewis acid strength (8). Although the mode of initiation in the B(pfp)3/H2U system has been addressed, many other aspects of polymerizations catalyzed by B(pfp)3 are not documented. This article preliminarily reports on several aspects of isobutylene homo- and copolymerizations catalyzed by B(pfp)3. The temperature and solvent polarity dependence as well as the determination of a first order chain transfer constant (ktr,M/kp) at -40°C are presented. Results and Discussion Homopolymerization of Isobutylene. A number of isobutylene homopolymerizations were run with B(pfp)3 using mostly water and 2-phenylpropan-2ol (PPOH) as initiators, two different temperatures, and solvents with different polarity. The results of these experiments appear in Table 1. A l l runs were stopped after 30 minutes of polymerization. Several observations can be made from these data. Polymerizations are generally inefficient in nonpolar solvents like hexane and toluene. Similar results have been reported for polymerization in methylcyclohexane (7). More polar solvents, like methylene chloride, permit higher conversion to polymer. In either case, molecular weight values are significantly higher than expected using [M]/[I] as a guide. As an example, experiment 10 should provide a polymer with a M of 101 kD, but the M of the prepared material is 908 kD. Although these polymerizations are not living, significantly higher M values suggest that slow initiation is responsible for the discrepancy. The use of a more efficient initiator, as in experiment 7, illustrates this point. Use of the t-alkyl chloride initiator T B D C C (see Table 1 for definition) provides for a polymer with a M of 13.2 kD which is significantly below that expected for this conversion ( M = 117 kD). Even though slow initiation contributes less to this polymerization, the lower M suggests the participation of chain transfer. Improving conversion by moving to more polar solvents suggests a lowering of the activation energy for cation formation. Both initiation and propagation may be more efficient in the polar solvents. Regardless of the solvent used, these polymerizations are slow and are colorless throughout, implying a low concentration of active chain ends. This observation is consistent with slow initiation and perhaps a reversible termination step that is competitive with propagation. Changes in solvent polarity are known to alter propagation equilibria as anticipated from the Winstein model (9,10). n
n
n
n
n
n
Kinetics of Homopolymerization. The kinetic character of water initiated polymerizations was further defined by analyzing the first order rate plot (Figure 1) and
In Cationic Polymerization; Faust, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
In Cationic Polymerization; Faust, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
[Cat] x l O (mol/L)
3 3
3
Table 1: B(pfp) IB Homopolymerizations initiator, Temp. Solvent (°C) [I] χ 1 0 (mol/L) Yield (%) M„
w
M /M„
2
2
M
a
2.2 159,300 24 PPOH, 1.8 -30 1 3.6 3.0 MeCl2 II 2.2 144,100 2 3.4 75 H 0 II II II 175,000 2.2 32 3 4.2 II II II 1.9 ·· 376,500 4 2.7 5.1 ·· ·· "
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