Article Cite This: Organometallics XXXX, XXX, XXX−XXX
pubs.acs.org/Organometallics
Catalyst Mileage in Olefin Polymerization: The Peculiar Role of Toluene Francesco Zaccaria, Christian Ehm,* Peter H. M. Budzelaar, Vincenzo Busico, and Roberta Cipullo* Università di Napoli Federico II, Dipartimento di Scienze Chimiche, Via Cintia, 80126 Napoli, Italy
Organometallics Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/30/18. For personal use only.
S Supporting Information *
ABSTRACT: Ti−C homolysis is generally regarded as a catalyst deactivation step in olefin polymerization catalyzed by titanium complexes. The present work demonstrates that for industrially relevant Ti catalysts in toluene and related solvents a “detour” via radical activation of the solvent can prolong catalyst lifetime and productivity, leading to chain transfer to solvents (CTS). For differently substituted phosphinimide half-titanocenes and other Ti catalyst classes, i.e. constrained geometry (CGC) and amidinate catalysts, CTS leads to formation of up to ∼60% benzylated chains. The efficiency of the reactivation pathway depends mostly on steric factors and correlates well with the percentage of buried volume, %VBur, as well with as DFT predictors. Thus, the solvent is far from innocent, and catalyst behavior in such solvents may not be representative of polymerizations in more innocent saturated hydrocarbons. On the other hand, it might be the case that, in truly innocent solvents, the monomer itself can play a less innocent role.
■
INTRODUCTION Molecular group IV catalysts for olefin polymerization have evolved out of niche applications in industry, currently holding a sizable market position.1−3 These single-center catalysts allow production of value-added materials via fine tuning of polymer architecture.2−4 Their successful development has been made possible by two key factors: i.e., the discovery of effective cocatalysts (above all, methylaluminoxane, MAO)5 and the identification of high-performance active species via more or less rational structural modifications of the precatalysts.6−8 Arguably, productivity is one of the most crucial performance indicators in polymerization catalysis. It is determined not only by the intrinsic ability of the catalyst to polymerize olefins but also by its tendency to form “dormant” or deactivated species.9−11 The identification of factors determining catalyst mileage would be highly desirable to improve catalyst productivity but has been hampered by the plethora of possible, often unexpected, side reactions competing with propagation.12 Some known examples of deactivation/decomposition reactions in olefin polymerization are intramolecular C−H activation of the ancillary ligand,13−15 α-H abstraction from Ti alkyl groups,16−18 solvent activation,19,20 (C6F5)− transfer from a borate counterion,21−23 and side reactions with “free” trimethylaluminum (TMA) in commercial MAO.24−26 For Ti catalysts, changes in oxidation state via Ti−C bond homolysis represent another common deactivation pathway, especially at high temperatures.27−29 This process is of particular commercial relevance, since titanium(IV) “post-metallocene” systems find major industrial applications in the production of commodity © XXXX American Chemical Society
polyolefins, often being employed in solution reactors operating at T > 120 °C.2,3 In an initial report, 30 we have shown that propene polymerization promoted by phosphinimide half-titanocenes (1-Cy and 1′-Ph in Figure 1, used for example for Nova Chemicals SURPASS polymers)3 in toluene leads to the formation of benzyl-initiated polymer chains. End group analysis clearly indicated that these are formed upon 2,1-propene insertion. DFT studies strongly suggested that Ti−sec-alkyl
Figure 1. Ti-based catalysts explored in this study (R = Ph, Cy, tBu). Received: June 8, 2018
A
DOI: 10.1021/acs.organomet.8b00393 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
(1′-R) and an aromatic (R = Ph)- or alkyl-substituted (R = Cy, tBu) phosphinimide ligand, providing deeper insight into the CTS mechanism. The generality of this process is then demonstrated using other industrially relevant Ti catalysts such as Cp-amidinate (2; Keltan ACE),31,32 ansa-cyclopentadienylamido (3; constrained geometry catalyst, CGC)33 and Cp-ketimide (4)34 (Figure 1). All catalysts were screened under a uniform set of experimental conditions (110 °C, 1 bar ppropene; see Table 1 for further details).35 MAO/BHT (BHT = 2,6-di-tert-butyl-4-methylphenol) was used as cocatalyst. The addition of BHT traps “free” TMA in commercial MAO solutions (AlMe3/BHT 1/2),36 preventing catalyst decomposition.24,25 Preliminary results suggested also that MAO/ BHT is particularly suitable to observe CTS.30 Screening of Phosphinimide Catalysts: Mechanistic Insight into CTS. The aforementioned set of phosphinimide catalysts allows us to investigate the two key aspects of the CTS mechanism (Scheme 1), (A) the ease of catalyst deactivation and (B) the extent of benzyl chain end formation (i.e., of catalyst reactivation via CTS), in greater detail. A summary of polymerization performance and polymer microstructural analysis is reported in Table 1 (see Table S1 in the Supporting Information for further details). Monomodal molecular weight distributions (MWD) were obtained for all catalysts. 1′-tBu produces very low Mw oligomers at 110 °C, which are difficult to collect. Therefore, only in this case a slightly lower temperature (100 °C) had to be used. Most phosphinimide catalysts show benzyl incorporation (5−21% Bn, Table 1) but for 1′-tBu, a high amount of allyl chain ends is instead observed. In all cases, significant amounts of nBu chain ends are observed, which can be indicative of frequent homolysis (vide infra).30 Chain Termination via Homolysis. The large amount of saturated nBu-type chain ends (Table 1) observed experimentally for the six phosphinimide catalysts indicates that the dominant chain termination event occurs after 2,1-propene insertion,30,37,38 but the very low amount of internal vinylidenes suggests that it is not connected to hydrogenolysis at a secondary growing alkyl chain (Table 1; see the Supporting Information for further details on chain ends analysis).39 Homolysis can become the dominant termination event at high temperature in propylene polymerization due to the weak Ti−sec-C bond formed during the polymerization reaction (Scheme 1).29,30 Chain termination can then coincide with catalyst deactivation, due to the formation of inactive Ti(III) complexes. We
bonds are very weak under polymerization conditions and that reduction of Ti(IV) active species via homolysis can be reversible due to an unprecedented chain transfer to solvent (CTS) process (Scheme 1).29,30 According to the proposed Scheme 1. Mechanism of Chain Transfer to Solvent (CTS)
mechanism, Ti−sec-alkyl bond homolysis leads to benzylic H abstraction from toluene by the polymeryl radical and formation of nBu chain ends; subsequent Ti(III)/benzyl radical recombination yields a Ti(IV)−Bn species that can restart chain growth (Scheme 1).30 A σ-bond metathesis mechanism could be ruled out in that work on the basis of experimental evidence and calculated barriers. The reversibility of homolytic decay processes may be crucial for some Ti catalysts to achieve high activities under forcing reaction conditions. In this paper, we explore the CTS process in more detail, combining polymerization results and DFT studies. Systematic screening of differently substituted phosphinimide systems in propene polymerization provide a better picture of the factors determining benzyl incorporation. The generality of homolytic CTS is demonstrated by expanding the study to other industrially relevant Ti catalyst classes (Figure 1). Solvent incorporation is found to occur even when toluene is diluted with saturated hydrocarbons.
■
RESULTS AND DISCUSSION In comparison to the initial report, the catalyst set was expanded to include six phosphinimide catalysts bearing Cp (1-R) or Cp*
Table 1. Phosphinimide Catalyst Performance in Propene Polymerization at High Temperaturea SATg entry 1 2 3i 4 5 6
UNSATg
cat.
activityb
Mnc
Mwc
PDI
∑REd
Int./Ter. REf
exptl ΔΔG⧧(HOM‑INS,Ti−Pr)e
total
iBu
nPr
nBuA+B
total
allyl
% Bnh
1′-Ph 1′-Cy 1′-tBu 1-Ph 1-Cy 1-tBu
2.6 × 10 2.0 × 102 4 50 2.2 × 102 1.4 × 102
3.3 1.6 1.4 6.4 4.1 2.3
6.4 2.7 1.6 13 7.8 3.9
2.0 1.7 1.2 2.0 1.9 1.8
10.4 7.4 6.4 9.7 8.1 11.6
6 3 3 21 7 5
1.2 0.8 0.8 2.3 1.5 1.2
3.0 4.8 3.6 1.8 2.6 4.0
0.8 3.4 2.6 0.8 1.0 1.1
0.7 0.3 0.3 0.6 0.7 1.2
1.5 1.1 0.7 0.4 0.9 1.7
0.31 1.92 2.30 0.16 0.29 0.66
0.27 1.85 2.25 0.02 0.06 0.20
21 9 n.d.j 5 14 9
2
Other experimental conditions: catalyst, 25 μmol; toluene, 200 mL; activator, MAO/BHT; Al/Ti = 400; ppropene = 1 bar; Tp = 110 °C; t = 15 min. Values are averages of duplicate experiments (see the Supporting Information). bIn kgPP mol−1 bar−1 h−1. cIn kDa. dSum of regioerrors ∑RE = 2,1 + 3,1 + 2 × PEP + iBu + nBuA + 2 × nBuB + 2-Bt; d). eRatio between internal regioerrors and terminal regioerrors arising from homolysis Int./Ter. RE = (2,1 + 3,1 + 2 × PEP + nBuB)/(nBuA + nBuB). fEstimated from Int./Ter. RE, in kcal/mol. gMole percent of saturated (SAT) and unsaturated (UNSAT) chain ends (total in Roman type and specific groups in italics; see the Supporting Information for details), determined via 13C NMR and 1 H NMR spectroscopy, respectively. hPercent of benzylated chains. iTp = 100 °C (see text). jn.d. = not detected. a
B
DOI: 10.1021/acs.organomet.8b00393 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
the steric bulk of the ancillary ligands and in predicted ΔΔG⧧HOM‑INS,Ti‑iPr values (Figure 2 and Table S2). These results highlight the importance of homolysis as a chain termination (and catalyst deactivation) pathway; it determines how likely it is that a catalyst can overcome regioerror formation and affects an important structural parameter, polymer MW. Reactivation via CTS. Rationalizing factors determining the incorporation of benzyl chain ends is not straightforward, as CTS is not only dependent on the ease of Ti−polymeryl homolysis but, among other things, also on the probability of H abstraction from toluene, Ti(III)/benzyl radical recombination, and chain reinitiation (Scheme 1). The probability of H abstraction from toluene and organic radical recombination, which prevents catalyst reactivation, are catalyst independent; Ti species are not directly involved. DFT predicts the barrier for H abstraction from toluene by an iPr radical to be 21.1 kcal/mol at 383 K; however, this includes an entropy contribution of ∼10 kcal/mol. This contribution is overestimated, as toluene is the solvent here, as discussed in ref 74. Under the applied experimental conditions, a barrier of 15−20 kcal/mol is easily accessible. Conversely, radical recombination of organic and metal fragments or of two metal fragments might be affected by the nature of the active species. For instance, dimerization of Cpphosphinimide Ti(III) cations, reported by Piers and coworkers for 1-tBu,47 is a plausible competing process for CTS but it can be prevented if bulky ligands are used (e.g., Cp* rather than Cp). High-dilution polymerization conditions should disfavor formation of homodinuclear species, but experimentally, a more intense green color of the polymerization mixture is observed for 1-R in comparison to 1′-R systems,30 indicating that it represents a non-negligible side reaction for less hindered Cp catalysts. Formation of a Ti(IV) benzyl cation does not guarantee generation of a benzyl chain end. Ti−Bn bond formation is reversible, and the result depends on the competition between olefin complexation/insertion and trapping of a benzyl radical by a variety of other processes. If we represent the first process by kINS and the second by kHOM, we find a fair correlation of ΔΔG⧧HOM‑INS,Ti‑Bn and the experimental percentage of benzylated chains for 1′-R systems, but for the 1-R systems, 1-Ph proved to be an outlier (Figure 3).
hypothesized that easier termination via homolysis could be expected with increasing steric hindrance of the active species.29 This interpretation was verified using the expanded phosphinimide catalyst set. The kinetic ratio of propagation/ termination after 2,1-insertion can be estimated experimentally by calculating the ratio between internal and terminal regioerrors (Int./Ter. RE, i.e. the number of 2,1-insertions until termination) in the polypropylene (PP) chain by 13C NMR (see Figure S2 in the Supporting Information).40 Figure 2 shows
Figure 2. Graphical comparisons of trends in Int./Ter. RE (light blue) and Mn (dark blue; in kDa) with percentage of buried volume %VBur (red). Experimental data for 1′-tBu are reported separately (see ref 41). Catalysts are ordered by decreasing Mn.
that trends in Int./Ter. RE follow those in the steric bulk of ancillary ligands,41 as estimated by the percentage of buried volume (%VBur, Table S2) in the catalyst active pocket:42 i.e., the higher the steric bulk, the more likely is chain termination after 2,1-insertion. Trends in Int./Ter. RE do not reflect the intrinsic regioselectivity of the catalysts; for instance, 1′-Ph and the less hindered 1-Ph give almost the same amount of overall regioerrors (∑RE ≈ 10%), but the former gives a much lower Int./Ter. RE ratio (6/21; entry 1 vs entry 4, Table 1) and hence a shorter polymer. Further evidence derives from DFT estimated relative Gibbs free energy differences between insertion TS (INS) and homolytic Ti−C bond cleavage TS (HOM) for Ti-iPr species (ΔΔG⧧HOM‑INS,Ti‑iPr), mimicking Ti-sec-polymeryl. Scott’s procedure was used to estimate the barrier for homolysis.43 A nice correlation between predicted barriers for homolysis, ΔΔG⧧HOM‑INS,Ti‑iPr, using the naked cation approximation, and experimental Int./Ter. RE can be observed (see Figure S3 and Table S2 in the Supporting Information). It has been shown earlier that calculating absolute ΔΔG⧧HOM‑INS,Ti‑iPr values requires inclusion of the anion in the calculations.29 This is particularly problematic in the case of MAO anions44,45 and also exceeds our current computational resources. We computationally explored counterion effects for the representative case of 1′-Ph by considering a relatively weakly coordinating anion, [MeB(C6F5)3]−, which has the advantage of directionally binding to the metal center with the Me fragment.46 The estimated ΔΔG⧧HOM‑INS,Ti‑iPr value drops from 22 kcal/mol in the case of the isolated cation to only 2 kcal/ mol for the ion pair (Table S3). Even for the very weakly coordinating B(C6F5)4−, an effect of >10 kcal/mol can be expected (Table S3). A pronounced anion effect is in agreement with previous estimations.29 In line with our interpretation, trends in number-average molecular weight (Mn) reflect those in
Figure 3. Graphical comparison between trends in experimental % Bn (green) and DFT calculated ΔΔG⧧HOM.‑INS,Ti‑Bn in kcal/mol (gray). Catalysts are grouped according to the type of Cp* vs Cp ancillary ligand. C
DOI: 10.1021/acs.organomet.8b00393 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 2. Phosphinimide Catalysts: DFT Insertion and Homolysis Barriers (kcal mol−1)a Ti−iPr agostic complex
Ti−Bn η2 complex
Ti−iPr olefin complex
ΔG⧧Ti‑iPr
ΔG⧧Ti‑iPr
ΔG⧧Ti‑Bn
catalyst
INS
HOM
ΔΔG⧧Ti‑iPr
INS
HOM
ΔΔG⧧Ti‑iPr
INS
HOM
ΔΔG⧧Ti‑Bn
1′-Ph 1′-Cy 1′-tBu 1-Ph 1-Cy 1-tBu
7.7 10.2 12.4 3.9 5.0 7.0
29.8 31.0 27.4 27.9 29.1 28.6
22.1 20.8 15.0 24.0 23.9 21.6
5.7 4.2 5.4 4.8 4.8 4.7
27.8 25.1 20.3 28.8 28.7 26.3
22.1 20.9 14.9 24.0 23.9 21.6
12.6 12.6 19.4 13.6 14.2 18.0
35.4 34.7 34.8 35.2 35.6 36.3
22.8 22.1 15.4 21.6 21.4 18.3
a
Conditions: level of theory M06-2X/TZ//TPSSTPSS/DZ, 383 K, 1 bar. Calculated using the naked cation approach.
Table 3. 1′-Ph Performance in Propene Polymerization at High Temperature with Different Cocatalystsa SATg entry 1 2i 3i 4 5 6
cocat. MAO/BHT B(C6F5)3/TiBA/ BHT TTB/TiBA/BHT MAO/BHT MAO/BHT MAO/BHT
UNSATg
AlR3/ BHTb
activityc
Mnd
Mwd
PDI
∑REe
Int./Ter. REf
total
iBu
nPr
nBuA+B
total
allyl
% Bnh
1/2 1/1
2
2.6 × 10 1.4 × 102
3.3 3.5
6.4 6.3
2.0 1.8
10.4 9.3
6 9
3.0 2.4
0.8 0.5
0.7 1.0
1.5 0.9
0.31 0.60
0.27 0.17
21 n.d.j
1/1 1/1 1/3 1/5
2.3 × 102
3.3
7.0
2.1
10.7
1.0
1.2
0.34
0.23
4
1.9 × 102 1.0 × 102
2.5 1.9
4.9 3.6
2.0 1.9
10.0 9.9
7 2.9 0.7 no polymer obtained 6 2.8 0.7 5 2.8 0.5
0.8 0.8
1.3 1.5
0.33 0.29
0.22 0.19
16 12
Other experimental conditions: catalyst, 25 μmol; toluene, 200 mL; activator, MAO/BHT; Al/Ti = 400; ppropene = 1 bar; Tp = 110 °C, t = 15 min. AlR3 = “free” TMA in MAO or Al(iBu)3 cIn kgPP mol−1 bar−1 h−1. dIn kDa. eSum of regioerror ∑RE = 2,1 + 3,1 + 2 × PEP + iBu + nBuA + 2 × nBuB + 2-Bt. fRatio between internal and terminal regioerrors Int./Ter. RE = (2,1 + 3,1 + 2 × PEP + nBuB)/(nBuA + nBuB). gMole percent of saturated (SAT) and unsaturated (UNSAT) chain ends (total in Roman type and specific groups in italics; see the Supporting Information for details) hPercent of benzylated chains. iAl/Ti = 25, B/Ti = 1. jn.d. = not detected. a
b
Table 4. Performance of Other Ti Catalysts in Propene Polymerization at High Temperaturea SATf entry 1 2 3 4h 5
cat. 2 3 4 2 2
b
solvent
activity
toluene toluene toluene mesitylene tol./sat.i
4.1 × 10 20 1.8 × 102 3.8 × 102 2.0 × 102 2
c
Mwc
PDI
∑RE
5.9 4.1 2.8 4.9 8.6
13 7.4 5.4 9.2 15
2.2 1.8 1.9 1.9 1.7
3.9 4.2 8.6 3.6 3.6
Mn
d
e
UNSATf
Int./Ter. RE
total
iBu
nPr
nBuA+B
total
allyl
% Bng
3 6
1.8 1.5 0.6 1.5 1.5
0.6 0.6 0.6 0.4 0.4
0.2 0.4 n.d.j 0.3 0.3
1.0 0.5 n.d. 0.8 0.8
0.27 0.49 0.30 0.28 0.10
0.24 0.14 n.d. 0.14 0.07
50 4 n.d. 58 15
3 3
Other experimental conditions: catalyst, 25 μmol; toluene, 200 mL; activator, MAO/BHT; Al/Ti = 400; ppropene = 1 bar; Tp = 110 °C, t = 15 min. In kgPP mol−1 bar−1 h−1. cIn kDa. dSum of regioerrors ∑RE = 2,1 + 3,1 + 2 × PEP + iBu + nBuA + 2 × nBuB + 2-Bt. eRatio between internal regioerrors and terminal regioerrors arising from homolysis Int./Ter. RE = (2,1 + 3,1 + 2 × PEP + nBuB)/(nBuA + nBuB). fMole percent of saturated (SAT) and unsaturated (UNSAT) chain ends (total in Roman type and specific groups in italics; see the Supporting Information for details). gPercent of benzylated chains. hCocatalyst = solid-MAO/BHT. iToluene/high boiling saturated hydrocarbons 1/9 v/v. jn.d. = not detected. a
b
investigating this hypothesis for catalyst 1′-tBu in greater detail. Results will be reported in due course. It appears that, in the case of Cp* (i.e., bulky) catalysts, trends in % Bn are largely related to the ease of chain reinitiation (Figure 3) rather than Ti−polymeryl homolysis. In less hindered Cp systems such as 1-Ph, which are more prone to dimerize, it is more difficult to identify a dominant contribution. Exploration of Cocatalyst Effects for 1′-Ph. Cocatalyst effects on the CTS were investigated for the representative catalyst 1′-Ph using typical molecular activators such as B(C6F5)3 and tritylborate, [Ph3C][B(C6F5)4] (TTB; entries 1−3, Table 2),46 in combination with a bulky trialkylaluminum, Al(iBu)3 (TiBA), and BHT.51,52 Table 3 shows that Int./Ter. RE and MW remain practically constant when the cocatalyst is varied from MAO/BHT (entries 1−3; see also Table S1 in the Supporting Information), indicating that the reactivity of the
The predicted homolysis barrier for 1′-tBu is already small without inclusion of anion effects. Experimentally, no benzyl incorporation is observed, suggesting that for this case homolysis has become easier than propagation. Experimentally, no benzyl incorporation but a surprisingly high amount of allyl-type chain ends is observed in propene polymerization (∼82% of allyl terminated chains). Allyl chain ends are commonly associated with β-methyl transfer reactivity, or with insertion in an allyltype M−(η3-CH2CHCH2) complex formed via monomer activation.48,49 We still observe high amounts of nBu chain ends, indicating that homolysis does occur with 1′-tBu. As the bond dissociation energies (BDEs) for allylic C−H bonds in propene (∼88 kcal/mol) are comparable to those for benzylic C−H bonds in toluene (∼89 kcal/mol),50 allyl formation might originate from radical activation of the monomer, analogously to what we discussed for CTS (Scheme 1). We are currently D
DOI: 10.1021/acs.organomet.8b00393 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
sample. This indicates that also Ti−n-alkyls can be susceptible to homolysis in some extreme cases under harsh reaction conditions.
active species is not influenced by the nature of the cocatalyst; however, a notable effect on chain end composition is observed. No or minimal amounts of benzylated chains are found with borane or borate activators: i.e., B(C6F5)3/TiBA/BHT < TTB/ TiBA/BHT ≪ MAO/BHT. Conversely, olefinic chain ends are increasing in the order B(C6F5)3/TiBA/BHT > TTB/TiBA/ BHT > MAO/BHT. The lack of Bn chain ends means that reinitiation via CTS after homolysis does not take place; this also leads to lower activity. Catalyst revival is hindered also in the absence of BHT: i.e., with solid MAO as activator/scavenger,30 we observed that activity was lower by more than 1 order of magnitude for catalysts 1-Cy and 1′-Ph. The potential role of BHT in the CTS mechanism was further investigated by performing polymerization experiments at varying AlMe3/BHT ratios (entries 1 and 4−6, Table 3). The phosphinimide system stays active if at least 2 equiv of BHT per TMA is added. At lower amounts (1 equiv; entry 5) TMA is likely not effectively trapped,36 leading to fast catalyst deactivation.24,25 Higher amounts of BHT (3−5 equiv; entries 5 and 6, Table 3) steadily decrease activity and the amount of benzylated chain ends. Free BHT is used as an antioxidant due to its radical stabilizer properties,53 and an increase in its concentration can trap polymeryl or benzyl radicals and prevent catalyst reactivation. Generality of CTS. A summary of polymerization performance and polymer microstructural analysis for catalysts 2-4 in toluene, as well as solvent and monomer effects is reported in Table 4 (see also Table S1 in the Supporting Information for further details). Amidinate catalyst 2 yields a remarkable amount of 50% benzylated chains (entry 1, Table 4), showing how competitive CTS can be for Ti-based postmetallocenes. The generality of the process is also demonstrated by the fact that CGC 3 (entry 2, Table 4), one of the very first examples of a commercially relevant molecular catalyst,33 produces 4% benzyl terminated PP.54 Catalyst 4 does not yield detectable amounts of benzyl chain ends. 4 gives a relatively large fraction of vinylidene chain ends, indicating an easier termination by β-H elimination (BHE) or transfer to monomer (BHTM).37 No nBu chain ends are found despite the low regioselectivity of 4, implying that homolysis is a less competitive process for this species. As catalyst 2 shows the highest benzyl incorporation, we selected this species to explore the effect of solvent changes on CTS (entries 4 and 5, Table 4). Replacing toluene with its trimethylated analogue, mesitylene, results in an even higher % Bn (∼60%), in line with previous observations on phosphinimide catalysts.30 Interestingly, even if toluene is diluted with high-boiling saturated hydrocarbons (1/9 v/v), a sizable fraction of 15% benzylated chains is still produced. Such a remarkable ease of benzyl incorporation prompted us to preliminarily explore CTS also in ethene homopolymerization with catalyst 2. In this case, no Ti−sec-alkyl species are expected, since ethene insertion only generates Ti−n-alkyl bonds, with the latter being less sterically strained and therefore generally less prone to give homolysis.29 The productivity of 2 at 1 bar partial pressure of ethene and 110 °C in toluene is extremely high,31 even on polymerizing with relatively low catalyst concentration (∼3 × 10−7 M), resulting in a broad MWD (PDI = 6.3). Although quantification is hampered by the multimodal MWD, 1H NMR characterization clearly shows the presence of benzyl chain ends in the resulting polyethylene
■
CONCLUSIONS Ti(III) species, generated by homolysis of Ti(IV)−polymeryl, are not necessarily permanently deactivated. In toluene and related solventscommonly used in academia to study olefin polymerizationradical attack on the solvent can lead to revived Ti(IV)−Bn species which can start a new, Bnterminated polymer chain (CTS). This phenomenon is not limited to phosphinimide catalysts and has now also been detected with CGC and Keltan-ACE catalysts. Thus, results obtained in toluene may not be representative of polymerizations carried out in the aliphatic solvents commonly used in industry. McConville has also pointed out that toluene is a noninnocent solvent in 1-hexene polymerization using diamide Ti complexes;55 in this case, however, deleterious effects on the polymerization activity were observed. In the series of phosphinimide catalysts, homolysis appears to be a major chain termination pathway, as indicated by MW and chain end analysis; trends correlate with %VBur and DFT estimated ΔΔG⧧HOM‑INS,Ti‑polymeryl and support the important role of steric factors. The efficiency of reactivation, leading to benzyl-terminated chains, is also highly sensitive to sterics. Trends in % Bn chain ends correlate with DFT estimates of ΔΔG⧧HOM‑INS,Ti‑Bn, indicating that insertion in a Ti−Bn species is the limiting factor in CTS.
■
EXPERIMENTAL SECTION
Polymerization. All manipulations were performed under an inert atmosphere of argon or nitrogen, employing Schlenk line techniques or MBraun LabMaster 130 gloveboxes. Precatalyst 1/1′-R,30,56 2,31,32 and 434 were synthesized according to previously established protocols, while catalysts 3 and 5 were purchased from MCat and used as received. Methanol and hydrochloric acid were purchased from Sigma-Aldrich and used as received. All other solvents were purchased from Romil and purified by passing through a mixed-bed activated-Cu/4Å molecular sieves column in an MBraun SPS-5 unit (final concentration of O2 and H2O
