Quantitative 13C NMR Analysis of Isotactic Ethylene–Propylene

Article pubs.acs.org/Macromolecules

Quantitative 13C NMR Analysis of Isotactic Ethylene−Propylene Copolymers Prepared with Metallocene Catalyst: Effect of Ethylene on Polymerization Mechanisms Antti Tynys,* Isa Fonseca,† Matthew Parkinson, and Luigi Resconi Borealis Polyolefine GmbH, St.-Peter-Straße 25, 4021 Linz, Austria S Supporting Information *

ABSTRACT: Low molecular weight homopolypropylene and ethylene−propylene copolymers with low ethylene content have been produced with a highly stereoselective metallocene catalyst in the presence of dihydrogen. Quantitative 13C NMR analysis allowed the detection of regioirregular (2,1-insertions) propylene units both in propylene homosequences and in ethylene− propylene sequences, and of different types of end groups. The microstructure of the polymers has been correlated with polymerization mechanisms, and the effect of ethylene on polymer regioregularity, termination and initiation steps has been investigated. The amount of 2,1-misinserted propylene units followed by one ethylene unit increases with increasing ethylene content in the polymer, which confirms that ethylene insertion after a 2,1-misinsertion is favored over propylene 1,2-insertion. Additionally, addition of ethylene reduces the frequency of secondary insertions. Under the investigated polymerization conditions, the dominating termination route was chain transfer to H2 after 1,2- or 2,1-inserted propylene unit. In the presence of ethylene, the relatively fast ethylene insertion decreased significantly the probability of chain termination after 2,1-inserted propylene unit. Chain initiation took place by propylene 1,2or 2,1-insertion into the Zr−H bond, and no signs of initiation by ethylene unit were detected. The initiation step was partly regioselective and favored 1,2-propylene insertion over 2,1-insertion.

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INTRODUCTION For microstructure analysis of polyolefins 13C NMR spectroscopy is a very powerful tool.1−3 In particular, due to their industrial significance, several in depth studies have been dedicated to the analysis of ethylene−propylene copolymers alone.4−12 Although such copolymers are only made from the relatively simple ethylene and propylene monomers, when polymerized a large number of different substructures become possible. Combined with the chemical similarity of the comonomers, resulting in a limited spectral dispersion, the 13 C NMR spectra are generally considered to be complex. From the perspective of spectral analysis such systems need to be considered as terpolymers, where not only different ethylene/ propylene sequences need to be considered, but also the tacticity of propylene units, as well as other microstructural features. For regio-irregular polypropylenes the three main types of regiodefects are easily detected and quantified by 13C NMR spectroscopy. The relative contents of 2,1-erythro (21e), 2,1threo (21t) and 3,1 (31) defects is characteristic to a specific catalytic system.3 For low molecular weight polymers, which have industrial relevance in melt blown application,13 end groups play an increasingly prominent role in the recorded spectra. With each © 2012 American Chemical Society

type of end group responsible for multiple resolved signals further spectral complexity is encountered. Similarly, if the catalyst is prone to making regioerrors during propylene incorporation, regio defects will be observed and need to be considered when analyzing the spectrum. The combination of individual microstructure elements further complicates the analysis of the NMR spectra, e.g., when end-groups are only formed by chain termination directly after secondary propylene insertion or if ethylene incorporation occurs directly after secondary propylene insertion. When correctly interpreted, such complexity allows a wealth of information about the chemical structure and mechanistic aspects of ethylene− propylene copolymerization to be understood. There have been several studies in the past regarding the assignment of NMR spectra of ethylene−propylene copolymers and rubbers.4−6,11,14 Among the many reports in the literature only a limited number consider the presence of secondary propylene insertion in their assignment9,10,24 and inclusion in microstructure quantification.6,15,24 The accepted assignments of Cheng9 and nomenclature of Carman4 are used in this work. The assignment of isolated regio defects and ethylene insertion Received: May 31, 2012 Revised: September 4, 2012 Published: October 1, 2012 7704

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BHT. To ensure a homogeneous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatary oven for at least 1 h. Upon insertion into the magnet, the tube was spun at 10 Hz. Standard single-pulse excitation was employed with NOE, a 3 s recycle delay and a bilevel WALTZ16 decoupling scheme.17,18 A total of 12288 (12k) transients were acquired per spectra. This setup was chosen primarily for the high resolution and sensitivity needed for detection of microstructure elements in low concentration. For quantitative measurements approximately 200 mg of material was dissolved in 3 mL of 1,2-tetrachloroethane-d2 (TCE-d2) stabilized with BHT along with chromium(III) acetylacetonate (Cr(acac)3) resulting in a 65 mM solution of relaxation agent in solvent30 using the dissolution process previously described. Upon insertion into the magnet the tube was spun at 10 Hz. Standard single-pulse excitation was employed without NOE using an optimum tip angle, a 1 s recycle delay and a bilevel WALTZ16 decoupling scheme.17,18 A total of 24576 (24k) transients were acquired per spectra. This setup was chosen primarily to ensure accurate quantification of microstructure elements in low concentration. Quantitative 13C{1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using custom automation programs. All chemical shifts were indirectly referenced to the central methylene groups of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present. Assignment was achieved through comparison to previously assigned spectra in the literature, empirical chemical shift calculations and quantitative analysis. The comonomer content was quantified using the method of Wang and Zhu15 through integration of multiple signals across the whole spectral region in the 13C{1H} spectra. This analysis method was chosen for its robust nature and ability to account for the presence of regio-defects when needed. Integral regions were slightly adjusted to increase applicability across a wider range of comonomer contents. Characteristic signals resulting from secondary insertion were also observed for all polymers.15,19,29 Three types of microstructure resulting from secondary insertion were seen: isolated 2,1-units resulting in 2,1-erythero regio defects, adjacent 2,1-units from 2,1-insertion directly followed by ethylene insertion and finally end 2,1-units from being the last inserted monomer before chain termination resulting in n-butyl end-groups. The isolated 2,1-units were quantified using the 21e6 and 21e8 sites at 17.35 and 17.71 ppm. The 2,1-units adjacent to ethylene were quantified using the 21E1 sites at 34.10 ppm and the end 2,1-units were quantified using the nBu4 sites at 17.13 ppm.15,20−22 With multiple modes of incorporation the total amount of secondary insertion related to both primary and secondary insertion could only be correctly calculated if each mode of secondary inserted propylene was considered. The total amount of propylene was quantified using the Pββ sites between 23.00 and 19.90 ppm to represent the primary inserted and accounting for signals not included in this region. The relative amount of each mode of 2,1insertion was quantified with respect to all inserted propylene according to:

after secondary insertions are based on other works of Busico and Lin, respectively.3,4,7,24 End groups assignments were made based upon the works of Cheng and Resconi.10,29 For low molecular weight ethylene−propylene copolymers the qualitative implications of 13C NMR based end-group analysis upon polymerization mechanism have previously been discussed by Cheng and Smith.10 Whereas Lin and Waymouth have described the mechanistic aspects for different catalyst types based on 13C NMR end-group and regio defect analysis.24 In this work a robust automated method for the analysis of complex ethylene−propylene copolymers 13C NMR spectra was developed. The method allowed quantitative analysis of regio-irregular polypropylene homopolymers and ethylene− propylene copolymers, where microstructure elements resulting from end-groups and secondary propylene insertion were observed in significant quantities. Through such a characterization method, we gained a deeper insight into the effect of ethylene on catalyst regioselectivity and mechanism of chain initiation and termination.

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EXPERIMENTAL SECTION

Three model polymers have been produced by liquid propylene polymerization under high H2 concentration. The catalyst used was a highly isospecific zirconocene complex, activated by methylalumoxane (1/MAO). Complex 1 is a metallocene developed and commercialized by Lummus Novolen Technology GmbH, having the formula shown in Chart 1.

Chart 1. rac-Methyl(cyclohexyl)silanediyl Bis(2-methyl-4-(4tert-butylphenyl)indenyl)zirconium Dichloride16

The polymers have been prepared under the conditions summarized in Table 1.

Table 1. Liquid Propylene Polymerization Conditions and Molecular Weight of the Samples variable

units

sample A

sample B

sample C

temperature [H2]/[C3] [C2]/[C3] Mn Mw

°C mol/kmol mol/kmol kg/mol kg/mol

75 0.47 − 28 69

70 0.33 4.00 25 70

2.29 2.65 27 64

[21e] = 100 ×

1 (21e6 + 21e8)/P 2

[21E] = 100 × 21E1/P [21nBu] = 100 × nBu4/P

The molecular weight (Mw) of the samples varied from 64 to 70 kg/ mol (Table 1), and the ethylene content ranged from 0 to 5.8 mol % (Table 3). Solution-state NMR spectroscopy was used to identify and quantify the microstructure of the polymers. All 13C{1H} NMR spectra were recorded using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for 1 H and 13C respectively. All spectra were recorded using a 13C optimized 10 mm extended temperature probehead at 125 °C using nitrogen gas for all pneumatics. For qualitative measurements approximately 200 mg of material was dissolved in 3 mL of 1,2-tetrachloroethane-d2 (TCE-d2) stabilized with

where

P = Pββ + 21e6 + 21e8 + 21E5 + nBu4 Regio defect contents are expressed in percent with respect to the total amount of 2,1-insertion quantified as the sum of each of the separate species i.e. [21] = [21e] + [21E] + [21nBu]. Because of the relatively low molecular weight, n-propyl, n-butyl, isobutyl, and 2,3-dimethylbutyl end-groups were observed. Quantification was achieved through integration of the representative nPr3, nBu4, iBu2, and dmBu1 signals which occurred at 39.90, 37.13, 26.00, 7705

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Figure 1. Qualitative 13C NMR spectra measured with NOE and without relaxation agent of the regio-irregular polypropylene (a) homopolymers and two ethylene−propylene copolymers with (b) 1.7 and (c) 5.8 mol % ethylene incorporation. Full assignments are given in Table 2, and assignment nomenclature is shown in Figure 2.

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and 17.76 ppm respectively (Figure 1). End-group contents were normalized to the number of backbone carbons as quantified using the bulk Sαα and Sαγ + Sαδ signals between 48.20 and 45.20 and 38.30− 37.0 ppm, respectively, correcting for signals not representative of primary inserted propylene and compensating for signals not represented in these regions. Thus, end-group contents were expressed as end groups per 1000 backbone carbons (kCbb−1) using the following relationship

RESULTS AND DISCUSSION The 13C NMR spectra of the three samples presented here clearly show the presence of regio-irregularity, ethylene incorporation and chain end-groups (Figure 1 and Table 2). It is clearly shown that A is a polypropylene homopolymer containing isolated secondary units (regiodefects), whereas B and C are propylene copolymers with low ethylene comonomer content, also containing minor amounts of secondary units. For all systems only 21e type regio-defects were observed. Further quantitative analysis of quantitative spectra (see Supporting Information) revealed that B and C had an ethylene content of 1.7 and 5.8 mol % respectively (Table 3). For both the ethylene−propylene copolymers, secondary insertion followed by ethylene and then primary propylene insertion was observed, resulting in 21E units. No indication of 2,1-threo or 3,1-insertion was observed. As polymerization was carried out in the presence of hydrogen, n-butyl end-groups were also expected resulting from chain transfer to hydrogen after secondary insertion. The relatively high content of isolated 2,1 defects in A shows that even though primary insertion after secondary insertion is known to be relatively slow,23 it can still compete with chain transfer to hydrogen. This observation is in good agreement with previous results reported by Lin and Waymouth.24 For the copolymers B and C both isolated 21e and 21E units were observed, with such ethylene insertion directly after secondary insertion in agreement with the previous work of Lin and Waymouth.24 As expected the amount of 21E units increased

[End] = 1000 × End/{2 × Sαα + 2 × (e6 + e8) + 4 × nPr3 + 2 × iBu4 + 5 × nBu4 + 3 × dmBu1 + 6 × 21E1 + 4 × (Sαγ + Sαδ)} where End represents the signal of a specific end-group End. The weight-average molecular weight Mw and the number-average molecular weight Mn were determined according to ISO 16014− 1:2003 and ISO 16014−4:2003. A Waters Alliance GPCV 2000 instrument, equipped with refractive index detector and online viscosimeter was used with 3 x High temperature PL Olexis columns and 1,2,4-trichlorobenzene (TCB, stabilized with 200 mg/L BHT) as solvent at 160 °C and a constant flow rate of 1 mL/min. 216.5 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration with 19 narrow MWD polystyrene (PS) standards in the range of 0.5 kg/mol to 11 500 kg/mol. All samples were prepared at concentration of approximately 1 mg/mL in stabilized TCB (same as mobile phase) by continuous shaking for 3 h until complete dissolution was achieved prior to injection into the GPC instrument. 7706

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Table 2. Assignments of Qualitative 13C NMR Spectra shown in Figure 1a δ (ppm)

assignment

δ (ppm)

assignment

11.63 14.25 14.63 15.76 16.49 16.80 17.35 17.71 17.76 18.87 20.30 20.72 20.89−21.97 21.32 22.80 23.33 23.99 24.63−25.08 26.00 27.15 27.35 28.19 28.78−29.13 29.37 29.54 30.00

decoupling sideband nBu1 nPr1 decoupling sideband dmb5 decoupling sideband 21e6 21e8 dmb1 decoupling sideband nPr2 dbm3 Pββ + Pβγ + Pβδ iBu6 iBu1 nBu2 iBu3 Sββ iBu2 decoupling sideband Sβδ decoupling sideband Tββ 21e10 nBu3 Sδδ

30.64 30.85 31.03 31.38 31.58 32.33 33.38 34.10 34.67 34.91 35.94 36.03 36.71 37.13 37.55−38.16 38.62 39.90 41.43 42.37 43.50 43.27 45.90−47.07 47.73 49.71 51.78

21e4 + nPr4 Sγγ Tβδ 21E5 (Tbg) 21e1 decoupling sideband Tδδ 21E1 (Tgg) 21E4 (Sab) + BHT 21E3 (Sab) 21e3 21e7 dmb4 nBu4 Sαγ + Sαδ 21e5 nPr3 decoupling sideband 21e9 decoupling sideband dmb6 Sαα iBu4 decoupling sideband decoupling sideband

3). Assuming this trend holds also for higher comonomer content it is suggested that isolated 2,1 regio-defects would not be formed above 7.2 mol %, based on the data points in Figure 3. A parallel study carried out in our laboratory supports this observation: we have found that a propylene-ethylene copolymer produced with the same catalyst system and containing 28.8 mol % of ethylene did not have 21e insertions in propylene sequences. In addition, also the sum of isolated 21e and 21E units, excluding n-butyl end groups, decreases with ethylene content. This is a new finding, that can be explained by assuming that insertion of a 2,1-coordinated propylene is relatively slow (for this type of metallocene) hence allowing exchange with an incoming ethylene unit.27 For propylene homo and copolymerization with ethylene, the mechanism of chain initiation and termination has been widely studied via characteristic chain end groups.24,28,29,31−33 In this study 13C NMR analysis revealed n-propyl, 2,3dimethylbutyl, n-butyl and isobutyl end groups (Figure 1). The amount and relative distribution of each of these end groups was quantified and is given in Table 3. The formation mechanisms of the identified end groups are illustrated in Figure 2. The isobutyl and n-butyl end groups arise from termination via chain transfer to H2 after 1,2- or 2,1-inserted propylene, respectively. The current data cannot fully exclude chain termination after ethylene insertion, which would result in n-propyl end group. In contrast, chain termination after ethylene is unlikely as further propagation is assumed to be significantly faster than termination. It is thus assumed that the observed n-propyl end groups arise solely from chain initiation by 1,2-insertion. Trace amounts of vinylidene end groups were also observed in the 1H NMR spectra of these systems (not shown). Such end groups are likely related to chain termination by β-H transfer to either the metal center or monomer. As the amount of vinylidene end groups was negligible in contrast to the other observed end groups it can be concluded that chain termination is dominated by chain transfer to H2 after 1,2- or 2,1-insertion of propylene with chain termination after ethylene insertion unlikely. With 3,6-dimethyl-heptyl end groups not observed, chain transfer to H2 after 2,1-propylene insertion followed by ethylene is not a relevant chain termination pathway. For the homopolymer A, the relative amount of the n-butyl end groups is nearly more than three times higher than the amount of the isobutyl end groups. This suggests termination after 2,1-insertion was more favored than termination after 1,2insertion, confirming the model of Busico of a slow primary propagation after secondary insertion, with termination being more probable. This observation is also supported by recent theoretical study carried out by Laine et al.,34 which shows that activation energy is particularly low for β-hydrogen elimination after secondary insertion in zirconocene-catalyzed olefin copolymerization. The presence of ethylene also has an effect on relative amounts of isobutyl end groups. The relative amount of isobutyl end groups increases with ethylene content. In the case of n-butyl end groups, the trend is not as clear. However, it seems that the relative amount of n-butyl end groups decreases at high ethylene content (Figure 4). The increase of isobutyl end groups shows that increased ethylene content favors the probability of termination after 1,2inserted propylene unit. This trend can be rationalized by faster ethylene insertion after 2,1-inserted propylene (leading to 21E units), which leads to lower termination probability after 2,1-

a

Minor chemical shift differences were observed between qualitative and quantitative spectra. Assignment nomenclature taken form Figure 2.

Table 3. Summary of the Microstructure Elements Quantified from the Quantitative 13C NMR Spectra variable ethylene content

21e (isolated) 21E (2,1 then E) 2,1 total n-propyl 2,3-dimethylbutyl n-butyl isobutyl

units

A

mol % − wt % − Secondary Insertions mol % 0.80 mol % − mol % 0.80 End Groups kCbb−1 (%) 0.33 (30) kCbb−1 (%) 0.19 (17) kCbb−1 (%) 0.44 (40) kCbb−1 (%) 0.13 (12)

B

C

1.74 1.16

5.79 3.94

0.60 0.10 0.70

0.16 0.17 0.33

0.28 0.16 0.51 0.22

(24) (14) (44) (19)

0.23 0.15 0.31 0.31

(23) (15) (31) (31)

with ethylene content, at the expense of isolated 2,1 units in propylene homosequences. Such behavior can be explained by favorable insertion of the smaller ethylene unit over a bulkier propylene unit at the stericaly hindered catalytic center resulting in more 21E units and less 21e units being produced. This observation is in good agreement with slow insertion of 1,2 propylene after secondary insertion. This is also in good agreement with the earlier work of Tsutsui et al. who showed that small amounts of ethylene increased catalyst activity in propylene polymerization.25 Consequently, due to the faster insertion of ethylene the likelihood for termination decreases resulting in increased molecular weight.26 When compared to the homopolymers, a clear trend is observed, showing decreased content of 21e units with ethylene content (Figure 7707

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Figure 2. Reaction scheme showing the possible initiation, propagation and termination reactions leading to specific microstructure elements. Assignment nomenclature are shown for relevant elements; i.e., 21e6 is the sixth carbon of the 2,1e-substructure. Methyl group numbering of the various dimethyl end-groups follows standard chemical nomenclature based on position from chain end along backbone.

2,1 misinsertion, termination of chain growth is likely to happen. However, termination probability decreases in the case of propylene/ethylene copolymerization because of faster insertion of ethylene monomer. Consequently it can be concluded that under the applied polymerization conditions, after a 2,1e unit, the relative rates of elementary steps follow the order: termination > ethylene insertion > 1,2 propylene insertion. The observation of n-propyl and 2,3-dimethylbutyl end groups showed that both chain initiation by 1,2- and 2,1propylene insertion respectively occurred (Figure 2). However, as neither ethyl nor 2,5-dimethylhexyl end groups were observed no evidence for chain initiation by ethylene or 2,1propylene followed by ethylene was found. Thus, under such polymerization conditions the probability of chain initiation by ethylene appears to be very low. This implies also that chain transfer to ethylene, otherwise a quite important chain termination mechanism in metallocene-catalyzed copolymerizations, is suppressed by H2. The observed amount of 2,3dimethylbutyl end groups was surprisingly high, and showed regio-selectivity of the first insertion propylene unit with a slight preference of 1,2- over 2,1-insertion. For the homopolymer (A) nearly two times more n-propyl end groups were observed than 2,3-dimethylbutyl end groups, with a similar trend observed for the copolymers. The relatively high amount of 2,3-dimethylbutyl end groups shows that initiation by 2,1insertion is competitive with that of 1,2-insertion. This observation provides additional support to the results of Moscardi et al. who concluded that secondary insertion into Zr−H bonds is competitive with primary insertion, even for highly regio-selective catalysts.35 In general, the end-group analysis suggested that termination after ethylene insertion is unfavorable due to fast propylene insertion into an ethylene terminus. This is in agreement with the work of Randall and Rucker who showed that ethylene initiation and chain termination after ethylene insertion is unlikely using Markovian analysis of various ethylene propylene copolymers.32

Figure 3. Correlation of the amount of isolated 2,1-units (21e) with ethylene content [E].

Figure 4. Correlation of the relative amount of n-butyl (circles) and isobutyl (squares) end-groups with ethylene content [E].

insertion. This observation is in agreement with the previous observation that the amount 21E units increased with ethylene content and with the indication that n-butyl end groups decreases with increasing ethylene content. These results show that ethylene has surprisingly pronounced effect on termination mechanism in the case of propylene/ethylene copolymerization. These results discussed here show that due to slow 1,2-insertion of propylene unit after 7708

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CONCLUSIONS The microstructure of metallocene catalyzed low molecular weight homopolypropylene and ethylene−propylene copolymers with low ethylene content were quantified using 13C NMR spectroscopy. Various polymerization mechanisms were investigated using the amount of specific microstructure elements present. In particular the effect of ethylene on the regioregularity, chain initiation and chain termination reactions was investigated. The amount of 2,1E units increased with ethylene content and confirmed that ethylene insertion after 2,1-insertion was favored over propylene 1,2-insertion. It was also shown that the presence of ethylene significantly suppressed the formation of isolated regio defects (21e), and proposed that the 2,1coordinated propylene undergo exchange with an incoming ethylene unit. Under the applied conditions chain termination was dominated by chain transfer to H2 after both 1,2 and 2,1insertion; in contrast after ethylene insertion no such termination was observed. The presence of ethylene did however have a marked effect upon the termination of the copolymers. Because of the slow propagation after 2,1 in the homopolymer termination after 2,1-insertion was likely. In contrast, when ethylene was present ethylene insertion after 2,1-insertion was faster resulting in a lower likelihood of chain termination after 2,1-insertion. Initiation was shown to take place by either propylene 1,2- or 2,1-insertion with no indication of ethylene initiation observed. The initiation step was shown to be less regioselective than propagation, only slightly favoring 1,2- over 2,1-insertion. The amount of 2,1-initiated chain ends was however competitive with that of 1,2-initated chain ends. This confirmed that secondary propylene insertion into Zr−H bond was competitive with primary insertion, even on the highly regioselective catalyst applied in this study. In general, this study further demonstrates that 13C NMR spectroscopy is an extremely powerful tool for the study of polymer microstructure, especially for ethylene propylene copolymers. The detailed information gained from a single spectrum allowed direct quantification of microstructure and insight into the mechanism of polymerization of state-of-the-art metallocene catalysts.

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research. The authors also thank Juha Paavilainen, Markku Vahteri, and Tuomas Noopila for preparing the sample polymers and Gerhard Hubner for undertaking some of the NMR measurements. Joachim Fiebig is thanked for valuable support and comments. The authors would also like to acknowledge the continued support of InnoTech Operational Support (IOS) management and, specifically, Jens Reussner, concerning the development and implementation of advanced polyolefin characterisation methods within Borealis.

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(1) Randall, J. C. J. Macromol. Sci.Rev. Macromol. Chem. Phys. 1989, C29(2 &3), 201−317. (2) Pooter, M. D.; Smith, P. B.; Dohrer, K. K.; Bennett, K. F.; Meadows, M. D.; Smith, C. G.; Schouwenaars, H. P.; Geerards, R. A. J. Appl. Polym. Sci. 1991, 42, 399−408. (3) Busico, V. Prog. Polym. Sci. 2001, 26, 443−533. (4) Carman, C. J.; Harrington, R. A.; Wilkes, C. E. Macromolecules 1977, 10, 536−544. (5) Ray, G. J.; Johnson, P. E.; Knox, J. R. Macromolecules 1977, 10, 773−778. (6) Randall, J. C. Macromolecules 1978, 11, 33−36. (7) Cheng, H. N. Anal. Chem. 1982, 54, 1828−1833. (8) Kakugo, M.; Naito, Y.; Mizunuma, K.; Miyatake, T. Macromolecules 1982, 15, 1150−1152. (9) Cheng, H. N. Macromolecules 1984, 17, 1950−1955. (10) Cheng, H. N. Macromolecules 1986, 19, 2065−2072. (11) Cheng, H. N.; Bennett, M. A. Makromol. Chem. 1987, 188, 135−148. (12) Martino, S. D.; Kelchtermans, M. J. Appl. Polym. Sci. 1995, 56, 1781−1787. (13) Tynys, A.; Fiebig, J., PCT patent application WO 2012/016928 to Borealis AG. (14) Smith, W. V. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 1573− 1585. (15) Wang, W.; Zhu, S. Macromolecules 2000, 33, 1157−1162. (16) Schottek, J.; Paczkowski, N. S.; Winter, A.; Sell, T. PCT patent application WO 2005/105863 to Novolen Technology Holdings. (17) Zhou, Z.; Kümmerle, R.; Qiu, X.; Redwine, D.; Cong, R.; Taha, A.; Baugh, D.; Winniword, B. J. Magn. Reson. 2007, 187, 225−233. (18) Busico, V.; Carbonniere, P.; Cipullo, R.; Pellechia, R.; Severn, J. R.; Talarico, G. Macromol. Rapid Commun. 2007, 28, 1128−1134. (19) Cheng, H. N. Macromolecules 1984, 17, 1950−1955. (20) Grassi, A.; Ammendola, P.; Longo, P.; Albizzati, E.; Resconi, L.; Mazzocchi, R. Gazz. Chim. Ital. 1988, 118, 539−543. (21) Busico, V.; Cipullo, R.; Talarico, G.; Segre, A. L.; Caporaso, L. Macromolecules 1998, 31, 8720−8724. (22) Zambelli, A.; Longo, P.; Ammendola, P.; Grassi, A. Gazz. Chim. Ital. 1986, 116, 731. (23) Busico, V.; Cipullo, R.; Romanelli, V.; Ronca, S.; Togrou, M. J. Am. Chem. Soc. 2005, 127, 1608−1609 and references therein.. (24) Lin, S.; Waymouth, R. M. Macromolecules 1999, 32, 8283−8290. (25) Tsutsui, T.; Ishimaru, N.; Mizuno, A.; Toyota, A.; Kashiwa, N. Polymer 1989, 30, 1350−1356. (26) Zeigler, R.; Rychlicki, H.; Resconi, L.; Piemontesi, F.; Baruzzi, G. Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 1997, 38, 847. (27) A parallel investigation in our laboratories, which showed that the amount of regioerrors decreases by increasing the temperature in propylene homopolymerisation, supports this mechanism: Resconi, L.; Reichelt, K. Presented at the 15th International Congress on Catalysis, July 2012, Munich, Germany. (28) Resconi, L.; Camurati, I.; Sudmeijer, O. Top. Catal. 1999, 7, 145−163. (29) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253−1345. (30) Singh, G.; Kothari, A. V.; Gupta, V. K. Polym. Test. 2009, 28, 475−479. (31) Böhm, L. L. Angew. Chem., Int. Ed. 2003, 42, 5010−5030.

ASSOCIATED CONTENT

S Supporting Information *

Figures showing quantitative 13C NMR spectra of samples A, B and C measured without NOE and with relaxation agent. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: +43 73269810. E-mail: antti.tynys@borealisgroup. com. Present Address †

BASF SE, Carl-Bosch Strasse, 67056 Ludwigshafen, Germany.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of Borealis Polyolefine GmbH and approval to publish this 7709

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(32) Randall, J. C.; Rucker, S. P. Macromolecules 1994, 27, 2120− 2129. (33) Tynys, A.; Saarinen, T.; Hakala, K.; Helaja, T.; Vanne, T.; Lehmus, P.; Löfgren, B. Macromol. Chem. Phys. 2005, 206, 1043−1056. (34) Laine, A.; Linnolahti, M.; Pakkanen, T. A.; Severn, J. R.; Kokko, E.; Pakkanen, A. Organometallics 2011, 30, 1350−1358. (35) Moscardi, G.; Piemontesi, F.; Resconi, L. Organometallics 1999, 18, 5264−5275.

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