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MgCl2 catalyst under very low monomer addition rate
Journal of Molecular Catalysis A: Chemical 363–364 (2012) 134–139
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Chain transfer reactions of propylene polymerization catalyzed by AlEt3 activated TiCl4 /MgCl2 catalyst under very low monomer addition rate Yue Yu, Zhisheng Fu, Zhiqiang Fan ∗ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 19 December 2011 Received in revised form 22 April 2012 Accepted 27 May 2012 Available online 9 June 2012 Keywords: Propylene polymerization MgCl2 -supported Ziegler–Natta catalyst Chain transfer reactions Secondary insertion
a b s t r a c t Chain transfer reactions of TiCl4 /MgCl2 –AlEt3 catalyzed propylene polymerization were investigated under conditions of severely starved monomer supply and suppressed chain transfer to cocatalyst. Slurry polymerization was conducted by injecting gaseous monomer at very low rate to the reactor containing preactivated catalyst. The preactivated solid catalyst has been washed by solvent to remove AlEt3 in the liquid phase, and chain transfer to AlEt3 was nearly completely suppressed. Besides 1-propen2-yl (vinylidene) end group formed by -H transfer after primary (1,2-)insertion, 1-propen-3-yl (allyl) end group formed by -Me transfer after 1,2-insertion and 2-buten-4-yl formed by -H transfer after secondary (2,1-)insertion were also detected in the polymer by 1 H NMR analysis. The monomer dependences of the chain transfer reactions were studied. Because of the unimolecular nature of -H transfer after a secondary insertion, the content of 2-buten-4-yl end group, which is too low to be detected in PP polymerized under conventional conditions, was markedly increased in the product of monomer-starved polymerization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It has been more than 40 years since the discovery of MgCl2 supported Ziegler–Natta catalyst, which nowadays still dominates in polypropylene (PP) production. Although it might be considered as a simple system judging by the composition, the reaction mechanism has not been adequately elucidated yet [1–3]. Since the 1990s, chain transfer reactions in olefin polymerization with metallocene catalysts have been studied in detail [4–13]. However, study of chain transfer reactions in MgCl2 supported catalyst systems was limited, partly because of the difficulties in tracing the low level of active sites in these heterogeneous systems. Studying chain-ends of polypropylene has been demonstrated a feasible way to investigate chain transfer reactions of MgCl2 -supported catalyst system [14,15]. In order to intensify the signals of the chain-ends, this approach requires decreasing the polymer’s molecular weight by either raising the polymerization temperature or adding large amount of hydrogen. The method will have limited accuracy and gives little information of mechanism when applied to systems forming high molecular weight product. As reported by Chadwick et al., chain transfer with monomer (or monomer assisted -H abstraction) is the dominant chain transfer reaction in propylene polymer-
∗ Corresponding author. Tel.: +86 571 87952400; fax: +86 571 87952400. E-mail address: [email protected] (Z. Fan). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.05.027
ization under conventional conditions [14,15]. They have also observed chain transfer with hydrogen after either primary or secondary insertions. However, they were unable to detect 2butenyl end groups that are formed by -H abstraction after a secondary propylene insertion [14,15]. Chain transfer with the alkylaluminum cocatalyst was found to be not so evident under conventional conditions, but becomes important at high temperature [14,15]. 13 C NMR was employed as a standard technique in studying the metallocene systems mentioned above, but it is not suitable for the MgCl2 -supported Ziegler–Natta system which has highly clean 13 C NMR spectra of the polymer (shown in Fig. 1) providing little information about chain transfers. For the analysis of olefinic end groups formed in many chain transfer reactions, 1 H NMR is much more sensitive than 13 C NMR. For this reason, 1 H NMR spectra have been taken as the main source of information in this work. In this work, we attempted to shed light on those chain transfer reactions that are greatly depressed by the overwhelming transfer reactions (transfer with the monomer and alkylaluminum) under conventional conditions. To achieve this, we have applied a new method of propylene slurry polymerization in order to relatively enhance the chain transfer reactions other than the chain transfer with monomer and with cocatalyst. Using a simple TiCl4 /MgCl2 catalyst preactivated with triethylaluminum (AlEt3 ), propylene polymerization was conducted by injecting the gaseous monomer into the reactor at very low rate. This enabled us to detect more
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Fig. 1. Typical 13 C NMR spectra of PP produced by TiCl4 /MgCl2 –AlEt3 under conditions described in the experimental part. The peak of [mmmm] methyl at 21.94 ppm was chosen as internal reference. (1) Crude product; (2) fraction of PP that is insoluble in n-octane at room temperature; (3) fraction of PP that is soluble in n-octane at room temperature.
types of olefinic end groups like 2-butenyl in the produced polymer by means of 1 H NMR. As this 2-butenyl end group can only be formed by -H transfer after 2,1-insertion, the method can be used to study the regioselectivity of the active species. Further more, chain transfer and isomerization reactions of active centers with a 2,1-inserted last unit can also be studied in this way. It is well known that regioselectivity strongly influences the reaction kinetics and formation of chain structure in propylene polymerization, like hydrogen response of the catalyst and melting temperature of the produced PP. Busico et al. measured the regioselectivity of MgCl2 -supported Ziegler–Natta catalysts through 13 C NMR analysis of the product of propylene/ethylene-[1-13 C] copolymerization [16–19]. However, possible chain transfers at propagation center with a 2,1 last-inserted monomeric unit can only be traced by characterizing the chain ends. It is still necessary to develop new approaches for better understanding this important aspect of Ziegler–Natta catalysis.
2. Experimental 2.1. Materials Polymerization-grade propylene was supplied by Yangzi Petrochemical Company (Nanjing, China), and further purified by passing through columns of 4A molecular sieves and deoxygenation catalyst. High purity nitrogen (99.999%) was used as a protective gas. Oxygen (99.6%) was used as quenching reagent without further purification. Anhydrous MgCl2 (Alfa Aesar Co.) and triethylaluminum (AlEt3 , Albemarle Corp.) were used as received. TiCl4 (Shanghai Meixing Chemical Industry Limited Company, Shanghai, China) was distilled under N2 . n-Heptane (Yixing No.2 Chemical Reagent Factory, Yixing, China) and n-octane (Shanghai Lingfeng Chemical Reagent Corporation, Shanghai, China) as solvent were refluxed over sodium and distilled.
2.2. Catalyst preparation 70 g anhydrous MgCl2 and 52 stainless steel balls (diameter 10 mm) were evenly distributed in 4 milling cans (inner diameter 53 mm × height 58 mm) and sealed under N2 , and then milled in a QM-1F planetary ball-mill at 250 r/min for 48 h. 35 mL TiCl4 and 39 g ball-milled anhydrous MgCl2 was placed into a 100-mL flask under N2 atmosphere. The mixture was stirred at 110 ◦ C for 6 h, then washed with 50 mL n-heptane, and finally vacuum-dried at 70 ◦ C for 30 min. Titanium content of the catalyst determined by conventional spectrophotometry method (colorized by H2 O2 ) was 3.4 wt%.
2.3. Polymerization procedure n-Octane, AlEt3 and 0.3 ± 0.01 g TiCl4 /MgCl2 catalyst were added in sequence into a 100-mL Schlenk flask at 60 ◦ C, with Al/Ti = 10 (mol/mol) and [AlEt3 ] = 0.05 mol/L. After stirring for 2 min, the suspension was left still for another 2 min to settle down the solid catalyst. The liquid phase was then siphoned out and 10 mL fresh n-octane was injected into the flask to wash the catalyst. The liquid phase was siphoned out again and another 10 mL fresh n-octane was injected into the flask. Polymerization was started by steadily inject gaseous propylene into the flask at rates of 10–100 mL/h through a long needle using a syringe pump. During the reaction, the reaction suspension was severely stirred and the tip of needle was placed at 1 cm above the liquid surface to ensure uniform dispersion of propylene in the liquid phase. After 1 h, the polymerization was terminated by injecting acidic water (1% hydrochloric acid) into the flask. Suspension of the product (fine solid particles) was repeatedly washed with acidic water and then distilled water. Polymer was obtained by vacuum-drying the suspension. Some polymerization runs were quenched by 100 mL oxygen before being terminated by acidic water to produce hydroxyl-capped polypropylene. A polymerization run with continuous supply of 1 atm propylene gas to the flask has also been made. 2.4. Characterization 1 H NMR spectra of PP were recorded on a Varian Mercury plus 300 spectrometer operated at 300 MHz in the pulse Fourier transform mode. S2pul pulse with acquisition time of 1.9 s and relaxation delay of 1 s was adopted, and 3000–6000 scans were accumulated depending on the spectra quality. The spectra were obtained at 120 ◦ C in o-dichlorobenzene-d4, using the peak at downfield of o-dichlorobenzene-d3 (7.14 ppm) as internal reference. 13 C NMR spectra of the PP samples were recorded at 120 ◦ C with the same instrument at 75 MHz. o-Dichlorobenzene-d4 was used as the solvent to prepare the polymer solution of 10 wt.%. Chromium triacetylacetonate (about 2 mg) was added in each sample to shorten the relaxation time. Broadband decoupling with a pulse delay of 3 s was employed. Typically, 5000 transients were collected. Molecular weight of the PP samples was measured by gel permeation chromatograph in a PL 220 GPC instrument (Polymer Laboratories Ltd.) at 150 ◦ C in 1,2,4-trichlorobenzene. Three PL mixed B columns (500–107 ) were used. Universal calibration against narrow polystyrene standards was adopted.
3. Results and discussion It has been reported that -hydride transfer to the monomer after a primary insertion is the most preferred way of chain
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Fig. 2. Effect of addition rate of monomer on molecular weight distribution of polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
Scheme 1. Possible formation mechanisms of the olefinic end groups.
termination in propylene polymerization with heterogeneous Ziegler–Natta catalyst [20–23]. Under conventional polymerization conditions, most of the olefinic chain-ends are 1-propen-2-yl (vinylidene) group formed in this pathway, and end groups like 2butenyl that is formed through -hydride transfer after a secondary insertion (see Eq. (3) of Scheme 1) cannot be detected, owing to very low probability of 2,1-insertion in these systems [17–19]. However, if the reaction forming 2-butenyl is an unimolecular process, one may increase the 2-butenyl/1-propen-2-yl ratio by greatly lowering the monomer concentration. Of course, the contents of other end groups from unimolecular reactions will also be increased. This method has been used in this article. As described in the experimental part, propylene was injected into the reactor at constant rate as low as 10 mL/h, or 19 mg/h. This made the system extremely starved of monomer, and the polymer yield equals to the weight of injected monomer within the experimental error. The weight of polymer can thus be estimated from the injection rate and duration of reaction. Eight polymerization runs with injection rate of 10, 20, 30, 40, 50, 60, 80, and 100 mL/h were made, and the PP yields agreed well with the weight of injected monomer. This means that both the chain propagation and chain transfer with monomer have been greatly retarded. 3.1. Suppression of chain transfer to cocatalyst In the monomer-starved polymerization system, chain transfer with cocatalyst (AlEt3 in this work) may become the dominant termination reaction in the absence of hydrogen [6,14,24,25], as the AlEt3 concentration will be much higher than [M]. As reported in literatures, the product of transfer with alkylaluminum, the
aluminum-terminated PP chains, can be converted to hydroxyterminated PP chains after quenching the polymerization system with oxygen and subsequent hydrolysis [24,25]. The hydroxy end groups can be easily detected in 1 H NMR spectrum of the polymer. Fig. 3 shows the effect of alkylaluminum on the distribution of end groups of PP synthesized under monomer-starved conditions. When AlEt3 is remained in the system after contacting with the catalyst, large amount of OH-terminated chain-ends was found, as shown in Fig. 3(d). The peaks around 3.4 ppm are assigned to ␣-H of CH(CH3 )CH2 OH. The fine structure of the triplet might indicate sterical inhomogeneity between the OH-terminated chain-ends. The intensity of this triplet is much stronger than the peaks of olefinic chain-ends at 4.5–6.0 ppm, indicating that chain transfer with AlEt3 could be a highly interfering factor to understanding the other chain transfer reactions. Fortunately, by removing most of the cocatalyst after catalyst activation, the triplets of OH-terminated chain-ends are largely diminished, and the peaks at 4.5–6.0 ppm are evidently intensified (see Fig. 3b and c). Chain transfer to cocatalyst was efficiently suppressed by washing the activated catalyst. Therefore, propylene polymerization under monomer-starved conditions and very low [AlEt3 ] may be suitable for studying chain transfer reactions other than the transfers with monomer and cocatalyst. By GPC analysis of the molecular weight of PP synthesized under monomer-starved conditions and very low [AlEt3 ], it was found that the total number of polymer chains by GPC agrees well with the sum of different olefinic end groups (1-propen-2-yl, 1-propen3-yl, 2-buten-4-yl, and 2-hexen-2-yl) within experimental errors (see Fig. 2 and Table 1). These results agree with Fig. 3 that implies absent of chain transfer with the cocatalyst. 3.2. Assignment of 1 H NMR signals and formation mechanism of the end groups Polypropylene obtained in this research has relatively low isotacticity. Fig. 4 shows the influence of monomer addition rate on tacticity of polymer which is expressed as content of meso diads [m] and racemic diads [r] estimated from signals of methylene in 1 H NMR spectra [26]. Unlike some examples of metallocene system [27], tacticity of polypropylene synthesized by washed TiCl4 /MgCl2 –AlEt3 catalyst is not sensitive to monomer concentration despite of the slight decrease of isotacticity with increase of monomer addition rate. Fig. 5 compares the 1 H NMR spectra of PP synthesized under starved conditions with that of a sample prepared under conventional mode, namely, with continuous supply of 1 atm propylene to the reactor. It is seen that the peaks at 5.0–5.4 ppm which are ignorable under 1 atm monomer pressure have been greatly intensified by using the new method. By comparing with similar signals in PP
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Table 1 Molecular weight of polypropylene. Monomer addition rate (mL/h) 30 50 80 a b c d
Mn a 7380 13,870 17,180
PDI
Oleb (mol/104 mol C3)
Mn c
17.43 9.57 11.66
24.09 29.31 25.65
17,440d 14,330 16,370
Number average molecular weight determined by GPC. Content of olefinic groups calculated from 1 H NMR data. Number average molecular weight of polypropylene calculated from Ole. This deviation might be attributed to presence of unidentified low-molecular-weight impurity in the sample.
Fig. 3. 1 H NMR spectra of polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10 and [Al] = 0.05 mol/L) at 60 ◦ C and monomer injection rate 30 mL/h for 1 h: (a) AlEt3 removed after catalyst activation and polymerization terminated with water; (b) AlEt3 removed after catalyst activation and polymerization quenched by O2 and then terminated by water; (c) AlEt3 removed after catalyst activation and polymerization (70 ◦ C) quenched by O2 and terminated by water; (d) AlEt3 remained after catalyst activation and polymerization quenched by O2 and terminated by water. 1H
prepared with metallocene catalysts [4,28,29], they were assigned to three kinds of internal olefinic groups. The strongest peak in this range, namely the one around 5.25 ppm can be assigned to 2-buten4-yl with olefinic hydrogen f and g, which should be formed by -hydride transfer after a secondary insertion. This kind of chainend has not been observed in PP synthesized with heterogeneous Ziegler–Natta catalysts before. The triplet centered at 5.07 ppm was assigned to 2-hexen-2-yl with olefinic hydrogen j according to literatures 18 and 19. This chain-end was found to be closely related to 2-buten-4-yl as shown in the next section, and tentative mechanistic pathways (Scheme 1) have been proposed to explain its origin. As illustrated in (3) and (4) of Scheme 1, 2-buten-4-yl might be produced by -H transfer after 2,1-insertion, and 2-hexen-2-yl might be produced by isomerization of 2-buten-4-yl in a manner of “chain-walking” reaction. This is supported by their concurrence and similar trends of content profiles which are discussed in the next section. According to Scheme 1(4), an intermediate chain-end, 3-buten-4-yl with olefinic hydrogen h and i should also exist. The
NMR signals of 3-buten-4-yl were most probably overlapped with the stronger signals of 2-buten-4-yl for the similar chemical environment of their olefinic hydrogen atoms. It is most likely that the signals from hydrogen h and i are much weaker than the 2-buten-4-yl. The doublet around 4.6 ppm was assigned to 1-propen-2-yl (vinylidene) [4] which should be produced by -H transfer after 1,2insertion as illustrated in Scheme 1(1). The triplet around 4.9 ppm and multiplet around 5.7 ppm were assigned to olefinic hydrogen
Fig. 4. Tacticity of polypropylene expressed as content of meso diads [m] and racemic diads [r] calculated from 1 H NMR data. Polypropylene was synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
Fig. 5. Olefinic region of 1 H NMR spectra of polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10) that was then washed, and polymerization was conducted at 80 ◦ C for 1 h: (1) polymerization was conducted under continuous 1 atm monomer supply; (2) monomer was injected at 60 mL/h.
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c, d and e of 1-propen-3-yl (allyl) which might be produced by Me transfer after 1,2-insertion [4]. This is supported by the clear presence of isobutyl in 13 C NMR spectra (peaks between 22 and 27 ppm [15]) of Fig. 1. Isobutyl could be produced by 1,2-insertion to Ti Me that is formed in methyl-transfer. The abundance of isobutyl end-group is in consistence with the strong intensity of 1-propen3-yl. It must be mentioned that, theoretically, -hydride transfer from the terminal CH3 of the propagation center after 2,1-insertion might also happen, as -hydride transfer from the methylene has been identified (Scheme 1(3)). However, the corresponding product, 1-buten-4-yl (4-butenyl) chain-end, was not detected in all the PP samples. Similar phenomenon has also been reported in several metallocene catalyzed polypropylene [4,13,30–32]. We will try to explain this phenomenon through further studies. 3.3. Monomer-dependence of chain transfer reactions In the chain transfer reactions depicted in Scheme 1, both the -H transfer after a 1,2-insertion (1) and -H transfer after a 2,1insertion (3) as well as the -Me transfer after a 1,2-insertion may be assisted by the monomer. In this case, the reaction becomes bimolecular process, and the rate of chain transfer (Rtr ) follows the equation Rtr = ktr [C*][M], where ktr is the chain transfer rate constant, [C*] is the concentration of active center and [M] is the monomer concentration. On the other hand, in -H transfer without monomer assistance, the reaction will be unimolecular and a first order kinetic law (Rtr = ktr [C*]) should be observed. When the chain propagation rate obeys the conventional equation: Rp = kp [C*][M], we can estimate the fraction of chain-ends in the polymer (Fe ) for the unimolecular transfer reactions: Fe =
ktr [C ∗ ] ktr = kp [C ∗ ][M] kp [M]
(1)
and for the bimolecular reactions: Fe =
ktr [C ∗ ][M] ktr = kp [C ∗ ][M] kp
(2)
where Fe is molar ratio of end groups to the monomer units of the polymer. It is clear that in unimolecular processes the chain-ends content will increase with decreasing monomer concentration, while in bimolecular processes the chain-ends content will keep at a constant level. Based on Eqs. (1) and (2), we tried to find the monomer dependences of the different chain transfer reactions by changing the monomer injection rate in the range of 10–100 mL/h. Even though all the polymerization runs are monomer-starved, relatively lower injection rate will still cause lower [M] in the system. The changes of chain-ends content vs. the monomer injection rate are shown in Fig. 6. The data of PP synthesized under 1 atm monomer pressure were also measured for reference. As the absolute [M] cannot be determined in such experiments, the results may be thought as qualitative. As illustrated in Fig. 6, the content of 1-propen-2-yl (vinylidene) was almost constant in the whole range of [M], implying that -H transfer after a 1,2-insertion is a bimolecular process, and this transfer can be identified as -H transfer to monomer. This is in agreement with literature observations on heterogeneous Ziegler–Natta catalytic systems [22,23]. The content of 1-propen-3-yl (allyl) decreased slowly with increasing [M], but still remained at a rather high level when the system was not severely starved of monomer (1 atm propylene). This behavior may be explained based on the multi-site character of the catalysis system. It is likely that some types of active centers undergo unimolecular -Me transfer, and some undergo
Fig. 6. Content and amount of each kind of olefinic end groups in polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
bimolecular -Me transfer reaction. Under 1 atm propylene pressure, most of the 1-propen-3-yl chain-ends may be formed via bimolecular process. It should be noted that -Me transfer in metallocene systems was considered as a unimolecular pathway [8,33]. The bimolecular character of a part of -Me transfer reactions in MgCl2 -supported Ziegler–Natta catalyst might reflect its difference with metallocene systems. The content of 3-buten-4-yl was neglected because of its low value compared to that of 2-buten-4-yl. So the content of 2-buten-4-yl in Fig. 6 was calculated from the integral value of all peaks in the region of 5.41–5.17 ppm. It’s changes with [M] implies unimolecular character of the -H transfer after a 2,1-insertion. However, comparing the signals of 5.2–5.4 ppm in PP by monomer-starved polymerization and by 1 atm polymerization (see Fig. 7), it is found that trans-2-buten-4-yl end groups (5.26 ppm) was predominant among the two isomers of this olefinic chain-end when the polymerization was severely monomer-starved, while the cis-2-buten-4-yl became the dominant isomer when the polymer was synthesized under 1 atm
Fig. 7. The variations of 2-buten-4-yl 1 H NMR signals with monomer addition rate. Polypropylene was synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
Y. Yu et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 134–139
propylene pressure. As proposed by Resconi et al., trans-2-buten-4yl is formed via unimolecular -H transfer to the transition metal after a 2,1-insertion, and cis-2-buten-4-yl is formed via bimolecular -H transfer to the monomer after a 2,1-insertion [31]. This mechanism is in agreement with the tendency of Fig. 6. Under monomer-starved conditions, the changes of 2-buten-4-yl content do follow the law of an unimolecular process. However, the emerging of cis-2-buten-4-yl at higher monomer concentration seems a sudden turnover and cannot be reasonably explained at present. The changes of 2-hexen-2-yl content with [M] (see Fig. 6) show the same trend as that of 2-buten-4-yl, though its number is always smaller than 2-buten-4-yl. This can be taken as an evidence supporting the formation of 2-hexen-2-yl by isomerization of 2-buten-4-yl. It is still not clear whether or not both isomers of 2-buten-4-yl could be involved in these isomerization reactions. Summarizing the results of chain transfer study through monomer-starved polymerization, the main difference with the conventional polymerization is strong enrichment of chain-ends formed by -H transfer after a 2,1-insertion. This enabled us not only investigate the mechanism of such transfer reactions, but also have an estimation of the frequency of secondary propylene insertion, which is a tough task in the conventional polymerization systems. This method can be used to study the regioselectivity of propylene polymerization reaction. 4. Conclusions Chain transfer reactions of TiCl4 /MgCl2 –AlEt3 -catalyzed propylene polymerization were investigated under conditions of severely starved monomer supply and suppressed chain transfer to cocatalyst. Besides 1-propen-2-yl end group formed by -H transfer after 1,2-insertion, 1-propen-3-yl end group formed by -Me transfer after 1,2-insertion and 2-buten-4-yl formed by -H transfer after 2,1-insertion also exist in the polymer. Small amount of 2-hexen-2-yl end group was also detected, which may be explained by isomerization of the 2-buten-4-yl chain-ends. The 1-propen-2-yl end group is produced by monomer-assisted H transfer after 1,2-insertion, the 1-propen-3-yl end group is produced by mixed bimolecular and unimolecular -Me transfer reactions after 1,2-insertion, and 2-buten-4-yl is mainly produced by unimolecular -H transfer after 2,1-insertion. Because of the unimolecular nature of -H transfer after 2,1-insertion, the content of 2-buten-4-yl end group, which is too low to be detected in PP polymerized under conventional conditions, can be markedly increased in the product of monomer-starved polymerization.
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Acknowledgements The authors thank Dr. Jun Ling (Zhejiang University) and Prof. Takeshi Shiono (Hiroshima University) for their suggestions about the assignments of 1 H NMR spectra. Support by the Major State Basic Research Programs (grant no. 2011CB606001) and National Natural Science Foundation of China (grant no. 20874084) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Chain transfer reactions of propylene polymerization catalyzed by AlEt3 activated TiCl4 /MgCl2 catalyst under very low monomer addition rate Yue Yu, Zhisheng Fu, Zhiqiang Fan ∗ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 19 December 2011 Received in revised form 22 April 2012 Accepted 27 May 2012 Available online 9 June 2012 Keywords: Propylene polymerization MgCl2 -supported Ziegler–Natta catalyst Chain transfer reactions Secondary insertion
a b s t r a c t Chain transfer reactions of TiCl4 /MgCl2 –AlEt3 catalyzed propylene polymerization were investigated under conditions of severely starved monomer supply and suppressed chain transfer to cocatalyst. Slurry polymerization was conducted by injecting gaseous monomer at very low rate to the reactor containing preactivated catalyst. The preactivated solid catalyst has been washed by solvent to remove AlEt3 in the liquid phase, and chain transfer to AlEt3 was nearly completely suppressed. Besides 1-propen2-yl (vinylidene) end group formed by -H transfer after primary (1,2-)insertion, 1-propen-3-yl (allyl) end group formed by -Me transfer after 1,2-insertion and 2-buten-4-yl formed by -H transfer after secondary (2,1-)insertion were also detected in the polymer by 1 H NMR analysis. The monomer dependences of the chain transfer reactions were studied. Because of the unimolecular nature of -H transfer after a secondary insertion, the content of 2-buten-4-yl end group, which is too low to be detected in PP polymerized under conventional conditions, was markedly increased in the product of monomer-starved polymerization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It has been more than 40 years since the discovery of MgCl2 supported Ziegler–Natta catalyst, which nowadays still dominates in polypropylene (PP) production. Although it might be considered as a simple system judging by the composition, the reaction mechanism has not been adequately elucidated yet [1–3]. Since the 1990s, chain transfer reactions in olefin polymerization with metallocene catalysts have been studied in detail [4–13]. However, study of chain transfer reactions in MgCl2 supported catalyst systems was limited, partly because of the difficulties in tracing the low level of active sites in these heterogeneous systems. Studying chain-ends of polypropylene has been demonstrated a feasible way to investigate chain transfer reactions of MgCl2 -supported catalyst system [14,15]. In order to intensify the signals of the chain-ends, this approach requires decreasing the polymer’s molecular weight by either raising the polymerization temperature or adding large amount of hydrogen. The method will have limited accuracy and gives little information of mechanism when applied to systems forming high molecular weight product. As reported by Chadwick et al., chain transfer with monomer (or monomer assisted -H abstraction) is the dominant chain transfer reaction in propylene polymer-
∗ Corresponding author. Tel.: +86 571 87952400; fax: +86 571 87952400. E-mail address: [email protected] (Z. Fan). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.05.027
ization under conventional conditions [14,15]. They have also observed chain transfer with hydrogen after either primary or secondary insertions. However, they were unable to detect 2butenyl end groups that are formed by -H abstraction after a secondary propylene insertion [14,15]. Chain transfer with the alkylaluminum cocatalyst was found to be not so evident under conventional conditions, but becomes important at high temperature [14,15]. 13 C NMR was employed as a standard technique in studying the metallocene systems mentioned above, but it is not suitable for the MgCl2 -supported Ziegler–Natta system which has highly clean 13 C NMR spectra of the polymer (shown in Fig. 1) providing little information about chain transfers. For the analysis of olefinic end groups formed in many chain transfer reactions, 1 H NMR is much more sensitive than 13 C NMR. For this reason, 1 H NMR spectra have been taken as the main source of information in this work. In this work, we attempted to shed light on those chain transfer reactions that are greatly depressed by the overwhelming transfer reactions (transfer with the monomer and alkylaluminum) under conventional conditions. To achieve this, we have applied a new method of propylene slurry polymerization in order to relatively enhance the chain transfer reactions other than the chain transfer with monomer and with cocatalyst. Using a simple TiCl4 /MgCl2 catalyst preactivated with triethylaluminum (AlEt3 ), propylene polymerization was conducted by injecting the gaseous monomer into the reactor at very low rate. This enabled us to detect more
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Fig. 1. Typical 13 C NMR spectra of PP produced by TiCl4 /MgCl2 –AlEt3 under conditions described in the experimental part. The peak of [mmmm] methyl at 21.94 ppm was chosen as internal reference. (1) Crude product; (2) fraction of PP that is insoluble in n-octane at room temperature; (3) fraction of PP that is soluble in n-octane at room temperature.
types of olefinic end groups like 2-butenyl in the produced polymer by means of 1 H NMR. As this 2-butenyl end group can only be formed by -H transfer after 2,1-insertion, the method can be used to study the regioselectivity of the active species. Further more, chain transfer and isomerization reactions of active centers with a 2,1-inserted last unit can also be studied in this way. It is well known that regioselectivity strongly influences the reaction kinetics and formation of chain structure in propylene polymerization, like hydrogen response of the catalyst and melting temperature of the produced PP. Busico et al. measured the regioselectivity of MgCl2 -supported Ziegler–Natta catalysts through 13 C NMR analysis of the product of propylene/ethylene-[1-13 C] copolymerization [16–19]. However, possible chain transfers at propagation center with a 2,1 last-inserted monomeric unit can only be traced by characterizing the chain ends. It is still necessary to develop new approaches for better understanding this important aspect of Ziegler–Natta catalysis.
2. Experimental 2.1. Materials Polymerization-grade propylene was supplied by Yangzi Petrochemical Company (Nanjing, China), and further purified by passing through columns of 4A molecular sieves and deoxygenation catalyst. High purity nitrogen (99.999%) was used as a protective gas. Oxygen (99.6%) was used as quenching reagent without further purification. Anhydrous MgCl2 (Alfa Aesar Co.) and triethylaluminum (AlEt3 , Albemarle Corp.) were used as received. TiCl4 (Shanghai Meixing Chemical Industry Limited Company, Shanghai, China) was distilled under N2 . n-Heptane (Yixing No.2 Chemical Reagent Factory, Yixing, China) and n-octane (Shanghai Lingfeng Chemical Reagent Corporation, Shanghai, China) as solvent were refluxed over sodium and distilled.
2.2. Catalyst preparation 70 g anhydrous MgCl2 and 52 stainless steel balls (diameter 10 mm) were evenly distributed in 4 milling cans (inner diameter 53 mm × height 58 mm) and sealed under N2 , and then milled in a QM-1F planetary ball-mill at 250 r/min for 48 h. 35 mL TiCl4 and 39 g ball-milled anhydrous MgCl2 was placed into a 100-mL flask under N2 atmosphere. The mixture was stirred at 110 ◦ C for 6 h, then washed with 50 mL n-heptane, and finally vacuum-dried at 70 ◦ C for 30 min. Titanium content of the catalyst determined by conventional spectrophotometry method (colorized by H2 O2 ) was 3.4 wt%.
2.3. Polymerization procedure n-Octane, AlEt3 and 0.3 ± 0.01 g TiCl4 /MgCl2 catalyst were added in sequence into a 100-mL Schlenk flask at 60 ◦ C, with Al/Ti = 10 (mol/mol) and [AlEt3 ] = 0.05 mol/L. After stirring for 2 min, the suspension was left still for another 2 min to settle down the solid catalyst. The liquid phase was then siphoned out and 10 mL fresh n-octane was injected into the flask to wash the catalyst. The liquid phase was siphoned out again and another 10 mL fresh n-octane was injected into the flask. Polymerization was started by steadily inject gaseous propylene into the flask at rates of 10–100 mL/h through a long needle using a syringe pump. During the reaction, the reaction suspension was severely stirred and the tip of needle was placed at 1 cm above the liquid surface to ensure uniform dispersion of propylene in the liquid phase. After 1 h, the polymerization was terminated by injecting acidic water (1% hydrochloric acid) into the flask. Suspension of the product (fine solid particles) was repeatedly washed with acidic water and then distilled water. Polymer was obtained by vacuum-drying the suspension. Some polymerization runs were quenched by 100 mL oxygen before being terminated by acidic water to produce hydroxyl-capped polypropylene. A polymerization run with continuous supply of 1 atm propylene gas to the flask has also been made. 2.4. Characterization 1 H NMR spectra of PP were recorded on a Varian Mercury plus 300 spectrometer operated at 300 MHz in the pulse Fourier transform mode. S2pul pulse with acquisition time of 1.9 s and relaxation delay of 1 s was adopted, and 3000–6000 scans were accumulated depending on the spectra quality. The spectra were obtained at 120 ◦ C in o-dichlorobenzene-d4, using the peak at downfield of o-dichlorobenzene-d3 (7.14 ppm) as internal reference. 13 C NMR spectra of the PP samples were recorded at 120 ◦ C with the same instrument at 75 MHz. o-Dichlorobenzene-d4 was used as the solvent to prepare the polymer solution of 10 wt.%. Chromium triacetylacetonate (about 2 mg) was added in each sample to shorten the relaxation time. Broadband decoupling with a pulse delay of 3 s was employed. Typically, 5000 transients were collected. Molecular weight of the PP samples was measured by gel permeation chromatograph in a PL 220 GPC instrument (Polymer Laboratories Ltd.) at 150 ◦ C in 1,2,4-trichlorobenzene. Three PL mixed B columns (500–107 ) were used. Universal calibration against narrow polystyrene standards was adopted.
3. Results and discussion It has been reported that -hydride transfer to the monomer after a primary insertion is the most preferred way of chain
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Fig. 2. Effect of addition rate of monomer on molecular weight distribution of polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
Scheme 1. Possible formation mechanisms of the olefinic end groups.
termination in propylene polymerization with heterogeneous Ziegler–Natta catalyst [20–23]. Under conventional polymerization conditions, most of the olefinic chain-ends are 1-propen-2-yl (vinylidene) group formed in this pathway, and end groups like 2butenyl that is formed through -hydride transfer after a secondary insertion (see Eq. (3) of Scheme 1) cannot be detected, owing to very low probability of 2,1-insertion in these systems [17–19]. However, if the reaction forming 2-butenyl is an unimolecular process, one may increase the 2-butenyl/1-propen-2-yl ratio by greatly lowering the monomer concentration. Of course, the contents of other end groups from unimolecular reactions will also be increased. This method has been used in this article. As described in the experimental part, propylene was injected into the reactor at constant rate as low as 10 mL/h, or 19 mg/h. This made the system extremely starved of monomer, and the polymer yield equals to the weight of injected monomer within the experimental error. The weight of polymer can thus be estimated from the injection rate and duration of reaction. Eight polymerization runs with injection rate of 10, 20, 30, 40, 50, 60, 80, and 100 mL/h were made, and the PP yields agreed well with the weight of injected monomer. This means that both the chain propagation and chain transfer with monomer have been greatly retarded. 3.1. Suppression of chain transfer to cocatalyst In the monomer-starved polymerization system, chain transfer with cocatalyst (AlEt3 in this work) may become the dominant termination reaction in the absence of hydrogen [6,14,24,25], as the AlEt3 concentration will be much higher than [M]. As reported in literatures, the product of transfer with alkylaluminum, the
aluminum-terminated PP chains, can be converted to hydroxyterminated PP chains after quenching the polymerization system with oxygen and subsequent hydrolysis [24,25]. The hydroxy end groups can be easily detected in 1 H NMR spectrum of the polymer. Fig. 3 shows the effect of alkylaluminum on the distribution of end groups of PP synthesized under monomer-starved conditions. When AlEt3 is remained in the system after contacting with the catalyst, large amount of OH-terminated chain-ends was found, as shown in Fig. 3(d). The peaks around 3.4 ppm are assigned to ␣-H of CH(CH3 )CH2 OH. The fine structure of the triplet might indicate sterical inhomogeneity between the OH-terminated chain-ends. The intensity of this triplet is much stronger than the peaks of olefinic chain-ends at 4.5–6.0 ppm, indicating that chain transfer with AlEt3 could be a highly interfering factor to understanding the other chain transfer reactions. Fortunately, by removing most of the cocatalyst after catalyst activation, the triplets of OH-terminated chain-ends are largely diminished, and the peaks at 4.5–6.0 ppm are evidently intensified (see Fig. 3b and c). Chain transfer to cocatalyst was efficiently suppressed by washing the activated catalyst. Therefore, propylene polymerization under monomer-starved conditions and very low [AlEt3 ] may be suitable for studying chain transfer reactions other than the transfers with monomer and cocatalyst. By GPC analysis of the molecular weight of PP synthesized under monomer-starved conditions and very low [AlEt3 ], it was found that the total number of polymer chains by GPC agrees well with the sum of different olefinic end groups (1-propen-2-yl, 1-propen3-yl, 2-buten-4-yl, and 2-hexen-2-yl) within experimental errors (see Fig. 2 and Table 1). These results agree with Fig. 3 that implies absent of chain transfer with the cocatalyst. 3.2. Assignment of 1 H NMR signals and formation mechanism of the end groups Polypropylene obtained in this research has relatively low isotacticity. Fig. 4 shows the influence of monomer addition rate on tacticity of polymer which is expressed as content of meso diads [m] and racemic diads [r] estimated from signals of methylene in 1 H NMR spectra [26]. Unlike some examples of metallocene system [27], tacticity of polypropylene synthesized by washed TiCl4 /MgCl2 –AlEt3 catalyst is not sensitive to monomer concentration despite of the slight decrease of isotacticity with increase of monomer addition rate. Fig. 5 compares the 1 H NMR spectra of PP synthesized under starved conditions with that of a sample prepared under conventional mode, namely, with continuous supply of 1 atm propylene to the reactor. It is seen that the peaks at 5.0–5.4 ppm which are ignorable under 1 atm monomer pressure have been greatly intensified by using the new method. By comparing with similar signals in PP
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Table 1 Molecular weight of polypropylene. Monomer addition rate (mL/h) 30 50 80 a b c d
Mn a 7380 13,870 17,180
PDI
Oleb (mol/104 mol C3)
Mn c
17.43 9.57 11.66
24.09 29.31 25.65
17,440d 14,330 16,370
Number average molecular weight determined by GPC. Content of olefinic groups calculated from 1 H NMR data. Number average molecular weight of polypropylene calculated from Ole. This deviation might be attributed to presence of unidentified low-molecular-weight impurity in the sample.
Fig. 3. 1 H NMR spectra of polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10 and [Al] = 0.05 mol/L) at 60 ◦ C and monomer injection rate 30 mL/h for 1 h: (a) AlEt3 removed after catalyst activation and polymerization terminated with water; (b) AlEt3 removed after catalyst activation and polymerization quenched by O2 and then terminated by water; (c) AlEt3 removed after catalyst activation and polymerization (70 ◦ C) quenched by O2 and terminated by water; (d) AlEt3 remained after catalyst activation and polymerization quenched by O2 and terminated by water. 1H
prepared with metallocene catalysts [4,28,29], they were assigned to three kinds of internal olefinic groups. The strongest peak in this range, namely the one around 5.25 ppm can be assigned to 2-buten4-yl with olefinic hydrogen f and g, which should be formed by -hydride transfer after a secondary insertion. This kind of chainend has not been observed in PP synthesized with heterogeneous Ziegler–Natta catalysts before. The triplet centered at 5.07 ppm was assigned to 2-hexen-2-yl with olefinic hydrogen j according to literatures 18 and 19. This chain-end was found to be closely related to 2-buten-4-yl as shown in the next section, and tentative mechanistic pathways (Scheme 1) have been proposed to explain its origin. As illustrated in (3) and (4) of Scheme 1, 2-buten-4-yl might be produced by -H transfer after 2,1-insertion, and 2-hexen-2-yl might be produced by isomerization of 2-buten-4-yl in a manner of “chain-walking” reaction. This is supported by their concurrence and similar trends of content profiles which are discussed in the next section. According to Scheme 1(4), an intermediate chain-end, 3-buten-4-yl with olefinic hydrogen h and i should also exist. The
NMR signals of 3-buten-4-yl were most probably overlapped with the stronger signals of 2-buten-4-yl for the similar chemical environment of their olefinic hydrogen atoms. It is most likely that the signals from hydrogen h and i are much weaker than the 2-buten-4-yl. The doublet around 4.6 ppm was assigned to 1-propen-2-yl (vinylidene) [4] which should be produced by -H transfer after 1,2insertion as illustrated in Scheme 1(1). The triplet around 4.9 ppm and multiplet around 5.7 ppm were assigned to olefinic hydrogen
Fig. 4. Tacticity of polypropylene expressed as content of meso diads [m] and racemic diads [r] calculated from 1 H NMR data. Polypropylene was synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
Fig. 5. Olefinic region of 1 H NMR spectra of polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10) that was then washed, and polymerization was conducted at 80 ◦ C for 1 h: (1) polymerization was conducted under continuous 1 atm monomer supply; (2) monomer was injected at 60 mL/h.
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c, d and e of 1-propen-3-yl (allyl) which might be produced by Me transfer after 1,2-insertion [4]. This is supported by the clear presence of isobutyl in 13 C NMR spectra (peaks between 22 and 27 ppm [15]) of Fig. 1. Isobutyl could be produced by 1,2-insertion to Ti Me that is formed in methyl-transfer. The abundance of isobutyl end-group is in consistence with the strong intensity of 1-propen3-yl. It must be mentioned that, theoretically, -hydride transfer from the terminal CH3 of the propagation center after 2,1-insertion might also happen, as -hydride transfer from the methylene has been identified (Scheme 1(3)). However, the corresponding product, 1-buten-4-yl (4-butenyl) chain-end, was not detected in all the PP samples. Similar phenomenon has also been reported in several metallocene catalyzed polypropylene [4,13,30–32]. We will try to explain this phenomenon through further studies. 3.3. Monomer-dependence of chain transfer reactions In the chain transfer reactions depicted in Scheme 1, both the -H transfer after a 1,2-insertion (1) and -H transfer after a 2,1insertion (3) as well as the -Me transfer after a 1,2-insertion may be assisted by the monomer. In this case, the reaction becomes bimolecular process, and the rate of chain transfer (Rtr ) follows the equation Rtr = ktr [C*][M], where ktr is the chain transfer rate constant, [C*] is the concentration of active center and [M] is the monomer concentration. On the other hand, in -H transfer without monomer assistance, the reaction will be unimolecular and a first order kinetic law (Rtr = ktr [C*]) should be observed. When the chain propagation rate obeys the conventional equation: Rp = kp [C*][M], we can estimate the fraction of chain-ends in the polymer (Fe ) for the unimolecular transfer reactions: Fe =
ktr [C ∗ ] ktr = kp [C ∗ ][M] kp [M]
(1)
and for the bimolecular reactions: Fe =
ktr [C ∗ ][M] ktr = kp [C ∗ ][M] kp
(2)
where Fe is molar ratio of end groups to the monomer units of the polymer. It is clear that in unimolecular processes the chain-ends content will increase with decreasing monomer concentration, while in bimolecular processes the chain-ends content will keep at a constant level. Based on Eqs. (1) and (2), we tried to find the monomer dependences of the different chain transfer reactions by changing the monomer injection rate in the range of 10–100 mL/h. Even though all the polymerization runs are monomer-starved, relatively lower injection rate will still cause lower [M] in the system. The changes of chain-ends content vs. the monomer injection rate are shown in Fig. 6. The data of PP synthesized under 1 atm monomer pressure were also measured for reference. As the absolute [M] cannot be determined in such experiments, the results may be thought as qualitative. As illustrated in Fig. 6, the content of 1-propen-2-yl (vinylidene) was almost constant in the whole range of [M], implying that -H transfer after a 1,2-insertion is a bimolecular process, and this transfer can be identified as -H transfer to monomer. This is in agreement with literature observations on heterogeneous Ziegler–Natta catalytic systems [22,23]. The content of 1-propen-3-yl (allyl) decreased slowly with increasing [M], but still remained at a rather high level when the system was not severely starved of monomer (1 atm propylene). This behavior may be explained based on the multi-site character of the catalysis system. It is likely that some types of active centers undergo unimolecular -Me transfer, and some undergo
Fig. 6. Content and amount of each kind of olefinic end groups in polypropylene synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
bimolecular -Me transfer reaction. Under 1 atm propylene pressure, most of the 1-propen-3-yl chain-ends may be formed via bimolecular process. It should be noted that -Me transfer in metallocene systems was considered as a unimolecular pathway [8,33]. The bimolecular character of a part of -Me transfer reactions in MgCl2 -supported Ziegler–Natta catalyst might reflect its difference with metallocene systems. The content of 3-buten-4-yl was neglected because of its low value compared to that of 2-buten-4-yl. So the content of 2-buten-4-yl in Fig. 6 was calculated from the integral value of all peaks in the region of 5.41–5.17 ppm. It’s changes with [M] implies unimolecular character of the -H transfer after a 2,1-insertion. However, comparing the signals of 5.2–5.4 ppm in PP by monomer-starved polymerization and by 1 atm polymerization (see Fig. 7), it is found that trans-2-buten-4-yl end groups (5.26 ppm) was predominant among the two isomers of this olefinic chain-end when the polymerization was severely monomer-starved, while the cis-2-buten-4-yl became the dominant isomer when the polymer was synthesized under 1 atm
Fig. 7. The variations of 2-buten-4-yl 1 H NMR signals with monomer addition rate. Polypropylene was synthesized with AlEt3 activated TiCl4 /MgCl2 (Al/Ti = 10). Polymerization was conducted at 60 ◦ C for 1 h after washing the activated catalyst.
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propylene pressure. As proposed by Resconi et al., trans-2-buten-4yl is formed via unimolecular -H transfer to the transition metal after a 2,1-insertion, and cis-2-buten-4-yl is formed via bimolecular -H transfer to the monomer after a 2,1-insertion [31]. This mechanism is in agreement with the tendency of Fig. 6. Under monomer-starved conditions, the changes of 2-buten-4-yl content do follow the law of an unimolecular process. However, the emerging of cis-2-buten-4-yl at higher monomer concentration seems a sudden turnover and cannot be reasonably explained at present. The changes of 2-hexen-2-yl content with [M] (see Fig. 6) show the same trend as that of 2-buten-4-yl, though its number is always smaller than 2-buten-4-yl. This can be taken as an evidence supporting the formation of 2-hexen-2-yl by isomerization of 2-buten-4-yl. It is still not clear whether or not both isomers of 2-buten-4-yl could be involved in these isomerization reactions. Summarizing the results of chain transfer study through monomer-starved polymerization, the main difference with the conventional polymerization is strong enrichment of chain-ends formed by -H transfer after a 2,1-insertion. This enabled us not only investigate the mechanism of such transfer reactions, but also have an estimation of the frequency of secondary propylene insertion, which is a tough task in the conventional polymerization systems. This method can be used to study the regioselectivity of propylene polymerization reaction. 4. Conclusions Chain transfer reactions of TiCl4 /MgCl2 –AlEt3 -catalyzed propylene polymerization were investigated under conditions of severely starved monomer supply and suppressed chain transfer to cocatalyst. Besides 1-propen-2-yl end group formed by -H transfer after 1,2-insertion, 1-propen-3-yl end group formed by -Me transfer after 1,2-insertion and 2-buten-4-yl formed by -H transfer after 2,1-insertion also exist in the polymer. Small amount of 2-hexen-2-yl end group was also detected, which may be explained by isomerization of the 2-buten-4-yl chain-ends. The 1-propen-2-yl end group is produced by monomer-assisted H transfer after 1,2-insertion, the 1-propen-3-yl end group is produced by mixed bimolecular and unimolecular -Me transfer reactions after 1,2-insertion, and 2-buten-4-yl is mainly produced by unimolecular -H transfer after 2,1-insertion. Because of the unimolecular nature of -H transfer after 2,1-insertion, the content of 2-buten-4-yl end group, which is too low to be detected in PP polymerized under conventional conditions, can be markedly increased in the product of monomer-starved polymerization.
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Acknowledgements The authors thank Dr. Jun Ling (Zhejiang University) and Prof. Takeshi Shiono (Hiroshima University) for their suggestions about the assignments of 1 H NMR spectra. Support by the Major State Basic Research Programs (grant no. 2011CB606001) and National Natural Science Foundation of China (grant no. 20874084) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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