Effects of Methylaluminoxane Modifications on Tuning the Bis(Imino)Pyridyl Iron-Catalyzed Oligomerization of Ethylene.
Linear [alpha]-olefins (LAO) are industrially highly demand chemicals, which are widely used for the production of polyolefins, plasticizers, detergents, synthetic lubricants, and many other important products. They are now produced mainly by the oligomerization of ethylene . Among the numerous catalysts for this reaction, bis(imino)pyridyl (BIP) iron catalysts [2-4] with small mono-ortho-alkyls on the N-aryl rings have exhibited extremely high activity and selectivity to the target LAOs. However, the resulting ethylene oligomers of such BIP-Fe catalysts are usually featured with a very broad distribution ranging from [C.sub.4] to [C.sub.30+]. And the proportion of polymers, can be higher than 40 wt% in the total products even the iron complexes with mono ortho-methyl substituted BIP ligands were employed [5, 6]. These undesired insoluble polymers would cause reactor fouling and pipeline blockage in a continuous process, which is a potential obstacle for the industrialization of the BIP-Fe catalysts. Therefore, it is of great importance to develop more selective oligomerization catalysts to reduce the simultaneous formation of insoluble polymers.
The nature of the active species is a decisive factor for the reaction behaviors and the product properties. It thus becomes important to solve the above-mentioned problem by mediating the nature or the micro-chemical environment of the active species. Tuning the ligand structure is believed a direct method for such purpose [7-10]. In order to reduce the polymer formation, the relative reaction ratio of chain propagation to chain termination should be decreased. A potential solution is to introduce electron-withdrawing groups into the ligand framework. Several BIP-Fe catalysts with fluorine atoms on the ortho-positions of the N-aryl rings were proved to trigger no ethylene to polymers [11-14]. This is due to the strong electron-withdrawing effect of the fluorine atoms, which reduces the electron density of iron centers and promotes the Fe-[H.sub.[beta]] interactions, thus accelerating the [beta]-H transfer.
Meanwhile, the nature of co-catalysts could also affect the active species [15, 16]. As we know, the BIP iron catalysts must be activated with cationizing co-catalysts for the active species to form. Methylaluminoxane (MAO) and modified MAOs serve as the most efficient co-catalysts at present. After the activation process, MAO would exist as a counter anion [[Me-MAO].sup.-], which has significant interactions with the cationic iron centers . Although the structure and detailed functioning principles of MAO still remain enshrouded by mystery, many modification methods can be applied to modify its structure, and further modulate the active ion pairs and their catalytic performance .
Recently, our group proposed an effective strategy for retarding the simultaneous polymer formation during ethylene oligomerization by MAO modification with phenolic compounds [19-21]. Owing to the reactions between phenols and Al[Me.sub.3 or the Al-Me moieties of MAO, these phenol-based treatments can effectively scavenge Al[Me.sub.3] in MAO and decorate phenoxy groups on MAO. As also evidenced by Busico and co-workers [22, 23]. such effects would efficiently tune the composition of MAO, and further make a pronounced change on its size, structure, Lewis acidity and reactivity. Most importantly, our results showed that the decoration of phenoxy groups would significantly reduce the product molecular weight and retard the polymer formation. The existence of phenoxy groups may lead to a better separation of the active cationanion ion pairs, a recent report on the pentafluorophenyl- or pentafluorophenoxy-modified solid polymethylaluminoxanes also mentioned this effect . Although this would be beneficial for the monomer insertion, the [beta]-H transfer would be accelerated at the same time, since the bimolecular [beta]-H transfer to monomer reaction has been proposed as the main termination process for the BIP-iron systems . In addition, a better ion-pair separation would decrease the steric hindrance on the active species, thus lowering the energy barriers for an attractive Fe-[H.sub.[beta]] interaction and the [beta]-H transfer, finally shifting the product distribution towards lighter olefins.
In this work, based on the previous results of MAO modification with various phenolic compounds, more MAO modifiers that have a phenol-like structure will be further investigated. Aiming at exploring the polymer-retarding mechanism of the modifiers and revealing the structural necessity for a suitable modifier, a detailed discussion on the relationship between the modifier structures and the oligomerization activity/product distribution will be present.
All manipulations involving air- or moisture-sensitive compounds were carried out under an atmosphere of nitrogen using standard Schlenk techniques or in a nitrogen glove-box. Solvents (toluene, n-heptane) were dried over 4 A molecular sieves for at least 10 days, and then further purified by solvent purification system of Innovative Technology (PS-400-5-SD, USA). Iron(III) acetylacetonate (Fe[(acac).sub.3], 99%), 2,6-diacetylpyridine (97%), 4-methoxy-2-methylaniline (98%), 1-naphthol (1-NaphOH) (99%), and 2-naphthol (2-NaphOH) (99%) were purchased from J&K Chemical Corp. (Shanghai, China); Phenol (99%), anisole (98%), benzyl alcohol (99%), and benzoic acid (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd.; Cyclohexanol (99%) and cyclohexyl carbinol (99%) were purchased from Aladdin Industrial Corp. (Shanghai, China). All of these chemicals were used as received. MAO (10 wt% solution in toluene) was purchased from Albermarle Chemical Corp. (Baton Rouge, LA) and stored at -18[degrees]C. High-purity nitrogen and polymerization-grade ethylene were obtained from SINOPEC Shanghai Corp. (Shanghai, China) and were passed through the oxygen- and moisture-scrubbing columns prior to use.
Synthesis of the Ligand
The BIP ligand (L) used in this work was synthesized according to the established [procedure,.sup.4] as shown in Scheme 1. 4-methoxy-2-methylaniline (2.06 g, 15.02 mmol) was added to a solution of 2,6-diacetylpyridine (1.20 g, 7.35 mmol) in absolute ethanol (30 mL). After the addition of three drops of acetic acid (glacial), the solution was refluxed for 48 h. Upon cooling to room temperature the product crystallized from ethanol, it was then filtered, washed with cold ethanol and dried in a vacuum oven (50[degrees]C) overnight. Yield: 77%. This ligand was then characterized with [sup.1]H NMR, MS, and elemental analyses, which can be referred to our previous work .
Preparation of the Catalyst Precursor
Throughout this study, we have employed an in situ formed catalyst system composed of Fe(acac)3 and the BIP ligand L. They should be premixed, and an optimal molar ratio is 1:1 according to our previous work . The homogeneous orangered mixture in toluene was therefore prepared, abbreviated as L-Fe[(acac).sub.3]. The concentration of the catalyst precursor in toluene, in terms of iron atoms [Fe], was 4 [micro]mol/mL.
Oligomerizations were carried out in a 250-mL glass reactor equipped with an ethylene inlet, vacuum line and magnetic stirrer. The reactor was heated by a heat gun for at least 15 min and purged with three cycles of nitrogen-vacuum. After cooling to the reaction temperature (50[degrees]C), 50 mL of toluene, the desired amount of MAO and modifiers were successively charged into the reactor, which were allowed to react for 20 min under [N.sub.2] protection. The reaction was monitored by the bubbling (mainly methane) via an oil-filled bubbler. An agitation speed around 700 rpm was adopted. Nitrogen was then removed after 20 min and a continuous flow of ethylene was introduced to make the system saturated with ethylene. To maintain a constant 1.0 bar pressure of ethylene over the reaction, the ethylene was allowed to exit via the oil-filled bubbler. The oligomerization was then initiated by injection of a solution of the catalyst precursor in toluene. At the end of a run, the reaction was terminated by addition of 3 mL of acidified ethanol and the reactor was cooled down to room temperature. The product mixture was then centrifuged to separate the oligomers in toluene and the insoluble polyethylene wax. The wax was first put on a filter paper and washed sufficiently with toluene to drip down the light fraction in the filtrate, which was then merged into the oligomers. Second, the wax was washed with ethanol to remove the residual MAO, then filtered, dried, and weighed. An internal standard (n-heptane, 1.0 mL) was injected into the resultant liquid phase.
The catalytic activity was quantified by the amount of products. For oligomers, individual olefins were identified by Gas chromatography-Mass spectrometry (GC-MS) (Nicolet HP GC6890/MS5973). [C.sub.12] olefin isomers were used to determine the linearity. Further quantitative analyses were performed by Agilent GC 6890 equipped with a HP-5 column (5% phenylmethylsiloxane, 30 m X 0.32 mm X 0.25 [micro]m) using n-heptane as the internal standard. The injector temperature was 320[degrees]C and the following temperature program was: 50[degrees]C/2 min, 50-300[degrees]C/20[degrees]C x [min.sup.-1], 300[degrees]C/10 min. The yield of volatile [C.sub.4] fraction was determined by extrapolation of [alpha] value. The [alpha] value is a characteristic coefficient of Schulz-Flory distribution, which can be calculated from the relative rate of chain propagation and chain termination or the quotient of the molar amounts of two subsequent oligomer fractions, [C.sub.14] and [C.sub.12] in this case (Eq. 1).
[alpha] = rate of propagation/rate of propagation + rate of termination = mol ([C.sub.n+2])/mol ([C.sub.n]) (1)
RESULTS AND DISCUSSION
The Necessity of Hydroxyl Group for Modifiers
The oligomerizations using pre-catalyst L-[Fe(acac).sub.3] were initiated by MAO activation employing a molar ratio Al/Fe = 1,000 and routinely performed at a temperature of 50[degrees]C over 30 min. As shown in Run 1, Table 1, the system L-[Fefacac).sub.3]/MAO triggered ethylene to LAO and polymers with exceptionally high activity. Noteworthy, the insoluble polymers accounted for a proportion higher than 30 wt% in the total products. To reduce the polymer formation, phenol (PhOH) was employed as a MAO modifier as reported in our previous work . Herein, the results of PhOH-mediated oligomerization are present in Table 1 (Runs 2-6) as a reference. It can be seen that as the molar ratio of phenol to MAO ([PhOH]/[Al]) increased from 0 to 0.7, the mass fraction of insoluble polymers in the total products decreased from 31.9 to 5.5 wt%. The activity for a-olefins, at the meantime, remained very high with a remarkable increase compared with that of the system without phenol. Moreover, the Schulz-Flory coefficient a was decreased upon addition of phenol, indicating that the phenol modification would enhance the chain termination, thus producing lower molecular weight products.
However, if anisole (PhOMe) was introduced as the MAO modifier, its influence on the oligomerization activity and product composition was much slighter. As evident from Runs 7-11, Table 1, with the molar ratio of [PhOMe]/[Al] increased, the activity and the polymer share remained high and comparable to the non-modifier system, only a slight decrease was observed. Therefore, the importance of hydroxyl group in the MAO modification process can be confirmed. As we have mentioned in the "Introduction section", PhOH can react with Al[Me.sub.3] and the Al-Me moieties of MAO, leading to Al[Me.sub.3] elimination and phenoxy-decoration on MAO, accompanied by the release of methane. These reactions make PhOH an efficient modifier in tuning the composition and structure of MAO. But for PhOMe, since there is no reactive hydrogen, the interactions between it and MAO would mainly rely on the non-covalent O [right arrow] Al bonds due to the electron-donating nature of the -OMe group. Such interactions are weak, unlike the phenoxy group directly grafted on MAO; the PhOMe on MAO can be easily liberated, making it difficult to well separate the active ion pairs. But at the same time, the PhOMe-mediated system remained high activity even when [PhOMe]/[Al] was up to 0.9 due to the less reactive -OMe group. While in the case of PhOH, a further increase of [PhOH]/[Al] to 0.9 greatly deactivated the catalyst, this may be due to the interaction between the free phenols and the active centers or a further decomposition of MAO by extra phenols. Therefore, the phenol modifier should be used with a controlled dosage.
Influence of the Aromatic Ring Size on Ethylene Oligomerization
In order to enhance the polymer-retarding effect of the modifiers under a controlled dosage, we found that bulkier alkyl groups could be introduced into the para-position of phenol . This was proved to effectively separate the active ion pairs when the corresponding phenoxy groups decorated on MAO and be beneficial for further reducing the polymer share in the total products. In this work, the influence of the aromatic ring size of the phenolic compounds will also be explored. The results of ethylene oligomerization mediated by 2-NaphOH are listed in Table 1.
Runs 12-15 give the effects of 2-NaphOH on the product distribution and oligomerization activity. First, as the molar ratio of [2-NaphOH]/[Al] increased from 0 to 0.7, the mass fraction of insoluble polymers was rapidly decreased. Such value was as low as ~1 wt% when the [2-NaphOH]/[Al] ratio was just reached 0.3. As shown in Fig. 1, it can be concluded that 2-NaphOH is a more effective polymer-retarding modifier than phenol. 2-NaphOH can retard the polymer formation to a much lower level with a lower dosage than phenol. Meanwhile, the olefin distribution was greatly shifted towards lower molecular weight olefins upon addition of 2-NaphOH. As depicted in Fig. 2, a significant increment of the [C.sub.4]-[C.sub.12] olefin contents was observed when the [2-NaphOH]/[Al] ratio increased, indicating that the relative rate of chain propagation to chain termination was reduced with the 2-NaphOH modification. These observations confirm that the phenolic compound with a bigger aromatic ring would have a stronger polymer-retarding effect.
The influence of 2-NaphOH in the product distribution is closely related to the interactions between the cationic iron centers and the anionic [[Me-MAO].sup.-] clusters. It can be imaged that the large anions would impose remarkable steric hindrance on the iron centers. While in ethylene oligo-/polymerization by BIP-Fe catalysts, the chain-transfer rate is known to be disfavored by steric hindrance in the axial positions of the propagating Fe-alkyl species [4, 11, 25, 28-31]. Concerning the addition of 2-NaphOH, it is possible that some of the phenolic hydroxyl groups would react with the Al-Me bonds on MAO, leading to the formation of NaphO-decorated MAO. As illustrated in Fig. 3, the modified MAO would turn into a novel [[NaphO-MAO-Me].sup.-] anion, which has a weaker coordinating ability and lesser tendency to aggregate with the cationic species due to the big naphthalene ring than the original [[Me-MAO].sup.-] anion after pre-catalyst activation, thus giving rise to a better separation of the active ion pairs. Therefore, the chain termination via [beta]-H transfer to form lower molecular weight products can be promoted.
Influence of Modifiers' Acidity on Ethylene Oligomerization
Apart from the influence on the product distribution, 2-NaphOH also greatly affected the oligoemrizaion activity. It was found that when the [2-NaphOH]/[Al] ratio was up to 0.3, the activity was decreased to nearly one third of the non-modifier system. A further increase of the [2-NaphOH]/[Al] ratio to 0.7 made the activity much lower. Although 2-NaphOH has a better polymer-retarding ability than phenol, its deactivation effect on the reaction activity is also more remarkable than that of phenol. In fact, from phenol to 2-NaphOH, not only the size of the aromatic rings is changed, but also the acidity of the phenolic hydroxyl groups. Therefore, 1-NaphOH was also employed as the modifier and the results are present in Runs 16-19, Table 1.
The oligomerization results of [1-NaphOH]/[Al] = 0.1-0.3 were comparable to those of the 2-NaphOH-modified systems with the same modifier dosage, but relatively lower values of polymer share and oligomerization activity were observed. In particular, when [1-NaphOH]/[Al] = 0.3, the insoluble polymers only accounted for 0.2 wt% in the total products, much lower than that of the PhOH-modified system. This verifies that the phenolic modifiers with a larger aromatic ring size have a better polymer-retarding effect. However, when the [1-NaphOH]/[Al] ratio was increased to 0.5, the catalytic activity was dramatically decreased. The deactivation effect on activity was even stronger upon addition of 1-NaphOH than 2-NaphOH.
Since the derealization of [pi] electrons on the naphthalene ring is not as uniform as that on the benzene ring, the electron density of the 1-position carbon and the 2-position carbon is different. The p-[pi] conjugation effect between the oxygen atom of 1-NaphOH and the naphthalene ring is stronger than that of 2-NaphOH. This leads to the stronger C-O bond and weaker O-H bond in 1-NaphOH, further resulting in the stronger acidity of 1-NaphOH. As shown in Table 2, the acidity of the modifiers follows the order: PhOH < 2-NaphOH < 1-NaphOH. When used in the catalytic system, the maximum molar ratio of [Mod.]/[Al] was gradually decreased from PhOH to 1 -NaphOH in order to remain the activity at a relatively high level. Upon addition of PhOH. high activity was observed at [PhOH]/[Al] = 0.7, while in the case of 1-NaphOH, relatively high activity was only observed al [l-NaphOH]/[Al] = 0.3. In other words, with the increase of the acidity of the phenolic modifiers, the dosage that can largely deactivate the catalytic system is reduced. Noteworthy, the ability to retard the production of insoluble polymers, at the same time, is enhanced. It can be speculated that the acidity of the modifiers plays an important role in tuning the oligomerization activity and product composition.
Generally, the reactions between phenols and MAO or Al[Me.sub.3] are parallel . a phenolic modifier with stronger acidity would thus allow that more active hydroxyl groups react with MAO, leading to more NaphO- groups being grafted on MAO. Besides, the decorated NaphO- groups are bulkier than the PhO- groups. Therefore, with the acidity increased from phenol to 2-NaphOH, and then to 1-NaphOH, a better separation of the active ion pairs can be expected, thus enhancing the polymer-retarding ability of the modifiers. However, the more aryloxy groups bound to MAO, at the same time, means that more methyl groups of MAO will be replaced. Meanwhile, the existence of NaphO- groups may shield the methyl groups nearby and make the modified MAO difficult to contact with the pre-catalyst. These effects will decrease the alkylating ability of the modified MAO at the stage of pre-catalyst activation, resulting in the rapid decline of activity with the increasing dosage of naphthols. Of cause, the reactions between naphthols and MAO are very complicated, it is also reasonable that the naphthols would cause the structure re-arrangement, collapse or decomposition of MAO.
To further study the relationship between the modifier's acidity and the catalytic behaviors, benzoic acid (PhCOOH) with similar structure to phenol was also employed as the modifier. The results are present Table 1 (Runs 20-23). As we can see, when the molar ratio of [PhCOOH]/[Al] was 0.1, the catalytic activity was significantly increased compared with that of the non-modifier system, which was similar to the results when phenol and naphthol were used with the same dosage. Meanwhile, the polymer share was decreased from 31.9 to 23.3 wt%. Concerning the polymer-retarding effect of various modifiers at [Mod.]/[Al] = 0.1 in Table 1, it can be concluded that a modifier with strong acidity able to reduce the polymer formation even when its dosage is low. When the [PhCOOH]/[Al] ratio was up to 0.3, nearly no polymers were produced. Meanwhile, as shown in Fig. 4, the olefin distribution was greatly shifted towards lower molecular weight olefins after the addition of benzoic acid. This indicates that the rate of chain termination is accelerated. However, as the [PhCOOH]/[Al] ratio increased, the activity was decreased rapidly. The activity of [PhCOOH]/[Al] = 0.3 was reduced to even <1,000 kg*[(mol-Fe*h).sup.-1]. Given that the pKa of PhCOOH is 4.20, the acidity is much stronger than phenol and naphthol, the result of PhCOOH-modified system was, to some extent, in good agreement with the relationship between the modifier's acidity and the catalytic behaviors discussed above. A modifier with strong acidity would be beneficial for retarding the polymer formation, but it may greatly deactivate the catalyst at the same time.
Noteworthy, when thinking about the case of PhCOOH, what we should keep in minds is that the carboxyl group is quite different from the hydroxyl group in their reactivities. According to the studies on the reactions of carboxyl acids and trialkylaluminum reported by Deffieux and co-workers [32-34]. even the reaction between PhCOOH and Al[Me.sub.3[ are more complicated than those between phenols and Al[Me.sub.3]. As shown in Scheme 2, both the OH and C=O bonds of PhCOOH can participate in the reactions with Al[Me.sub.3], four molecules of Al[Me.sub.3 can be consumed by one molecule of PhCOOH. Therefore, it can be imaged that the reactions between PhCOOH and MAO would be much more complex, the high reactivity and acidity of PhCOOH may lead to a big change in the structure and composition of MAO, which can be unfavorable for the catalytic system.
Influence of Phenol-like Alcohols on Ethylene Oligomerization
Herein, we also tried to explore the effects of alcohol modifiers, cyclohexanol (CH-OH) bearing a phenol-like structure was then studied. The results are listed in Runs 24-28 Table 3. When the [CH-OH]/[Al] ratio was 0.1, the activity was as high as 4,280 kg* [(mol-Fe*h).sup.-1], higher than that of the system with no modifiers. This indicates that a small amount of CH-OH has a similar effect as the phenols, it can react with Al[Me.sub.3] and eliminate Al[Me.sub.3 from the catalytic system, thus increasing the activity. However, when the [CHOH]/[Al] ratio was increased to 0.3, the system was largely deactivated. Therefore, different [CH-OH]/[Al] ratios between 0.1 and 0.3 are further explored. It was found that the CH-OH indeed served as a polymer-retarding modifier, the corresponding CH-O- groups could also be grafted on MAO. With the [CH-OH]/[Al] ratio increased from 0 to 0.2, the polymer share was reduced. However, the activity decreased rapidly at the same time. Another two alcohol modifiers, cyclohexyl carbinol (CH-C[H.sub.2]OH) and benzyl alcohol (Ph-C[H.sub.2]OH), were also investigated (Table 3). The results of these two modifiers were similar to CH-OH, high activity was only observed when the dosage was low. An increase of the [Mod.]/ [Al] ratio to 0.3 would remarkably deactivate the catalytic system. It was found that the deactivation effect of these modifiers was stronger than their polymer-retarding effect, making it difficult to obtain a satisfactory low polymer formation with the catalytic system remaining a high activity. Although their structures are more or less similar to the structure of phenol, their chemical nature is still alcohols. When reacting with MAO, they tend to destroy the MAO, rather than providing a suitable modification for MAO.
In this article, various chemicals like phenol, anisole, naphthols, benzoic acid, and phenol-like alcohols were employed as the MAO modifiers to tune the oligomerization activity and product distribution of the BIP-Fe catalyst. By means of exploring the necessity of the hydroxyl group, the influence of the aromatic ring size, the acidity, and the phenolic/non-phenolic framework of the modifiers, the optimal structures for a suitable modifier were discussed. It was found that the modifiers could react with the Al[Me.sub.3] containing in MAO and provide MAO with pronounced surface and structure modifications. This would further modify the nature of the active species and effectively tune the catalytic behaviors. Of cause, a structure re-arrangement or even decomposition of MAO would occur when the amount of modifiers was increased, thus leading to the deactivation of the catalysts. We can conclude from these results that the phenolic compounds are better modifiers, various substituent modifications on their framework can be achieved to provide them suitable steric size and acidity. This work is part of our project on screening the efficient polymer-retarding modifiers for the BIP-iron ethylene oligomerization catalysts. Such strategy based on MAO modifications is simple and effective, and can largely tune the product composition without complicated modifications on the BIP ligands. More studies on the relationship between the nature of the modifiers and the structure and reactivity of MAO are in progress.
The authors gratefully acknowledge the support and encouragement of China Postdoctoral Science Foundation (2018 M632466) and National Natural Science Foundation of China (21176208 and U1663222).
[1.] P.-A.R. Breuil, L. Magna, and H. Olivier-Bourbigou, Catal. Lett., 145, 173 (2015).
[2.] G.J.P. Britovsek, V.C. Gibson, B.S. Kimberley, P.J. Maddox, S. J. McTavish, G.A. Solan, A.J.P. White, and D.J. Williams, Chem. Commun., 849 (1998).
[3.] B.L. Small and M. Brookhart, J. Am Chem. Soc., 120. 7143 (1998).
[4.] G.J.P. Britovsek, S. Mastroianni, G.A. Solan, S.P.D. Baugh, C. Redshaw, V.C. Gibson, A.J.P. White, D.J. Williams, and M. R.J. Elsegood, Chem.-Eur. J., 6, 2221 (2000).
[5.] B. Su and G. Feng, Polym. Int., 59, 1058 (2010).
[6.] Z. Boudene, A. Boudier, P.-A.R. Breuil, H. Olivier-Bourbigou, P. Raybaud, H. Toulhoat, and T. de Bruin, J. Catal., 317, 153 (2014).
[7.] A. Boudier, P.A. Breuil, L. Magna, H. Olivier-Bourbigou, and P. Braunstein, Chem. Commun., 50, 1398 (2014).
[8.] B.L. Small, Acc. Chem. Res., 48, 2599 (2015).
[9.] Z. Flisak and W.-H. Sun, ACS Catal., 5, 4713 (2015).
[10.] Z. Wang, G.A. Solan, W. Zhang, and W.-H. Sun, Coord. Chem. Rev., 363, 92 (2018).
[11.] Y. Chen, C. Qian, and J. Sun, organometallics, 22, 1231 (2003).
[12.] Z. Zhang, J. Zou, N. Cui, Y. Ke, and Y. Hu, J. Mol. Catal. A Chem., 219, 249 (2004).
[13.] Z. Zhang, S. Chen, X. Zhang, H. Li, Y. Ke, Y. Lu, and Y. Hu, J. Mol. Catal. A Chem., 230, 1 (2005).
[14.] G. Xie, T. Li, and A. Zhang, Inorgan. Chem. Commun., 13, 1199 (2010).
[15.] E.Y.-X. Chen and T.J. Marks, Chem. Rev., 100, 1391 (2000).
[16.] M. Bochmann, organometallics, 29, 4711 (2010).
[17.] K.P. Bryliakov, E.P. Talsi, N.V. Semikolenova, and V. A. Zakharov, organometallics, 28, 3225 (2009).
[18.] H.S. Zijlstra and S. Harder, Eur. J. Inorgan. Chem., 2015, 19 (2015).
[19.] J. Ye, B.B. Jiang, Y.C. Qin, W. Zhang, Y.M. Chen, J.D. Wang, and Y.R. Yang, RSC Adv., 5, 95981 (2015).
[20.] W. Zhang, J. Ye, B.B. Jiang, J.D. Wang, Z.W. Liao, Z.L. Huang, Y. R. Yang, and Z.B. Ye, Macromol. React. Eng., 12, 1700061 (2018).
[21.] W. Zhang, B.B. Jiang, J. Ye, Z.W. Liao, Z.L. Huang, J.D. Wang, and Y.R. Yang, Chin. J. Polym. Sci., 36, 1207 (2018).
[22.] V. Busico, R. Cipullo, F. Cutillo, N. Friederichs, S. Ronca, and B. Wang, J. Am. Chem. Soc., 125, 12402 (2003).
[23.] L. Rocchigiani, V. Busico, A. Pastore, and A. Macchioni, Dalton Trans., 42, 9104 (2013).
[24.] A.F.R. Kilpatrick, N.H. Rees, S. Sripothongnak, J.-C. Buffet, and D. O'Hare, Organometallics, 37, 156 (2018).
[25.] L. Deng, P. Margl, and T. Ziegler, J. Am. Chem. Soc., 121, 6479 (1999).
[26.] J. Ye, B.B. Jiang, J.D. Wang, Y.R. Yang, and Q. Pu, J. Polym. Sci. A Polym. Chem., 52, 2748 (2014).
[27.] J.D. Wang, W. Li, B.B. Jiang, and Y.R. Yang, J. Appl. Polym. Sci., 113, 2378 (2009).
[28.] G.J.P. Britovsek, M. Bruce, V.C. Gibson, B.S. Kimberley, P. J. Maddox, S. Mastroianni, S.J. McTavish, C. Redshaw, G. A. Solan, S. Stromberg, A.J.P. White, and D.J. Williams, J. Am. Chem. Soc., 121, 8728 (1999).
[29.] Y. Chen, R. Chen, C. Qian, X. Dong, and J. Sun, organometallics, 22, 4312 (2003).
[30.] M.E. Bluhm, C. Folli, and M. Doring, J. Mol. Catal. A Chem., 212, 13 (2004).
[31.] C. Bianchini, G. Giambastiani, I.R. Guerrero, A. Meli, E. Passaglia, and T. Gragnoli, organometallics, 23, 6087 (2004).
[32.] H. Cramail, K. Radhakrishnan, and A. Deffieux, C. R. Chimie, 5, 49 (2002).
[33.] T. Dalet, H. Cramail, and A. Deffieux, Macromol. Chem. Phys., 205, 1394 (2004).
[34.] J. Tudella, M.R. Ribeiro, H. Cramail, and A. Deffieux, Macromol. Chem. Phys., 208, 815 (2007).
Jian Ye (iD), (1) Yu Li, (1) Binbo Jiang (iD), (1) Wei Zhang, (1) Jingdai Wang, (1,2) Zuwei Liao, (1,2) Zhengliang Huang, (1) Yongrong Yang, (1,2) Zhibin Ye (3)
(1) Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
(2) State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
(3) Department of Chemical and Materials Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada
Correspondence to: B. Jiang; e-mail: email@example.com
Contract grant sponsor: China Postdoctoral Science Foundation; contract grant number: 2018M632466. contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 21176208; U1663222.
Caption: SCHEME 1. Synthesis of 2,6-bisf 1 -(4-methoxy-2-methylphenylimino)ethyllpyridine (L).
Caption: FIG. 1. Influence of the [-OH]/[All ratio on the mass fraction of insoluble polymers (PhOH and 2-NaphOH as the modifier respectively). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 2. Olefin distributions obtained with 2-NaphOH as the modifier. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. Proposed active ion-pair species of L-[Fe(acac).sub.3]/MAO system with or without the modification of 2-NaphOH (MAO is referred to as a linear chain structure for the sake of simplicity). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 4. Olefin distributions obtained with PhCOOH as the modifier. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: SCHEME 2. Reactions between PhCOOH and Al[Me.sub.3. [321.
TABLE 1. Influence of various modifiers on oligomerization promoted by L-Fe[(acac).sub.3]/MAO (a). Run Modifier [mod.]/[Al] (mod.) [Mol]/[Mol] 1 0 2 0.1 3 0.3 4 0.5 5 0.7 6 0.9 7 0.1 8 0.3 9 0.5 10 0.7 11 0.9 12 0.1 13 0.3 14 0.5 15 0.7 16 0.1 17 0.3 18 0.5 19 0.7 20 0.1 21 0.3 22 0.5 23 0.7 Oligomer Run [Y.sub.o] (c) [alpha] % LAO (d) [g] 1 2.35 0.76 98 2 2.62 0.77 98 3 3.03 0.79 97 4 3.71 0.75 97 5 2.90 0.73 98 6 0.12 0.73 99 7 1.96 0.79 98 8 2.64 0.76 98 9 2.28 0.78 98 10 2.12 0.77 99 11 2.05 0.75 99 12 2.59 0.75 97 13 1.00 0.70 97 14 1.31 0.70 98 15 0.33 0.69 98 16 2.47 0.74 97 17 1.11 0.69 97 18 0.02 0.69 82 19 -- -- -- 20 3.02 0.78 98 21 0.71 0.70 98 22 0.25 0.70 97 23 0.27 0.70 97 Polymer Run [Y.sub.p] (e) wt% PE (f) Act. (b) [g] 1 1.10 31.9 3,450 2 1.33 33.7 3,950 3 1.26 29.4 4,290 4 0.52 12.3 4,230 5 0.17 5.5 3,070 6 Trace -- 120 7 1.09 35.7 3,050 8 1.21 31.4 3,850 9 1.02 30.9 3,300 10 0.85 28.6 2,970 11 0.78 27.6 2,830 12 1.11 30.0 3,700 13 0.011 1.1 1,011 14 0.013 1.0 1.323 15 0.002 0.6 332 16 0.69 21.8 3,160 17 0.002 0.2 1,112 18 Trace -- 20 19 -- -- -- 20 0.92 23.3 3,940 21 Trace -- 710 22 Trace -- 250 23 Trace -- 270 (a) General conditions: pre-cat.: [Fe] = 2 [micro]mol; Al/Fe = 1,000; solvent: toluene, 50 mL; T = 50CC; P = 1 bar; t = 30 min. (b) Total Activity: kg x [(mol-Fe).sup.-1] x [h.sup.-1]. (c) Yield of the soluble olefins, determined by GC. (d) % LAO content in oligomers determined by GC. (e) Yield of the insoluble polymers. (f) The mass fraction of insoluble polymers in the total products. TABLE 2. Relationship between the acidity of the modifiers and the mass fraction of polymers. Modifier pKa (a) Max. [mod.]/[Al] (b) wt% PE PhOH 9.98 0.7 5.5 2-NaphOH 9.51 0.5 1.0 1-NaphOH 9.34 0.3 0.2 (a) Acid dissociation constant, pKa = -[log.sub.10]([[H.sup.+]][[A.sup.-]]/[HA]). (b) The highest ratio of [Mod.]/[Al] when activity >1,000 kg x [(mol-Fe x h).sup.-1] TABLE 3. Influence of phenol-like modifiers on oligomerization promoted by L-Fe[(acac).sub.3]/MAO (a). Run Modifier [mod.]/[Al] (mod.) [Mol]/[Mol] 1 None 0 24 0.1 25 0.15 26 0.18 27 0.2 28 0.3 29 0.1 30 0.2 31 0.3 32 0.1 33 0.2 34 0.3 Oligomer Run [Y.sub.o] (c) [alpha] % LAO (d) [g] 1 2.35 0.76 98 24 2.72 0.75 98 25 2.26 0.72 98 26 0.47 0.70 99 27 0.17 0.70 98 28 0.07 0.70 97 29 2.20 0.80 98 30 2.68 0.72 98 31 0.04 0.71 99 32 1.84 0.77 98 33 2.39 0.74 98 34 0.23 0.70 99 Polymer Run [Y.sub.p] (e) wt% Act. (b) [g] PE (f) 1 1.10 31.9 3,450 24 1.56 36.4 4,280 25 0.47 17.2 2,730 26 0.06 11.3 530 27 0.002 1.2 172 28 Trace -- 70 29 1.81 45.1 4,010 30 0.55 17.0 3,230 31 -- -- 40 32 0.80 30.3 2,640 33 0.57 19.3 2,960 34 0.001 0.4 231 (a) General conditions: pre-cat.: [Fe] = 2 [micro]mol; Al/Fe = 1,000; solvent: toluene, 50 mL; T = 50[degrees]C; P = 1 bar; t = 30 min. (b) Total Activity: kg x [(mol-Fe).sup.-1] x [h.sup.-1]. (c) Yield of the soluble olefins, determined by GC. (d) % LAO content in oligomers determined by GC. (e) Yield of the insoluble polymers. (f) The mass fraction of insoluble polymers in the total products.
|Printer friendly Cite/link Email Feedback|
|Author:||Ye, Jian; Li, Yu; Jiang, Binbo; Zhang, Wei; Wang, Jingdai; Liao, Zuwei; Huang, Zhengliang; Yang, Yon|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2019|
|Previous Article:||Specific Influence of Polyethersulfone Functionalization on the Delamination Toughness of Modified Carbon Fiber Reinforced Polymer Processed by Resin...|
|Next Article:||Electron Beam Irradiation to Recrosslink Devulcanized Sulfur Cured Rubber Blended Polypropylene.|