Next Article in Journal
Tissue Engineering Bionanocomposites Based on Poly(propylene fumarate)
Next Article in Special Issue
Use of Vanadium Complexes Bearing Naphthalene-Bridged Nitrogen-Sulfonate Ligands as Catalysts for Copolymerization of Ethylene and Propylene
Previous Article in Journal
Preparation and Property Evaluation of Conductive Hydrogel Using Poly (Vinyl Alcohol)/Polyethylene Glycol/Graphene Oxide for Human Electrocardiogram Acquisition
Previous Article in Special Issue
Isothermal and Non-Isothermal Crystallization Studies of Long Chain Branched Polypropylene Containing Poly(ethylene-co-octene) under Quiescent and Shear Conditions
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Polymers
Volume 9
Issue 7
10.3390/polym9070262
polymers-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Homo- and Copolymerizations of Ethylene and Norbornene Using Bis(β-ketoamino) Titanium Catalysts Containing Pyrazolone Rings

by
Lixia Pei
1,2,
San He
1,
Jie Gao
3,
Heng Liao
3 and
Haiyang Gao
3,*
1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610550, China
2
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
3
School of Materials Science and Engineering, PCFM Lab, GD HPPC Lab, Sun Yat-sen University, Guangzhou 510275, China
*
Author to whom correspondence should be addressed.
Polymers 2017, 9(7), 262; https://doi.org/10.3390/polym9070262
Submission received: 14 June 2017 / Revised: 27 June 2017 / Accepted: 28 June 2017 / Published: 30 June 2017
(This article belongs to the Special Issue Olefin Polymerization and Polyolefin)
Browse Figures
Versions Notes

Abstract

A series of bis(β-ketoamino) titanium complexes containing pyrazolone rings (1–3) have been synthesized, characterized, and used as precursors for homo- and copolymerization of ethylene and norbornene. The titanium complexes activated with methylaluminoxane (MAO) exhibited good activities for homopolymerization of ethylene (E) to produce linear polyethylenes (PEs). Ethylene–norbornene copolymers (E–N) were also prepared by these catalysts with moderate activities, and influences of ligand substituents and norbornene addition on copolymer microstructure were studied in detail. Microstructure analysis of the E–N copolymers by 13C NMR and differential scanning calorimetry (DSC) techniques showed that alternating (ENEN) and isolated (ENEE) norbornene predominately appeared in the copolymer chain, and the NN dyad and NNN triad sequences were also present in the copolymers obtained by the less bulky catalyst 1.

Graphical Abstract

1. Introduction

Cyclic olefin copolymers (COCs) of ethylene (E) with cyclic olefins, especially norbornene (N), representing a new class of amorphous materials has attracted great interest over the past decade because of their remarkable properties, such as high vapor and thermal resistance, excellent optical transparency, and high refractive indexes [1,2,3,4,5]. In fact, these material properties can be precisely controlled by varying monomer composition, sequence distribution, and the chain stereoregularity, which closely depends on the employed catalyst structure. Both the ligand environment and the center metal atom type play a major role in the copolymerization of ethylene and norbornene.
Driven by industrial applications, copolymerizations of norbornene and ethylene have been performed using various transition metal catalysts including titanium [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27], zirconium [28,29], nickel [30,31,32,33,34,35,36,37,38], palladium [39,40,41,42,43,44,45], chromium [46,47,48], vanadium [49], and rare earth metal catalysts [50,51]. Group IV metal catalysts, especially titanium based catalysts, receive more attention among these different metal based catalysts. For instance, ethylene–norbornene (E–N) copolymers were first obtained by Kaminsky with metallocene/methylaluminoxane (MAO) [6,7,8], and then half sandwich and constrained-geometry titanium catalysts (CGCs) developed by Dow Chemical were widely used in the copolymerization of ethylene and norbornene [9,10,11,12,13,14]. Usually, random E–N copolymers with high norbornene incorporations up to 70 mol % are obtained with these titanium catalysts. Non-metallocene titanium complexes activated by MAO or modified methylaluminoxane (MMAO) are also used to effectively catalyze copolymerization of ethylene and norbornene [15,16,17,18,19,20,21,22,23,24,25]. In particular, titanium catalysts bearing bis(pyrrolide-imino) [16], bis(imino-indolido) [18], and bis(β-enaminoketonato) ligands [19] are reported to catalyze ethylene and norbornene copolymerization in a living fashion. Although non-metallocene titanium catalysts usually produce alternating E–N copolymers, the bis(α-alkyloxoimine) titanium catalyst facilitates random copolymers with high norbornene incorporation up to 76 mol %, because the electron-negative alkoxide group makes the titanium center prone to be attacks by the norbornene monomer [27]. These differences of the E–N copolymer microstructure highlight the powerful ligand influence on the reactivity of titanium catalysts and the potential reward for continued efforts to uncover new type of ligands and catalysts.
Schiff base derivatives containing pyrazolone are important members of the β-ketoamines ligand family because of their ease of preparation and modification of both steric and/or electronic effects. β-Ketoamine ligands containing the pyrazolone ring are easily coordinated to various metal atoms, and nickel-, cobalt-, and copper-based complexes have been synthesized and used to catalyze olefin polymerizations in the presence of MAO [52,53,54,55,56,57,58,59]. To the best of our knowledge, early transition metal complexes chelating β-ketoamine ligands containing pyrazolone rings have not been synthesized and studied for their catalytic properties. Herein, we have reported novel bis(β-ketoamino) titanium precursors containing pyrazolone rings for homo- and copolymerizations of ethylene and norbornene. The influences of catalyst structure and norbornene addition on copolymerization activity and incorporation of norbornene in copolymers have been investigated in detail.

2. Experiment

All manipulations involving air- and moisture sensitive compounds were performed under dried and purified nitrogen (99.999%) using standard vacuum-line, Schlenk, or dry glovebox techniques.

2.1. Materials

All solvents were purified using standard procedures. n-Butyllithium (n-BuLi) solution in hexane (2.2 M) was purchased from Aldrich (Saint Louis, MO, USA). Norbornene (bicyclo[2.2.1] hept-2-ene; Acros) was purchased from Acros (Koblenz, Germany). and purified by distillation over potassium metal and used as a solution in toluene. Methylaluminoxane (MAO) solution (10 wt % in toluene) was purchased from Acros. Titanium tetrachloride (TiCl4) was distilled prior to use. Ethylene gas (polymerization grade) was further purified by passage through columns of molecular sieves. Other commercial reagents were purchased and directly used without purification. Ligands HL1 (5-methyl-2-phenyl-4-[(2-phenylamino)-phenylmethylene]pyrazol-3(2H) -one), HL2 (5-methyl-2-phenyl-4-[(2-o-tolyl)-phenylmethylene]pyrazol-3(2H)-one), and HL3 (5-methyl-2-phenyl-4-[(2-α-naphthylamino)-phenylmethylene]pyrazol-3(2H)-one) were prepared according the reported method [57].

2.2. Measurements

Elemental analyses were performed on a Vario EL microanalyzer (Elementar, Hanau, Germany). Mass spectra for the titanium complexes were measured on a Thermo LCQ DECA XP liquid chromatography (Thermo, San Diego, CA, USA)—mass spectrometry using electrospray ionization. 1H NMR spectra were carried out on Mercury-plus 300 MHz NMR spectrometers (Varian, Salt Lake, UT, USA) at room temperature in CDCl3 solution for organic compounds. 13C NMR spectra of polymers were carried out on a Bruker 500 MHz (Bruker, Rheinstetten, Germany) at 120 °C o-C6D4Cl2 solution using solvent as a reference. The norbornene incorporations in the copolymer were calculated from the 13C NMR spectra (Bruker, Rheinstetten, Germany). The molecular weight and molecular weight distribution (PDI = Mw/Mn) of the polymers at 150 °C were performed on a high-temperature gel permeation chromatography (GPC), PL-GPC 220 instrument (PL, Shropshire, UK) equipped with a differential refractive index (RI) detector. Differential scanning calorimetry (DSC) analyses were conducted with a Perkin Elmer DCS-7 system (Perkin Elmer, Waltham, MA, USA). The DSC curves were recorded at second heating curves at a heating rate of 10 °C/min and a cooling rate of 10 °C/min.

2.3. Crystal Structure Determination

The crystal of Ti complex 2 was mounted on a glass fiber and transferred to a Bruker CCD platform diffractometer (Karlsruhe, Germany). Data obtained with the ω-2θ scan mode was collected on a Bruker SMART 1000 CCD diffractometer (Karlsruhe, Germany) with graphite-monochromated Cu Kα radiation (λ = 1.54178 Å) at 293 K. The structure of Ti complex 2 was solved by direct methods using the program SHELXS97 (Göttingen Univeristy, Germany), while further refinement with full-matrix least squares against F2 was obtained with the SHELXL97 program package. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were introduced in calculated positions with the displacement factors of the host carbon atoms.

2.4. Ethylene Polymerization

A round-bottom Schlenk flask with stirring bar was heated for 3 h at 150 °C under vacuum and then cooled to room temperature. The flask was pressurized to 1.5 atm of ethylene (absolute pressure) and vented three times. The appropriate MAO solution as a cocatalyst and toluene was added into the glass reactor under 1.5 atm of ethylene. The system was continuously stirred for 5 min, and then toluene and 1 mL of a solution of Ti complex in toluene were added sequentially by syringe to the well-stirred solution, and the total reaction volume was kept at 35 mL. The ethylene pressure was kept constant at 1.5 atm (absolute pressure) by continuous feeding of gaseous ethylene throughout the reaction. The other reaction temperatures were controlled with an external oil bath or a cooler in polymerization experiments. The polymerizations were terminated by the addition of 200 mL of acidic methanol (95:5 methanol/HCl) after continuously stirring for the desired time. The resulting precipitated polymers were collected by filtration, washed with methanol three times, and then dried in vacuum at 60 °C to a constant weight.

2.5. Copolymerization of Ethylene and Norbornene

A Schlenk glass flask with stirring bar was heated for 3 h at 150 °C under vacuum and then slowly cooled to room temperature. The flask was vented three times using ethylene gas. Then the glass flask was charged with toluene, the prescribed amount of MAO solution, and norbornene at initialization temperature of 30 °C. The system was continuously stirred for 5 min, and then the titanium complex solution was charged into the glass flask. The pressure of 1.5 atm (absolute pressure) was maintained by continuously feeding ethylene gas and the polymerization reaction was performed for the desired time. Polymerization was terminated by the addition of acidic methanol (methanol/HCl, 95:5). The resulting precipitated polymers were collected by filteration, washing with methanol three times, and drying under vacuum at 60 °C to a constant weight.

2.6. Synthesis of Bis(β-ketoamino) Titanium Complexes

Under nitrogen, to a stirred solution of ligand (2.2 mmol) in 40 mL toluene at −78 °C was added a 2.2 M n-BuLi hexane solution (1.0 mL, 2.2 mmol) dropwise. The mixtures were allowed to warm to room temperature slowly and stirred for 2.5 h. TiCl4 (1.2 mmol) in 20.0 mL toluene was dropped into the resulting mixture at −78 °C with stirring over 0.5 h. The mixtures were allowed to warm to room temperature slowly and further stirred for 16 h. The resulting filtrate was further concentrated in vacuum to ~5 mL after filteration. Dried hexane (30 mL) was added into the concentrated solution, and the mixture was stirred for a certain time and then filtered. The residual solid was washed three times with dried hexane and dried in vacuum. Further recrystallizations of the obtained solid products in hexane/toluene solution afforded the pure titanium complexes 1–3. 1H NMR spectroscopy of titanium complexes 1–3 were shown in Figures S1–S3.
Ti complex 1 ((L1)2TiCl2): yield: 65%. 1H NMR (C6D6), δ (ppm): 8.82 (d, 2H), 7.36 (t, 2H), 7.02 (t, 1H), 6.89–6.64 (m, 8H), 6.52 (d, 2H), 1.54 (s, 3H). EI–MS (m/z): 787 (M–Cl); 354 (Ligand+H). Anal. calcd. for C46H36N6Cl2TiO2: C, 67.08; H, 4.41; N, 10.20. Found: C, 67.58; H, 4.31; N, 10.06.
Ti complex 2 ((L2)2TiCl2): yield: 72%. 1H NMR (C6D6), δ (ppm): 8.77 (d, 2H), 7.35 (t, 2H), 7.27 (t, 2H), 7.13–6.53 (m, 6H), 6.41 (t, 2H), 2.23 (s, 3H), 1.55 (s, 3H). EI–MS (m/z): 815 (M–Cl); 368 (Ligand+H). Anal. calcd. for C48H40N6Cl2TiO2: C, 67.69; H, 4.73; N, 9.87. Found: C, 67.43; H, 4.61; N, 10.08.
Ti complex 3 ((L3)2TiCl2): yield: 80%. 1H NMR (C6D6), δ (ppm): 8.85 (d, 2H), 7.77(t, 1H), 7.58 (m, 1H), 7.36 (m, 3H), 7.20 (m, 2H), 7.12–6.69 (m, 9H), 6.36–6.61 (m, 2H), 1.59 (s, 3H). EI–MS (m/z): 887 (M–Cl); 404 (Ligand+H). Anal. calcd. for C54H40N6Cl2TiO2: C, 70.21; H, 4.36; N, 9.10. Found: C, 70.42; H, 4.24; N, 9.01.

3. Results and Discussion

3.1. Synthesis of Bis(β-ketoamino) Titanium Complexes

The synthetic route of the titanium complex is shown in Scheme 1. β-Ketoamine ligands containing pyrazolone rings were prepared following a previously reported method [57]. After the β-ketoamine ligands were treated with n-butyllithium in toluene, 0.5 equiv of TiCl4 was added to the solutions and the titanium complexes 1–3 were obtained as dark red solids. The structures of bis(β-ketoamino) titanium complexes 1–3 were confirmed by elemental analysis and EI–MS and 1H NMR spectroscopy. In comparison with 1H NMR spectra of ligand compounds HL, no labile protons were observed at ~13 ppm for titanium complexes (see Figures S1–S3 in Supplementary Materials), suggesting that titanium complexes were successfully synthesized.
A crystal of the titanium complex 2 suitable for X-ray crystallography was grown from hexane/toluene solution by slow evaporation. The crystallographic data, data collection, and refinement are summarized in Tables S1–S5. As shown in Figure 1, the molecular structure of the titanium complex 2 belongs to the C2-symmetric system with a distorted octahedral geometry in the solid state. The two oxygen donor atoms are situated in the trans position with the O(1)–Ti–O(2) angle, of 164.93°, while the two nitrogen donor atoms are situated in the cis position with the N(1)–Ti–N(4) angle of 90.61°. The two chlorine atoms are situated in the cis position with the Cl(1)–Ti–Cl(2) angle of 92.40°. The Ti–O (1.8739 Å), Ti–N (2.209 Å), and Ti–Cl (2.2695 Å) bond distances are also typical for bis(phenoxy-imino) titanium and bis(β-enaminoketonato) titanium complexes [19,60].

3.2. Ethylene Homopolymerization

In the presence of MAO, bis(β-ketoamino) titanium complexes 1–3 were firstly investigated as precursors for ethylene polymerization. The polymerization data summarized in Table 1 clearly showed that the substituent effect of the ligand played an important role in the catalytic performances. The order of catalytic activities for ethylene polymerization is 1 > 2 > 3 at the optimized temperatures, suggesting that bulky substituent on N-aryl moiety decreased catalytic activity for ethylene polymerization. Additionally, increasing the steric hindrance of the ligand also led to a decrease in polymer molecular weight. This steric effect of o-aryl substituents on ethylene polymerization activity conflicts with previous observations made using bis(β-diketiminato) titanium catalysts [23]. An appropriate reason is that the crowded space around titanium metal center provided by ligands with three aryl substituents may slow down the coordination and insertion of ethylene, thus decreasing molecular weight and catalytic activity.
The polymerization results in Table 1 also demonstrated the effect of reaction temperature on ethylene polymerization. It was found that the catalytic activity increased with increasing reaction temperature and reached a maximal value. A decrease in activity was observed at higher temperature, and this was probably related to the thermal instability of the active species at higher temperatures. Molecular weight decreased uniformly with the increase of polymerization temperature, suggesting acceleration of chain transfer/termination. This is a common behavior for most of the non-metallocene Ti catalyst systems [61]. Besides, steric substituents influence the thermal stability of titanium catalysts. Although catalyst 1 was the most highly active at −10 °C, it showed the lowest activity at high temperature of 50 °C. A huge drop of 84% in activity was observed for catalyst 1 when the temperature was increased from −10 to 50 °C, whilst moderately decreased activity was observed for catalyst 3. This observation can be attributed to the protection and stabilization of the nickel metal center provided by bulky substituents.
High-temperature 13C NMR analysis of the obtained polyethylene showed a peak at 30.0 ppm, and no signals originating from branching carbons were observed (Figure 2). The melting points (Tm) of polymers measured by the differential scanning calorimetry (DSC) were in the 130–138 °C region (Figure 3), indicating that the produced polymers possessed linear structure with virtually no branching.

3.3. Copolymerization of Ethylene and Norbornene

Copolymerizations of ethylene and norbornene were also carried out with bis(β-ketoamino) titanium complexes 1–3 activated with MAO under the conditions of 30 °C and 1.5 atm ethylene pressure, and copolymerization data are summarized in Table 2. Molecular weight distributions (Mw/Mn) of the polymeric products are close to 2 and appear as a single modal in GPC chromatogram, which indicates that copolymerization occurs at the single active site and the polymerization products are “true” copolymers instead of the blend of homopolymers. Copolymerization results in Table 2 clearly demonstrate that the order of catalytic activities for copolymerization is 1 > 2 > 3 under the same conditions, which is same trend to ethylene homopolymerization. Less bulky catalyst 1 with phenyl groups showed the highest copolymerization activity and afforded the copolymer with the highest molecular weight. The same trend to ethylene homopolymerization can be attributed to extremely low activities of bis(β-ketoamino) titanium catalysts for norbornene polymerization. Hence, substituent effect on copolymerization activity and molecular weight is a result of steric effect. However, the highest norbornene incorporation in copolymers was achieved using catalyst 1 among three catalysts. Substituent effect on norbornene incorporation can also be interpreted to steric effect of o-aryl substituents, because of bulky norbornene monomer.
Catalyst 1 was selected to further investigate the effects of the norbornene addition on copolymerization of ethylene and norbornene because of high catalytic activity. Polymerization results (entries 1–5 in Table 2) show that increasing norbornene addition from 1 to 12 g leads to a decrease in copolymerization activity and molecular weight. This resulted from extremely low activity of catalyst 1 for norbornene polymerization. However, incorporation of norbornene increased with an increase in norbornene addition and then was gradually close to a constant value of 50 mol %. Generally, the obtained E–N copolymers had similar norbornene incorporation (<50 mol %) to alternating E–N copolymers obtained by other non-metallocenes titanium catalysts [19,23].
Microstructures of the E–N copolymers obtained at the same conditions were further determined by 13C NMR technique in order to investigate the effect of catalyst structure on copolymer microstructure. The 13C NMR spectroscopy of the copolymer containing 28.7 mol % norbornene incorporation obtained by catalyst 3 showed the “classical” resonances of alternating structure. Eight major peaks which are characteristics of alternating (ENEN) and isolated (ENEE) norbornene sequences of E–N copolymer were observed in Figure 4C. These signals were assigned according to the previous literature: 29.9, 30.4, 30.9 ppm to C5/C6 and successive ethylene sequences; 33.1 ppm to C7; 41.7, 42.2 ppm to C1/C4 and 47.4, 48.0 ppm to C2/C3 [19,23,25].
The E–N copolymers obtained by catalysts 1 and 2 displayed the higher levels of norbornene incorporation, and 13C NMR spectra of copolymers were slightly different. In addition to eight major peaks, several minor signals were observed. As shown in Figure 4B, several minor signals at 28.3, 31.4, 33.6, 41.2 ppm appeared for the copolymer containing 32.0 mol % norbornene incorporation obtained by catalyst 2. As described in previous assignment of E–N copolymers, two signals at 28.3 and 31.4 ppm are assigned to carbons C5/C6 in meso- and racemic-ENNE sequences, the resonance at 33.6 ppm is assigned to C7 in ENN, and the resonance at 41.2 ppm is assigned to C1/C4 of racemic-ENNE sequence [23,25]. This strongly suggests the presence of the NN dyad sequences in the E–N copolymer is obtained by catalyst 2. In the spectroscopy of the copolymer containing 38.7 mol % norbornene incorporation obtained by catalyst 2, additional weak signals at 31.9, 34.2–37.6 ppm, 39.0–40.1 ppm, and 48.6–53.0 ppm can be assigned to NNN triad sequences in the E–N copolymer [23,25]. Additionally, the decreased intensity of successive ethylene sequences at 29.9 ppm was observed because more norbornene units are homogeneously distributed on the copolymer chain.
Increased norbornene addition from 2 to 4 g can improve norbornene incorporation in copolymer. Though norbornene incorporation increased, bulky catalyst 3 still afforded the alternating E–N copolymers without NN dyad sequences (Figure 5C). For the E–N copolymers obtained by catalyst 1 and 2, more intensive NN dyad and NNN triad resonances appeared (Figure 5A,B). Hence, o-substituent of ligand predominantly determines copolymer microstructure.
Catalyst 1 was chosen to investigate the effect of norbornene addition on copolymer microstructure. As shown in Figure 6A, the E–N copolymer that obtained low norbornene addition of 1 g had 27.0 mol % norbornene incorporation, and only displayed eight characteristic signals of alternating (ENEN) and isolated (ENEE) norbornene sequences. With an increase in norbornene addition, norbornene incorporation increased and NN dyad sequences appeared in the copolymer (Figure 5A and Figure 6A). For the copolymer that obtained 12 g of norbornene addition with the highest norbornene incorporation of 48.6%, more intensive resonances assigned to the NN dyad and NNN triad sequences appeared in the E–N copolymer (Figure 6B) [23,25]. This suggests that increased norbornene addition not only improved norbornene incorporation in copolymer, but also enhanced insertion frequency of norbornene into the Ti–N bond, thus forming successive norbornene sequences.
The E–N copolymers obtained by bis(β-ketoamino) titanium catalysts had a glass transition temperature (Tg) in a range from 58–115 °C, determined by means of the differential scanning calorimetry (DSC) analysis. The Tg value of 58 °C was detected for the copolymer with 27.0 mol % norbornene incorporation, while the highest Tg value of 115 °C was determined for the copolymer with 48.6 mol % norbornene incorporation. The Tg value was closely dependent upon norbornene incorporation in the copolymers, and the Tg value increased with increasing norbornene incorporation in copolymers.

4. Conclusions

In this study, we have successfully synthesized and characterized novel bis(β-ketoamino) titanium complexes containing pyrazolone rings 1–3. Ethylene polymerization using 1–3 activated with MAO produced linear polyethylene with high activities. The substituent influence on ethylene polymerization was largely attributed to the steric effect of o-aryl substituents. Copolymerizations of ethylene and norbornen using catalyst 1–3/MAO produced E–N copolymers with high acitvities. The substituent influence on copolymerization activity, copolymer molecular weight, and norbornene incorporation was attributted to steric effect. Although increased norbornene addition improved norbornene incorporation in the copolymers, o-substituents of ligand predominantly determined copolymer microstructure. All of the copolymers showed the Tg in a temperature range from 58 to 115 °C, which increased with additional incorporation of norbornene. The bis(β-ketoamino) titanium complex containing pyrazolone rings is a promising candidate for tuning the microstructure of E–N copolymers with the design of the o-substituents on the aryl ring.

Supplementary Materials

Supplementary Materials are available online at www.mdpi.com/2073-4360/9/7/262/s1.

Acknowledgments

The financial support of this research was by Open Fund of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University) (PLN1407), Natural Science Foundation of Guangdong Province (1414050000552), Science and Technology Planning Project of Guangzhou City (201510010070), and the Fundamental Research Funds for the Central Universities (17lgjc02), who are all gratefully acknowledged.

Author Contributions

Lixia Pei and Haiyang Gao conceived and designed the experiments; Lixia Pei, San He, Jie Gao, and Heng Liao performed the experiments and analyzed the data; Haiyang Gao contributed reagents/materials/analysis tools; Lixia Pei and Haiyang Gao wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Kaminsky, W.; Engehausen, R.; Kopf, J. A tailor-made metallocene for the copolymerization of ethene with bulky cycloalkenes. Angew. Chem. Int. Ed. 1995, 34, 2273–2275. [Google Scholar] [CrossRef] [Scilit]
  2. Rische, T.; Waddon, A.J.; Dickinson, L.C.; MacKnight, W.J. Microstructure and morphology of cycloolefin copolymers. Macromolecules 1998, 31, 1871–1874. [Google Scholar] [CrossRef] [Scilit]
  3. Kaminsky, W. Olefin polymerization catalyzed by metallocenes. Adv. Catal. 2001, 46, 89–159. [Google Scholar]
  4. Blank, F.; Janiak, C. Metal catalysts for the vinyl/addition polymerization of norbornene. Coord. Chem. Rev. 2009, 253, 827–861. [Google Scholar] [CrossRef] [Scilit]
  5. Ma, R.; Hou, Y.B.; Gao, J.; Bao, F. Recent progress in the vinylic polymerization and copolymerization of norbornene catalyzed by transition metal catalysts. Polym. Rev. 2009, 49, 249–287. [Google Scholar] [CrossRef] [Scilit]
  6. Kaminsky, W.; Bark, A.; Arndt, M. New polymers by homogenous zirconocene/aluminoxane catalysts. Makromol. Chem. Macromol. Symp. 1991, 47, 83–93. [Google Scholar] [CrossRef] [Scilit]
  7. Kaminsky, W.; Bark, A.; Steiger, R. Stereospecific polymerization by metallocene/aluminoxane catalysts. J. Mol. Catal. 1992, 74, 109–119. [Google Scholar] [CrossRef] [Scilit]
  8. Kaminsky, W.; Noll, A. Copolymerization of norbornene and ethene with homogenous zirconocenes/methylaluminoxane catalysts. Polym. Bull. 1993, 31, 175–182. [Google Scholar] [CrossRef] [Scilit]
  9. Ruchatz, D.; Fink, G. Ethene–norbornene copolymerization using homogenous metallocene and half-sandwich catalysts:  Kinetics and relationships between catalyst structure and polymer structure. 2. comparative study of different metallocene- and half-sandwich/methylaluminoxane catalysts and analysis of the copolymers by 13C nuclear magnetic resonance spectroscopy. Macromolecules 1998, 31, 4674–4680. [Google Scholar] [PubMed]
  10. Ruchatz, D.; Fink, G. Ethene–norbornene copolymerization with homogeneous metallocene and half-sandwich catalysts:  Kinetics and relationships between catalyst structure and polymer structure. 3. copolymerization parameters and copolymerization diagrams. Macromolecules 1998, 31, 4681–4683. [Google Scholar] [CrossRef] [Scilit] [PubMed]
  11. Ruchatz, D.; Fink, G. Ethene–norbornene copolymerization with homogeneous metallocene and half-sandwich catalysts:  Kinetics and relationships between catalyst structure and polymer structure. 4. development of molecular weights. Macromolecules 1998, 31, 4684–4686. [Google Scholar] [CrossRef] [Scilit] [PubMed]
  12. Bergstrom, C.H.; Sperlich, B.R.; Ruotoistenmaki, J.; Seppala, J.V. Investigation of the microstructure of metallocene-catalyzed norbornene–ethylene copolymers using NMR spectroscopy. J. Polym. Sci. A 1998, 36, 1633–1638. [Google Scholar] [CrossRef] [Scilit]
  13. McKnight, A.L.; Waymouth, M.R. Ethylene/norbornene copolymerizations with titanium CpA catalysts. Macromolecules 1999, 32, 2816–2825. [Google Scholar] [CrossRef] [Scilit]
  14. Lee, B.Y.; Kim, Y.H.; Won, Y.C.; Han, J.W.; Suh, W.H.; Lee, I.S.; Chung, Y.K.; Song, K.H. Synthesis of [2,2’-methylenebis(1,3-dimethylcyclopentadienyl)] zirconium dichloride and its reactivity in ethylene−norbornene copolymerization. Organometallics 2002, 21, 1500–1503. [Google Scholar] [CrossRef] [Scilit]
  15. Wendt, R.A.; Fink, G. Ethene–norbornene copolymerizations using two different homogeneous metallocene catalyst systems and investigations of the copolymer microstructure. J. Mol. Catal. A 2003, 203, 101–111. [Google Scholar] [CrossRef] [Scilit]
  16. Yoshida, Y.; Saito, J.; Mitani, M.; Takagi, Y.; Matsui, S.; Ishii, S.; Nakano, T.; Kashiwa, N.; Fujita, T. Living ethylene/norbornene copolymerisation catalyzed by titanium complexes having two pyrrolide–imine chelate ligands. Chem. Commun. 2002, 12, 1298–1299. [Google Scholar] [CrossRef] [Scilit]
  17. Nomura, K.; Tsubota, M.; Fujiki, M. Efficient Ethylene/norbornene copolymerization by (aryloxo)(indenyl)titanium(IV) complexes−MAO catalyst system. Macromolecules 2003, 36, 3797–3799. [Google Scholar] [CrossRef] [Scilit]
  18. Yoshida, Y.; Mohri, J.; Ishii, S.; Mitani, M.; Saito, J.; Matsui, S.; Makio, H.; Nakano, T.; Tanaka, H.; Onda, M.; et al. Living copolymerization of ethylene with norbornene catalyzed by bis(pyrrolide−imine) titanium complexes with MAO. J. Am. Chem. Soc. 2004, 126, 12023–12032. [Google Scholar] [CrossRef] [Scilit] [PubMed]
  19. Li, X.F.; Dai, K.; Ye, W.P.; Pan, L.; Li, Y.S. New titanium complexes with two β-enaminoketonato chelate ligands:  Syntheses, structures, and olefin polymerization activities. Organometallics 2004, 23, 1223–1230. [Google Scholar] [CrossRef] [Scilit]
  20. Vijayakrishna, K.; Sundararajan, G. Non-Cp type titanium precatalysts for ethylene/norbornene copolymerization. Polymer 2006, 47, 8289–8296. [Google Scholar] [CrossRef] [Scilit]
  21. Wang, C.; Sun, X.L.; Guo, Y.H.; Gao, Y.; Liu, B.; Ma, Z.; Xia, W.; Shi, L.P.; Tang, Y. Novel titanium catalysts bearing an [O, N, S] tridentate ligand for ethylene homo- and copolymerization. Macromol. Rapid. Commun. 2005, 26, 1609–1614. [Google Scholar] [CrossRef] [Scilit]
  22. Gao, M.L.; Wang, C.; Sun, X.L.; Qian, C.T.; Ma, Z.; Bu, S.Z.; Tang, Y.; Xie, Z.W. Ethylene–norbornene copolymerization by new titanium complexes bearing tridentate ligands. sidearm effects on catalytic activity. Macromol. Rapid. Commun. 2007, 28, 1511–1516. [Google Scholar] [CrossRef] [Scilit]
  23. Li, Y.F.; Gao, H.Y.; Wu, Q. Homo- and copolymerization of ethylene and norbornene with bis(β-diketiminato) titanium complexes activated with methylaluminoxane. J. Polym. Sci. A 2008, 46, 93–101. [Google Scholar] [CrossRef] [Scilit]
  24. Hu, H.; Gao, H.Y.; Song, K.M.; Liu, F.S.; Long, J.M.; Zhang, L.; Zhu, F.M.; Wu, Q. Novel bis(benzoin) titanium catalyst for homo- and copolymerization of norbornene with ethylene: Synthesis, characterization and catalytic properties. Polymer 2008, 49, 4552–4558. [Google Scholar] [CrossRef] [Scilit]
  25. Hasan, T.; Ikeda, T.; Shiono, T. Ethene–norbornene copolymer with high norbornene content produced by ansa-fluorenylamidodimethyltitanium complex using a suitable activator. Macromolecules 2004, 37, 8503–8509. [Google Scholar] [CrossRef] [Scilit]
  26. He, L.; Liu, J.; Li, Y.; Liu, S.; Li, Y. High-temperature living copolymerization of ethylene with norbornene by titanium complexes bearing bidentate [O, P] ligands. Macromolecules 2009, 42, 8566–8570. [Google Scholar] [CrossRef] [Scilit]
  27. Hu, H.; Gao, H.Y.; Wu, Q. High norbornene incorporation in ethylene–norbornene copolymerization with a bis(α-alkyloxoimine) titanium-MAO catalyst. Sci. China Chem. 2010, 53, 1634–1640. [Google Scholar]
  28. Benedikt, G.M.; Elce, E.; Goodall, B.L.; Kalamarides, H.A.; McIntosh, L.H.; Rhodes, L.F.; Selvy, K.T.; Andes, C.; Oyler, K.; Sen, A. Copolymerization of ethene with norbornene derivatives using neutral nickel catalysts. Macromolecules 2002, 35, 8978–8988. [Google Scholar] [CrossRef] [Scilit]
  29. Tritto, I.; Boggioni, L.; Zampa, C.; Ferro, D.R. Ethylene−norbornene copolymers by Cs-symmetric metallocenes:  Fetermination of the vopolymerization parameters and mechanistic considerations on the basis of tetrad analysis. Macromolecules 2005, 38, 9910–9919. [Google Scholar] [CrossRef] [Scilit]
  30. Gao, H.Y.; Chen, Y.; Zhu, F.M.; Wu, Q. Copolymerization of norbornene and styrene catalyzed by a novel anilido–imino nickel complex/methylaluminoxane system. J. Polym. Sci. A 2006, 44, 5237–5246. [Google Scholar] [CrossRef] [Scilit]
  31. Gao, H.Y.; Guo, W.J.; Bao, F.; Gui, G.Q.; Zhang, J.K.; Zhu, F.M.; Wu, Q. Synthesis, molecular structure, and solution-dependent behavior of nickel complexes chelating anilido–imine donors and their catalytic activity toward olefin polymerization. Organometallics 2004, 23, 6273–6280. [Google Scholar] [CrossRef] [Scilit]
  32. Gao, H.Y.; Zhang, J.K.; Chen, Y.; Zhu, F.M.; Wu, Q. Vinyl-polymerization of norbornene with novel anilido–imino nickel complexes/methylaluminoxane: Abnormal influence of polymerization temperature on molecular weight of polynorbornenes. J. Mol. Catal. A 2005, 240, 178–185. [Google Scholar] [CrossRef] [Scilit]
  33. Gao, H.Y.; Pei, L.X.; Li, Y.F.; Zhang, J.K.; Wu, Q. Vinyl polymerization of norbornene with nickel catalysts bearing [N,N] six-membered chelate ring: Important influence of ligand structure on activity. J. Mol. Catal. A 2008, 280, 81–86. [Google Scholar] [CrossRef] [Scilit]
  34. Gao, H.Y.; Ke, Z.F.; Pei, L.X.; Song, K.M.; Wu, Q. Drastic ligand electronic effect on anilido–imino nickel catalysts toward ethylene polymerization. Polymer 2007, 48, 7249–7254. [Google Scholar] [CrossRef] [Scilit]
  35. Gao, H.Y.; Liu, Y.; Li, G.L.; Xiao, Z.F.; Liang, G.D.; Wu, Q. Catalytic synthesis of polyethylene–block–polynorbornene copolymers using a living polymerization nickel catalyst. Polym. Chem. 2014, 5, 6012–6018. [Google Scholar] [CrossRef] [Scilit]
  36. Chen, M.; Zou, W.; Cai, Z.; Chen, C. Norbornene homopolymerization and copolymerization with ethylene by phosphine-sulfonate nickel catalysts. Polym. Chem. 2015, 6, 2669–2676. [Google Scholar] [CrossRef] [Scilit]
  37. Li, Y.F.; Jiang, L.; Wang, L.Y.; Gao, H.Y.; Zhu, F.M.; Wu, Q. Nickel(II) complexes supported by a fluorinated β-diketiminate backbone ligand: Synthesis, catalytic activity toward norbornene polymerization, and the oxygenated species. Appl. Organomet. Chem. 2006, 20, 181–186. [Google Scholar] [CrossRef] [Scilit]
  38. Li, Y.F.; Wang, L.Y.; Gao, H.Y.; Zhu, F.M.; Wu, Q. Novel nickel(II) complexes chelating β-diketiminate ligands: Synthesis and simultaneous polymerization and oligomerization of ethylene. Appl. Organomet. Chem. 2006, 20, 436–442. [Google Scholar] [CrossRef] [Scilit]
  39. Long, J.M.; Gao, H.Y.; Song, K.M.; Liu, F.S.; Hu, H.; Zhang, L.; Zhu, F.M.; Wu, Q. Synthesis and characterization of Ni-II and Pd-II complexes bearing N,N,S tridentate ligands and their catalytic properties for norbornene polymerization. Eur. J. Inorg. Chem. 2008, 27, 4296–4305. [Google Scholar] [CrossRef] [Scilit]
  40. Kiesewetter, J.; Kaminsky, W. Ethene/norbornene copolymerization with palladium(II) α-diimine catalysts: From ligand screening to discrete catalyst species. Chem. Eur. J. 2003, 9, 1750–1758. [Google Scholar] [CrossRef] [Scilit] [PubMed]
  41. Kiesewetter, J.; Arikan, B.; Kaminsky, W. Copolymerization of ethene with norbornene using palladium(II) α-diimine catalysts: Influence of feed composition, polymerization temperature, and ligand structure on copolymer properties and microstructure. Polymer 2006, 47, 3302–3314. [Google Scholar] [CrossRef] [Scilit]
  42. Tian, J.W.; He, X.H.; Liu, J.Y.; Deng, X.D.; Chen, D.F. Palladium(II) and copper(II) chloride complexes bearing bulky α-diimine ligands as catalysts for norbornene vinyl-addition (co)polymerization. RSC Adv. 2016, 6, 22908–22916. [Google Scholar] [CrossRef] [Scilit]
  43. He, X.H.; Liu, Y.M.; Chen, L.; Chen, Y.W.; Chen, D.F. Ni(II) and Pd(II) complexes bearing benzocyclohexane–ketoarylimine for copolymerization of norbornene with 5-norbornene-2-carboxylic ester. J. Polym. Sci. A 2012, 50, 4695–4704. [Google Scholar] [CrossRef] [Scilit]
  44. Chen, Y.; Mandal, S.; Sen, A. Synthesis of (N−O)-ligated palladium(II) complexes and their use in ethene homopolymerization and norbornene copolymerizations. Organometallics 2010, 29, 3160–3168. [Google Scholar] [CrossRef] [Scilit]
  45. Ravasio, A.; Boggioni, L.; Tritto, I. Copolymerization of ethylene with norbornene by neutral aryl phosphine sulfonate palladium catalyst. Macromolecules 2011, 44, 4180–4186. [Google Scholar] [CrossRef] [Scilit]
  46. Peuckert, U.; Heitz, W. Vinylic polymerization by homogeneous chromium(III) catalysts. Macromol. Rapid. Commun. 1998, 19, 159–162. [Google Scholar] [CrossRef]
  47. Peuckert, U.; Heitz, W. Vinylic Polymerization and copolymerization of norbornene and ethene by homogeneous chromium(III) catalysts. Macromol. Chem. Phys. 2001, 202, 1289–1297. [Google Scholar] [CrossRef] [Scilit]
  48. Pei, L.X.; Tang, Y.; Gao, H.Y. Homo- and copolymerization of ethylene and norbornene with anilido–imine chromium catalysts. Polymers 2016, 8, 69. [Google Scholar] [CrossRef] [Scilit]
  49. Tang, L.M.; Wu, J.Q.; Duan, Y.Q.; Pan, L.; Li, Y.G.; Li, Y.S. Ethylene polymerizations, and the copolymerizations of ethylene with hexene or norbornene with highly active mono(β-enaminoketonato) vanadium(III) catalysts. J. Polym. Sci. A 2008, 46, 2038–2048. [Google Scholar] [CrossRef] [Scilit]
  50. Li, X.; Baldamus, J.; Hou, Z. Alternating ethylene–norbornene copolymerization catalyzed by cationic half-sandwich scandium complexes. Angew. Chem. Int. Ed. 2005, 44, 962–965. [Google Scholar] [CrossRef] [Scilit] [PubMed]
  51. Ravasio, A.; Zampa, C.; Boggioni, L.; Tritto, I.; Hitzbleck, J.; Okuda, J. Copolymerization of ethylene with norbornene catalyzed by cationic rare-earth metal half-sandwich complexes. Macromolecules 2008, 41, 9565–9569. [Google Scholar] [CrossRef] [Scilit]
  52. Bao, F.; Lu, X.Q.; Gao, H.Y.; Gui, G.Q.; Wu, Q. Vinylic and ring-opening metathesis polymerization of norbornene with bis(β-ketoamine) cobalt complexes. J. Polym. Sci. A 2005, 43, 5535–5544. [Google Scholar] [CrossRef] [Scilit]
  53. Bao, F.; Lu, X.Q.; Qiao, Y.Q.; Gui, G.Q.; Gao, H.Y.; Wu, Q. Nickel and cobalt complexes bearing β-ketoamine ligands: Syntheses, structures and catalytic behavior for norbornene polymerization. Appl. Organometal. Chem. 2005, 19, 957–963. [Google Scholar] [CrossRef] [Scilit]
  54. Bao, F.; Ma, R.; Lu, X.Q.; Gui, G.Q.; Wu, Q. Structures and styrene polymerization activities of a series of nickel complexes bearing ligands of pyrazolone derivatives. Appl. Organometal. Chem. 2006, 20, 32–38. [Google Scholar] [CrossRef] [Scilit]
  55. Bao, F.; Lu, X.Q.; Kang, B.S.; Wu, Q. Vinyl polymerization of norbornene catalyzed by a series of bis(β-ketoiminato)nickel (II) complexes in the presence of methylaluminoxane. Eur. Polym. J. 2006, 42, 928–934. [Google Scholar] [CrossRef] [Scilit]
  56. Bao, F.; Ma, R.; Jiao, Y.H. Homo- and copolymerization of norbornene with styrene catalyzed by a series of copper(II) complexes in the presence of methylaluminoxane. Appl. Organometal. Chem. 2006, 20, 368–374. [Google Scholar] [CrossRef] [Scilit]
  57. Lu, X.Q.; Bao, F.; Kang, B.S.; Wu, Q.; Liu, H.Q.; Zhu, F.M. Syntheses, structures and catalytic activity of copper(II) complexes bearing N,O-chelate ligands. J. Organomet. Chem. 2006, 691, 821–828. [Google Scholar] [CrossRef] [Scilit]
  58. Bao, F.; Lü, X.Q.; Chen, Y. Copolymerization of norbornene and styrene catalyzed by a series of bis(β-ketoamine) nickel(II) complexes in the presence of methylaluminoxane. Polym. Bull. 2007, 58, 495–502. [Google Scholar] [CrossRef] [Scilit]
  59. Bao, F.; Ma, R.M.; Ma, R.; Jiao, Y.H. A copper complex with β-ketoamine ligand based on pyrazolone derivative: Synthesis, crystal structure and catalysis for norbornene polymerization. J. Coord. Chem. 2007, 60, 557–566. [Google Scholar] [CrossRef] [Scilit]
  60. Saito, J.; Mitani, M.; Matsui, S.; Tohi, Y.; Makio, H.; Fujita, T. A new titanium complex having two phenoxy-imine chelate ligands for ethylene polymerization. Macromol. Chem. Phys. 2002, 203, 59–65. [Google Scholar] [CrossRef] [Scilit]
  61. Sudhakar, P. Appreciable norbornene incorporation in the copolymerization of ethylene/norbornene using titanium catalysts containing trianionic N[CH2CH(Ph)O]33− ligands. J. Polym. Sci. A 2008, 46, 444–452. [Google Scholar] [CrossRef] [Scilit]
Polymers 09 00262 sch001
Scheme 1. Synthesis of titanium complexes.
Scheme 1. Synthesis of titanium complexes.
Polymers 09 00262 sch001
Polymers 09 00262 g001
Figure 1. Molecular structure of Ti complex 2 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms and toluene were omitted for clarity.
Figure 1. Molecular structure of Ti complex 2 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms and toluene were omitted for clarity.
Polymers 09 00262 g001
Polymers 09 00262 g002
Figure 2. 13C NMR spectrum of the polyethylene obtained from 1/MAO (entry 4).
Figure 2. 13C NMR spectrum of the polyethylene obtained from 1/MAO (entry 4).
Polymers 09 00262 g002
Polymers 09 00262 g003
Figure 3. DSC curves of polyethylenes obtained by 1/MAO at different temperatures (entries 1–5 in Table 1).
Figure 3. DSC curves of polyethylenes obtained by 1/MAO at different temperatures (entries 1–5 in Table 1).
Polymers 09 00262 g003
Polymers 09 00262 g004
Figure 4. 13C NMR spectra of the polyethylenes obtained from 1–3/MAO at 2 g norbornene addition (entries 2, 6, and 8 in Table 2). (* represents signals of NN dyad, and ■ represents signals of NNN triad).
Figure 4. 13C NMR spectra of the polyethylenes obtained from 1–3/MAO at 2 g norbornene addition (entries 2, 6, and 8 in Table 2). (* represents signals of NN dyad, and ■ represents signals of NNN triad).
Polymers 09 00262 g004
Polymers 09 00262 g005
Figure 5. 13C NMR spectra of the polyethylenes obtained from 1–3/MAO at 4 g norbornene addition (entries 3, 7, and 9 in Table 2).
Figure 5. 13C NMR spectra of the polyethylenes obtained from 1–3/MAO at 4 g norbornene addition (entries 3, 7, and 9 in Table 2).
Polymers 09 00262 g005
Polymers 09 00262 g006
Figure 6. 13C NMR spectra of the polyethylenes obtained from 1/MAO at different norbornene additions (entries 1 and 5 in Table 2).
Figure 6. 13C NMR spectra of the polyethylenes obtained from 1/MAO at different norbornene additions (entries 1 and 5 in Table 2).
Polymers 09 00262 g006
Table 1. Ethylene polymerizations with 1–3/ methylaluminoxane (MAO).
Table 1. Ethylene polymerizations with 1–3/ methylaluminoxane (MAO).
EntryCat.Tp (°C)Yield (g)Activity (kg PE /(mol·Ti·h))Mw a (kg/mol)Mw/Mn aTm b (°C)
11−200.612447151.45138
21−101.485926501.56137
3101.114445561.61136
41300.652602401.71135
51500.261041601.82130
62−200.522086141.41138
72−101.425685901.49137
8201.174684351.52136
92300.983923451.65135
102500.451802851.76132
113−200.251005881.49138
123−100.391565041.50135
13300.732924051.63134
143300.562243401.77132
153500.411642901.84131
Polymerization conditions: Ti complex, 5 μmol; Al/Ti = 200; ethylene pressure, 1.5 atm; reaction time, 30 min, solvent, toluene; total volume: 35 mL; a Determined by high temperature gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C; b Determined by differential scanning calorimetry (DSC).
Table 2. Copolymerizations of ethylene and norbornene with 1–3/MAO.
Table 2. Copolymerizations of ethylene and norbornene with 1–3/MAO.
EntryCat.NB Addition (g)Yield (g)Activity aMw b (kg/mol)Mw/Mn bXN c mol %Tg (°C)
1111.471475311.6827.058
2121.101104491.7438.789
3140.74743721.8646.997
4180.39392701.7847.4106
51120.24241841.8048.6115
6220.90903791.9032.060
7240.64643032.0242.391
8320.55553121.9528.746
9340.48482642.1434.671
Polymerization conditions: Ti complex, 10 μmol; Al/Ti = 200; reaction time, 60 min, temperature, 30 °C, solvent, toluene; total volume: 35 mL. a In unit of (kg P(E–N) (mol·Ti·h)−1); b Determined by high temperature gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C; c Determined by 13C NMR.

Share and Cite

MDPI and ACS Style

Pei, L.; He, S.; Gao, J.; Liao, H.; Gao, H. Homo- and Copolymerizations of Ethylene and Norbornene Using Bis(β-ketoamino) Titanium Catalysts Containing Pyrazolone Rings. Polymers 2017, 9, 262. https://doi.org/10.3390/polym9070262

AMA Style

Pei L, He S, Gao J, Liao H, Gao H. Homo- and Copolymerizations of Ethylene and Norbornene Using Bis(β-ketoamino) Titanium Catalysts Containing Pyrazolone Rings. Polymers. 2017; 9(7):262. https://doi.org/10.3390/polym9070262

Chicago/Turabian Style

Pei, Lixia, San He, Jie Gao, Heng Liao, and Haiyang Gao. 2017. "Homo- and Copolymerizations of Ethylene and Norbornene Using Bis(β-ketoamino) Titanium Catalysts Containing Pyrazolone Rings" Polymers 9, no. 7: 262. https://doi.org/10.3390/polym9070262

APA Style

Pei, L., He, S., Gao, J., Liao, H., & Gao, H. (2017). Homo- and Copolymerizations of Ethylene and Norbornene Using Bis(β-ketoamino) Titanium Catalysts Containing Pyrazolone Rings. Polymers, 9(7), 262. https://doi.org/10.3390/polym9070262

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop