Thermal and mechanical properties of ultrahigh-molecular-weight ethylene/1-hexene copolymers prepared by living polymerization with fluorinated bis(phenoxy-imine) titanium(IV) catalyst
Graphical abstract
Introduction
With great progresses of the single-site catalyst technology, olefin copolymers having well controlled molecular weight and high comonomer content have attracted much attention in polyolefin industries [1], [2], [3]. In particular, ultrahigh molecular weight (UHMW, >106 g/mol) copolymers have become a type of promising emerging polyolefin materials. Such polymers have extremely extended chain lengths and exhibit distinctly novel and superior performances [4], [5]. However, the synthesis of UHMW olefin copolymers or poly(α-olefin)s represents a great challenge because it requires a catalyst having good ability in both chain propagation and comonomer incorporation against chain termination and transfer side reactions.
Several metallocene and post-metallocene catalysts showed high potential in the preparation of UHMW polyolefins. Fries et al. [6] reported poly(1-hexene) of 3.38 × 106 g/mol, with (C5HMe4)2HfCl2 in a high pressure compressed solution. Ishii et al. [7] employed a bis(phenoxy-imine)-Zr complex and synthesized ethylene/propylene (EP) copolymers of 10.2 × 106 g/mol with comonomer incorporation (F2) of 20 mol%. Rieger et al. [8] employed rac-[1-(9-η5-fluorenyl)-2- (5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane] dimethyl Hf and prepared polypropylene elastomers having Mw of 4.9 × 106 g/mol. Fujita et al. [9] had poly(1-butene) of 9.2 × 106 g/mol, poly(1-hexene) of 5.72 × 106 g/mol and poly(1-octene) of 6.05 × 106 g/mol, with thiobis(phenoxy) Ti dichloride. Reybuck et al. [10] reported an UHMW ethylene/1-hexene (EH) copolymer having Mw of 1.19 × 106 g/mol and 1-hexene content of 33.2 mol%, prepared by (2-PhInd)2ZrCl2. The sample had a broad MWD with PDI of 3.7, showing multiple populations of polymer chains. Kiesewetter et al. [11] synthesized an UHMW EH copolymer having Mw of 2.31 × 106 g/mol and 1-hexene content of 17 mol% by a bis(phenolate)-Hf catalyst. Klosin et al. reported that the imino-enamido hafnium and amidoquinoline hafnium catalysts could be used to prepare UHMW ethylene/1-octene (EO) copolymers at 120 °C. The resulting copolymer had Mw of 1.1 × 106 g/mol and F2 of 7.0 mol% [12], [13]. Weiser et al. studied an olefin living polymerization with fluorinated bis(phenoxy-imine) titanium catalyst and prepared EP copolymer having Mw of 2.2 × 106 g/mol, F2 of 20 mol% and PDI of 1.9, as well as di-block copolymer having Mw of 2 × 106 g/mol, F2 of 8 mol% and PDI of 1.6 [14], [15].
Thermo-mechanical properties of ethylene/α-olefin copolymers with regular molecular weights were comprehensively studied [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. However, an examination of the literature found few studies on those of UHMW olefin copolymers. None of the above works provided a systematical analysis of mechanical performance of the synthesized polymers, probably because of lack of the sample masses. Recently, we prepared a series of UHMW EH copolymers by the living coordination copolymerization with bis[N-(3-propylsalicylidene)-pentafluoroanilinato] titanium(IV) dichloride (PFI) [26], [27]. The molecular weights of the resultant copolymers reached over 1.4 × 106 g/mol. The 1-hexene contents F2 were over 25 mol% with a random comonomer distribution along chain backbones. In this work, we investigate the thermo-mechanical properties of these copolymer samples, and compare their performance against a commercial 1-octene based POE sample as a reference.
Section snippets
Materials
The EH copolymer samples were prepared via a solution process of living coordination polymerization of ethylene and 1-hexene with PFI as catalyst. The detailed information of the preparation can be found in the ref [26].
Characterization
Melting temperature (Tm) and glass transition temperature (Tg) of the polymer samples were determined by DSC with a TA Q200 differential scanning calorimeter. The temperature was programmed as follows: the sample was first heated to 160 °C at a rate of 30 °C/min and held for
Results and discussion
Table 1 summarizes the molecular weight, composition, melting temperature and glass temperature data of the prepared ethylene/1-hexene copolymer samples. The polymers had very narrow molecular weight distributions with PDIs between 1.06 and 1.34, suggesting that the polymerization was indeed living and well controlled. The molecular weights were high, with the Mw values ranged from 2 × 105 up to 1.4 × 106 g/mol. The 1-hexene incorporation was good with F2 values between 8 and 25 mol%. A
Conclusion
A series of ethylene-co-1-hexene copolymer samples were prepared by a living coordination polymerization with fluorinated bis(phenoxy-imine) titanium(IV) catalyst. Thermal and mechanical properties of the polyolefin materials were systematically analyzed. The EH copolymer had random sequence distributions, which were highly correlated to the melting temperatures from DSC measurements. The EH copolymers having ultrahigh molecular weight and high 1-hexene incorporation exhibited excellent
Acknowledgments
The work is supported by the National Basic Research Program of China (2011CB606001), the National Natural Science Foundation of China (No. 21420102008), the Special Research Fund for the Doctoral Program of Higher Education of China (No. 20100101110041), Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (No. SKL-ChE-14Z), and the Fundamental Research Funds for the Central Universities (No. 2015FZA4023).
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