Copolymers of ethylene with monoalkenyl- and monoalkenyl(siloxy)silsesquioxane (POSS) comonomers – Synthesis and characterization

European Polymer Journal 90 (2017) 368–382

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Copolymers of ethylene with monoalkenyl- and monoalkenyl (siloxy)silsesquioxane (POSS) comonomers – Synthesis and characterization

MARK

⁎

Paweł Grocha, Katarzyna Dziubeka, , Krystyna Czajaa, Beata Dudziecb, Bogdan Marciniecb,c a b c

Opole University, Faculty of Chemistry, Oleska 48, 45-040 Opole, Poland Adam Mickiewicz University in Poznan, Faculty of Chemistry, Umultowska 89B, 61-614 Poznan, Poland Centre for Advanced Technologies, Adam Mickiewicz University in Poznan, Umultowska 89C, 61-614 Poznan, Poland

AR TI CLE I NF O

AB S T R A CT

Keywords: Copolymerization Ethylene Polyhedral oligomeric silsesquioxane (POSS) Metallocene

The hybrid ethylene/POSS copolymers were obtained using the rac-Et(Ind)2ZrCl2 catalyst activated by MAO. A series of monoalkenyl- and monoalkenyl(siloxy)silsesquioxanes derivatives with different structures of reactive alkenyl substituent and types of non-reactive groups attached to the T8 POSS cage was used as comonomers. The kind and concentration of the POSS comonomer in the reaction feed as well as extended reaction time were found to strongly influence the catalyst efficiency and incorporation of POSS units into polymer chains. The comonomer reactivity was significantly dependent on the length of the alkenyl reactive substituent in the POSS molecule and it was highest for the POSS structures with mediumlength of the alkenyl substituent due to their steric and inductive effects of the silicon-oxygen cage. The molecular weight as well as the kind and the content of unsaturated end groups in copolymers were dependent on the kind and amount of POSS comonomer in the reaction feed. The change in melting temperature, crystallinity degree, crystallization temperature and morphology of copolymers was observed as compared to neat polyethylene (PE).

1. Introduction The hybrid organic-inorganic materials have attracted much attention for more than ten years, both from the academic and industrial points of view, because of their interesting physicochemical and mechanical properties. Such materials combine the features of organic and inorganic materials and thus could turn out to be useful in various application areas [1–3]. Polyhedral oligomeric silsesquioxanes (POSS) make one of the most interesting examples of the organic-inorganic compounds, and their advantageous features have contributed to the rapid progress in the research on their synthesis and applications. A POSS contains an inorganic core consisting of the silicon and oxygen atoms which usually has the form of a cage. Various substituents may be attached to each corner silicon atom, thus the properties of POSS can be modified in the wide range by the structural control [4]. Developing new POSS-containing polymeric materials with improved thermal stability and resistance to oxidation [5–10] has been one of the most promising application fields for POSS compounds. Significant opportunities have been provided by copolymerization of alkenylsilsesquioxanes with olefins. However, the number of literature data concerning coordination (co) polymerization of such monomers with the use of organometallic catalysts is still limited [5–10]. It is thus very difficult to determine ⁎

Corresponding author. E-mail address: [email protected] (K. Dziubek).

http://dx.doi.org/10.1016/j.eurpolymj.2017.03.038 Received 28 September 2016; Received in revised form 13 February 2017; Accepted 15 March 2017 Available online 18 March 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. Molecular structures of POSS comonomers used; monoalkenyl(siloxy)silsesqioxanes: (a) POSS-(i-Bu)7(OSi(CH3)2(CH2CH]CH2), (b) POSS-(i-Bu)7(OSi (CH3)2C4H8CH] CH2), (c) POSS-(i-Bu)7(OSi(CH3)2(C8H16CH]CH2), (d) POSS-(Cy)7(OSi(CH3)2(C8H16CH] CH2), and monoalkenylsilsesquioxanes: (e) POSS-(iBu)7(CH]CH2), (f) POSS-(i-Bu)7(CH2CH]CH2), (g) POSS-(i-Bu)7(C4H8CH]CH2), (h) POSS-(i-Bu)7(C8H16CH]CH2), (i) POSS-(Cy)7(CH2CH] CH2), and (j) POSS(Cy)7(C4H8CH]CH2).

even a generic correlation between the structure of the POSS comonomer and the properties of the obtained hybrid copolymers. The purpose of the present work was to comprehensively study the influence of the structure of monofunctional silsesquioxane comonomers on the performance of the ethylene/POSS copolymerization process in which the rac-ethylenebis(indenyl)zirconium dichloride catalyst (rac-Et(Ind)2ZrCl2) activated by methylaluminoxane (MAO) was used. Monoalkenyl- and monoalkenyl(siloxy) silsesquioxanes (without or contained the dimethylsiloxy spacer which connected the alkenyl substituent with the POSS cage, respectively) with different substituents attached to the corner silicon atoms in the T8 POSS cage were applied as comonomers (Fig. 1). The studied POSS derivatives contained seven iso-butyl (i-Bu) or cyclohexyl (Cy) substituents as non-reactive groups. In turn, the reactive n-alkenyl substituents were different from each other in the length. It should be noted that, to the best of our knowledge, that type POSS derivatives have not been used until now in copolymerization with ethylene over metallocene catalysts. Moreover, there is no comparative report available, which would present the relation between the POSS structure and the activity of a catalytic system, (co)monomer reactivity in copolymerization in ethylene, (co)monomer incorporation degree as well as physicochemical properties of the copolymer products obtained.

2. Experimental 2.1. Materials Chlorosilanes (Aldrich), anhydrous magnesium sulphate (Aldrich), calcium hydride (Aldrich), triethylamine (Fluka), silica gel 60 (Fluka), 1,3,5,7,9,11,14-heptaisobutyltricyclo-[7.3.3.15,11]heptasiloxane-endo-3,7,14-triol (trisilanolisobutyl POSS) (Hybrid Plastics), methylaluminoxane (MAO, 10 wt.%, Sigma-Aldrich), rac-ethylenebis-(1-η5-indenyl)-zirconium dichloride (rac-Et (Ind)2ZrCl2, Sigma-Aldrich), 1,2-dichlorobenzene-d4 (POCH Gliwice), chloroform-d (99.8%, Deutero GmbH), hydrochloric acid (35–38%, POCH Gliwice) and methanol (POCH Gliwice) were used as purchased. Pure grade n-pentane (Chempur), n-hexane (Chempur) and THF (Chempur) were dried prior to use over CaH2 and stored under argon. Ethylene (Grade 3.5, Air Liquide) and nitrogen (Messer) were used after passing through a column with the sodium metal supported on Al2O3. Toluene (POCH Gliwice) was refluxed over sodium. 1,2,4-Trichlorobenzene (TCB, 99 wt.%) (Aldrich) was purified by distillation.

2.2. Synthesis and NMR characterization of 1-alkenyl- and 1-alkenyl(dimethylsiloxy)-3,5,7,9,11,13,15-hepta(iso-butyl)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxanes (monoalkenyl- and monoalkenyl(siloxy)silsesquioxanes) All syntheses were conducted under argon atmosphere using standard Schlenk-line and vacuum techniques. Depending whether the alkenyl group was attached directly to the POSS core (monoalkenylsilsesquioxanes) or via eSiOe spacer (monoalkenyl(siloxy)silsesquioxanes), the methodology for their syntheses varied. It was based on hydrolytic condensation of incompletely condensed trisilanolisobutyl POSS or consecutive hydrolytic condensation followed by hydrolysis of its intermediate chlorosubstituted derivative and second condensation reaction with the respective alkenylchlorosilane. This synthetic methodology for the synthesis of monofunctional silsesquioxanes is well-documented [2]. It was applied in the modified version described by Marciniec et al. [11] and analytically pure compounds were isolated with the yield of 89–97%. The structure and synthetic route to 1alkenyldimethylsiloxy-3,5,7,9,11,13,15-hepta(iso-butyl)pentacyclo-[9.5.1.13,9.15,15.17,13]octasiloxanes is shown in Scheme 1 [12]. All synthesized compounds were characterized by NMR analysis. 369

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Scheme 1. Reaction path for the synthesis of 1-alkenyl- and 1-alkenyl(dimethylsiloxy)-3,5,7,9,11,13,15-hepta(iso-butyl)pentacyclo-[9.5.1.13,9.15,15.17,13]octasiloxanes.

2.2.1. NMR spectral data for monoalkenylsilsesquioxanes 1-Vinyl-3,5,7,9,11,13,15-hepta(iso-butyl)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, POSS-(i-Bu)7(CH]CH2); isolation yield: 97%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.60–0.63 (m, 14H eCH2e (i-Bu)), 0.95–0.96 (m, 42H, eCH3e (i-Bu)), 1.82–1.90 (m, 7H, eCHe (i-Bu)), 5.87 (dd, JH,H = 14.04 Hz, 20.45 Hz, 1H, H2C]CHe), 5.98 (dd, JH,H = 4.88 Hz, 20.45 Hz, 1H, H2C]CHe), 6.04 (dd, JH,H = 4.58 Hz, 14.34 Hz, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): 22.66–22.76 (i-Bu), 24.10 (i-Bu), 25.91–26.02 (iBu), 130.19 (H2C]CH), 136.06 (H2C]CHe). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): −67.40, −67.86; −81,54 (eSiVi). 1-Allyl-3,5,7,9,11,13,15-hepta(iso-butyl)-pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, POSS-(i-Bu)7(CH2CH]CH2); isolation yield: 96%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.59–0.61 (m, 14H, eCH2e (i-Bu)), 0.95–0.97 (m, 42H, eCH3 (i-Bu)), 1.60 (d, JH,H = 7.63 Hz, 2H, H2C]CHeH2Ce), 1.81–1.90 (m, 7H, eCHe (i-Bu)), 4.88–4.96 (m, 2H, H2C]CHe), 5.72–5.81 (m, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): 19.75 (H2C]CH-H2Ce), 22.38–22.50 (i-Bu), 23.83 (i-Bu), 25.67–25.71 (i-Bu), 114.76 (H2C]CHe), 132.28 (H2C]CHe). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): −67.51, −67.90, −68.24; −71.80 (eSiAll). 1-Allyl-3,5,7,9,11,13,15-hepta(cyclohexyl)-pentacyclo[9.5.1.1 3,9.15,15.17,13]octasiloxane, POSS-(Cy)7(CH2CH]CH2); isolation yield: 95%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.73–0.77 (m, 7H, eCHe (Cy)), 1.23–1.26 (m, 35H, eCH2e (Cy)), 1.61 (d, JH,H = 7.63 Hz, 2H, H2C]CHeH2Ce), 1.73 (m, 35H, eCH2e (Cy)), 4.90–4.98 (m, 2H, H2C]CHe), 5.74–5.83 (m, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): 19.70 (H2C]CHeH2Ce), 23.09–23.15 (Cy), 26.56–26.63 (Cy), 26.89 (Cy), 27.46–27.49 (Cy), 114.71 (H2C]CHe), 132.60 (H2C]CH). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): −68.67; −68.68; −71.03 (eSieAll). 1-Hexenyl-3,5,7,9,11,13,15-hepta(iso-butyl)-pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, POSS-(i-Bu)7(C4H8CH]CH2), isolation yield: 95%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.59–0.61 (m, 16H, eH2Ce (hexenyl), eCH2e (i-Bu)), 0.95–0.96 (m, 42H, eCH3 (i-Bu)), 1.41–1.43 (m, 4H, eH2Ce (hexenyl)), 1.82–1.90 (m, 7H, eCHe (i-Bu)), 2.03–2.05 (m, 2H, eH2Ce (hexenyl)), 4.92–5.01 (m, 2H, H2C]CHe), 5.76–5.84 (m, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): 11.87 (eH2Ce), 22.20 (eH2Ce), 22.49–22.52 (iBu), 23.86–23.88 (i-Bu), 25.68–25.69 (i-Bu), 31.85 (eH2Ce), 33.44 (eH2Ce), 114.16 (H2C]CHe), 138.97 (H2C]CHe). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): −67.17 (Si-hexenyl); −67.74, −67.77, −67.90. 1-Hexenyl-3,5,7,9,11,13,15-hepta(cyclohexyl)-pentacyclo[9.5.1.1 3,9.15,15.17,13]octasiloxane, POSS-(Cy)7(C4H8CH]CH2); isolation yield: 92%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.59–0.63 (m, 2H, eH2Ce (hexenyl)), 0.73–0.77 (m, 7H, eCHe (Cy)), 1.22–1.26 (m, 35H, eCH2e (Cy)), 1.42–1.45 (m, 4H, eH2Ce (hexenyl)), 1.72–1.74 (m, 35H, eCH2e (Cy)), 2.04–2.06 (m, 2H, eH2Ce (hexenyl)), 4.92–5.02 (m, 2H, H2C]CHe), 5.77–5.85 (m, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): 11.70 (eH2Ce), 22.34 (eH2Ce), 23.14–23.16 (Cy), 26.61–26.64 (Cy), 26.90 (Cy), 27.48–27.50 (Cy), 31.80 (eH2Ce), 33.41 (eH2Ce), 114.15 (H2C]CHe), 138.98 (H2C]CHe). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): −66.29 (Si-hexenyl), −68.67, −68.69, −68.83. 1-Decenyl-3,5,7,9,11,13,15-hepta(iso-butyl)-pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, POSS-(i-Bu)7(C8H16CH]CH2); isolation yield: 90%. 1 H NMR (300.2 MHz, CDCl3, δ, ppm): 0.57–0.61 (m, 16H, eCH2e (decenyl), eCH2e (i-Bu)), 0.94–0.96 (m, 42H, eCH3 (i-Bu)), 1.35–1.39 (m, 12H, eCH2e(decenyl), 1.79–1.92 (m, 7H, eCHe (i-Bu)), 2.00–2.07 (m, 2H, eH2Ce(decenyl)), 4.91–5.02 (m, 2H, H2C]CHe), 5.75–5.88 (m, 1H, H2C]CHe). 13C NMR (100.6 MHz, CDCl3, δ, ppm): 12.30 (eH2Ce), 22.82–22.84 (i-Bu), 22.96 (eH2Ce), 24.18–24.20 (i-Bu), 26.01 (i-Bu), 29.30 (eH2Ce), 29.50 (eH2Ce), 29.56 (eH2Ce), 29.67 (eH2Ce), 32.95 (eH2Ce), 34.15 (eH2Ce), 114.40 (H2C]CHe), 139.56 (H2C]CHe). 29Si NMR (79.5 MHz, CDCl3, δ, ppm): −67.00 (Si-decenyl); −67.77, −67.91.

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2.2.2. NMR spectral data for monoalkenyl(siloxy)silsesquioxanes 1-(Dimethylallylsiloxy)-3,5,7,9,11,13,15-hepta(iso-butyl)-pentacyclo[9.5.1.13,9.15,15.17,13]-octasiloxane, POSS-(i-Bu)7(OSi(CH3)2(CH2CH]CH2); isolation yield: 93%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.12 (s, 6H, SiCH3), 0.59–0.62 (m, 14H, eCH2e (i-Bu)), 0.95–0.97 (m, 42H, eCH3 (i-Bu)), 1.61 (d, JH,H = 8.85 Hz, 2H, H2C]CHeH2CeSie), 1.81–1.91 (m, 7H, eCHe (i-Bu)), 4.84–4.90 (m, 2H, H2C]CHe), 5.76–5.84 (m, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): −0.83 (CH3Si), 22.37–22.48 (i-Bu), 23.81–23.84 (i-Bu), 25.68 (i-Bu, H2C]CHeH2CeSie), 113.52 (H2C]CHe), 133.97 (H2C]CHe). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): 8.13 (eOSi(CH3)2e), −67.02, −67.85, −109.67 (O4Si). 1-(Dimethylhexenylsiloxy)-3,5,7,9,11,13,15-hepta(iso-butyl)-pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, POSS-(i-Bu)7(OSi (CH3)2C4H8CH]CH2); isolation yield: 92%. 1 H NMR (500.3 MHz, CDCl3, δ, ppm): 0.10 (s, 6H, SiCH3), 0.56–0.62 (m, 16H, eCH2e (hexenyl), eCH2e (i-Bu)), 0.95–0.97 (m, 42H, eCH3 (i-Bu)), 1.37–1.41 (m, 4H, eCH2e (hexenyl)), 1.83–1.90 (m, 7H, eCHe (i-Bu)), 2.04–2.06 (m, 2H, eCH2e (hexenyl)), 4.91–5.01 (m, 2H, H2C]CHe), 5.77–5.85 (m, 1H, H2C]CHe). 13C NMR (125.8 MHz, CDCl3, δ, ppm): −0.07 (CH3Si), 17.91 (eH2Ce), 22.65–22.79 (i-Bu, eH2Ce), 24.06–24.09 (i-Bu), 25.94 (i-Bu), 32.88 (eH2Ce), 33.81 (eH2Ce), 114.34 (H2C]CHe), 139.36 (H2C]CHe). 29Si NMR (99.4 MHz, CDCl3, δ, ppm): 11.33 (eOSi(CH3)2e), −67.07, −67.86, −109.68 (O4Si). 1-(Dimethyldecenylsiloxy)-3,5,7,9,11,13,15-hepta(iso-butyl)-pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane, POSS-(i-Bu)7(OSi (CH3)2(C8H16CH]CH2); isolation yield: 91%. 1 H NMR (400.2 MHz, CDCl3, δ, ppm): 0.09 (s, 6H, SiCH3), 0.54–0.63 (m, 16H, eH2Ce (decenyl), eCH2e (i-Bu)), 0.95–0.97 (m, 42H, eCH3 (i-Bu)), 1.29–1.41 (m, 12H, eH2Ce (decenyl)), 1.81–1.91 (m, 7H, eCHe (i-Bu)), 2.02–2.07 (m, 2H, eH2Ce (decenyl)), 4.91–5.02 (m, 2H, H2C]CHe), 5.76–5.86 (m, 1H, H2C]CHe). 13C NMR (100.6 MHz, CDCl3, δ, ppm): −0.31 (CH3Si), 17.86 (eH2Ce), 22.41–22.50 (i-Bu), 23.02 (eH2Ce), 23.82–23.86 (i-Bu), 25.69 (i-Bu), 28.96 (eH2Ce), 29.23 (eH2Ce), 29.40 (eH2Ce), 29.51 (eH2Ce), 33.49 (eH2Ce), 33.85 (eH2Ce), 114.10 (H2C]CHe), 139.23 (H2C]CHe). 29Si NMR (79.5 MHz, CDCl3, δ, ppm): 11.40 (eOSi(CH3)2e), −67.08, −67.87, −109.68 (O4Si). 1-(Dimethyldecenylsiloxy)-3,5,7,9,11,13,15-hepta(cyclohexyl)-pentacyclo[9.5.1.1 3,9.15,15.17,13]octasiloxane, POSS-(Cy)7(OSi (CH3)2(C8H16CH]CH2); isolation yield: 89%. 1 H NMR (400.2 MHz, CDCl3, δ, ppm): 0.12 (s, 6H, SiCH3), 0.56–0.60 (m, 2H, eH2Ce (decenyl)), 0.73–0.78 (m, 7H, eCHe (Cy)), 1.22–1.39 (m, 47H, eH2Ce (decenyl), eCH2e (Cy)), 1.73–1.76 (m, 35H, eCH2e (Cy)), 2.01–2.07 (m, 2H, eH2Ce (decenyl)), 4.91–5.01 (m, 2H, H2C]CHe), 5.76–5.86 (m, 1H, H2C]CHe). 13C NMR (100.6 MHz, CDCl3, δ, ppm): −0.20 (CH3Si), 17.92 (eH2Ce), 23.05–23.15 (eH2Ce, (Cy)), 26.53–26.63 (Cy), 26.88 (Cy), 27.45–27.49 (Cy), 28.96 (eH2Ce), 29.23 (eH2Ce), 29.41 (eH2Ce), 29.51 (eH2Ce), 33.50 (eH2Ce), 33.84 (eH2Ce), 114.07 (H2C]CHe), 139.23 (H2C]CHe). 29Si NMR (79.5 MHz, CDCl3, δ, ppm): 11.13 (eOSi(CH3)2e), −68.13, −68.66, −68.69, −108.48 (O4Si). 2.3. Homopolymerization of ethylene All the operations were performed under dry and oxygen-free conditions using the inert nitrogen environment. The ethylene polymerization reactions were performed in the Büchi reactor (500 cm3) equipped with the magnetic stirrer and heating-cooling jacket. The reactor was purged with nitrogen and heated up to 55 °C for 30 min before polymerization. Toluene (150 cm3), the required amount of MAO (nMAO = 6.02 · 10−3 mol) and the catalyst solution in toluene (nZr = 7 · 10−7 mol) were introduced into the reactor. When the required temperature (50 °C) was reached, the ethylene gas was fed. The ethylene feed pressure (0.5 MPa) and the temperature were kept constant through the runs. After the prescribed polymerization time, the ethylene gas feed was stopped, the pressure in the reactor was reduced to 0.1 MPa and the obtained mixture was transferred to a dilute solution of hydrochloric acid in methanol. Polyethylene was filtered and purified by stirring for 2 h with the 5% HCl solution in methanol, then with methanol, and dried to constant weight. 2.4. Copolymerization of ethylene with POSS Copolymerization of ethylene with POSS comonomers was carried out in the same way as polymerization of ethylene. Toluene (140 cm3), the required amounts of MAO (nMAO = 3.01–9.03 · 10−3 mol), the POSS solution in 10 cm3 toluene ([POSS] = 0.67; 1.67; 3.33; 6.67·10−3 mol/dm3 in the reaction mixture), and the catalyst solution in toluene (nZr = 7·10−7 mol) were introduced into the reactor. When the required temperature (50 °C) was reached, the ethylene gas was fed. The ethylene feed pressure (0.5 MPa) and the temperature were kept constant through the runs. After the prescribed polymerization time, the ethylene gas feed was stopped, the pressure in the reactor was reduced to 0.1 MPa and the obtained mixture was transferred to a dilute solution of hydrochloric acid in methanol. The copolymer was filtered, washed with hexane three times, with 5% HCl solution in methanol, and then with methanol. The copolymer product was dried to constant weight after successive steps of washing. In order to evaluate the purification of ethylene/POSS copolymers, FT-IR was used to detect the residual of the unreacted POSS comonomer. 2.5. Characterization The FT-IR analysis was accomplished using a Nicolet Nexus 2002 FT-IR spectrometer from 4000 to 400 cm−1 with the 2 cm−1 resolution. The (co)polymer samples were pressed into tablets form with KBr. The relative content of POSS in copolymers were estimated from the intensity ratio of the absorption band with the maximum at 1120 cm−1 for the SieO bonds to the absorption band 371

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at 2020 cm−1 for the methylene e(CH2)ne groups. The relative content of vinyl, vinylidene, trans-vinylidene end groups were estimated from the intensity ratio of the absorption band at 908, 888 and 965 cm−1, respectively, to the absorption band at 2020 cm−1. The calculations were performed using the OMNIC software. 1 H, 13C, 29Si NMR spectra of POSS comonomers were recorded on Brucker Avance 400 MHz and 500 MHz in CDCl3 as a solvent (chemical shifts are reported in ppm with reference to the residual CHCl3 solvent peaks for 1H and 13C and to TMS for 29Si). The 1H NMR of the copolymers were recorded with the Ultrashield Bruker spectrometer (400 MHz). The copolymers were analyzed in 1,2-dichlorobenzene-d4 at 120 °C. The peak of the residual undeuterated solvent at 7.249 ppm was chosen as the internal reference for the 1H NMR analysis. The crystallinity degrees, crystallization temperatures and melting temperatures of polyethylene and copolymers were estimated with a DSC 2010 TA Instruments at the heating rate of 10 °C/min. The sample was melted, then recrystallized and again heated up to about 170 °C in the inert gas atmosphere – in nitrogen. The polymer crystallinity degree was calculated from the equation:

χ=

ΔHf ΔHt ,c

·100%

where χ – crystallinity degree (%), ΔHf – heat of fusion for the sample (J/g), ΔHt,c – heat of fusion for standard (290 J/g) [13]. The average molecular weight (Mw) and molecular weight distribution (Mw/Mn) of each (co)polymer sample was determined by gel permeation chromatography using the Alliance 135 GPCV 2007 apparatus. The sample to be analyzed was dissolved at 135 °C for 24 h. The measurements were performed at 135 °C, with TCB as the solvent, and at the flow rate of 1.0 cm3/min. The data were analyzed using polystyrene calibration curves. The scanning electron microscope (SEM) experiments of the polyethylene and copolymers samples were carried out on the Hitachi model TM 3000 electron microscope. The samples were fixed on an aluminium sample stubs and coated with gold by the conventional sputtering techniques. The employed accelerating voltage was 15 kV for SEM. 3. Results and discussion 3.1. Activity of catalytic system Monofunctional silsesquioxanes with various substituents attached to the corner silicon atoms in the T8 POSS cages were applied as comonomers in copolymerization with ethylene, using the rac-Et(Ind)2ZrCl2/MAO catalytic system. The POSS derivatives differed from each other in the length of the reactive alkenyl substituent, in the absence or presence of the dimethylsiloxy spacer between the silicon-oxygen cage and alkenyl chain (monoalkenyl- and monoalkenyl(siloxy)silsesquioxanes, respectively) as well as in the type of seven non-reactive substituents: iso-butyl (i-Bu) or cyclohexyl (Cy) (Fig. 1). The results of the study of influence of the structures of monoalkenyl- and monoalkenyl(siloxy)silsesquioxanes comonomers on the activity of the catalytic system in ethylene/POSS copolymerization were presented in Tables 1 and 2. The catalytic system was found less efficient in copolymerization of ethylene with POSS versus ethylene homopolymerization (23,000 kgPE/molZr·0.5 h), irrespective to the kind of the POSS comonomer applied. This probably was due to the presence of a bulky comonomer in the copolymerization environment which significantly hindered the access of ethylene to the active site [14–18]. The copolymerization reaction was found to be strongly influenced by the structure of POSS derivatives which were used as comonomers. The catalytic system showed lower maximum activity in the case of copolymerization of ethylene with POSS comonomers which contained shorter alkenyl substituents and the same type of non-reactive groups at the silicon-oxygen cage. For example, in the case of iso-butyl substituted POSS, depending on the length of the alkenyl substituent, the maximum activity of the catalytic system increased as follows: POSS-(i-Bu)7(OSi(CH3)2(CH2CH]CH2) < POSS-(i-Bu)7(OSi(CH3)2(C4H8CH]CH2) < POSS(i-Bu)7(OSi(CH3)2(C8H16CH]CH2) and POSS-(i-Bu)7(CH]CH2) < POSS-(i-Bu)7(CH2CH]CH2) < POSS-(i-Bu)7(C4H8CH]CH2) < POSS-(i-Bu)7(C8H16CH]CH2). The kind of the non-reactive substituent at the silicon-oxygen cage of POSS comonomers also affected the performance of copolymerization process. In the case of POSS-(i-Bu)7(CH2CH]CH2) and POSS-(Cy)7(CH2CH]CH2), with a relatively short alkenyl substituent, a higher maximum activity was observed for cyclohexyl-substituted POSS. However, in copolymerization of ethylene with POSS which contained longer alkenyl groups: POSS-(i-Bu)7(C4H8CH]CH2) and POSS-(Cy)7(C4H8CH]CH2), as well as with POSS-(i-Bu)7(OSi(CH3)2(C8H16CH]CH2) and POSS-(Cy)7(OSi(CH3)2(C8H16CH]CH2), higher maximum activity values were obtained for the catalytic system when iso-butyl substituted POSS comonomers were used. It should be also noted that, in general, a higher maximum activity was obtained in the case of application of monoalkenylsilsesquioxane comonomers which contained no dimethylsiloxy spacer connected the alkenyl substituent with the silicon-oxygen cage. The literature reports indicate that the concentration of silsesquioxane comonomers in the reaction feed has a significant influence on the efficiency of copolymerization of olefins with POSS – the catalytic activity of the system deceases with the increasing amount of POSS introduced into the feed [5,8]. Our studies showed that the trend of changes in the activity of the catalytic system for the increasing concentration of the POSS comonomer (from 0.67 to 6.67 · 10−3 mol/dm3) was dependent on the structure of silsesquioxane derivative used. In the case of copolymerization of ethylene with POSS which contained short-chains (POSS-(i-Bu)7(OSi(CH3)2(CH2CH]CH2), POSS-(i-Bu)7(CH]CH2)) as well as the longest alkenyl reactive substituent (POSS-(i-Bu)7(OSi(CH3)2(C8H16CH]CH2), POSS372

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Table 1 The effects of the structure and concentration of monoalkenyl(siloxy)silsesquioxane on the performance of ethylene/POSS copolymerization over the rac-Et (Ind)2ZrCl2/MAO catalytic system. Item

(Co)monomers

[POSS] (10−3 mol/dm3)

Activitya

A1120 b A2020

POSS content in (co) polymerc mol%

wt.%

d

d

Tm (°C)

Xc (%)

d

Tc (°C)

Mw · 103 (g/ mol)e

Mw e Mn

1

E

0

22,920

0

0

0

132.0

64.2

117.5

165

1.9

2 3 4 5 6

E + POSS-(i-Bu)7(OSi(CH3)2 (CH2CH]CH2)

0.67 1.67 3.33 6.67 6.67f

5653 7161 6331 4210 8210

1.18 2.22 4.35 5.54 7.31

0.008 0.016 0.030 0.036 0.043

0.27 0.53 0.99 1.18 1.41

135.9 135.4 134.3 133.8 133.6

65.1 65.3 65.8 63.7 63.4

118.5 118.2 117.8 117.5 115.1

238 231 219 200 195

1.9 2.1 2.0 1.6 1.6

7 8 9 10 11

E + POSS-(i-Bu)7(OSi(CH3)2 (C4H8CH]CH2)

0.67 1.67 3.33 6.67 6.67f

3049 6566 7174 8274 9416

0.66 1.61 3.43 11.08 4.87

0.003 0.019 0.026 0.058 0.035

0.10 0.66 0.90 1.98 1.20

135.5 134.7 134.0 132.0 134.2

64.3 64.2 63.9 58.7 61.5

116.7 117.2 117.7 117.5 115.7

279 277 267 234 197

1.5 1.4 1.4 1.5 1.5

12 13 14 15 16

E + POSS-(i-Bu)7(OSi(CH3)2 (C8H16CH]CH2)

0.67 1.67 3.33 6.67 6.67f

8667 8329 7453 7050 14,274

0.98 2.65 5.82 8.02 12.29

0.006 0.020 0.035 0.041 0.074

0.22 0.73 1.27 1.49 2.65

135.4 134.3 134.2 133.5 131.7

64.8 64.3 63.4 63.1 62.8

116.7 115.6 116.1 116.4 118.4

316 298 258 243 202

1.4 1.5 1.4 1.4 2.2

17 18 19 20 21

E + POSS-(Cy)7(OSi(CH3)2 (C8H16CH]CH2)

0.67 1.67 3.33 6.67 6.67f

8054 8107 8044 7880 11,467

2.12 5.77 7.63 10.91 13.07

0.011 0.036 0.039 0.056 0.075

0.47 1.66 1.41 2.37 3.15

135.0 133.0 132.7 132.6 132.0

64.1 63.1 62.5 59.1 58.9

119.1 117.3 117.0 115.2 114.3

274 254 242 241 239

1.6 1.3 1.5 1.6 2.2

Reaction time: 30 min. a kg(co)polymer/molZr. b Relative content of POSS in copolymers – data were obtained by FT-IR. c Determined by: 1H NMR. d DSC. e GPC. f Reaction time: 60 min.

(Cy)7(OSi(CH3)2C8H16CH]CH2)), the activity of the catalytic system generally decreased with the increasing concentration of POSS in the feed. However, when ethylene was copolymerized with POSS with medium lengths of the alkenyl substituent (POSS-(iBu)7(CH2CH]CH2), and POSS-(i-Bu)7(OSi(CH3)2C4H8CH]CH2)), the activity was observed to increase gradually for the increasing concentration of POSS in the feed. Meanwhile, in copolymerization of ethylene with POSS derivatives which contained the longest alkenyl reactive substituent, the highest activity was obtained at the POSS concentration of 1.67 · 10−3 mol/dm3 (in the case of copolymerization of ethylene with POSS-(i-Bu)7(C4H8CH]CH2), POSS-(Cy)7(C4H8CH]CH2) and POSS-(i-Bu)7(C8H16CH]CH2)). It should also be emphasized that extended reaction time contributed to the increased yield of copolymerization, regardless of the POSS comonomer structure (Fig. 2). The results proved high stability of the catalyst in ethylene/POSS copolymerization which is atypical for homogeneous metallocene systems in ethylene polymerization but which is specific for the supported ones [19,20]. Moreover, these results indicated that the presence of POSS comonomers in the reaction medium did not cause any catalyst deactivation and the reaction time extension could be a convenient method to enhance the copolymerization yield. It should be also noted that, to our knowledge, the influence of the reaction time on the olefin/POSS copolymerization has not been studied until now. 3.2. Incorporation of POSS comonomer The successful incorporation of POSS units into polymer chains was confirmed by the FT-IR spectra. Fig. 3 shows the selected FT-IR spectra of neat polyethylene and copolymers of ethylene with POSS-(i-Bu)7(OSi (CH3)2(C8H16CH]CH2) which were obtained at different concentrations of POSS in the reaction feed. All FT-IR spectra exhibit the bands which are specific for polyethylene: (i) bands in the region of 2921–2869 cm−1 associated with stretching vibrations of the (sp3) CeH bond; (ii) band at 2020 cm−1 due to the presence of a long hydrocarbon chain in the (co) polymer structure; (iii) bands in the region of 1460–1370 cm−1 which correspond to the (sp3) C–H deformation vibrations in CH3, CH2 and CH groups; (iv) bands at 730 cm−1 and 719 cm−1 associated with rocking vibrations of CeH in CH2 in the crystalline and amorphous phases, respectively [21]. In addition, the bands at 1120 and 1229 cm−1 associated with stretching vibrations of SieO and SieC in the POSS cage were also identified in the FT-IR spectra for ethylene/POSS copolymers [7,22]. It should be also mentioned that the intensity of the band at 1120 cm−1 increased with the increasing concentration of the POSS comonomer in the reaction feed. Thus, it can be assumed that the number of POSS units incorporated into the copolymer increased. 373

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Table 2 The effects of the structure and concentration of monoalkenylsilsesquioxane on the performance of ethylene/POSS copolymerization over the rac-Et(Ind)2ZrCl2/MAO catalytic system. Item

(Co)monomers

[POSS] (10−3 mol/ dm3)

Activitya

A1120 b A2020

POSS content in (co) polymerc (mol%)

(wt.%)

d

Tm (°C)

d

Xc (%)

d

Mw · 103 (g/ mol)e

Mw e Mn

Tc (°C)

1 2 3 4 5

E + POSS-(i-Bu)7 (CH]CH2)

0.67 1.67 3.33 6.67 6.67f

7286 4953 3107 3113 3937

1.28 2.43 2.78 3.01 4.33

0.007 0.012 0.014 0.016 0.023

0.21 0.36 0.42 0.48 0.69

134.4 134.2 134.2 134.2 133.8

61.5 59.9 62.4 62.1 64.6

117.8 117.9 118.4 118.6 118.1

157 153 149 145 136

2.2 1.9 1.6 1.7 2.0

6 7 8 9 10

E + POSS-(i-Bu)7 (CH2CH]CH2)

0.67 1.67 3.33 6.67 6.67f

3369 3530 5086 8399 10,084

0.77 4.12 5.13 11.28 12.48

0.004 0.021 0.025 0.045 0.051

0.12 0.64 0.76 1.36 1.54

134.3 134.2 133.9 133.2 133.7

64.6 60.5 63.6 62.2 62.4

118.4 118.4 117.9 116.9 115.6

170 162 148 141 138

1.7 1.6 2.0 2.1 1.9

11 12 13 14 15

E + POSS-(i-Bu)7 (C4H8CH]CH2)

0.67 1.67 3.33 6.67 6.67f

8899 12,289 11,264 9820 14,214

2.01 6.86 10.74 12.30 14.76

0.008 0.027 0.057 0.062 0.074

0.26 0.86 1.80 1.95 2.32

133.2 133.5 133.6 132.8 132.0

65.5 65.7 63.7 64.9 64.3

118.2 115.7 115.2 115.6 115.1

148 104 93 89 83

2.3 2.2 2.4 2.7 2.4

21 22 23 24 25

E + POSS-(i-Bu)7 (C8H16CH]CH2)

0.67 1.67 3.33 6.67 6.67f

9721 13,507 10,594 7481 21,767

3.27 7.59 8.95 9.78 11.08

0.002 0.039 0.051 0.055 0.062

0.07 1.31 1.71 1.84 2.07

133.7 132.3 132.6 132.4 132.1

63.6 63.4 63.5 61.5 59.4

117.0 116.5 116.9 116.1 116.9

202 184 182 179 170

3.1 2.8 2.3 3.2 2.5

26 27 28 29 30

E + POSS-(Cy)7 (CH2CH]CH2)

0.67 1.67 3.33 6.67 6.67f

9826 7940 13,614 9087 19,519

1.37 2.34 4.64 8.21 12.04

0.003 0.005 0.011 0.022 0.032

0.11 0.19 0.41 0.81 1.17

135.8 135.2 134.3 134.1 133.0

66.2 65.6 65.0 64.4 63.3

117.8 116.1 117.2 114.9 116.0

114 71 64 61 60

2.3 2.0 2.1 2.4 2.2

31 32 33 34 35

E + POSS-(Cy)7 (C4H8CH]CH2)

0.67 1.67 3.33 6.67 6.67f

7096 11,700 10,352 7397 14,817

2.01 3.57 6.90 7.96 14.73

0.004 0.008 0.016 0.031 0.058

0.15 0.31 0.61 1.18 2.19

137.2 133.1 133.0 132.5 131.2

66.6 67.3 67.1 65.7 62.8

117.4 117.5 118.0 118.0 118.5

188 127 97 73 56

2.4 2.1 2.1 2.1 2.4

Reaction time: 30 min. a kg(co)polymer/molZr. b Relative content of POSS in copolymers – data were obtained by FT-IR. c Determined by: 1H NMR. d DSC. e GPC. f Reaction time: 60 min.

Fig. 2. Influence of reaction time on the performance of copolymerization of ethylene with POSS comonomers over the rac-Et(Ind)2ZrCl2/MAO catalytic system.

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Fig. 3. FT-IR spectra for (a) polyethylene and (b–d) copolymers of ethylene with POSS-(i-Bu)7(OSi(CH3)2(C8H16CH]CH2) obtained at different concentrations of POSS in the reaction feed: (b) 0.67; (c) 1.67 and (d) 6.67 · 10−3 mol/dm3.

Therefore, FT-IR spectroscopy proved to be useful for estimation of the relative content of POSS units in copolymers: ratio of band intensities at 1120 cm−1 and at 2020 cm−1 in FT-IR spectra were determined. It was found that the relative content of the POSS units in a copolymer increased with the increasing concentration of POSS in the reaction feed, regardless of the type of the POSS comonomer used (Tables 1 and 2). Interestingly, the POSS incorporation degree in the copolymer chain was considerably determined by the structures of silsesquioxane derivative. Among the studied POSS derivatives with different lengths of the reactive alkenyl substituents, the highest relative content of POSS units in the copolymer product was generally achieved after using POSS with the medium length of alkenyl substituent. This could be attributed to the influence of the inductive effect of this comonomer on the reactivity in copolymerization with ethylene. It should be also noted that the kind of non-reactive substituents attached to the POSS cage influenced the relative content of POSS units in copolymers. By comparing the results obtained for copolymers E/POSS-(i-Bu)7(CH2CH]CH2) and E/POSS(Cy)7(CH2CH]CH2) as well as E/POSS-(i-Bu)7(C4H8CH]CH2) and E/POSS-(Cy)7(C4H8CH]CH2), it was demonstrated that the copolymers with iso-butyl substituted POSS derivatives were characterized by a higher content of POSS units. However, in the case of ethylene copolymers with POSS-(i-Bu)7(OSi(CH3)2(C8H16CH]CH2) and POSS-(Cy)7(OSi(CH3)2(C8H16CH]CH2) which contained the dimethylsiloxy spacer at the long-chain alkenyl substituent, a higher relative content of POSS units was obtained for the product with cyclohexyl-substituted POSS. In both cases the probable explanation is the influence of the steric effect of the bulky cyclohexyl group. For POSS derivatives with short-chain alkenyl substituents and bulky Cy groups, coordination of vinyl groups to the active center could be sterically hindered. In the second case, the cyclohexyl groups probably more effectively limited the conformational freedom of the reactive long alkenyl substituents. It should be mentioned that the time of copolymerization also proved an important factor which influenced incorporation of the POSS comonomer into the copolymer. It turned out that the relative content of POSS units in the copolymer increased with the extension of the copolymerization time, regardless of the kind of the POSS comonomer used. Thus, it could be concluded that the extended reaction time did not limit incorporation of the POSS comonomer into the copolymer chain and that could be the effective way to increase not only the copolymer yield but also the content of POSS in the copolymer. The compositions of copolymers were also determined by 1H NMR spectroscopy. In the case of copolymers of ethylene with isobutyl substituted POSS, the POSS content was calculated based on the ratio of the integral intensities of the signals for methyl protons (1H) of the iso-butyl group in the POSS unit (δ = 0.9 ppm) and the methylene protons (4H) in the main polymer chain (δ = 1.3 ppm) (Fig. 4). As regards copolymers which contained cyclohexyl-substituents POSS, the signals for methylene protons (2 and 3H) of cyclohexyl groups (δ = 1.6 and 1.8 ppm) were taken into account (Fig. 5) [5,8,22]. Tables 1 and 2 summarize the absolute contents of POSS in the copolymers as estimated from 1H NMR spectra. It should be noted that the results of the 1H NMR analysis were in line with those from the FT-IR data for ethylene/POSS copolymers. Thus, it was also confirmed that the content of POSS incorporated into the copolymer chain increased with the increasing concentration in the reaction feed as well as for longer copolymerization times. On the basis of the results obtained, it could be also concluded that the highest efficiency of ethylene/POSS copolymerization – the highest maximum activity and high content of POSS in the copolymer product (up to 3.15 wt.%) – could be provided by the use of POSS which contain long-chain alkenyl reactive substituents. 3.3. Reactivity ratios and copolymer composition The compositions of ethylene/POSS copolymers were also evaluated by determining the reactivity ratios and structural 375

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Fig. 4. 1H NMR spectrum of ethylene/POSS-(i-Bu)7(OSi(CH3)2(CH2CH]CH2) copolymer.

parameters which characterize the copolymers. In the case of ethylene/POSS copolymers, due to their low contents of POSS comonomer, the copolymer composition equation as used in the Fineman-Ross method was replaced by the simplified equation of Böhm: mM/mE = [1/rE] × [MM/ME], where mE and mM are mole fractions of ethylene (E) and the comonomer (M); MM and ME are concentrations of ethylene and comonomer (POSS) in the copolymerization feed; and rE is the reactivity ratio of ethylene [23,24]. The results as presented in Table 3 confirmed that the structure of POSS comonomer significantly affected its reactivity. It was assumed that the reactivity of comonomer decreases when the reactivity ratio of ethylene rE increases. Thus, the ability to copolymerize with ethylene for POSS comonomers with the same type of non-reactive groups generally increased with the increasing lengths of the reactive alkenyl chains attached to the POSS cage, ranging in chain length from ethylene to hexylene, but a decrease was observed in the case of POSS comonomers with the decenyl reactive substituent. The explanation of this phenomenon is a strong inductive effect which was observed in the case of POSS molecules with shortchain reactive substituents. That could be confirmed by the presence of the stretching band at 1632 cm−1 associated with the vinyl group in the FT-IR spectra (Fig. 6). It results from a shift in electron density from the vinyl group to the electron-withdrawing silicon atoms from the silicon-oxygen cage of POSS. For longer alkenyl substituents on the POSS cage, the inductive effect became weaker in the hydrocarbon chain which consisted of single bonds only. Thus, the band corresponding to C]C was shifted towards higher wavenumbers (Fig. 6). No additional increase in the force constant of the double bond in the POSS molecule was observed for POSS with the longest alkenyl substituent (Fig. 6). The inductive effect did not influence reactivity of this POSS comonomer due to sufficient separation of its silicon-oxygen cage from the vinyl group in the alkenyl substituent, and its decreased ability to copolymerize may result from the conformational freedom of the long-chain alkenyl substituents. Moreover, the cyclohexyl-substituted POSS derivatives were characterized by the lowest reactivity in copolymerization with ethylene as compared to iso-butyl substituted ones. It could be caused by the steric hindrance of cyclohexyl groups as well. It was also confirmed that the length of the average sequence of ethylene also increased with the increasing reactivity ratio for

Fig. 5. 1H NMR spectrum of ethylene/POSS-(Cy)7(OSi(CH3)2(C8H16CH] CH2) copolymer.

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Table 3 Reactivity ratios rE and structural parameters for copolymers of ethylene with POSS comonomers. Item

(Co)monomers

rE

Molar fraction of comonomer into copolymer POSS

E

Average sequence length of E

Monomer dispersity

1 2b

E + POSS-(i-Bu)7(OSi(CH3)2(CH2CH] CH2)

57.1

0.00016 0.00036

0.99994 0.99964

6250 2778

100.0 100.0

3a 4b

E + POSS-(i-Bu)7(OSi(CH3)2(C4H8CH]CH2)

29.8

0.00019 0.00058

0.99981 0.99942

5263 1724

100.0 99.9

5a 6b

E + POSS-(i-Bu)7(OSi (CH3)2(C8H16CH] CH2)

47.8

0.00020 0.00041

0.99980 0.99959

5000 2439

100.0 100.0

7a 8b

E + POSS-(Cy)7(OSi(CH3)2(C8H16CH] CH2)

40.4

0.00036 0.00056

0.99964 0.99944

2778 1786

100.0 99.9

9a 10b

E + POSS-(i-Bu)7(CH]CH2)

222.1

0.00012 0.00016

0.99988 0.99984

8333 6250

100.0 100.0

11a 12b

E + POSS-(i-Bu)7(CH2CH] CH2)

42.0

0.00021 0.00045

0.99979 0.99955

4762 2222

100.0 100.0

13a 14b

E + POSS-(i-Bu)7(C4H8CH]CH2)

30.1

0.00027 0.00062

0.99973 0.99938

3704 1613

100.0 99.9

15a 16b

E + POSS-(i-Bu)7(C8H16CH] CH2)

35.9

0.00039 0.00055

0.99961 0.99945

2564 1818

100.0 99.9

17a 18b

E + POSS-(Cy)7(CH2CH] CH2)

80.3

0.00005 0.00022

0.99995 0.99978

20,000 4546

100.0 100.0

19a 20b

E + POSS-(Cy)7(C4H8CH]CH2)

57.0

0.00008 0.00031

0.99992 0.99969

12,500 3226

100.0 100.0

a

Concentration of POSS in reaction feed: a 1.67 · 10−3 mol/dm3. b 6.67 · 10−3 mol/dm3.

ethylene (Table 3). In turn, the incorporation degree of POSS units into the polymer chain increased with the increase in the POSS ability to copolymerize with ethylene. However, the calculated values of monomer dispersity (MD) were close to the value of 100 [25]. This is indicative for isolated POSS units in the copolymer chain, irrespective of the kind and the content of POSS comonomers in the polymer. 3.4. Molecular weight characterization The results presented in papers [5,8] show that incorporation of POSS comonomers into PE or PP chains decrease the molecular weight of copolymers obtained. In this study it was found that the average molecular weights, Mw were depended on the kind and concentration of POSS

Fig. 6. FT-IR spectra for monoalkenyl(siloxy)silsesquioxanes: (a) POSS-(i-Bu)7(OSi(CH3)2(CH2CH] CH2), (b) POSS-(i-Bu)7(OSi(CH3)2(C4H8CH]CH2), and (c) POSS-(iBu)7(OSi(CH3)2(C8H16CH]CH2).

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Fig. 7. The GPC curves of polyethylene, E/POSS-(i-Bu)7(OSi(CH3)2(C4H8CH]CH2) (a) and E/POSS-(i-Bu)7(C4H8CH]CH2) (b) copolymers produced using rac-Et (Ind)2ZrCl2/MAO catalytic system.

comonomer used. In the case of ethylene copolymers with monoalkenyl(siloxy)silsesquioxanes the Mw values were higher (up to 316 · 103 g/mol, Table 1) than for polyethylene (165 · 103 g/mol). In turn, for copolymers with monoalkenylsilsesquioxanes the Mw values were in generally lower (Table 2) in comparison to PE. However, the molecular weights of ethylene/POSS copolymers decreased as the POSS concentration increased in the feed and at extended reaction times, irrespective of the kind of the POSS comonomer used. It should be noted, too, that the increasing concentration of POSS comonomer in the feed and extended copolymerization time resulted in the increased content of POSS in a copolymer at the same time. Hence, it can be assumed that the ratio of the propagation rate to the chain termination rate was decreased in the presence of a POSS comonomer in the reaction feed. In the case of ethylene copolymers with monoalkenylsilsesquioxanes it could be concluded that the POSS comonomer act as a chain transfer agent. In contrast, for ethylene copolymers with monoalkenyl(siloxy)silsesquioxanes, it could be assumed with the POSS comonomer not affected on the chain termination reaction. Moreover, in this case, the decrease in Mw values with the increasing POSS concentration in the feed is due to steric effect of comonomer. Those phenomenons may be explained more detailed by different chain termination reaction determined by the kind of POSS comonomer used. It is also worth emphasizing that the molecular weight distribution in the copolymers obtained was narrow (Mw/Mn = 1.3–3.1, Fig. 7, Tables 1 and 2) which is typical for (co)polymers prepared by a single-site catalyst [26,27]. 3.5. Analysis of unsaturated end groups The unsaturated end groups in selected ethylene/POSS copolymer samples were analyzed through FT-IR and 1H NMR spectra. As can be seen in the exemplary FT-IR spectra (Fig. 8), various types of unsaturated end group were formed in the copolymer chains: vinyl (bands at 908 cm−1 and 990 cm−1), trans-vinylene (at 965 cm−1), vinylidene (at 888 cm−1) and tri-substituted end groups (at 820 cm−1). Based on the spectra for all ethylene/POSS copolymers, the relative contents of specific unsaturated groups were determined (Table 4). It was found that copolymer chains contained mainly vinyl end groups and their relative contents increased with the increasing contents of POSS units in the polymer chains. Interestingly, in the case of ethylene copolymer with monoalkenyl(siloxy)silsesquioxanes which contained the dimethylsiloxy spacer in the reactive alkenyl substituent, the vinyl end groups were dominant and other types of unsaturation were hardly observed (Fig. 8a).

Fig. 8. The vinyl region in the FT-IR spectra of ethylene/POSS-(i-Bu)7(OSi(CH3)2(C4H8CH]CH2) (a) and ethylene/POSS-(Cy)7(C4H8CH]CH2) (b); polyethylene (1) and copolymers (2 and 3) at 1.67 and 6.67 · 10−3 mol/dm3 concentrations of POSS in the reaction feed.

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Table 4 Relative contents of unsaturated end groups in copolymers of ethylene with POSS comonomers. Item

(Co)monomers

POSS content in copolymerc

A965/A2020d

A908/A2020d

A888/A2020d

(mol%)

(wt.%)

E + POSS-(i-Bu)7 (OSi(CH3)2(CH2CH]CH2)

0.016 0.036

0.53 1.18

0 0

0.425 0.487

0 0.064

3a 4b

E + POSS-(i-Bu)7(OSi(CH3)2(C4H8CH]CH2)

0.021 0.058

0.64 1.98

0 0

0.478 0.499

0 0

5a 6b

E + POSS-(i-Bu)7 (OSi(CH3)2(C8H16CH]CH2)

0.020 0.041

0.73 1.49

0 0

0.454 0.544

0 0

7a 8b

E + POSS-(Cy)7 (OSi(CH3)2(C8H16CH]CH2)

0.036 0.056

1.54 2.37

0 0

0.584 0.756

0 0

9a 10b

E + POSS-(i-Bu)7(CH]CH2)

0.012 0.016

0.36 0.48

0.042 0.048

0.404 0.421

0.032 0.046

11a 12b

E + POSS-(i-Bu)7(CH2CH]CH2)

0.021 0.045

0.64 1.36

0.034 0.006

0.337 0.361

0.010 0.033

13a 14b

E + POSS-(i-Bu)7(C4H8CH]CH2)

0.027 0.062

0.86 1.95

0.033 0.045

0.429 0.565

0.013 0.043

15a 16b

E + POSS-(i-Bu)7(C8H16CH] CH2)

0.038 0.053

1.28 1.78

0.033 0.060

0.497 0.572

0.011 0.022

17a 18b

E + POSS-(Cy)7(CH2CH]CH2)

0.005 0.022

0.19 0.81

0 0

0.406 0.648

0.250 1.740

19a 20b

E + POSS-(Cy)7(C4H8CH] CH2)

0.008 0.031

0.31 1.18

0.009 0.015

0.574 0.631

0.389 1.023

a

1 2b

Concentration of POSS in reaction feed: a 1.67 · 10−3 mol/dm3. b 6.67 · 10−3 mol/dm3. c Determined by 1H NMR. d Data were obtained by FT-IR.

Moreover, in the case of copolymers with monoalkenylsilsesqioxanes without the dimethylsiloxy spacer, the increase of the intensity of the vinylidene absorption band at 888 cm−1, trans-vinylene absorption band at 965 cm−1 and tri-substituted end groups at 820 cm−1 were visible (Fig. 8b). It should be noted, however, that the content of those types of unsaturated end groups were clearly lower than the content of vinyl groups (Table 4). The presence of unsaturated groups was also identified in the 1H NMR spectrum by the characteristic signals at about 4.6 for vinylidene, at 4.9 and 5.8 for vinyl, and at 5.4 ppm for tri-substituted end groups. The latter signal overlaps with the signal for the trans-vinylene end groups occurring in the same range (Fig. 9). Similarly to FT-IR results, in the NMR spectra, signals assigned to the vinyl end groups were primarily observed, for all the studied copolymers regardless of the type of the POSS comonomer. Moreover, no presence of other unsaturated end group was noticed in the NMR spectra of copolymers which contained monoalkenyl(siloxy)silsesqioxanes (Fig. 9a). It should be noted that the signals from the end vinylidene groups and trans-vinylene could be seen in the spectra in the case of monoalkenylsilsesqioxanes (Fig. 9b). The high content of vinyl end groups in the copolymers indicates that the main mechanism of the polymer chain termination is β-

Fig. 9. 1H NMR spectra of ethylene/POSS-(i-Bu)7(OSi(CH3)2(C4H8CH] CH2) copolymer (a) and ethylene/POSS-(Cy)7(C4H8CH]CH2) (b) at 6.67 · 10−3 mol/dm3 of POSS in the reaction feed.

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hydrogen elimination or β-hydrogen transfer to the (co)monomer [17,28]. However, a presence of other types of unsaturated end groups, which were additionally detected only in the case of ethylene copolymers with monoalkenylsilsesqioxanes, shows that the growth of these macromolecules can also be terminated in other mechanisms. Vinylidene end groups may arise from the chain termination reaction: transfer to the POSS comonomer followed by 1,2insertion. A similar chain termination mechanism has been also observed for metallocene-catalyzed ethylene copolymerization with 1,5-hexadiene and 7-methyl-1,6-octadiene [29]. In turn, the trans-vinylene groups, which are also present in the analyzed copolymers, could be formed as a result of the 2,1-insertion and then elimination of β-hydrogen or isomerization of a vinyl to a vinylene group [30]. Finally, the tri-substituted end groups are formed when POSS is the last monomer which is inserted, and that reaction is followed by isomerization and β-hydrogen transfer to the metal center [31]. The observed effect of the dimethylsiloxy spacer in reactive alkenyl substituent of monoalkenyl(siloxy)silsesquioxane comonomers on the polymer chain-growth termination process, which was resulted in formation of the vinyl end groups, could be explained by the high conformational lability of this kind of substituent. Thus, the interaction of silicon-oxygen cage of POSS with the metallic active center of catalyst is possible, causing the hindrance of the elimination of β-hydrogen atom attached to the tertiary carbon atom from POSS comonomer molecule being incorporated as a terminal unit of polymer chain. While for copolymers of ethylene with monoalkenylsilsesquioxanes, the highest content of vinylidene end groups was observed in the case of copolymers with POSS which contained the shortest alkenyl substituent. This is probably because of the inductive effect caused by silicon atoms in silicon-oxygen POSS cage, which facilitate the elimination of β-hydrogen atom attached to the tertiary carbon atom from POSS comonomer incorporated into polymer chain during copolymerization. However, the influence of inductive effect decreased as the length of alkenyl substituent increases in POSS comonomer. This results in decrease in the relative content of vinylidene end groups in copolymer obtained (Table 4).

3.6. Properties of ethylene/POSS copolymers The melting temperatures (Tm) for ethylene/POSS copolymers were within the range of 131.2–137.2 °C (Tables 1 and 2) and, in most cases, they were slightly higher than that for neat PE (132.0 °C). The Tm values did not vary significantly for different kinds of POSS comonomers used. However, it was also observed that the values of Tm decreased with the increase of POSS content in copolymer. The selected DSC thermograms are presented in Fig. 10. It should be noted that this results are in very good agreement with literature data [5,7,8]. The structures of POSS influenced the values of the crystallization temperatures (Tc) of copolymers (Table 1 and 2) which were usually slightly lower in comparison with that for neat PE (117.5 °C). Moreover, the Tc values decreased with the increasing content of POSS in a copolymer, irrespective of the kind of the POSS derivative used. These results indicate that the presence of POSS probably caused a decrease in the polymer chain molecular order. However, Tc was observed to increase for copolymers with a low content of POSS derivatives with a short-chain alkenyl substituent. Thus, it could be concluded that POSS units can act as nucleating agents for polyethylene in those cases. The structure of the POSS comonomer did not significant influence the crystallinity degree (χ) of copolymers obtained (Table 1 and 2). The χ values varied in the range of 56.0–67.3% (χ was 64.2% for neat PE). However, the crystallinity degree values for copolymers generally decreased with the increasing amount of POSS in the feed and extension of the reaction time, thereby with the increasing content of POSS units in a copolymer. POSS units incorporated into the polymer chain changed the morphological properties of copolymers, in comparison to neat polyethylene. Fig. 11 shows selected SEM images of the polyethylene sample and samples of ethylene copolymer with POSS-(iBu)7(OSi(CH3)2C4H8CH]CH2). For all copolymers, the surfaces of granules were porous and irregular, resembling “pumice”, and they became more and more porous with the increase of the POSS concentration in the reaction feed.

Fig. 10. The DSC curves of PE, E/POSS-(i-Bu)7(OSi(CH3)2(C4H8CH] CH2) (a) and E/POSS-(i-Bu)7(C4H8CH]CH2) (b) copolymers synthesized with rac-Et(Ind)2ZrCl2/ MAO catalytic system.

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Fig. 11. SEM images for polyethylene (a) and for ethylene/POSS-(i-Bu)7(OSi(CH3)2C4H8CH]CH2) copolymers obtained at different concentrations of the POSS copolymer in the reaction feed: 0.67 · 10−3 mol/dm3 (b), 3.33 · 10−3 mol/dm3, (c) and 6.67 · 10−3 mol/dm3 (d).

4. Conclusions The ethylene copolymers with monoalkenyl- and monoalkenylsilsesquioxanes were synthesized using the rac-Et(Ind)2ZrCl2/MAO catalytic system. The POSS derivatives with different kinds of non-reactive substituents and different structures of the reactive alkenyl substituents attached to the T8 POSS cage were used as comonomers. The structure and concentration of the POSS comonomer in the reaction feed were found to significantly influence the efficiency of the catalytic system and the content of POSS units incorporated into the copolymer, probably because of the steric and inductive effects of substituents. It was found that the copolymerization yield and incorporation of POSS units into the polymer chain could also be enhanced by the extended reaction time which indicated that POSS comonomers were not involved in deactivation of the catalyst. The reactivity of the POSS comonomers was dependent mainly on the length of their reactive alkenyl substituents attached to the POSS cage – the ability to copolymerize with ethylene was higher for POSS derivatives which contained medium-length alkenyl groups. It was also found that the POSS units would be isolated in the copolymer chain. The molecular weight (Mw) of ethylene/POSS copolymers decreased with the growing amount of the POSS comonomer incorporated into the polymer. It was observed that monoalkenylsilsesquioxane comonomers could act as a chain transfer agent. In turn, when the monoalkenyl(siloxy)silsesquioxanes not affected on the chain termination reaction and the decrease in Mw values with the increasing POSS concentration in the feed could be due to steric effect of comonomer. Different chain termination reactions, which dependent on the kind of POSS comonomer were confirmed by the analysis of the type and content of unsaturated end groups The melting temperature values, crystallinity degrees and crystallization temperatures for ethylene/POSS copolymers varied slightly depending on the structure and amount of the POSS comonomer used. Incorporation of POSS in the polymer chain affected the morphology of the copolymer granules.

Acknowledgments This work was financially supported by National Science Centre of Poland (NCN), under OPUS 4 research scheme, grant number: DEC-2012/07/B/ST5/03042.

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