Study on the synthesis of hexene-1 catalyzed by Ziegler-Natta catalyst and polyhexene-1 applications

2.1 Reagents and instruments

The Z-N118 catalyst (Ti content: 2.6%) was purchased from Basell. The n-hexane (Tianjin No.3 Chemical Reagent Factory), cyclohexane (Tianjin Kemiou Chemical Reagent Co., Ltd.), n-heptane (Tianjin Kemiou Chemical Reagent Co., Ltd.), and hexene-1 (J&K Scientific Co., Ltd) were all analytical grade and pretreated with 5A molecular sieves before use. Triethylaluminium (1.0 mol⋅L^-1 in hexane solution) was purchased from Albemarle. Nitrogen (99.99% purity) was from Daqing Xuelong Gas Co., Ltd. Analytical-grade cyclohexyl methyl dimethoxy silane (CHMMS) was purchased from J&K Scientific Co., Ltd.

Experiment apparatuses included a two-necked Schlenk flask (Synthwave); a 500 mL three-necked flask (Tianjin Shengbo Glass Instrument Factory); a DF-101S thermostatic oil bath in which the sample can be magnetically stirred (Tianjin Yuhua Instrument Co., Ltd.); Bruker-Tonsor27 FT-IR spectrometer (Bruker, Germany); a GPC220CV high-temperature gel permeation chromatography apparatus from WATERS, USA; and a Bruker DRX-400 nuclear magnetic resonance (¹H and ¹³C-NMR) spectrometer from Bruker, USA.

2.2 Experimental procedures

2.2.1 Synthesis of external donors

All reactions were carried out under argon using Schlenk standard technology. The synthesis of the external donors is shown in the following schematic diagram (15) (Scheme 1).

Scheme 1

The reaction structures of DPPDMS (a) and DPRDMS (b).

1. Preparation of DPPDMS

   The reaction needs to add 80 mL of n-heptane to a 500 mL five-necked flask followed by 23 mL of 0.2 mol/L piperidine. After the drop-wise addition of 133 mL 1.66 mol/L n-butyllithium in n-heptane, it needs slowly cooling the mixture with water at 25°C (room temperature) for 1 h. After further drop-wise addition of tetramethoxysilane (15 mL of a 0.1 mol/L (0.1 M), the mixture was again cooled with water. Upon completion of the additions, the resultant mixture was cooled at 25°C (room temperature) for 6 h and then incubated for a further 12 h. The supernatant liquid was transferred under argon into a 250 mL three-necked flask, and the precipitate washed three times with 30 mL portions of n-heptane. A colorless transparent liquid (DPPDMS) was obtained (yield: 53.1%) after the solvent and unreacted reagents were removed by reduced pressure distillation (-0.1 MPa). ¹H NMR (400 MHz, CDCl₃): δ 1.4(m, 2H), 1.6(m, 2H), 2.9(m, 2H), 3.5(s, 3H) (Figure 1).

2. Preparation of DPRDMS

   The reaction conditions were identical to those used for the preparation of DPPDMS as described in Section 2.2.1 except that nafoxidine was used in place of piperidine. After reduced pressure distillation, the product DPRDMS, a colorless transparent liquid was obtained in 40.1% yield. ¹H NMR (400 MHz, CDCl₃); δ 1.7(m, 2H), 3.1(m, 2H), 3.5(s, 3H) (Figure 2).

Figure 1

¹H NMR spectrum of DPPDMS.

Figure 2

¹H NMR spectrum of DPRDMS.

2.3 Test and characterization

3.1 Fourier transform infrared (FTIR) spectra of PHe

PHe sample was characterized by FTIR spectroscopy. Figure 3 shows a C–H stretching vibration band over the range of 2855~2955 cm^-1; there were two other stretching bands at 1377 and 1456 cm^-1, respectively. These bands may be a C–H in-plane bending of the methyl group. The weak band at 725 cm^-1 may result from the butyl side chain. The weakened C–H stretching at 1395 cm^-1 and reduced C=C stretching vibration at 1640 cm^-1 of vinyl indicate the formation of PHe.

Figure 3

The FTIR of polyhexene-1.

3.2 ¹³C NMR spectra of PHe and calculation of its isotacticity

The PHe elastomer samples with isotacticities of 71% and 93% were characterized by ¹³C NMR, as shown in Figure 4. The chemical shift values of the six carbons of poly(1-hexene) observed are in good agreement with other reports (16). The isotactic calculation method is from C3 to C2 (between 36 ppm and 34 ppm), and the ratio of the isotactic part mm divided by mm+mr+rr is the isotacticity of polyhexene-1. The peak heights were uniform. The methyl signal was observed over the range of 13.00~17.00 ppm. The carbon atoms of the CH₂ group in the branch appeared at 22.70~23.18 ppm. The resonance of the carbon atoms at ca 28 and 40 ppm suggests the presence of the methyl side chain with 1,3 and 1,6 couplings.

Figure 4

The ¹³C NMR of polyhexene-1 with an isotacticity of 93% (a) and 71% (b).

3.3 Effects of External Donors (EDs) on the polymerization of hexene-1

We characterized the effects of different EDs on the polymerization reaction. The data in Table 1 show that the EDs alter the efficiency of the catalyst, and also change the tacticity of the product. Regarding catalytic efficiency, the silane-based EDs, DPPDMS and DPRDMS, both outperform CHMMS, possibly because of steric hindrance and an electronic effect. CHMMS, commonly used in industry, has only one hexamethylene group, while DPPDMS and DPRDMS contain two piperidyl and two pyrrole groups, respectively. Therefore, the steric hindrance provided by these groups in DPPDMS and DPRDMS would facilitate electron donation and result in

higher isotacticity of the product. The conjugation effect due to the introduction of piperidyls and pyrrolyls would also lead to enhanced electron donating ability and higher catalytic activity. Table 1 also shows that in the presence of both EDs, (DPPDMS and DPRDMS), the polymerization reaction gives optimal results. Hence, the ED synthesized by the complex formulation of DPPDMS and DPRDMS (mass ratio = 1:1) was used for all of our polymerizations.

3.4 Effects of reaction time on the hexene-1 polymerization reaction

Table 2 displays the effects of reaction time on the catalytic activity of the system and the isotacticity of hexene-1 homopolymer. Figure 5 shows that the amount of hexene-1 homopolymer formed increased until it reached a maximum. With longer reaction times, the dispersion of catalyst in hexene-1 became more uniform with stirring, and the active sites of the catalyst were more accessible so that maximum catalytic activity (899 g/g Cat·h^-1) was obtained. Extended reaction time led to a balanced state between the available active sites of the catalyst and the hexene-1 molecules so that no more active sites were available for catalysis of hexene-1 after 2 h. The polymer isotacticity reached 93.7% in 2 h. After another hour the polymer isotacticity reached its peak value (94.2%), while the catalytic activity decreased slightly. Therefore, the best reaction time is 2 h.

Figure 5

Effect of reaction time on catalyst activity and isotacticity.

3.5 Effects of reaction temperature on hexene-1 polymerization

Table 3 shows the effects of reaction temperature on the catalytic activity of the system and the isotacticity of

hexene-1 homopolymer at a 2 h reaction time. Figure 6 shows that the reaction rate increased and then decreased with the increasing reaction temperatures, with optimal product formation at 30°C yielding 899 g/g Cat·h^-1. The increased reaction temperature leads to the formation of more catalyst active sites, and the rapid formation of active sites within a short time leads to higher catalytic activity. In this reaction system, the reaction capacity of alkylaluminium is also increased at elevated temperatures, which raises the number of active sites. This effect also explains the increase in catalytic activity. Further temperature increases lead to further dissolution of PHe in cyclohexane, which leads to a decreased hexene-1 concentration in these active sites and thus to lower catalytic activity. For these reasons, there is an optimum reaction temperature at 30°C.

Figure 6

Effect of temperature on catalyst activity and isotacticity.

3.6 Effects of catalyst dosage on the hexene-1 polymerization reaction

Table 4 shows the effect of catalyst dosage on the rate of the reaction and isotacticity of hexene-1 homopolymer at 30°C at a reaction time of 2 h. Figure 7 shows that an increase in catalyst dosage increases the catalytic activity

Figure 7

Effect of catalyst dosage on catalyst activity and isotacticity.

until it reaches a maximum at 10 mg. At low catalyst concentrations, the increase in catalyst dosage enables more hexene-1 molecules to contact the active sites so the catalytic activity increases. However, after the catalyst dosage exceeds 10 mg, the catalytic activity reaches a plateau. Although more catalysts contributed to more active sites availability, a saturation state is reached for the hexene-1 monomer concentration, so chain growth is not improved any further. We observed the best results using 10 mg catalyst in the reaction mix.

3.7 Effects of solvent type and dosage on hexene-1 polymerization

Table 5 shows the effects of solvent type and dosage on the catalytic activity in the reaction, and on the isotacticity, molecular weight and molecular weight distribution of the hexene-1 homopolymer product at 30°C after a reaction time of 2 h. All properties are improved after solvent addition. The catalyst was premixed in solution, which contributed to more uniform catalyst distribution and sufficient contact between active sites and hexene-1 molecules. (The catalyst is a solid substance and is mixed

with a solvent to form a mixture.) The catalytic activity, polymer isotacticity, molecular weight and molecular weight distribution were generally improved with magnetic stirring. We studied three solvents commonly used in industry: n-hexane, cyclohexane, and n-heptane. These data show that the optimum catalytic activity and isotacticity occur in n-heptane. However, we found poor molecular weight distribution for the polymer when using n-heptane as the solvent. This distribution was detrimental to subsequent polymer processing. The best solvent is cyclohexane.

3.8 Effects of alkylaluminium dosage on the polymerization of hexene-1

Table 6 displays the effects of alkylaluminium dosage on the formation and isotacticity of hexene-1 homopolymer product at 30°C after a reaction time of 2 h (10 mg catalyst; 0.5 mol hexene-1 (62 mL); 50 mL hexane; n(Al)/n(CHMMS) = 20). Figure 7 shows that as the mole ratio of co-catalyst to the main catalyst, AlEt₃, increases, the catalytic activity first increases and then decreases. The AlEt₃ at the beginning of the reaction reacted with impurities. Thus, some titanium (Ti) failed to be activated, and the reaction resulted in lower average polymerization activity. Increases in AlEt₃ dosage lead to more Ti activation and contribute to higher polymerization activity. However, when the ratio of n(Al)/n(Ti) exceeds 200, further increase in AlEt₃ dosage leads to decreased polymerization activity. This effect can be explained as follows (17, 18, 19):

1. The alkylaluminium competes with the monomer for the adsorption onto active sites.

2. A certain amount of catalyst active sites are present in this system. Further increases in the catalyst dosage do not lead to more active sites. Thus, the catalyst activity curve at higher alkylaluminium amounts in the reaction mixture reaches a maximum and does not show further change.

3. With increasing AlEt₃ dosage, the propagating chains shift to AlEt₃, so the effective catalytic ability of AlEt₃ decreases the polymerization activity.

4. Excessive AlEt₃ reduces Ti⁴⁺ to Ti²⁺ which leads to the loss of polymerization activity. As the mole ratio of n(Al)/n(Ti) increases, the polymer isotacticity shows an overall decrease, perhaps because the increase in alkylaluminium dosage increases the probability of transfer to alkylaluminium, and decreases the polymer isotacticity. The best value for catalytic activity is observed at n(Al)/n(Ti) of 200 (Figure 8).

Figure 8

Effect of Al/Ti mole ratio on catalyst activity and polymer isotaticity.

3.9 Application of PHe produced

The linear low density polyethylene (LLDPE) 7042 was blended with the PHe produced (PE/PHe = 0.05, 0.1, 0.15 and 0.2) synthesized in this study. The molecular weight of LLDPE7042 was 10.7857 and the molecular weight distribution was 3.75. The mechanical properties of the blends were investigated by the extrusion and pelletizing test using a twin screw extruder, and the results are shown in the following table.

Table 7 shows that the elongation at break and the impact strength of the polyethylene added with PHe are improved, and gradually increase with the increase of the PHe content. This is mainly due to the existence of four carbon chains on the side chain of PHe, which makes the polymer chains slide between each other and therefore results in elastic properties. However, the tensile strength and yield strength all decrease, since the lack of double bonds in the PHe structure makes it unable to form crosslinks.