2. Experimental Results and Discussion

2.1 Effects of catalyst structure

The effect of catalyst structure on the polymerization behavior of 1-octene is shown in Table 1. The main difference in Cat. 1 ~ Cat. 5 is that the synthetic raw materials are 2,4,6-trimethylphenol, 4-methylphenol, 4-fluorophenol, 2,6-difluorophenol, and 2,4,6-trifluorophenol, respectively.

From Table 1, it can be seen that as the number of electron pushing groups on the catalyst ligand increases, the monomer conversion rate and the Mη of the polymer decrease; When the substituent group is an electron withdrawing group or the number of electron withdrawing groups increases, the polarity of the chemical bond between the ligand and the transition metal Ti atom can be strengthened. The monomer conversion rate and polymer’s Mη are both relatively improved, among which Cat. 4 using 2,6-difluorophenol as raw material and Cat. 5 using 2,4,6-trifluorophenol as raw material have ideal monomer conversion rates (greater than 96.9%), and the Mη of the obtained polymer is greater than 3.55×106.

Polymerization conditions：0 ℃，24 h at 1st stage，5 ℃，144 h at 2nd stage，methyl aluminoxane(MAO) as cocatalyst，n(Al)∶n(Ti)=50∶1，n(1-octene)∶n(Ti)=2 000∶1.

The experimental results indicate that adjusting the electron absorption ability of substituents on the catalyst ligand can regulate the electronic effects and coordination environment of the catalyst's active sites, thereby regulating the catalytic performance of the catalyst and obtaining ultra-high molecular weight polymers, that is the electron withdrawing ability of the substituents on the catalyst ligand increases or the number of electron withdrawing substituents increases, the electron cloud density around the Ti atom in the catalyst decreases, the electropositivity increases, and it is easier to bind with the 1-carbon atom of the negatively charged a-olefin monomer double bond due to polarization, leading to coordination, 1,2-insertion, and chain growth, thus obtaining a higher molecular weight polymer. Due to the fact that carbon atoms with negative charges in olefin monomers are more likely to bind and coordinate with Ti atoms with higher positive charges, the monomer conversion rate is higher and the catalyst activity is higher within the same polymerization time.

Considering that the price of 2,4,6-trifluorophenol is higher than that of 2,6-difluorophenol, it is preferable to explore the effect of polymerization conditions on the polymerization behavior of 1-octene in Cat. 4.

2.2 Effect of polymerization conditions

2.2.1 Effect of temperature

The effect of polymerization temperature in the first stage on 1-octene polymerization is shown in Table 2.

According to Table 2, when the polymerization temperature in the first stage increases from -10 ℃ to 0 ℃, the conversion rate of 1-octene gradually increases, and the Mη of the polymer increases. Perhaps due to an appropriate increase in temperature, the viscosity of the polymerization system (1-octene liquid-phase bulk polymerization) will decrease, which is conducive to monomer Brownian motion and thus increases the polymerization rate. That is a slight increase in the polymerization temperature in the first stage is beneficial for 1-octene polymerization, but when the polymerization temperature in the first stage increases from 0 ℃ to 25 ℃, the conversion rate of 1-octene and polymer’s Mη decrease, indicating that when the polymerization temperature in the first stage is high, ultra-high molecular weight poly 1-octene cannot be obtained, possibly due to the chain transfer reaction caused by the high temperature.

Polymerization conditions：1st stage polymerization time 24 h，2nd stage polymerization temperature 5 ℃，2nd stage polymerization time 144 h，MAO as cocatalyst，n(Al)∶n(Ti)=50∶1，n(1-octene)∶n(Ti)=2 000∶1.

2.2.2 Effect of polymerization time

The effect of polymerization time of 1-octene in the second stage on monomer conversion rate and polymer’s Mη is shown in Table 3.

From Table 3, it can be seen that with the extension of polymerization time in the second stage, the conversion rate of 1-octene increases. But when the polymerization time reaches 144 hours, the conversion rate of 1-octene tends to stabilize. After 144 hours of polymerization, the polymerization system became a block solid, and the unreacted monomers had difficulty in mass transfer, making it difficult to undergo polymerization reactions. Therefore, it is not advisable to extend the reaction time after 144 hours of polymerization. The polymer’s Mη increases with the extension of the second stage polymerization time, i.e. the reaction time is extended from 144 hours to 168 hours, and the polymer’s Mη is still increasing. This indicates that the stability of the Cat.4/MAO catalyst system is good.

Polymerization conditions：1st stage polymerization temperature 0 ℃，1st stage polymerization time 24 h，2nd stage polymerization temperature 5 ℃，MAO as cocatalyst，n(Al)∶n(Ti)=50∶1，n(1-octene)∶n(Ti)=2 000∶1.

2.2.3 Effect of types of co catalysts and molar ratio of Aluminum to Titanium

The effect of co catalysts on the conversion rate of 1-octene and the Mη of polymer is shown in Table 4.

From Table 4, it can be seen that MAO is the most suitable co catalyst. Compared with AlEt3 and AlEt2Cl, the Cat.4/MAO catalytic system was used to catalyze the liquid-phase bulk polymerization of 1-octene, resulting in the highest conversion rate of 1-octene and the highest Mη of the polymer; The polymerization effect is the worst when AlEt2Cl is used as a co catalyst.

Polymerization conditions：1st stage polymerization temperature 0 ℃，1st stage polymerization time 24 h，2nd stage temperature 5 ℃，2nd stage polymerization time 144 h，n(1-octene)∶n(Ti)=2 000∶1.

Different co catalysts have different forms of stable existence. Although MAO is a hydrolysis product of trimethylaluminum (TMA) and an oligomer with a repetitive structure of Al (Me2) O, it is the small amount of TMA present in MAO that truly plays a role in alkylation and chain transfer. Due to the rapid reaction of TMA, the alkylation reaction with the active center of the catalyst is also rapid. Another function of MAO is to combine and stabilize the dissociated chloride ions from the main catalyst to form complex counter ions, which is beneficial for the stability of the active center. The Al (Me2) O structure of MAO may also facilitate the formation of an environment for coordination, insertion, and chain growth between 1-octene monomers and catalyst active sites.

From Table 4, it can also be seen that when AlEt3 is used as a co catalyst, the conversion rate of 1-octene and the Mη of the polymer decrease with the increase of AlEt3 dosage. This may be due to the excessive reduction of titanium oxide (IV) in the catalyst with an increase in the amount of AlEt3 to cheaper Ti, such as Ti (II), which is not suitable for 1-octene polymerization, resulting in a decrease in catalyst activity. Therefore, the conversion rate of 1-octene decreases. The increase in the addition of AlEt3 increases the probability of chain transfer reaction from the polymer growth chain to AlEt3, resulting in a decrease in polymer’s Mη. Compared with AlEt3, due to the presence of Cl with strong electronegativity in AlEt2Cl, the Al atom exhibits strong positivity (i.e. the charge around the Al atom tends towards Cl), and the binding strength between Al and Et increases. Therefore, it is difficult to lose Et in AlEt2Cl. The catalyst system composed of AlEt2Cl and Cat. 4 catalyzes the polymerization of 1-octene with different results compared to AlEt3. From Table 4, it can be seen that when AlEt2Cl is used as a co catalyst, the conversion rate of 1-octene and the Mη of the polymer increase with the increase of AlEt2Cl addition. But the polymerization effect is worse than using MAO as a co catalyst.

In summary, the suitable conditions for 1-octene polymerization are: catalytic system Cat.4/MAO; The first stage polymerization temperature is 0 ℃ and the polymerization time is 24 hours; The second stage polymerization temperature is 5 ℃ and the polymerization time is 144 hours; n（Al）∶n（Ti）=50∶1，n（1-octene）∶n（Ti）=2 000∶1. Under this condition, the monomer conversion rate is 96.9%, and the product Mη of poly (1-octene) 3.55×106.

2.3 Polymer Structure and Performance Characterization

2.3.1 FTIR Characterization results

The FTIR spectrum of poly (1-octene) is shown in Figure 1.

From Figure 1, it can be seen that the absorption peak at 2856 cm-1 corresponds to the C-H bond stretching vibration of CH2; The absorption peak at 2918 cm-1 corresponds to the C-H bond stretching vibration of CH3; The absorption peak at 1456 cm-1 corresponds to the shear and asymmetric vibration of CH2; The absorption peak at 1373 cm-1 is the bending vibration of the C-H bond; The absorption peak at 723 cm-1 is CH2 oscillation; The absorption peak at 772 cm-1 belongs to the - (CH2) - CH3 side group attached to the main chain. The above absorption peaks correspond to the saturated C-C single bonds of the polymer main chain and side groups. There are no absorption peaks at 990, 3080, and 1640 cm-1 respectively corresponding to C-H bond vibration, stretching vibration, and C=C bond stretching vibration in Figure 1. Perhaps due to the extremely high molecular weight of the polymer, the proportion of C=C bond ends generated by β-H chain transfer is extremely low, and FTIR cannot recognize it.

2.3.2 1H NMRCharacterization results

The 1H NMR spectrum of poly (1-octene) is shown in Figure 2. 

As shown in Figure 2, the absorption peaks at δ=1.68，1.40，0.95 are attributed to the H atoms in poly 1-octene CH, CH2, and CH3, respectively. There is no signal peak of hydrogen atom in CH=CH in Figure 2, indicating that the double bond content in the obtained poly (1-octene) molecular chain is extremely low.