Synthesis of Ultrahigh Molecular Weight Polymers Containing Reactive Functionality with Low PDIs by Polymerizations of Long-Chain α-Olefins in the Presence of Their Nonconjugated Dienes by Cp*TiMe2(O-2,6-iPr2C6H3)–Borate Catalyst

Copolymerizations of 1-decene (DC) with 1,9-decadiene (DCD), 1-dodecene (DD) with 1,11-dodecadiene (DDD), and 1-tetradecene (TD) with 1,13-tetradecadiene (TDD), using Cp*TiMe2(O-2,6-iPr2C6H3) (1)–[Ph3C][B(C6F5)4] (borate) catalyst in the presence of AliBu3/Al(n-C8H17)3 proceeded in a quasi-living manner in n-hexane at −30 to −50 °C, affording ultrahigh molecular weight (UHMW) copolymers containing terminal olefinic double bonds in the side chain with rather low PDI (Mw/Mn) values. In the DC/DCD copolymerization, the resultant copolymer prepared at −40 °C possessed UHMW (Mn = 1.40 × 106 after 45 min) with low PDI (Mw/Mn = 1.39); both the activity and the PDI value decreased at low polymerization temperature (Mn = 5.38 × 105, Mw/Mn = 1.18, after 120 min at −50 °C). UHMW poly(TD-co-TDD) was also obtained in the copolymerization at −30 °C (Mn = 9.12 × 105, Mw/Mn = 1.51, after 120 min), using this catalyst.


Introduction
Transition metal catalyzed olefin polymerization is the core technology in the polyolefin industry, and the recent progress in the catalyst development provides new possibilities for the synthesis of new polymers [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Homopolymers of long-chain (higher) α-olefins are branched macromolecules with a high graft density, and the polymers are thus recognized as the simplest bottlebrush polymers, with their backbone and side chains consisting of alkanes [15,16]. Amorphous poly(α-olefin)s are used in hot-melt applications due to their high melt-flow rate with low density, and the ultrahigh molecular weight (UHMW) polymers possess highly entangled bottlebrush architectures and are used as drag-reducing agents (DRAs) in pipeline transport methods for crude oil and petroleum products for improvement of piping system capacity [17][18][19][20]. Recent reports revealed their melt structure, linear rheology, and interchain friction mechanism, including effect of side-chain length toward their linear viscoelastic response and melt microstructure [15,16].
Since, as described above, UHMW polymers are simple bottlebrush polymers prepared by polymerization of these higher α-olefins via the grafting-through approach, we thus have an interest in synthesis of the UHMW polymers containing terminal olefinic double bond by copolymerization of DC with 1,9-decadiene (DCD), DD with 1,11-dodecadiene (DDD), and TD with 1,13-tetradecadiene (TDD), using 1-borate catalyst [46]. We thus, herein, wish to introduce our explored results for synthesis of

Materials and Methods
All experiments were conducted in a dry box, under a nitrogen atmosphere, unless otherwise specified. All chemicals of reagent grade were purified by the standard purification protocols. The n-Hexane or toluene (anhydrous grade, Kanto Kagaku Co. Ltd., Tokyo, Japan) was stored in a bottle containing molecular sieves (mixture of 3A and 4A 1/16, and 13X) in the dry box, and was used without further purification. The 1-Decene, 1-dodecene, 1-tetradecene, 1,9-decadiene, 1.11dodecadiene, and 1,13-tetradecadiene (reagent grades, TCI Co., Ltd., Tokyo, Japan) were stored in bottles, in the dry box, and were passed through an alumina short column prior to use. Cp*TiMe2(O-2,6i Pr2C6H3) (1) was prepared according to our previous report [46], and [Ph3C][B(C6F5)4] (Asahi Glass Co. Ltd., Tokyo, Japan) was used as received.
All 1 H and 13 C NMR spectra were recorded on a Bruker AV 500 spectrometer (500.13 MHz for 1 H; 125.77 MHz for 13 C, Bruker Japan K.K., Tokyo, Japan) at 25 °C, and all chemical shifts in the spectra were recorded in ppm (reference SiMe4). Samples for the measurement were prepared by dissolving the polymers in 1,1,2,2-tetrachloroethane-d2 solution. Gel-permeation chromatography (GPC) were conducted for analysis of molecular weights (based on the calibration with standard polystyrene samples as the standard procedure) and the distributions. HPLC grade THF (degassed prior to use) was used for GPC analysis, and the GPC analysis was performed at 40 °C on a Shimadzu SCL-10A, using a RID-10A detector (Shimadzu Co., Ltd.), using degassed prior to use in THF (containing 0.03 wt.% of 2,6-di-tert-butyl-p-cresol, flow rate 1.0 mL/min). GPC columns (ShimPAC GPC-806, 804, and 802, 30 cm × 8.0 mm diameter, spherical porous gel made of styrene/divinylbenzene copolymer, ranging from <10 2 to 2 × 10 7 MW).
Typical polymerization procedures were as follows: in the dry box, 1-decene (30.0 mL), 1,9decadiene (0.5 mL), n-hexane (30.0 mL), and Al i Bu3 and Al(n-C8H17)3 (prescribed amount) were added into a 100 mL round-bottom flask, which was connected to three-way valves. The flask was taken out from the dry box, and a toluene solution containing 1 (2.0 µmol/mL), which was pretreated with 2.0 eq. of Al i Bu3 at −30 °C, was then added into the mixture precooled at −30 °C under N2 atmosphere. The polymerization was started by the addition of a prescribed amount of toluene solution containing

Materials and Methods
All experiments were conducted in a dry box, under a nitrogen atmosphere, unless otherwise specified. All chemicals of reagent grade were purified by the standard purification protocols. The n-Hexane or toluene (anhydrous grade, Kanto Kagaku Co. Ltd., Tokyo, Japan) was stored in a bottle containing molecular sieves (mixture of 3A and 4A 1/16, and 13X) in the dry box, and was used without further purification. The 1-Decene, 1-dodecene, 1-tetradecene, 1,9-decadiene, 1.11-dodecadiene, and 1,13-tetradecadiene (reagent grades, TCI Co., Ltd., Tokyo, Japan) were stored in bottles, in the dry box, and were passed through an alumina short column prior to use. Cp*TiMe 2 (O-2,6-i Pr 2 C 6 H 3 ) (1) was prepared according to our previous report [46], and [Ph 3 C][B(C 6 F 5 ) 4 ] (Asahi Glass Co. Ltd., Tokyo, Japan) was used as received.
All 1 H and 13 C NMR spectra were recorded on a Bruker AV 500 spectrometer (500.13 MHz for 1 H; 125.77 MHz for 13 C, Bruker Japan K.K., Tokyo, Japan) at 25 • C, and all chemical shifts in the spectra were recorded in ppm (reference SiMe 4 ). Samples for the measurement were prepared by dissolving the polymers in 1,1,2,2-tetrachloroethane-d 2 solution. Gel-permeation chromatography (GPC) were conducted for analysis of molecular weights (based on the calibration with standard polystyrene samples as the standard procedure) and the distributions. HPLC grade THF (degassed prior to use) was used for GPC analysis, and the GPC analysis was performed at 40 • C on a Shimadzu SCL-10A, using a RID-10A detector (Shimadzu Co., Ltd.), using degassed prior to use in THF (containing 0.03 wt.% of 2,6-di-tert-butyl-p-cresol, flow rate 1.0 mL/min). GPC columns (ShimPAC GPC-806, 804, and 802, 30 cm × 8.0 mm diameter, spherical porous gel made of styrene/divinylbenzene copolymer, ranging from <10 2 to 2 × 10 7 MW).
Typical polymerization procedures were as follows: in the dry box, 1-decene (30.0 mL), 1,9-decadiene (0.5 mL), n-hexane (30.0 mL), and Al i Bu 3 and Al(n-C 8 H 17 ) 3 (prescribed amount) were added into a 100 mL round-bottom flask, which was connected to three-way valves. The flask was taken out from the dry box, and a toluene solution containing 1 (2.0 µmol/mL), which was pretreated with 2.0 eq. of Al i Bu 3 at −30 • C, was then added into the mixture precooled at −30 • C under N 2 atmosphere. The polymerization was started by the addition of a prescribed amount of toluene solution Polymers 2020, 12, 3 4 of 10 containing [Ph 3 C][B(C 6 F 5 ) 4 ] (2.0 µmol/mL). A certain amount (3.0 mL) of the reaction solution was taken out via a syringe from the reaction mixture, to monitor the time course; the sample solution was then quickly poured into i PrOH (150 mL) containing HCl (10 mL). The resultant polymer as precipitates was collected, adequately washed with i PrOH, and then dried in vacuo, for further analysis.

Results and Discussion
On the basis of our previous reports for polymerizations of 1-decene (DC), 1-dodecene (DD), and 1-tetradecene (TD) [22], and of 1,7-octadiene [38], Cp*TiMe 2 (OAr) (1, Ar = 2,6-i Pr 2 C 6 H 3 ) was chosen as the catalyst precursor, and [Ph 3 C][B(C 6 F 5 ) 4 ] (borate) was chosen as the cocatalyst in the presence of Al i Bu 3 and Al(n-C 8 H 17 ) 3 [22,29]. Copolymerizations of DC with 1,9-decadiene (DCD) were conducted in n-hexane at −30 to −50 • C, in the presence of Al cocatalyst [Al(n-C 8 H 17 ) 3 /Al i Bu 3 /Ti = 400/100/1.0 (at −30 and −40 • C) or 300/200/1.0 (at −50 • C), molar ratio]; the ratios were used on the basis of the homo polymerization results [22]. As reported previously [22,29,[47][48][49], use of Al(n-C 8 H 17 ) 3 , weak reagent for alkylation, and/or chain transfer was effective to proceed without catalyst deactivation, probably not only due to a role as a scavenger, but also due to the fact that the Al alkyl would contribute to the stabilization of the catalytically active species by preventing the decomposition from further reaction with borate [50][51][52]. The results in the DC/DCD copolymerization are summarized in Table 1.  As observed in the polymerization of DC, the copolymerization of DC with DCD proceeded with high catalytic activities (2750-7820 kg-polymer/mol-Ti·h within 20 min), even at −30 • C, affording high molecular weight polymers with rather narrow molecular weight distributions (run 1, M n = 3.24 × 10 5 -7.53 × 10 5 , M w /M n = 1.43-1.47). The M n value increased over the time course, without significant changes in the PDI values. It turned out that the PDI values decreased at a low temperature, with a decrease in the catalytic activity; the resultant copolymer prepared at −40 • C possessed UHMW (run 2, M n = 1.40 × 10 6 after 45 min), with low PDI (M w /M n = 1.39), and the PDI value became low when the copolymerization was conducted at −50 • C (run 3, M n = 5.38 × 10 5 , M w /M n = 1.18, after 120 min at −50 • C). As shown in Figure 1a, linear relationships between the M n values and the polymer yields (turnover numbers, TON) were observed, suggesting that these polymerizations proceeded in a quasi-living manner, as reported in the polymerization of DC [22]. As shown in Figure 2b (shown below), the resultant copolymers contain terminal olefinic double bonds by incorporation of Polymers 2020, 12, 3 5 of 10 DCD. The content of DCD estimated by 1 H NMR spectra slightly decreased gradually due to rather high consumption of DCD (rather high conversion of DCD and changes in the DCD concentration in the reaction solution) during the polymerization time course. This would suggest the possibility of (rather) gradient composition, although we do not have the firm elucidation at this moment.
yields (turnover numbers, TON) were observed, suggesting that these polymerizations proceeded in a quasi-living manner, as reported in the polymerization of DC [22]. As shown in Figure 2b (shown below), the resultant copolymers contain terminal olefinic double bonds by incorporation of DCD. The content of DCD estimated by 1 H NMR spectra slightly decreased gradually due to rather high consumption of DCD (rather high conversion of DCD and changes in the DCD concentration in the reaction solution) during the polymerization time course. This would suggest the possibility of (rather) gradient composition, although we do not have the firm elucidation at this moment.   Table 2 summarizes results in the DD/DDD copolymerization conducted at −40 and −50 °C. As observed in the DC/DCD copolymerization, the Mn value increased over the time course, without significant changes in the PDI values, and the PDI became low at −50 °C (run 5). The resultant copolymer prepared at −50 °C possessed high molecular weight with low PDI value (run 5, Mn = 5.46 × 10 5 , Mw/Mn = 1.28 after 120 min). As shown in Figure 1b, a linear relationship between the Mn values and the polymer yields (turnover numbers, TON) was observed in the polymerization at −50 °C, suggesting a possibility of a quasi-living manner, as observed in the DC/DCD copolymerization.  Table 2 summarizes results in the DD/DDD copolymerization conducted at −40 and −50 • C. As observed in the DC/DCD copolymerization, the M n value increased over the time course, without significant changes in the PDI values, and the PDI became low at −50 • C (run 5). The resultant copolymer prepared at −50 • C possessed high molecular weight with low PDI value (run 5, M n = 5.46 × 10 5 , M w /M n = 1.28 after 120 min). As shown in Figure 1b, a linear relationship between the M n values and the polymer yields (turnover numbers, TON) was observed in the polymerization at −50 • C, suggesting a possibility of a quasi-living manner, as observed in the DC/DCD copolymerization.   Table 3 summarizes results in TD/TDD copolymerization conducted at −30 • C. Due to a difficulty of polymerization at low temperature (the n-hexane solution would be heterogeneous due to the freezing of TD), the polymerization could be conducted only at −30 • C, under rather diluted conditions. As observed in Tables 1 and 2, the M n value increased over the time course, without significant changes in the PDI values. The resultant copolymer possessed high molecular weight, with unimodal molecular weight distribution (run 6, M n = 9.12 × 10 5 , M w /M n = 1.51 after 120 min). As also shown in Figure 2a, a linear relationship between the M n values and the polymer yields (turnover numbers, TON) was clearly observed. The results thus also suggest that the TD/TDD copolymerization proceeded in a quasi-living manner. As shown in Figure 2b, the resultant polymers possessed a terminal olefinic double bond, as observed in poly(1-octene-co-1,7-octadiene) and poly(ethylene-co-1-octene-co-1,7-octadiene) [38], as well as in poly(ethylene-co-styrene-co-1,7-octadiene) [44] prepared by Cp*TiCl 2 (O-2,6-i Pr 2 C 6 H 3 )-MAO catalyst, and no resonances ascribed to protons in the internal olefins were observed (additional 1 H NMR spectra are shown in the Supplementary Materials) [53]. The resultant polymers are highly soluble in toluene, THF, chloroform, dichloromethane, etc., without any difficulties (as seen in poly(1,5 hexadiene) containing partial cross-linking prepared by Cp 2 ZrCl 2 -MAO catalysts even under diluted conditions [54]). The results thus suggest that the resultant polymers were poly(DC-co-DCD)s and poly(TD-co-TDD)s containing terminal olefins in the side chain, as expected on the basis of our previous results [22,38].

Conclusions
We have shown that synthesis of ultrahigh molecular weight (UHMW) highly branched (bottlebrush) polymers that contain terminal olefinic double bonds in the side chain with rather low PDI (M w /M n ) values has been attained by polymerization of long-chain (higher) α-olefins (1-decene (DC), 1-dodecene (DD), and 1-tetradecene (TD)) in the presence of corresponding nonconjugated dienes (1,9- , and UHMW poly(TD-co-TDD) was also obtained in the TD/TDD copolymerization at −30 • C (M n = 9.12 × 10 5 , M w /M n = 1.51, after 120 min). As described in the introduction, these polymers should possess highly branched bottlebrush architectures, and the present results strongly suggest a possibility of introduction of reactive functionality (terminal olefins) into the side chain (outside of the cylindrical structure). Moreover, as described in the introductory, as well as reported previously [38], an introduction of hydroxy group by treatment of the terminal olefinic double bonds with BBN and the subsequent grafting (by living ring opening polymerization of ε-caprolactone) would be possible. One issue we have not yet clarified clearly is the effect of diene monomers on the monomer reactivity ratio. We thus believe that the results could demonstrate providing new materials (functionalized polyolefin bottlebrush) based on polyolefins, and more details including further analysis and applications will be introduced in the future.