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Direct Synthesis of Ultrahigh Molecular Weight Functionalized Isotactic Polypropylene

2023; Chinese Chemical Society; Volume: 5; Issue: 11 Linguagem: Inglês

10.31635/ccschem.023.202202621

2096-5745

Guanglin Zhou, Hongliang Mu, Xin Ma, Xiaohui Kang, Zhongbao Jian,

Synthesis and properties of polymers

Open AccessCCS ChemistryRESEARCH ARTICLES30 Oct 2023Direct Synthesis of Ultrahigh Molecular Weight Functionalized Isotactic Polypropylene Guanglin Zhou, Hongliang Mu, Xin Ma, Xiaohui Kang and Zhongbao Jian Guanglin Zhou State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Hongliang Mu State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 , Xin Ma State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Xiaohui Kang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Pharmacy, Dalian Medical University, Dalian 116044 and Zhongbao Jian *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 https://doi.org/10.31635/ccschem.023.202202621 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ultrahigh molecular weight functionalized isotactic polypropylene (f-UHMW-iPP) through the direct copolymerization of propylene with polar monomers is highly desirable but has not been accessed thus far because it involves challenging regio- and stereochemistry along with usually reduced molecular weight. Herein, in contrast to the unsuccessful catalyst strategy, a polar monomer-assisted strategy is used to access the above material. The introduction of O- or S-functionalized long-chain polar olefins into the hafnium-catalyzed copolymerization of propylene (and bulkier α-olefins) significantly increases the copolymer molecular weight with a maximum observed increase of +488%. f-UHMW-iPP and functionalized isotactic poly(α-olefin)s (Mw < 2000 kDa, [mmmm]: 99%) are thus prepared at ambient conditions. The incorporation of 1 mol % of polar monomer improves the surface property and significantly increases the long-sought toughness (860%) of brittle iPP, without reducing the tensile strength (42 MPa) due to the key achievement of ultrahigh molecular weight. A discussion of the mechanism involved in the beneficial effects of incorporating the polar monomer is herein presented by an in-depth density functional theory calculation. Download figure Download PowerPoint Introduction Polyolefin plastics are both the most important of the synthetic polymers and ubiquitous in modern society. Polyethylene (PE), polypropylene (PP), and isotactic PP (called iPP) are the most commonly used polymers because they exhibit beneficial mechanical properties with low cost and facile processing. Improving polyolefin for use in advanced and value-added applications is of great interest to both academia and industry.1–4 Introducing a polar functional group into PE and iPP would allow for the synthesis of functionalized polyolefins with improved polymer properties, including dyeability, compatibility, rheology, and adhesion, while retaining the performance of the parent polyolefins.5–8 The coordination-insertion mechanism allows for facile and atom-economical copolymerization of olefins and polar monomers to produce functionalized polyolefins, albeit with the usually reduced catalytic activity and polymer molecular weight. Numerous studies have demonstrated the preparation of functionalized PE through the copolymerization of ethylene and polar monomers.9–24 It is underdeveloped for the copolymerization of propylene (and α-olefin) with polar monomers due to the challenges involving uncontrolled regio- (1,2- and 2,1-) and stereo- (isotactic and syndiotactic) chemistry and the usually reduced catalytic activity and molecular weight.25–27 Ultrahigh molecular weight, functionalized polyolefins, and isotactic polyolefins are sought via this reaction as an academic and industrial challenge. Group 4 early-transition metal catalysts such as zirconium and hafnium (Hf) catalysts have exhibited control over tacticity and regiochemistry during propylene polymerization.28 Pioneering examples of polar olefins copolymerized with propylene include alkyl boron, alkyl aluminum pretreated alcohol/amino, bis(trimethylsilyl) preprotected amino, bulky nPr2N, iPr2N, and Ph2N-substituted α-olefins (Figure 1a).29–48 Alkyl aluminum or trimethylsilyl preprotected O- and S-functionalized polar olefin monomers have been employed in propylene copolymerization,34,35,48 but monosubstituted O- and S-functionalized polar olefin monomers have attracted less attention than bulky disubstituted N-functionalized polar olefin monomers (Figure 1a). High catalytic activity and high molecular weight N-functionalized iPP have been achieved using group 4 catalysts, but ultrahigh molecular weight functionalized isotactic polypropylene (f-UHMW-iPP) remains elusive, and a mechanistic pathway is needed to reach such an ultrahigh molecular weight. Figure 1 | Advances on the copolymerization of propylene and polar monomers by using: (a) group 4 early transition metal catalysts, (b) group 10 late-transition metal catalysts, (c) ultrahigh molecular weight, functionalized, and isotactic polyolefins in this work. Download figure Download PowerPoint Additionally, the less oxophilic group 10 late-transition metal catalysts such as nickel and palladium catalysts have been shown to promote the copolymerization of propylene with polar olefins including acrylates, which are groundbreaking although their tacticity ([mm] ≤ 74%), catalytic activity, and molecular weight (Mw ≤ 33 kDa) need to be improved to be useful for application (Figure 1b).49–52 The usually detrimental effects of the polar monomer on olefin copolymerization have led to an increased interest in methods to mitigate those effects. Herein, we present a simple synthesis method and the underlying mechanism that exhibits a beneficial effect when using polar monomers to produce functionalized iPP (and also isotactic poly(α-olefin)s) (Figure 1c). The introduction of O- and S-functionalized olefins into the copolymerization of propylene (and α-olefins) significantly increases the copolymer molecular weight. This produces f-UHMW-iPPs (and poly(α-olefin)s) under mild conditions for the first time, which importantly balance the tensile strength and toughness of iPP along with improved surface property. Experimental Methods All syntheses involving air- and moisture-sensitive compounds were carried out using standard Schlenk-type glassware (or in a glove box) under an atmosphere of nitrogen. All solvents were purified from the MBraun SPS system (M. Braun Inertgas-Systeme GmbH, Munich, Germany). NMR spectra for the ligands, complexes, and polymers were recorded on a Bruker AV400 (Bruker Corporation, Billerica, USA; 1H: 400 MHz, 13C: 100 MHz) or a Bruker AV500 (Bruker Corporation, Billerica, USA; 1H: 500 MHz, 13C: 125 MHz). NMR assignments were confirmed by 1H–1H correlation spectroscopy, 1H–13C heteronuclear singular quantum correlation (HSQC), and 1H–13C heteronuclear multiple bond correlation (HMBC) experiments when necessary. The molecular weights and molecular weight distributions (Mw/Mn) of the polymers were measured by gel permeation chromatography (GPC) on a polymer laboratories-gel permeation chromatography (PL-GPC) 220-type high-temperature chromatograph equipped with three PL-gel 10 μm Mixed-B light scattering (LS) type columns at 150 °C, a PL-GPC 220-type chromatograph equipped with three PL-gel 10 μm Mixed-B LS type columns at 35 °C. For the f-UHMW-iPP samples, the usual filtering method is impossible, and thus the PolymerChar was used for filtering. Melting points (Tm) of polymers and copolymers were measured through differential scanning calorimetry (DSC) analyses, which were carried out on a METTLER TOPEM DSC (Mettler-Toledo, Zurich, Switzerland) or a Q2000 DSC (TA Instruments, New Castle, USA) under a nitrogen atmosphere at heating and cooling rates of 10 °C/min (temperature range: 50 °C–200 °C). The water contact angle (WCA) of each sample was obtained by using a contact angle goniometer DSA100 (KRÜSS GmbH, Hamburg, Germany) at room temperature at least five times by water drop of 5 μL. Stress/strain experiments were performed on an Electromechanical Universal Testing Machine (E43.104) at room temperature (5 mm/min). Polymers were melt-pressed at 200 °C to obtain the test specimens, which have 12 mm gauge length, 2 mm width, and 0.8 mm thickness. At least three specimens of each polymer were tested. All detailed crude data can be found in Supplementary Information. Computational Methods All quantum chemical computations were performed by using the Gaussian 16 package of programs. Each structure was optimized at the BP86/BSI level and subsequently characterized as a minimum (Nimag = 0) or a transition state (Nimag = 1) by harmonic vibration frequencies which provided thermodynamic data. The transition state structures were shown to connect the reactant and product on either side via intrinsic reaction coordinate following. In the BSI, the C, H, O, and N were calculated by the 6-31G* basis set, and the Hf atoms were treated by the quasi-relativistic LANL2DZ ECP effective core potential. To obtain more reliable relative energies, the single-point calculations of optimized structures were carried out at the level of BP86-D3 (BP86 with Grimme's density functional theory (DFT)-D3 correction)/basis sets II (BSII), taking into account the solvation effect of toluene with the SMD (solvation model based on solute electron density) solvation model. In BSII, the 6-311G(d, p) basis set was used for C, H, O, and N atoms and the atom Hf was treated by the same basis set as that for the structure optimization. Therefore, unless otherwise mentioned, the free energy (ΔG, 298.15 K, 1 bar) in solution, which was used for description of energy profiles, was obtained from the solvation single-point calculation and the gas-phase Gibbs free energy correction. For details and references, see Supplementary Information. Results and Discussion Catalysts and polar monomers Precise control over tacticity and regiochemistry is important for the copolymerization of propylene (and α-olefins) with polar monomers. Five group 4 early-transition metal catalysts 1–5 (Figure 2) that display atactic, syndiotactic-rich, isotactic-rich, or isotactic selectivity in propylene polymerization were synthesized according to known procedures.53–57 Polar monomers are key in stereoselective propylene copolymerization. Figure 2 shows the O- and S-functionalized polar monomers used in propylene (and α-olefins) copolymerizations including C4OPh and C4SPhMe58 compared to C4CPh and the bulky C4OiPr2Ph (cf. Supporting Information for details). For the selection of polar monomers, three factors were considered. The O- and S-heteroatoms had a weaker interaction with the active center than the N-heteroatom. The use of the aryl substituent on the heteroatom avoided the protection of alkyl aluminum. The long methylene units (CH2)n enabled the comonomer as a branch, while favoring the polymerization of the polar monomer. Figure 2 | Catalysts and polar monomers. Early transition metal catalysts, olefin monomers, O- and S-functionalized polar monomers used in the polymerization. Download figure Download PowerPoint Stereoselective copolymerization of α-olefins and polar monomer The precatalysts and polar monomers were tested for the copolymerization of α-olefins under mild conditions (25 °C) to select the optimal catalyst (Table 1). Without the addition of alkyl aluminum (AliBu3), the precatalyst 1 activated by trityl borate [Ph3C][B(C6F5)4] mediated the atactic polymerization of 1-hexene (hex.) at 25 °C in 5 mL of toluene with a low conversion (22.7%) and polymer molecular weight (12 kDa). Adding 50 equiv of C4OPh to the 1-hexene polymerization stopped the reaction (Table 1, entries 1 vs 2). The copolymerization of hex. (900) with C4OPh (100) using the precatalyst 2 produced the syndiotactic-rich ([rrrr] = 50%) functionalized poly(1-hexene) without an increase in weight-average molecular weight (Mw) (Table 1, entries 3 vs 4). A similar trend was observed in the 3-promoted isotactic-rich copolymerization of hex. (950) with C4OPh (50) (Table 1, entries 5 vs 6). The nonmetallocene precatalyst 4 terminated the reaction of hex. (900) with C4OPh (100) in the same way as the metallocene precatalysts, even though it enabled a highly isotactic ([mmmm] = 99%) polymerization of hex. (Table 1, entries 7 vs 8). These results indicate challenges in achieving both an efficient copolymerization of hex. with C4OPh and the increase in Mw of copolymers versus homopolymers. Table 1 | Compared Data of α-Olefin Polymerization and Copolymerization of α-Olefin with Polar Monomer Using 1–5a Entry Cat. M1 (equiv)/M2 (equiv) t (h) Conv. (%)b Mw (103)c Increase on Mw PDIc X (mol %)d Tacticityd 1 1 Hex. (1000)/- (0) 5 22.7 12 – 2.17 – Atactic 2 1 Hex. (950)/C4OPh (50) 12 Trace – – – – – 3 2 Hex. (1000)/- (0) 12 83.0 59 – 1.65 – 54% (rrrr) 4 2 Hex. (900)/C4OPh (100) 12 46.9 59 +0% 1.65 8.72 50% (rrrr) 5 3 Hex. (1000)/- (0) 1 49.8 39 – 1.87 – 74% (mmmm) 6 3 Hex. (950)/C4OPh (50) 1 47.2 40 +3% 2.01 3.22 69% (mmmm) 7 4 Hex. (1000)/- (0) 5 22.4 392 – 2.02 – 99% (mmmm) 8 4 Hex. (900)/C4OPh (100) 30 Trace – – – – – 9 5 Hex. (1000)/- (0) 1 82.1 428 – 2.39 – 99% (mmmm) 10 5 Hex. (980)/C4OPh (20) 1 83.1 1083 +153% 2.42 1.04 99% (mmmm) 11 5 Hex. (950)/C4OPh (50) 5 66.5 1762 +312% 1.71 3.51 99% (mmmm) 12 5 Hex. (900)/C4OPh (100) 5 39.1 2160 +405% 1.93 4.52 99% (mmmm) 13 5 Hex. (850)/C4OPh (150) 12 18.3 1365 +219% 2.18 6.64 99% (mmmm) 14 5 Hex. (800)/C4OPh (200) 12 9.8 1144 +167% 2.16 9.67 99% (mmmm) 15 5 Hex. (950)/C4CPh (50) 3 87.0 402 −6% 2.38 4.04 99% (mmmm) 16 5 Hex. (900)/C4CPh (100) 3 90.7 508 +19% 2.68 8.17 99% (mmmm) 17 5 Oct. (1000)/- (0) 5 87.4 390 – 2.03 – 99% (mmmm) 18 5 Oct. (950)/C4OPh (50) 3 79.2 1346 +245% 2.28 2.54 99% (mmmm) 19 5 4M1P (1000)/- (0) 0.5 84.4 202 – 2.26 – 99% (mmmm) 20 5 4M1P (950)/C4OPh (50) 3 49.3 1004 +397% 1.74 2.83 99% (mmmm) aStandard reaction conditions: catalyst (10 μmol), [Ph3C][B(C6F5)4] (10.5 μmol), toluene (5 mL), 25 °C. All entries are based on at least two runs, unless noted otherwise. bWeight of polymers obtained/total weight of monomers fed. cDetermined by GPC in tetrahydrofuran at 35 °C (entries 1–18) and in 1,2,4-trichlorobenzene at 150 °C (entries 19 and 20). dDetermined by 1H and 13C NMR spectroscopy in CDCl3 at 25 °C (entries 1–18) and in C2D2Cl4 at 110 °C (entries 19 and 20). The Hf precatalyst 5 with the activation of [Ph3C][B(C6F5)4] in the absence of AliBu3 under identical conditions mediated polymerization of 1-hexene to produce a highly isotactic ([mmmm] = 99%) poly(1-hexene) at a high conversion of 82.1% in 1 h, so this was used for the rest of this study. Polar monomer usually shows a detrimental effect on Mw in olefin copolymerization. However, the reaction of hex. (980) with C4OPh (20) using 5 provided the functionalized poly(1-hexene) with a significantly higher Mw, without compromising catalytic activity and isotacticity. Mw of the copolymer increased by +153% from 428 kDa (homopolymer) to 1083 kDa (copolymer) (Table 1, entries 9 vs 10), which suggests that an ultrahigh molecular weight-functionalized isotactic poly(1-hexene) was produced by this facile method. Decreasing the ratio of hex. versus C4OPh from 980∶20, 950∶50, 900∶100, 850∶150, to 800∶200 led to an increase in the incorporation of C4OPh from 1.04 to 9.67 mol %, without compromising the isotacticity (Table 1, entries 10–14; Figure 3a,b). The Mw of all obtained 1-hexene copolymers were at an ultrahigh level and the largest increase in Mw was +405% (2160 vs 428 kDa). Figure 3 | Isotactic copolymerization of 1-hexene with C4OPh, and the compared C4CPh at varied ratios: (a) GPC curves and (b) changes in molecular weight and comonomer incorporation. Download figure Download PowerPoint The nonpolar comonomer C4CPh ("O" was replaced with "CH2") and was further copolymerized with 1-hexene under otherwise identical conditions to corroborate the importance of the heteroatom (O) in enhancing molecular weight. No significant change in Mw was observed with varying ratios of hex. versus C4CPh from 1000∶0 (428 kDa), 950∶50 (402 kDa), to 900∶100 (508 kDa) (Table 1, entries 15–16; Figure 3). This is indicative of the polar monomer increasing the molecular weight that can be applied to other α-olefins. Adding C4OPh to 1-octene (oct.) and 4-methyl-1-pentene (4M1P) reactions, respectively, resulted in an increase of +245% (1346 vs 390 kDa) and +397% (1004 vs 202 kDa) in Mw (Table 1, entries 17–20). This clearly shows that polar monomers are an alternative to the time-intensive synthesizing and screening of catalysts previously used to produce ultrahigh molecular weight isotactic poly(α-olefin)s. The precatalyst 5 cannot generate ultrahigh molecular weight isotactic poly(α-olefin)s but can be used to produce ultrahigh molecular weight functionalized isotactic poly(α-olefin)s including poly(1-hexene), poly(1-octene), and poly(4-methyl-1-pentene). Isotactic copolymerization of propylene and polar monomer The high molecular weight method of production described above has also been applied to prepare high molecular weight, functionalized, and isotactic polypropylene from isotactic copolymerization of propylene and polar monomers. In fact, the precatalyst 5 activated with [Ph3C][B(C6F5)4] or B(C6F5)3 was enough to promote propylene or α-olefin polymerization (Table 1).4,25 However, because a large amount of solvents (toluene: 150 mL in Table 2 vs 5 mL in Table 1) was used in propylene (co)polymerizations, a small amount of AliBu3 (less than the amount of added polar monomer) was added as a scavenger to remove impurities and thus improve catalytic activities as reported previously.40,41,44 Under 1 bar of propylene at 25 °C in 150 mL of toluene, 5/[Ph3C][B(C6F5)4] showed a high activity for producing highly isotactic ([mmmm] = 99%) polypropylene with a molecular weight of 341 kDa in 20 min (Table 2, entry 1). Adding C4OPh (25 equiv to 5) to 1 bar of propylene atmosphere (and otherwise identical conditions) led to a significant increase of +262% (1233 kDa) in the copolymer molecular weight (Table 2, entry 2). This showed that f-UHMW-iPP was synthesized by introducing a polar monomer, which mirrored the results shown for copolymerization of α-olefins. Increasing the amount of comonomer increased the incorporation of C4OPh (0.08–1.43 mol %) and reduced the catalytic activity (Table 2, entries 2–6; Figure 4a,b). The molecular weights of all propylene copolymers were higher than those of the propylene homopolymer (570–1233 vs 341 kDa). Both the yield and Mw of the functionalized iPPs increased with reaction time (10, 20, 40, and 60 min) at a consistent amount of C4OPh (100 equiv), (Table 2, entries 3 and 7–9; Figure 4c,d). The same trend was observed with time and varied amounts of C4OPh (Table 2, entries 10–12). The highest Mw of the functionalized iPP was 2151 kDa, an increase of +488% relative to 341 kDa of iPP. Table 2 | Isotactic Copolymerization of Propylene with Polar Monomersa Entry Comon. (equiv) Time (min) Yield (g) Act. (103)b Mw (103)c Increase on Mw PDIc X (mol %)d Tme 1 – (–) 20 8.30 2490 341 – 1.81 – 157.6 2 C4OPh (25) 20 2.43 729 1233 +262% 2.09 0.08 153.8 3 C4OPh (100) 20 1.09 327 972 +185% 2.01 0.35 148.3 4 C4OPh (150) 20 0.81 243 714 +109% 1.73 0.55 145.0 5 C4OPh (200) 20 0.20 60 570 +67% 1.48 0.86 140.6 6 C4OPh (300) 20 0.18 54 590 +73% 1.57 1.43 133.8 7 C4OPh (100) 10 0.53 318 731 +114% 2.47 0.40 146.7 8 C4OPh (100) 40 1.87 281 1597 +368% 1.66 0.33 147.5 9 C4OPh (100) 60 2.67 267 2151 +488% 1.80 0.33 146.6 10 C4OPh (150) 180 2.58 86 1809 +430% 1.90 0.52 143.5 11 C4OPh (200) 480 2.01 25 1521 +346% 1.71 0.66 141.7 12 C4OPh (300) 480 0.87 11 1387 +307% 1.52 1.00 136.1 13 C4CPh (100) 20 5.58 1674 163 −52% 1.99 0.78 152.7 14 C4SPhMe (100) 20 3.73 1119 1116 +227% 1.79 0.54 145.0 15 C4OiPr2Ph (50) 20 8.19 2457 488 +43% 2.69 0.16 151.9 16 C4OiPr2Ph (100) 20 8.32 2496 442 +30% 2.41 0.44 151.0 17 C4OiPr2Ph (150) 20 5.67 1701 508 +49% 2.52 0.93 138.1 aStandard reaction conditions: catalyst 5 (10 μmol), [Ph3C][B(C6F5)4] (10.5 μmol), AliBu3 (50 equiv) as a scavenger, propylene (1 bar), toluene (150 mL), 25 °C. All entries are based on at least two runs, unless noted otherwise. bg mol−1 h−1. cDetermined by GPC in 1,2,4-trichlorobenzene at 150 °C. dDetermined by 1H NMR spectroscopy in C2D2Cl4 at 110 °C. eDetermined by DSC (second heating). Figure 4 | Isotactic copolymerization of propylene with C4OPh, C4SPhMe, C4OiPr2Ph, and the compared C4CPh: (a) GPC curves, (b) changes in molecular weight and comonomer incorporation at varied ratios, (c) time dependence on yield and molecular weigh, and (d) molecular weight and catalytic activity using different comonomer. Download figure Download PowerPoint The comonomer C4CPh was tested in propylene copolymerization as a control experiment and displayed high catalytic activity with a decrease of 52% in the copolymer Mw (Table 2, entry 13). This reveals the key role of the heteroatom in affecting the copolymer Mw. Replacing O with S allowed for the facile synthesis of ultrahigh molecular weight (1116 kDa) S-functionalized iPP displaying an increase of +227% in Mw (Table 2, entry 14) with a high copolymerization activity of 1.1 × 106 g mol−1 h−1. Compared to studies on the synthesis of ultrahigh molecular weight polyethylene (UHMWPE)59 and functionalized UHMWPE,60,61 ultrahigh molecular weight isotactic polypropylene (UHMW-iPP), as a kind of thermoplastic engineering plastic, has rarely been investigated,62–65 and the functionalized UHMW-iPP is formidable. Although high molecular weight functionalized iPP is available,40,41,44 this work now presents the first example of accessing f-UHMW-iPP. Another important parameter we need to improve is catalytic activity, for which a bulky substituent was used to reduce the O-coordinating interaction with the metal active species. Copolymerizations of C4OiPr2Ph with propylene produce activities (1.7–2.5 × 106 g mol−1 h−1) comparable to thse of propylene homopolymerization (Table 2, entries 15–17 vs 1) with an increase of +30% to +49% in the Mw of copolymers relative to the propylene homopolymer. This suggests that use of the polar monomers in copolymerization with propylene can produce the desired catalytic activity and molecular weight. As a comparison with propylene homopolymerization, the copolymerization of propylene with C4OPh produced the significantly higher Mw copolymer accompanied by a significant decrease in activity; however, the copolymerization of propylene with C4OiPr2Ph produced the higher Mw copolymer with almost unchanged activity. The difference between C4OPh copolymerization and C4OiPr2Ph copolymerization should be attributed to the greater repulsion between iPr groups and the fact that the auxiliary ligand in the case of C4OiPr2Ph weakened the O-backbiting interaction, which generated the lower suppressing effect on the chain transfer of β-H elimination but favored the chain growth of the next monomer (see DFT part). Copolymer microstructures and properties The produced functionalized isotactic polypropylenes and poly(α-olefin)s (cf. Supporting Information for details) were characterized by GPC, DSC, and 1H/13C NMR spectroscopy ( Supporting Information Figures S2–S91). Figure 5a shows that the 13C NMR spectrum of iPP exhibits three resonances at 46.09, 28.46, and 21.58 ppm, indicating a highly isotactic selectivity ([mmmm] = 99%). Sharp singlets at the same chemical shifts were observed in the copolymers of propylene + C4OPh, propylene + C4SPhMe, and propylene + C4OiPr2Ph, indicating that the high isotacticity was retained. The generation of functionalized iPPs is suggested by resonances at 43.41 ppm (13C)/4.04 ppm (1H, t), 43.38 ppm (13C)/2.94 ppm (1H, t), and 43.49 ppm (13C)/3.83 ppm (1H, t) that correspond to methylene groups originating from –CH2OPh, –CH2SPhMe, and –CH2OiPr2Ph, respectively. The 1H NMR spectrum can be used to calculate the amount of comonomer incorporated into the material (cf. Supporting Information for details), which can affect the molecular weight and melting point of a polymer. A measurement of the WCA can be used to evaluate the influence of polar functional groups on the surface of iPP (Figure 5b). The results show that the WCA of iPP decreases with increasing comonomer incorporation. The WCA is observed to decrease 11° with the incorporation of 1 mol % of polar monomer. Figure 5 | Copolymer microstructures and properties in Table 2: (a) 13C NMR spectra, (b) WCAs, (c) stress–strain curves of iPP and isotactic copolymers of propylene+C4OPh, propylene+C4SPhMe, propylene+C4OiPr2Ph. Download figure Download PowerPoint Broader applications of iPP are limited because it is a rigid and brittle plastic, independent of its molecular weight. The introduction of long-chain branches (e.g., long-chain polar monomers) increases toughness but usually reduces tensile strength40,41,44 because too many branches reduce the crystallinity of iPP. For instance, a contradiction between toughness and tensile strength always occurs in pioneering N-functionalized iPPs originating from the copolymerization of N-functionalized polar monomer with propylene (Figure 1a).40,41,44 The incorporation of a polar monomer can provide a suitable number of branches to increase toughness with minimal change to the tensile strength. Figure 5c shows the tensile testing of pure iPP with a high tensile strength of 29 MPa and a low elongation to failure of 13%, which is indicative of a typically rigid and brittle material. The f-UHMW-iPP (the copolymer of propylene + C4OPh) with an incorporation of C4OPh of 1 mol % exhibits a tensile strength of 42 MPa and a significant increase in the elongation to failure (860%) that is 66 times higher than that of pure iPP. This suggests that 1 mol % of polar monomer incorporation is enough to significantly improve the desired toughness of iPP, without compromising the tensile strength of the parent iPP due to the key achievement of ultrahigh molecular weight. A lower incorporation (0.54 mol %) of polar monomer demonstrates similar improvement in tensile strength (31 MPa) and elongation to failure (590%) in f-UHMW-iPP (the copolymer of propylene+C4SPhMe). Mechanistic insights Observation of the beneficial effect of a polar compound for either enhancing tacticity or elevating molecular weight in propylene copolymerization41,44,45 and for assisting the reactions in ethylene or styrene copolymerization18,22,24,66–68 has been described. However, a mechanistic insight into the increased molecular weight in propylene or α-olefin copolymerization enabled by the polar monomer remains elusive, which should involve the comonomer effect, the chain transfer pathway, and so on. Since the copolymerizations of both α-olefin/polar monomer and propylene/polar monomer mediated by 5/[Ph3C][B(C6F5)4] give increased molecular weights, a selected DFT calculational study of the copolymerization of propylene and C4OPh was carried out to further understand the effect of polar monomer on the Mw of copolymer and the catalytic activity using the precatalyst 5 that leads to the isotactic polymerization of propylene. The stereoselective polymerization of propylene is computationally discussed in Figure 6. The propylene polymerization was computed based on a real active species ( P1Ar), obtained by the first insertion of propylene into the Hf–Aryl bond as previously reported.40,69,70 The results show that, for the first insertion of propylene, the Hf–Aryl bond is more favored than that of the Hf–Me bond ( Supporting Information Figure S1, free-energy barrier: 4.1 vs 12.4 kcal mol−1, and energy release: −20.3 vs −14.9 kcal mol−1). The second propylene insertion with the re-face into the Hf−Me bond shows a slightly lower energy barrier of 13.9 kcal mol−1 than the si-model (14.5 kcal mol−1). Successive re-insertions of propylene are required to overcome an energy barrier of 13.8