Ethylene/Polar Monomer Copolymerization by [N, P] Ti Complexes: Polar Copolymers with Ultrahigh-Molecular Weight.

A series of novel titanium complexes (2a-2e) bearing [N, P] aniline-chlorodiphenylphosphine ligands (1a-1e) featuring CH3 and F substituents have been synthesized and characterized. Surprisingly, in the presence of polar additive, the complexes (2a-2e) all displayed high catalytic activities (up to 1.04 × 106 gPolymer (mol·Ti)-1·h-1 and produced copolymer with the ultrahigh molecular weight up to 1.37 × 106 g/mol. The catalytic activities are significantly enhanced by introducing electron-withdrawing group (F) into the aniline aromatic ring. Especially, the increase in activity based on different complexes followed the order of 2e > 2d > 2c > 2b > 2a. Simultaneously, density functional theory (DFT) calculations have been performed to probe the polymerization mechanism as well as the electronic and steric effects of various substituents on the catalyst backbone. DFT computation revealed that the polymerization behaviors could be adjusted by the electronic effect of ligand substituents; however, it has little to do with the steric hindrance of the substituents. Furthermore, theoretical calculation results keep well in accordance with experimental measurement results. The article provided an appealing design method that the employment of fluorine atom as electron-withdrawing to be studied is the promotive effect of transition-metal coordination polymerization.

Significantly Tunable Foaming Behavior of Blowing Agent for the Polyethylene Foam Resin with a Unique Designed Blowing Agent System.

The chemical blowing agent plays a crucial role in enhancing the performance of the polyethylene (PE) foaming resin during the rotational foaming process. Previously, the conventional blowing agent of the PE resin commonly used pure azodicarbonamide (AZ). It had the unavoidable drawbacks of releasing NH3 and exhibiting strong reactions during the rotational foaming process. Meantime, pure AZ had a relatively high decomposition temperature, resulting in a sharp foaming process. To address the above issues, this work developed a uniquely designed blowing agent system. In this study, a novel blowing agent for the PE resin was successfully synthesized by a one-pot method. This blowing agent consisted of an activator and AZ, which exhibited a lower decomposition temperature and a milder decomposition rate than AZ. The activator was constituted of small-sized ammonium dihydrogen phosphate on the AZ surface, which could be decomposed properly and deliver phosphoric acid and H2O during the foaming process. Then, AZ reacted with H2O under phosphoric acid catalysis. Also, this reaction generated CO2 emission while reducing the emission of NH3 through recombination with phosphoric acid. Moreover, phosphoric acid catalysis caused a decrease in the AZ decomposition temperature. Meantime, the thermal coupling appeared during the foaming process, which could further reduce the decomposition rate. Consequently, the small activator played a key role in regulating cell formation and diffusion. Compared to AZ, the novel blowing agent system significantly reduced the cell diameter of the PE foam resin and enhanced its flexural modulus by 50%. Furthermore, the novel blowing agent facilitated better demolding performance and improved the surface morphology of the PE foam product. This research provides significant foaming behavior regulation for the PE resin during industrial applications.

Model Simulation and Rheological Research on Crosslinking Behavior of Polyethylene Resin.

The crosslinking behavior of polyethylene (PE) determines its exceptional performance and application. In this study, we investigated the crosslinking behaviors of different PE resins through model simulation and rheological methods. Specifically, the mathematical equation of "S" model was established for PE resin. According to this equation, the optimal maximum gel content for high-density polyethylene (HDPE) was found to be around 85%. Moreover, the maximum crosslinking degrees for different PE resins depended largely on their density and molecular weight. The melt viscosities before crosslinking in PE resins were highly influenced by their melt index. The higher melt indexes resulted in the lower storage moduli, improving melt processability during processing. In addition, the crosslinking rates of PE resins were strongly influenced by peroxide concentration, independent of PE resin structures. For high molecular weight and low-density PE resins, they exhibited decreased ti values, increased A0 values, and decreased k6 values. However, there were no noticeable variations in the values of k2 and phi among different PE resins. All simulated modeling outcomes showed remarkable consistency with the experimental rheological data. These findings are of strong significance in the industrial manufacture of PE resin.

Improved Rotational Foam Molding Properties of Tailored Polyethylene Blends with Higher Crystallization Temperature and Viscosity-Temperature Sensitivity.

Rotational foam molding has attracted more and more attention due to the inexpensive machines required, low residual stresses, and flexible design for special and high-value-added applications. However, it is a great challenge to control cell sizes and morphology because of its coalesce and collapse during prolonged heating or at different temperatures. A novel tailored polyethylene blend with a unique chain structure for rotational foam molding was creatively proposed and demonstrated, and the effects of crystallization temperature and viscosity-temperature sensitivity on foaming were also investigated. The polyethylene blends with few chain branches in the low-molecular-weight part and many chain branches in the high-molecular-weight part effectively improved the crystallization temperature and the viscosity-temperature sensitivity for better prevention of coalesce and collapse during the foam-shaping stage.

Band Spacing in Poly(3-hydroxybutyrate) and Its Blends with Poly(propylene carbonate): Dependence on Thermal Processing.

The band spherulites grown in neat poly(3-hydroxybutyrate) (PHB) and its blends with poly(propylene carbonate) (PPC) were observed by polarized optical microscopy. For the spherulites in neat PHB, it is evident that the band spacing increases first and then decreases with melting time. As the melting time is within 7 min, the band spacing increases continuously, which should be attributed to increasing mobility of polymer chains or decreasing viscosity of the melt. When the melting time is prolonged, evident thermal degradation of PHB occurs and results in a great deal of noncrystalline fractions, which is similar with addition of miscible amorphous polymers in the melt, and the band spacing decreases accordingly. The thermal degradation of PHB cannot, however, be detected by a thermogravimetric analyzer because of less volatile productions. An evident decrease of molecular weight can be measured by gel permeation chromatography, indicating occurrence of serious degradation. The decrease of crystallization and melting temperature revealed by differential scanning calorimetry (DSC) also prove the thermal degradation. For spherulites in PHB/PPC blends, however, the variation of band spacing differs from that in neat PHB. The band spacing increases continuously when melting time is within 15 min. The crystallization and melting behaviors are not influenced greatly by prolonging melting time in PHB/PPC blends. The variations of Mw for PHB/PPC are slighter than those of the neat PHB and PPC upon heating at 190 °C. Combined with the corresponding DSC results, it is conjectured that blending may prohibit the degradation of PHB to some extent. An intermolecular interaction can be detected between PHB and PPC via Fouriertransform infrared spectra and should help to avoid degradation of PHB to a certain degree. The present results may help widen the applications of PHB and shed some light on understanding the formation mechanism of the band for aliphatic polyester polymers.

Study on Hydrogen Sensitivity of Ziegler⁻Natta Catalysts with Novel Cycloalkoxy Silane Compounds as External Electron Donor.

Two novel cycloalkoxy silane compounds (ED1 and ED2) were synthesized and used as the external electron donors (EEDs) in Ziegler⁻Natta catalysts with diethyl 2,3-diisopropylsuccinate as internal electron donor. The results indicated that the Ziegler⁻Natta catalysts using ED1 and ED2 as EEDs had high catalytic activities and good stereoselectivities. The melt flow rate (MFR) and gel permeation chromatography (GPC) results revealed that the obtained polypropylene has higher MFR and lower average molecular weights than the commercial EED cyclohexyl methyl dimethoxysilane. The differential scanning calorimetry (DSC) results indicated that new isospecific active centers formed after the introduction of new external donors. The work implied that the novel EEDs could improve the hydrogen sensitivities of the catalyst system and obtain polymers with high melt flow rate.

Highly Active Copolymerization of Ethylene and N-Acetyl-O-(ω-Alkenyl)-l-Tyrosine Ethyl Esters Catalyzed by Titanium Complex.

A series of N-acetyl-O-(ω-alkenyl)-l-tyrosine ethyl esters were synthesized by the reaction of vinyl bromides (4-bromo-1-butene, 6-bromo-1-hexene, 8-bromo-1-octene and 10-bromo-1-decene) with N-acetyl-l-tyrosine ethyl ester. ¹H NMR, elemental analysis, FT-IR, and mass spectra were performed for these N-acetyl-O-(ω-alkenyl)-l-tyrosine ethyl esters. The novel titanium complex can catalyze the copolymerization of ethylene and N-acetyl-O-(ω-alkenyl)-l-tyrosine ethyl esters efficiently and the highest catalytic activity was up to 6.86 × 104 gP·(molTi)-1·h-1. The structures and properties of the obtained copolymers were characterized by FT-IR, (¹H)13C NMR, GPC, DSC, and water contact angle. The results indicated that the obtained copolymers had a uniformly high average molecular weight of 2.85 × 105 g·mol-1 and a high incorporation ratio of N-acetyl-O-(but-3-enyl)-l-tyrosine ethyl ester of 2.65 mol % within the copolymer chain. The units of the comonomer were isolated within the copolymer chains. The insertion of the polar comonomer into a copolymer chain can effectively improve the hydrophilicity of a copolymer.

Synthesis and structures of some heterometallic [(Li, Y)2, (M3, Ce) (M = Li or Na), (Li, Zr2) and (Li, Zr)4] oligomeric diamides derived from 1,2-bis(neopentylamino)benzene.

The following crystalline, X-ray-characterised heterometallic oligomeric diamides have been prepared in good yield under mild conditions in diethyl ether from the dilithio or disodio derivative of the N,N'-dineopentyl-1,2-diaminobenzene [{N(H)(CH2Bu(t))}2C6H4-1,2] (abbreviated as H2L):[Y(L)(mu-Cl)2Li(OEt2)2]2 (1), [Li(OEt2)2Li(mu2-Cl)4(mu3-Cl)2{Zr(L)}2]2 (2), [Zr(L)2(mu-Cl){Li(OEt2)2}(mu2-Cl)2Zr(L)] (3), [Ce{(mu-L)M}3(OEt2)(1/2)] (3M = Li(1.82)Na(1.18)) (4), [Ce{(mu-L)Na}3(OEt2)] (5) and [Ce{(mu-L)Na}3] (6). Compounds 1-3 were obtained from Li2(L) and YCl3 (the colourless 1) or ZrCl4 (the red 2 and 3), while the red 4 and 5 were isolated from CeCl3 and M2(L) (3M = Li(1.82)Na(1.18)) (4) or Na2(L) (5). Attempted oxidation of 5 with Br2 in hexane yielded the black 6. The ligand is N,N'-chelating to each of the d- or f-block metals in 1-6; and in 4-6 L is also acting as a bridge between Ce and the alkali metal, to which L is thus also chelating.

The coordination chemistry of the C1-symmetric bis(silyl)methyl ligand [CH(SiMe3){SiMe(OMe)2}]- revisited: Li/M- (M = Zn, Tl, Ce), Li4- or Ce2-methoxy-bridged alkyls.

The following crystalline oligonuclear metal alkyls have been synthesised under mild conditions and structurally characterised: [(THF)Li(mu-A)(mu-Cl)(mu3-OMe)Zn]2, [Li(mu-A)2Tl]2(4 and 4'), [Li4(mu-A)3(micro3-OMe)]5, [(mu-A)Li2(mu-A)2(mu3-OMe)Ce(A)](6) and [Ce(A)(mu2-OMe){mu2-OS(O)(CF3)O}]2(11)[= CH(SiMe3){SiMe(OMe)2}]. Compounds 2-6 were obtained from [Li(mu-A)]infinity(1) and ZnCl2(3), TlCl (4 and 4' and 5) and CeCl3(6), and 11 was isolated from K(A)(prepared from 1 + KOBu(t)) and cerium(III) triflate Ce(OTf)3. The principal novel features are (i) and (ii) as follows. As for (i), the diversity of ligand-to-metal bonding is noteworthy, the ligand being (a)C,O-bridging in 3{as in the known compounds 1 and in [Li2Mg5(mu3-OMe)6(mu2-OMe)2(mu2-A)4](2)}; (b)C,O,O'-bridging and O,O'-chelating in 4 and 4'; (c)C,O,O'-bridging in 5; (d)C,O,O'-bridging and C,O-chelating in 6; and (e)C,O-chelating in 11. Regarding (ii), it is interesting that the ligand [A]- is surprisingly ready to undergo fragmentation by Si-OMe cleavage and thereby present bridging methoxy ligands (mu2-OMe)2 to a pair of Ce3+ ions in 11, or mu3-OMe acting as a cap for triangular arrays of three hard metal ions (Mg3 in 2, LiZn2 in 3, Li3 in 5, and Li2Ce in 6).