Unimodal Molecular Weight Distribution Research Articles

High−molecular weight poly(2-substituted 1,3-diene)s using iminopyridine iron precatalysts at high polymerization temperature and low iron loading

The synthesis of a series of iron(II) dichloride complexes bearing iminopyridine (ImPy) ligands, differing in the substituents at the pyridine, bridging carbon and N−aryl ring is described (1-LHPh−7-LHS). The X-ray structures of some of them are presented. All the iron compounds and, for comparison, the already reported 8-LHPhP, that features bulky ortho isopropyl N−aryl substituents, have been investigated as precatalysts for the polymerization of 2-substituted 1,3-dienes, i.e., isoprene and β-myrcene, using a low excess of methylaluminoxane (MAO) as alkylating reagent, iron loading as low as 0.025 mol% and variable temperature up to 100 °C. The goal is to have insights into ligand effects on catalyst reactivity and selectivity and to recognize the most robust iron precatalysts to access high-molecular weight (MW) polymers using low iron loading and high polymerization temperature, which meets current demands of mild, operationally simple approach. Particularly, the ketimine 4-LMePh and 5-LPhPh are more stable and least prone to chain termination than all the aldimine parents. 4-LMePh and 5-LPhPh produce cis-1,4/3,4 ultra-high-MW poly(isoprene)s and solid high-MW poly(β-myrcene)s, while maintaining narrow and unimodal molecular weight distribution. In this study we intentionally kept the monomer conversion low to provide a valuable assessment of catalyst stability, uncorrupted by mass or heat transport limitations. Furthermore, the synthesis of the unsubstituted aminopyridine (AmPy) iron(II) dichloride complex 9-LAPh, analogue to 1-LHPh, is described. 9-LAPh has been demonstrated to be a valuable probe for stability studies. Given the variety of catalytic transformations and substrates for which (ImPy)FeCl2 precatalysts have promising reactivity and selectivity, those developed in this study offer promising applications also beyond the polymerization of 1,3-dienes.

Enhanced nucleation of bimodal molecular weight distribution polymers: A molecular dynamics study

We perform coarse-grained molecular dynamics (CGMD) simulations to study the homogeneous nucleation of bimodal and unimodal molecular weight distribution polymers with equivalent average molecular weight. First, a statistical method is proposed to determine the critical nuclei and thus calculate the free energy barrier of nucleation. From the temperature dependence of diffusion coefficient, we also determine the activation energy of diffusion. Then we calculate the nucleation rate and find that it is consistent with the classical nucleation theory for homogeneous nucleation in semi-crystalline polymers. Compared with unimodal system, the bimodal system exhibits lower interfacial free energy and consequently lower free energy barrier for nucleation, while the two systems have similar activation energy for diffusion. This suggests that the promoted nucleation rate of bimodal molecular weight distribution polymer is a result of the reduction of interfacial free energy, which is eventually a consequence of chain-folding nucleation of long chain component.

Flexible Axial Shielding Strategy for Improving Ethylene (Co)polymerization with 8-Cycloalkylnaphthyl α-Diimine Catalysts.

8-aryl or alkyl-naphthyl substituents are widely used as an effective axial shielding strategy for the suppression of chain transfer in late-transition metal-catalyzed ethylene (co)polymerization to yield high molecular weight polyethylene and copolymers. In this study, two 8-cycloalkylnaphthyl acenaphthene-based α-diimine ligands and the corresponding four nickel and palladium complexes were designed and synthesized to explore the effect of axial flexible shielding on ethylene (co)polymerization. In ethylene polymerization, the nickel complexes displayed high activities (up to 1.99 × 106 g mol-1 h-1) and generated lightly branched (34-54/1000 C) polyethylenes with high molecular weights (up to Mn = 1075 kg/mol), whereas the corresponding palladium complexes exhibited moderate activities (level of 104 g mol-1 h-1), producing highly branched (111-125/1000 C) polyethylenes with high molecular weights (up to Mn = 37.6 kg/mol). Highly branched (110-123/1000 C) E-MA copolymers with moderate insertion ratios (1.97-5.56 mol %) were produced by these palladium complexes in ethylene/methyl acrylate (MA) copolymerization. In addition, the size of the 8-cycloalkyl ring in these α-diimine catalysts strongly influences the ethylene (co)polymerization. Compared to cyclopentyl groups, cyclohexyl groups are more effective in suppressing chain transfer reactions in the polymerization of ethylene and the copolymerization of ethylene and MA, leading to higher molecular weight polyethylene and E-MA copolymers. Most interestingly, compared to the reported rigid planar 8-arylnaphthyl catalysts, the flexible 8-cyclohexylnaphthyl catalysts exhibited higher activity and produced higher molecular weight polyethylene in ethylene polymerization. Moreover, in nickel-catalyzed ethylene polymerization, the cyclohexyl catalyst produced significantly reduced branched polyethylene, while in palladium-catalyzed ethylene (co)polymerization, the cyclohexyl catalyst produced more highly branched polyethylene and copolymers. In contrast to the previously reported flexible 8-butylnaphthyl nickel catalysts, the 8-cycloalkylnaphthyl catalysts reported in this work yielded polyethylene with narrow unimodal molecular weight distributions.

Molecular weight distribution shape approach for simultaneously enhancing the stiffness, ductility and strength of isotropic semicrystalline polymers based on linear unimodal and bimodal polyethylenes

Bimodal polyethylenes (PEs) have captured broad attention in recent years, while little is known about the contribution of bimodal molecular weight distribution (MWD) shape on their properties compared with unimodal MWD shape. In this work, a comparative investigation of MWD shape-mechanical property relationship of isotropic semicrystalline polymers based on linear unimodal and bimodal PEs has been carried out. This first systematic study indicates that MWD shape influences the chain arrangements in crystalline and amorphous phases which affect their mechanical performance. PEs with bimodal shape exhibit higher crystallinity, lower entanglements and higher fraction of tie molecules than unimodal shape, contributing to simultaneously enhanced Young's modulus, yield strength, elongation at break and tensile strength. This work demonstrates that the modulation of MWD from unimodal to bimodal shape could overcome the trade-off among stiffness, strength and ductility of semicrystalline polymers without altering chain structure, chemical composition or average molecular weight.

Molecular Weight Distribution Shape Dependence of the Crystallization Kinetics of Semicrystalline Polymers Based on Linear Unimodal and Bimodal Polyethylenes

The manipulation of the crystallization kinetics and crystallinity of polymers without altering their chemical composition, chain structure, or average molecular weight is challenging, while little attention is paid to the role of molecular weight distribution (MWD) shape. In this work, a series of well-defined linear unimodal and bimodal polyethylenes (PEs) with molecular weights ranging from 300 k to 1200 k g/mol were synthesized to serve as a model system to study the impact of bimodal MWD shape on the crystallization kinetics of semicrystalline polymers in comparison with unimodal MWD shape at the same weight-average molecular weight (Mw). It is shown that PEs with a bimodal shape exhibit a faster nucleation rate and crystallization rate with a smaller lamellar width at low isothermal temperatures. A higher crystallization enthalpy of bimodal PEs is shown in non-isothermal experiments. The mechanism behind this is elucidated, which suggests that MWD shapes mainly affect the small-scale nucleation process without altering the large-scale growth process. This first systematic comparative study on the crystallization kinetics of linear unimodal and bimodal PEs gives insight into tailoring the crystallization behavior of semicrystalline polymers from an MWD shape perspective.

Depolymerization of Polyesters by Transesterification with Ethanol Using (Cyclopentadienyl)titanium Trichlorides

Exclusive chemical conversions of polyesters [poly(ethylene adipate) (PEA), poly(butylene adipate) (PBA), poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT)] to the corresponding monomers (diethyl adipate, diethyl terephthalate, ethylene glycol, 1,4-butane diol) by transesterification with ethanol using Cp’TiCl3 (Cp’ = Cp, Cp*) catalyst have been demonstrated. The present acid-base-free depolymerizations by Cp’TiCl3 exhibited completed conversions (>99%) of PET, PBT to afford diethyl terephthalate and ethylene glycol or 1,4-butane diol exclusively (selectivity >99%) without formation of any other by-products in the NMR spectra (150–170 °C, Ti 1.0, or 2.0 mol%). The resultant reaction mixture after the depolymerization of PBA with ethanol via the CpTiCl3 catalyst (1.0 mol%, 150 °C, 3 h), consisting of diethyl adipate and 1,4-butane diol, was heated at 150 °C in vacuo for 24 h to afford high molecular weight recycled PBA with unimodal molecular weight distribution (Mn = 11,800, Mw/Mn = 1.6), strongly demonstrating a possibility of one-pot (acid-base-free) closed-loop chemical recycling.

Insight of polypropylene synthesis with high performance multidentate internal donor catalyst system

AbstractPropylene polymerization using magnesium dichloride (MgCl2) supported Ziegler–Natta (Z–N) system is a robust and preferred catalytic process for producing polypropylene (PP) grades. Herein, synthesis of polypropylene products is reported using MgCl2 supported multidentate (3,3,3′,3′‐tetramethyl‐2,2′,3,3′‐tetrahydro‐1,1′‐spirobiindane‐5,5′,6,6′‐tetracarbonate) Z–N pre‐catalyst. The catalyst performance evaluation over propylene polymerization found that multidentate Z–N pre‐catalyst has better hydrogen response with controlled isotacticity of PP products as compared to conventional bidentate Z–N pre‐catalysts. Melt rheology analysis showed higher loss modulus and storage modulus for ultra‐high molecular weight isotactic polypropylene (UHMWiPP) as compared to PP resins with medium molecular weight. Further, GPC analysis reveal medium to broad unimodal molecular weight distribution (MWD) of PP products produced with multidentate Z–N pre‐catalyst system.

Synthesis of High Molecular Weight Water-Soluble Polymers as Low-Viscosity Latex Particles by RAFT Aqueous Dispersion Polymerization in Highly Salty Media.

We report the synthesis of sterically-stabilized diblock copolymer particles at 20% w/w solids via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of N,N′-dimethylacrylamide (DMAC) in highly salty media (2.0 M (NH4)2SO4). This is achieved by selecting a well-known zwitterionic water-soluble polymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), to act as the salt-tolerant soluble precursor block. A relatively high degree of polymerization (DP) can be targeted for the salt-insoluble PDMAC block, which leads to the formation of a turbid free-flowing dispersion of PDMAC-core particles by a steric stabilization mechanism. 1H NMR spectroscopy studies indicate that relatively high DMAC conversions (>99%) can be achieved within a few hours at 30 °C. Aqueous GPC analysis indicates high blocking efficiencies and unimodal molecular weight distributions, although dispersities increase monotonically as higher degrees of polymerization (DPs) are targeted for the PDMAC block. Particle characterization techniques include dynamic light scattering (DLS) and electrophoretic light scattering (ELS) using a state-of-the-art instrument that enables accurate ζ potential measurements in a concentrated salt solution. 1H NMR spectroscopy studies confirm that dilution of the as-synthesized dispersions using deionized water lowers the background salt concentration and hence causes in situ molecular dissolution of the salt-intolerant PDMAC chains, which leads to a substantial thickening effect and the formation of transparent gels. Thus, this new polymerization-induced self-assembly (PISA) formulation enables high molecular weight water-soluble polymers to be prepared in a highly convenient, low-viscosity form. In principle, such aqueous PISA formulations are highly attractive: there are various commercial applications for high molecular weight water-soluble polymers, while the well-known negative aspects of using a RAFT agent (i.e., its cost, color, and malodor) are minimized when targeting such high DPs.

Thermal‐stable asymmetric α‐diimine nickel(II) catalysts: Synthesis, characterization, and its norbornene (co)polymerization behavior

Four asymmetric α‐diimine dibromide nickel(II) complexes bearing both 2,6‐diisopropyl phenyl and substituted triphenyl {[(C3H7)2C6H3NC(C10H6)CN(R2C6H3)2C6H3]NiBr2, R = CH3,Catal.1; CF3,Catal.2; OCH3,Catal.3; C6H5,Catal.4} were synthesized and characterized. In the presence of B(C6F5)3, these newly synthesized nickel complexes showed high activities toward norbornene (NBE) homopolymerization (0.84–4.88 × 105 gpolymer/molNi‧h) under elevated temperatures, yielding high molecular weight polymers (106.5–657.9 kDa). These Ni/B catalytic systems also efficiently promoted the copolymerization of NBE withn‐butyl methacrylate (n‐BMA) and di(n‐butyl) itaconate (n‐DBI) under 70°C, respectively, affording copolymers with high molecular weight (n‐BMA: 95.0–235 kDa;n‐DBI:35.4–52.5 kDa) and moderate polar monomer incorporations (n‐BMA:1.2–6.4 mol%;n‐DBI:0.8–2.8 mol%). The copolymers with polar groups showed improved solubilities. The unimodal molecular weight distribution of the resultant polymers implied the single‐site catalytic behavior of these nickel catalysts toward NBE (co)polymerization. In addition, both the obtained PNB and the copolymers showed high glass transition temperature (289–345°C) and excellent thermal stability.

Analysis of Ethylene Copolymers with Long-Chain α-Olefins (1-Dodecene, 1-Tetradecene, 1-Hexadecene): A Transition between Main Chain Crystallization and Side Chain Crystallization.

A series of ethylene copolymers with long-chain α-olefins [LCAOs, 1-dodecene (DD), 1-tetradecene (TD), 1-hexadecene (HD)] and various LCAO contents were prepared, and their thermal properties, including effects of LCAO content and side chain length, were explored. The Cp*TiCl2(O-2,6-iPr2-4-SiEt3-C6H2)–MAO catalyst system afforded rather high-molecular-weight copolymers with unimodal molecular weight distributions and uniform compositions (confirmed by DSC thermograms). In addition to the melting temperatures (Tm values) corresponding to the so-called main chain crystallization (samples with low LCAO contents, the Tm value decreased upon increasing the LCAO content) and the side chain crystallization [polymer samples with high LCAO contents, by intermolecular interaction of side chains as observed in poly(DD), poly(TD), and poly(HD)], the other Tm value was observed, especially in poly(ethylene-co-HD)s (assumed to be due to co-crystallization of the branch and the main chain through an interaction of the main chain and the long side chains). The presence of another crystalline phase in poly(ethylene-co-HD)s was also suggested by a wide-angle X-ray diffraction (WAXD) analysis. These Tm values in poly(ethylene-co-TD)s and poly(ethylene-co-DD)s with rather high TD or DD contents were affected by the heating conditions in the measurement of DSC thermograms (5 or 10 °C/min), suggesting that the driving force for formation of the crystal packing (observed as Tm) is weak and affected by the alkyl side chain lengths.

Vanadium-Catalyzed Terpolymerization of α,ω-Dienes with Ethylene and Cyclic Olefins: Ready Access to Polar-Functionalized Polyolefins

α,ω-Dienes are an important class of monomers due to their utility in the synthesis of cyclopolyolefins and reactive polyolefin intermediates. In this contribution, the terpolymerization of two α,ω-dienes (i.e., 1,5-hexadiene and 1,7-octadiene) with ethylene and various cyclic olefins [i.e., norbornene (NB), 5-ethylidene-2-norbornene (ENB), and dicyclopentadiene (DCPD)] catalyzed by a chelated imido vanadium complex has been examined. The ENB and DCPD diene termonomers provide additional sites for post-polymerization functionalization. Vanadium-catalyzed terpolymerization of the investigated α,ω-dienes yields polyolefins with a high molecular weight (Mw up to 200 × 103 g mol–1), unimodal and narrow molecular weight distribution, subambient glass transition temperatures (−30 < Tg °C < −3), and a proper content of C═C bonds. Comprehensive NMR investigation of the obtained polymers revealed that subtle changes in the α,ω-diene size have important effects on the numerous combinations of insertion paths (ring closure vs ring opening), from which different repeating units with a C═C bond in the side or main polymer chain and cyclic units are installed. Finally, the poly(ethylene-ter-1,5-hexadiene-ter-NB) was subjected to thiol-ene addition using thioglycolic acid, methyl thioglycolate, and N-acetyl-l-cysteine to access polar-functionalized polyolefins with a degree of functionalization and properties dependent on the thiol substitution.

Synthesis and characterization of new proton‐exchange membranes based on poly‐1‐vinyl‐1,2,4‐triazole doped with phenol‐2,4‐disulfonic acid

Poly-1-vinyl-1,2,4-triazole (PVT) was synthesized in isolated yield by free radical polymerization of 1-vinyl-1,2,4-triazole. The number average molecular weight and weight average molecular weight of the synthesized PVT, determined by gel permeation chromatography, was 88 567 and 153 309 Da, respectively. PVT had a unimodal molecular weight distribution with a polydispersity coefficient of 1.7. Mixing PVT with phenol-2,4-disulfonic acid (PDSA) in the presence of a polyvinyl alcohol cross-linked with oxalic acid produces proton exchange membranes having different PVT:PDSA ratios. The composition and structure of the membranes were established by elemental analysis, energy-dispersive X-ray spectroscopy (EDS), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance spectroscopy (NMR). FT-IR and 15N NMR studies have shown that the interaction of PVT with PDSA was accompanied by the formation of acid-base complexes due to proton transfer from PDSA to the triazole rings of PVT. Formation of acid-base complexes PVT-PDSA additionally stabilizes membranes and increases their thermal stability. The SEM study showed that the membrane surface was homogeneous. The properties of membranes, such as thermal and oxidative stability, ion-exchange capacity, proton conductivity, water uptake, and their mechanical properties, have been studied. The commercial membrane Nafion 212 was used as a comparison. The proton conductivity of membranes increases with an increase in the content of PDSA in their composition and for membranes 1-3 was 59.80, 7.30, 6.23 mS/cm, respectively. The activation energies for proton transfer across the membranes were 19.5, 21.6, and 38.2 kJ/mol for the membranes 1-3, respectively. Water uptake, ion-exchange capacity, and proton conductivity depend on the content of PDSA in the membrane. The membranes obtained were tested in a test cell of an ElectroChem hydrogen fuel cell (USA) at a temperature of 25°C with the supply of humidified hydrogen and air.

Flexible "Sandwich" (8-Alkylnaphthyl α-Diimine) Catalysts in Insertion Polymerization.

8-Arylnaphthyl substituents are privileged motifs frequently integrated into late-transition-metal catalysts, endowing them with an ability to retard chain transfer in ethylene polymerization. In this contribution, we disclose a sort of novel α-diiminenickel and -palladium complexes containing flexible 8-alkylnaphthyl in lieu of rigid 8-arylnaphthyl and their catalytic performance in ethylene polymerization. An interesting feature of these 8-alkylnaphthyl-substituted α-(diimine)PdMeCl complexes is that they present as a mixture of syn and anti isomers (syn:anti = ca. 1:1 ratio, determined by 1H and 13C NMR spectroscopy). In ethylene polymerization, these nickel complexes displayed high activity (up to 3.37 × 106 g mol-1 h-1) and generated branched polyethylenes with broad or bimodal molecular weight distributions (4.6-29.3), while the corresponding palladium complexes exhibited moderate activity, producing highly branched polyethylenes with unimodal and narrow molecular weight distributions (<1.8). In ethylene (E)/methyl acrylate (MA) copolymerization, highly branched E-MA copolymers with considerable MA incorporations were achieved by these palladium complexes. Most interestingly, compared to rigid 8-arylnaphthyl-substituted α-diiminenickel and -palladium complexes, the flexible 8-alkylnaphthyl ones showed significantly improved activity and generated lower or comparable molecular weight polyethylenes or E-MA copolymers.

NEt3-Triggered Synthesis of UHMWPE Using Chromium Complexes Bearing Non-innocent Iminopyridine Ligands

Chromium complexes bearing non-innocent iminopyridine ligands were investigated as precatalysts for the polymerization of ethylene using different aluminum cocatalysts and Lewis base NEt3 as additive. Beyond confirming the key role of the chromium to ligand synergy in accessing the active complex, other factors that play a crucial role are (i) the nature of the aluminum activator that influences the ion pair generated, (ii) the presence of the additive that boosts the synthesis of UHMWPE with narrow and unimodal molecular weight distribution even at 40 °C, and (iii) the polymerization temperature that affects the polymerization catalysis and the polymer molecular weight. UV–vis–NIR and Fourier transform infrared (FT-IR) spectroscopic techniques have been applied to inspect the activation process and to understand the mode of action by which NEt3 affects the catalytic conversion of ethylene. NEt3 interacts with the aluminum cocatalyst (rather than with the chromium complex) to form an Et3N*AlRx adduct, thus affecting the catalytic ion pair.

Properties of Linear Low Density Polyethylene

As result of the modification of polyolefin, composite materials based on them were obtained for the use in special equipment in order to entrain their heat resistance. Data analysis DTA (differential thermal analysis) of the DTA curve of this polyethylene sample suggests a bimodal nature of their MWD (molecular weight distribution) which differs from polyethylene with unimodal MWD and a series of endo oxidation effects with a maximum temperature of 245, 335, 358 and 435 °С. X-ray structural studies showed that the crystal system and the size of the unit cells of the crystal lattice of LLDPE practically does not differ from those of LDPE. LLDPE as well as LDPE and HDPE has a layered structure with dense packing of macromolecules. In terms of crystallinity and crystallite sizes, LLDPE are on par with HDPE and significantly differ from LDPE. These data are in agreement with published data. The parameters of the unit cells of the crystal structure of UHMWPE are close to those of LLDPE, and by crystallinity it occupies middle ground between HDPE and LLDPE.

Chemoselective radical polymerization of N-allylmethacrylamide with an aid of complexation with Li+ cation

Radical polymerization of N-allylmethacrylamide (NAlMAAm) was conducted in CH3CN in the presence or absence of lithium bis(trifluoromethanesulfonyl)imide (LiNTf2). The addition of LiNTf2 accelerated the polymerization and affected the molecular weight distribution of the polymers obtained. Polymers with unimodal molecular weight distributions were obtained in the presence of LiNTf2, whereas those with multimodal distributions were obtained in the absence of LiNTf2. This suggested that LiNTf2 suppressed side reactions such as cross-linking reaction, commonly observed in polymerization of divinyl monomers, and allylic hydrogen abstraction reaction. The 1H NMR, IR and MALDI-TOF mass analyses of the polymers obtained suggested that the radical polymerization proceeded in a chemoselective manner, leading to preferential formation of linear polymers having pendant allyl groups.

Design, synthesis and characterization of higher poly(alkyl lactate acrylate)s

Homologous series of alkyl lactate and alkyl lactate acrylates starting from hexyl to dodecyl were synthesized and then polymerized by solution polymerization using AIBN as the free radical initiator. Lactic acid derivatives such as alkyl lactates and its acrylates were synthesized by azeotropic method. The formation of monomers and polymers were confirmed using Electron Impact-Mass Spectra, Fourier Transform Infrared spectroscopy and proton and proton decoupled carbon nuclear magnetic resonance spectroscopy. Inherent viscosity of poly(alkyl lactate acrylate)s was in the range of 0.12–0.26 dL/g. GPC curves of polymers exhibited unimodal molecular weight distribution. The thermal stability of the poly(alkyl lactate acrylate)s were determined using thermogravimetric analysis (TGA).

Phosphazene/Lewis Acids as Highly Efficient Cooperative Catalyst for Synthesis of High‐Molecular‐Weight Polyesters by Ring‐Opening Alternating Copolymerization of Epoxide and Anhydride

ABSTRACTThe synthesis of high‐molecular‐weight polyesters via the ring‐opening alternating copolymerization (ROAC) of cyclic anhydrides and epoxides using organocatalysts remains a major challenge. In this context, the organic cyclic trimeric phosphazene base (CTPB) was used as an efficient catalyst for the ROAC of phthalic anhydride (PA) with epoxides. It was found that the activity and PA conversion dramatically increased with the addition of triethyl borane (TEB) as a cocatalyst, even a small amount of TEB, for example, 5 mol % relative to CTPB. The molecular structures of obtained polyesters were characterized by nuclear magnetic resonance (1H NMR, 13C NMR) and matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI TOF), which demonstrated the formation of perfectly alternating polyesters with unimodal and narrow molecular weight distributions. Different molecular weight polyesters (up to 118.5 kg mol−1) were facilely prepared by reducing the loading of CTPB and TEB, while the polymerizations were fast (turnover frequency, TOF up to 500 h−1). Thermal analysis of the resulting polymers indicated that the alternating polyesters were amorphous, and their Tgs increased with the increment of molecular weights. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 803–810

Synthesis and characterization of aminophenolate-ligated rare-earth metal amide complexes and their catalytic activity for lactides polymerization

Amine elimination of Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 with aminophenol H[ON] {H[ON] = 2-(CH2NC5H10)-4,6-tBu2-C6H3OH} in 1:2 molar ratio in THF gave the monometallic rare-earth metal amide complexes [ON]2LnN(SiMe3)2 (Ln = Yb (1), Y (2), Gd (3), Sm (4), Nd (5)) in 57%—73% isolated yields. All these complexes were characterized by elemental analysis. The molecular structures of complexes 1–4 were determined by single crystal X-ray diffraction. These complexes are highly active for l-lactide polymerization to give high molecular weight polymers with unimodal molecular weight distributions. In addition, these complexes can also initiate rac-lactide polymerization with high activity to afford heterotactic-rich polylactides.

Disentangling and Lamellar Thickening of Linear Polymers during Crystallization: Simulation of Bimodal and Unimodal Molecular Weight Distribution Systems.

We have performed coarse-grained molecular dynamics simulations to study the isothermal crystallization of bimodal and unimodal molecular weight distribution (MWD) polymers with equivalent average molecular weight (Mw). By using primitive path analysis, we can monitor the entanglement evolution during the process of crystallization. We have discovered a quantitative correlation between the degree of disentanglement and crystallinity, indicating that chain disentanglement permits the process of crystallization. In addition, the crystalline stem length also displays a linear relation with the degree of disentanglement at different temperatures. Based on the observation in our simulations, we can build a scenario of the whole process of chain disentangling and lamellar thickening on the basis of chain sliding diffusion. Furthermore, we have enough evidence to infer that the temperature dependence of crystalline stem length is basically a result of temperature dependence of chain sliding diffusion. Our observations are also in agreement with Hikosaka's sliding diffusion theory. Compared to the unimodal system, the disentanglement degree of the bimodal system is more delayed than its crystallinity due to the slower chain sliding of the long-chain component; the bimodal system reaches a larger crystalline stem length at all temperatures due to the promotion of higher chain sliding mobility of the short-chain component.