Metallocene-mediated synthesis of chain-end functionalized polypropylene and application in PP/clay nanocomposites

This paper summarizes our research in the preparation of chain end functionalized isotactic polypropylene (PP) having a terminal functional group, such as Cl, OH, and NH₂. The chemistry involves metallocene-mediated propylene polymerization using rac-Me₂Si[2-Me-4-Ph(Ind)]₂ZrCl₂/MAO complex in the presence of styrene derivatives (St-f) and hydrogen, which serve as the chain transfer agents. The molecular weight of the resulting PP polymers with a terminal Cl, OH and NH₂ group (i.e., PP-t-Cl, PP-t-OH and PP-t-NH₂) are inversely proportional to the molar ratio of [St-f]/[propylene]. Despite the extremely low concentration of functional group, the high molecular weight chain end functionalized PP-t-OH and exhibit a distinctive advantage over other functional PP polymers containing side chain functional groups or long functional blocks. The terminal hydrophilic OH and cations, with good mobility and reactivity, effectively hydrogen bond and ion-exchange the cations (Li⁺, Na⁺, etc.) located between the clay interlayers, respectively. Such interactions anchor the PP chain to the clay surfaces. On the other hand, the remaining rest of the unperturbed end-tethered high molecular weight PP tail exfoliates the clay layers. This exfoliated structure is maintained even after further mixing of the PP-bearing platelets with pure neat PP polymers.     

Synthesis of exfoliated isotactic polypropylene/functional alkyl-triphenylphosphonium-modified clay nanocomposites by in situ polymerization

Diphenyl (4-hydroxyphenyl) hexadecyl phosphonium bromide (POH) -modified montmorillonite (POHMMT) was used to prepare a novel TiCl₄/MgCl₂/POHMMT compound catalyst and exfoliated iPP/POHMMT nanocomposites were prepared by the in situ intercalative polymerization of propylene with the TiCl₄/MgCl₂/POHMMT compound catalyst. The POH surfactants don’t change the catalytic characteristic of the Z-N catalyst and the obtained PP presents high isotacticity, normal molecular weight and molecular weight distribution. The WAXD, SAXS and TEM results demonstrate the highly exfoliated iPP/POHMMT nanocomposites were produced by the in situ polymerization with this novel catalyst, while the intercalated iPP/Na⁺MMT nanocomposites were produced with the TiCl₄/MgCl₂/Na⁺MMT compound catalyst. Through this approach, in situ propylene polymerization can actually take place between the silicate layers and lead not only to PP with high isotacticity and molecular weight, but also to highly exfoliated PP nanocomposites.     

Preparation of polypropylene/clay nanocomposites by <Emphasis Type="Italic">in situ</Emphasis> polymerization with TiCl<Subscript>4</Subscript>/MgCl<Subscript>2</Subscript>/clay compound catalyst

TiCl₄/MgCl₂/clay compound catalyst was prepared by chemical reaction.Exfoliated polypropylene（PP）/clay nanocomposites were synthesized by in situ polymerization with this compound catalyst.Effects of polymerization temperature,polymerization time,propylene pressure,solvent consumption and pre-treat time of catalyst on catalyst activity and catalytic stereospecificity were studied.Under optimal conditions,activity of the nano-compound catalyst is about 88.3 kg/（mol Ti·h）.Isotacticity of PP obtained in the nanocomposites is in the range of 89%-99%,and its melting temperature is about 159℃.The weight-average molecular weight of PP can reach 6.7×10⁵ - 7.8×10⁵,and the molecular weight distribution is between 7.7 and 7.9.     

Isothermal crystallization kinetics of PP in PP/Mg(OH)2 composites

The crystallization and melting behavior of PP/Mg(OH)₂ composites was investigated, and the crystallization kinetic parameters and thermal characteristics were investigated according to the Avrami method. Optical polarizing microscope (POM) analysis suggested that the presence of Mg(OH)₂ particles gave rise to an increase in the number of nuclei and a decrease in PP spherulitic size. The Avrami exponent n of the PP and composites increased with increasing crystallization temperature, and markedly deceased with the addition of low Mg(OH)₂ content. A significant increase in crystallization kinetic constant, and a decrease in crystallization half time of PP were observed in the presence of Mg(OH)₂ particles, indicating a heterogeneous nucleating effect of Mg(OH)₂ upon crystallization of PP. The melting temperature and equilibrium melting temperature of PP in the composites decreased with increasing the Mg(OH)₂ content, which is directly related to the size of the PP crystals. The difference of PP melting enthalpies in the PP and composites demonstrated that the presence of Mg(OH)₂ can effectively enhance the crystalline of PP. The crystallization thermodynamics of PP and composites were studied according to the Hoffman theory. Surface free energy of PP chain folding for crystallization of PP/Mg(OH)₂ composites was lower than that of PP, confirming the heterogeneous nucleation effect of Mg(OH)₂. However, the evaluation of the nucleation activation energy of PP suggested the presence of a large amount of Mg(OH)₂ particles in the PP matrix reduced the mobility of PP segments and restricted the development of PP nucleation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1914–1923, 2005     

Synthesis of ethylene/propylene elastomers containing a terminal reactive group: The combination of metallocene catalysis and control chain transfer reaction

This article discusses a chemical route to prepare new ethylene/propylene copolymers (EP) containing a terminal reactive group, such as ?‐CH₃ and OH. The chemistry involves metallocene‐mediated ethylene/propylene copolymerization in the presence of a consecutive chain transfer agent—a mixture of hydrogen and styrene derivatives carrying a CH₃ (p‐MS) or a silane‐protected OH (St‐OSi). The major challenge is to find suitable reaction conditions that can simultaneously carry out effective ethylene/propylene copolymerization and incorporation of the styrenic molecule (St‐f) at the polymer chain end, in other words, altering the St‐f incorporation mode from copolymerization to chain transfer. A systematic study was conducted to examine several metallocene catalyst systems and reaction conditions. Both [(C₅Me₄)SiMe₂N(^t‐Bu)]TiCl₂ and rac‐Et(Ind)₂ZrCl₂, under certain H₂ pressures, were found to be suitable catalyst systems to perform the combined task. A broad range of St‐f terminated EP copolymers (EP‐t‐p‐MS and EP‐t‐St‐OH), with various compositions and molecular weights, have been prepared with polymer molecular weight inversely proportional to the molar ratio of [St‐f]/[monomer]. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1858–1872, 2005     

Study of ions with excess kinetic energy. III—Mass spectra of excess kinetic energy ions of aliphatic amines

The mass spectra obtained from ions having excess kinetic energy (named the excess kinetic energy mass spectra) formed by electron impact of several aliphatic amines have been studied. Comparison with the excess kinetic energy mass spectra of the analogous alcohols shows many similarities. For example, the intensity of the [CH₂OH]⁺ ion of an aminoalcohol is about the same as the intensity of the [CH₂NH₂]⁺ ion. The excess kinetic energy mass spectra give useful information about the presence or absence of various functional groups, such as CH₂NH₂, CH₂OH, CH₃CHNH₂ and alkyl groups, and also about rearrangements. The fragmentation mechanisms are straightforward and applicable to a variety of classes of compounds. The molecular weight dependence of excess kinetic energy mass spectra is described.     

Benefits of near-infrared spectroscopy for characterization of selected organo-montmorillonites

The potential of near infrared (NIR) spectroscopy in characterization of organically modified clay minerals is introduced. Selected organo-clays, possibly perspective fillers in clay polymer nanocomposites, were prepared from Na-montmorillonite and different surfactants containing octylammonium chain(s), hexadecylammonium chain(s) or a benzene ring with or without a reactive double bond. Based on the stretching (ν) and bending (δ) vibrations observed in the middle IR (MIR) region, the first overtone (2νXH) and combination (ν + δ)XH modes of XH groups (X = O, C, N) are identified. The effect of larger alkylammonium cations on the vibrations of Si-O and OH bonds in montmorillonite layers is observed. The changes in the intensity of the (ν + δ)H₂O band near 5250 cm⁻¹ allows for comparison of the amount of water adsorbed on the montmorillonite surface. The water content decreases with the size of the organic cation reflecting increasing hydrophobicity of the montmorillonite surface. The NIR region shows the 2νCH₃ and 2νCH₂ bands in the 5900-5500 cm⁻¹ region, an upward shift is observed for the complex band due to 2νCH(Ar) of aromatic benzene ring. The NIR spectra are extremely useful in identification of NH₂⁺, NH⁺ and vinyl groups, which are difficult to recognize in the MIR spectra of organo-clays due to overlapping with other absorption bands. The intense bands corresponding to overtones and combination vibrations of NH₃⁺ and NH₂⁺ groups are found in the 6600-6050 cm⁻¹ and 5000-4600 cm⁻¹ regions, the (ν + δ)NH⁺ is unambiguously identified near 4750 cm⁻¹. The characteristic band assigned to 2νCH₂ in H₂CC is detected near 6130 cm⁻¹.     

Synthesis of azide end‐functionalized isotactic polypropylene building block and renewed modular synthesis of diblock copolymers of isotactic polypropylene and poly(ε‐caprolactone)

This article details a synthesis of azide end‐functionalized isotactic polypropylene (i‐PP), a unique polymeric building block that can engage in Huisgen's 1,3‐dipolar cycloaddition of azide and alkyne (click reaction) to construct well‐defined i‐PP‐based polymer architecture. Controlled, consecutive chain transfer reaction to 1,2‐bis(4‐vinylphenyl)ethane and hydrogen in metallocene‐mediated propylene polymerization catalyzed by rac‐Me₂Si(2‐Me‐4‐Ph‐Ind)₂ZrCl₂/MAO resulted in styryl‐terminated i‐PP (i‐PP‐t‐St) of controlled molecular weight. Following a regioselective hydrochlorination reaction, the terminal styryl groups were quantatively transformed to 1‐chloroethylbenzene groups, which was further reacted with NaN₃ to give i‐PP terminated with an azide group (i‐PP‐t‐N₃). Structural monitoring of the polymers through the whole transformation process using ¹H NMR and FTIR as well as GPC and DSC reveals a clean and clear formation of i‐PP‐t‐N₃ (M_n in between 10,000 and 40,000 g/mol). This clickable i‐PP building block was applied to a renewed, modular synthesis of amphiphilic i‐PP‐b‐PCL (poly(ε‐caprolactone)) diblock copolymers. Composition‐diversified, structure‐well defined diblock copolymers were obtained in high yields, confirming both the high end group selectivity as well as high reactivity of azide the clickable moiety in the i‐PP building block and the effectiveness of azide‐alkyne click reaction in constructing new i‐PP architecture. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and degradation of end‐group‐functionalized polylactide

Three kinds of OH‐terminated polylactides were synthesized by the ring‐opening polymerization of lactide, with an alcohol such as dodecanol, glycerol, or pentaerythritol, in the presence of stannous octoate. Moreover, Cl‐, NH₂‐, and COOH‐terminated polylactides were synthesized from OH‐terminated polylactides. The end groups of the polylactides were identified by ¹H NMR and ¹³C NMR. According to thermal analysis, the cold crystallization temperatures of Cl‐, NH₂‐, and COOH‐terminated polylactides were higher than those of OH‐terminated polylactides. The thermal stability of OH‐terminated polylactides was poor, whereas NH₂‐ and Cl‐terminated polylactides were more resistant to thermal degradation. In a hydrolysis degradation test, the mass and molecular weight loss of COOH‐terminated polylactides were high, whereas those of Cl‐ and NH₂‐terminated polylactides were much lower. These end‐group effects were increased with an increasing number of chain arms. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 973–985, 2001     

Long-range interfacial electron transfer and electrocatalysis of molecular scale <Emphasis Type="Italic">Prussian</Emphasis> Blue nanoparticles linked to Au(111)-electrode surfaces by different chemical contacting groups

We have explored interfacial electrochemical electron transfer (ET) and electrocatalysis of 5–6 nm Prussian Blue nanoparticles (PBNPs) immobilized on Au(111)-electrode surfaces via molecular wiring with variable-length, and differently functionalized thiol-based self-assembled molecular monolayers (SAMs). The SAMs contain positively (?NH₃ ⁺) or negatively charged (–COO–) terminal group, as well an electrostatically neutral hydrophobic terminal group (–CH₃). The surface microscopic structures of the immobilized PBNPs were characterized by high-resolution atomic force microscopy (AFM) directly in aqueous electrolyte solution under the same conditions as for electrochemical measurements. The PBNPs displayed fast and reversible interfacial ET on all the surfaces, notably in multi-ET steps as reflected in narrow voltammetric peaks. The ET kinetics can be controlled by adjusting the length of the SAM forming linker molecules. The interfacial ET rate constants were found to depend exponentially on the ET distance for distances longer than a few methylene groups in the chain, with decay factors (β) of 0.9, 1.1, and 1.3 per CH₂, for SAMs terminated by ?NH₃ ⁺,–COO–, and–CH₃, respectively. This feature suggests, first that the interfacial ET processes follow a tunneling mechanism, resembling that of metalloproteins in a similar assembly. Secondly, the electronic contact of the SAM terminal groups that anchor non-covalently the PBNP are crucial as reported for other types of molecular junctions. Highly efficient PBNP electrocatalysis of H₂O₂ reduction was also observed for the three linker groups, and the electrocatalytic mechanisms analyzed.     

Evaluation of ammonium terminated PMMA as compatibilizers for monomer casting polyamide6/clay nanocomposites

The compatibilization effects provided by ammonium terminated PMMA(PMMA‐t‐NH₃⁺) on monomer casting polyamide6 (MCPA6)/clay(pristine sodium montmorillonite) nanocomposites were studied in this article. PMMA‐t‐NH₃⁺ used in this study was prepared by radical polymerization using 2‐aminoethanethiol hydrochloride as chain transfer agent. MCPA6/clay/PMMA‐t‐NH₃⁺ nanocomposites were prepared by in situ anionic ring‐opening polymerization of ε‐caprolactam. X‐ray diffraction and transmission electron microscopy plus rheological measurement were used to characterize those nanocomposites. The results indicated that PMMA‐t‐NH₃⁺ would be a good compatibilizer for this system. With PMMA‐t‐NH₃⁺ content increasing, a better dispersion of clay was successfully achieved in the MCPA6 matrix. Furthermore, analysis using differential scanning calorimetry indicated that well dispersed clay layers limited the mobility of the MCPA6 molecule chains to crystallize, reduce the crystalline degree, and favor the formation of the γ‐crystalline form of the MCPA6 matrix. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1802–1810, 2008     

Chemical Modification of Poly(epichlorohydrin) Using Montmorillonite Clay

Cationic ring opening polymerization of epichlorohydrin (1) and acetic anhydride in the presence of Maghnite- H (Mag-H＋) as a catalyst afforded, ω-diacetylated poly(epichlorohydrin) (P1) in a moderate yield and molecular weight without formation of side products and degradation. P1 was chemically modified with morpholine (2), piperidine (3) and pyrrolidine (4) into the corresponding new functional poly(epichlorohydrin)s (P2—P4) in a moderate reaction conversion . The conversion of P1 into P2—P4 was confirmed by using FTIR and NMR spectroscopy and the yield was calculated from the elemental analysis data according to the mole fraction concept. The obtained functional polymers were further characterized by thermal analysis which showed a substantial increase of the glass transition temperature (Tg). Thus, the chemical modification of α,ω-acetylated PECH using Mag-H＋ offers a simple method for obtaining functional polymers. Mag-H＋ is a montmorillonite sheet silicate clay exchanged with proton.     

Terpolymerization of propylene oxide,carbon dioxide,and trimethylene carbonate

High-molecular-weight polymers with different contents of propylene carbonate (PC), and trimethylene carbonate (TMC) units in the polymer chain were synthesized by the coordination anionic copolymerization of carbon dioxide, propylene oxide (PO), and TMC in supercritical carbon dioxide (scCO₂). Zinc adipate (ZnAd) was used as a catalyst. The terpolymerization products were characterized by ¹H and ¹³C NMR, IR spectroscopy, GPC, and DSC. The effect of the reaction conditions on the yield, composition, structure, and molecular weight and thermal characteristics of the terpolymers was studied. The phase behavior of the synthesized polymers and mixtures of polypropylene carbonate with polytrimethylene carbonate was examined.

     

Preparation and chain extension of C100dicarboxylic acid

The selective degradation of polyethylene crystals with fuming nitric acid has been used to prepare a C₁₀₀ chain with terminal functional groups. After treating this product with concentrated sulphuric acid, it is shown by average molecular weight, titration and infra-red spectroscopy measurements that each molecule contains two carboxylic acid groups. This C₁₀₀ dicarboxylic acid is chain extended with several low molecular weight difunctional coupling agents. Block copolymers of this acid and poly(propylene glycol) have been prepared.     

Platin(0)‐Komplexe mit amino‐substituierten Alkinen: Neue Metallorganische Bausteine für supramolekulare Architekturen und „Kristall‐Engineering”︁

Platinum(0) Complexes with Amino‐Substituted Alkynes: Novel Organometallic Building Blocks for Supramolecular Architectures and “Crystal Engineering” Homoleptic Bis(alkyne)platinum(0) compounds containing either NH₂‐ or NH₂‐/OH‐substituents are formed by reaction of Pt(cod)₂ with alkynes as stable compounds. They can be used as variable building blocks for supramolecular networks. The crystal structure analyses of Bis(2‐amino‐2,5dimethyl‐5‐hydroxy‐hex‐3‐yne)platinum(0) ( 1 ) and of Bis(1(3‐amino‐3‐methyl‐but‐1‐inyl)‐cyclohexane‐1‐ol)platinum(0) ( 2 ) exhibit that the low‐valent Pt atom is tetrahedrally surrounded by the four sp‐hybridizated carbonatoms of the alkynes. Despite the fact that the bond lengths and ‐angles of the PtC₄ units are equal, the supramolecular structures are different. While in 1 polymer strands are formed in which the bis(alkyne)‐Pt⁰ units are connected by (OH)₂(NH₂)₂‐ tetrahedrons, 2 yields only a dimer containing a network of four OH‐ and two NH₂‐groups. Platinum(0) complexes with cationic alkynes bearing ammonium substituents can be isolated as thermal stable compounds. The X‐ray structures of [Cl( FH ⁺)Pt(cod)]₄ ( 8 ) reveals that four molecular units form a cube with both four NH₃⁺ groups and Cl^– at the corners connected by hydrogen bridges. In the bis(alkyne)Pt⁰ complex [Cl_1.5( FH ⁺)_1.5( F )_0.5Pt] ( 9 ) only 1,33 of two NH₂ groups are protonated and a hydrogen bridged network connects four bis(alkyne)Pt⁰ units (cod: cycloocta‐1.5‐diene, F : 1‐(trimethylsilylethinyl)‐1‐amino‐cyclohexane).     

Cationic polymerization of L,L‐lactide

Cationic bulk polymerization of L ,L‐ lactide (LA) initiated by trifluromethanesulfonic acid [triflic acid (TfA)] has been studied. At temperatures 120–160 °C, polymerization proceeded to high conversion (>90% within ～8 h) giving polymers with M_n ～ 2 × 10⁴ and relatively high dispersity. Thermogravimetric analysis of resulting polylactide (PLA) indicated that its thermal stability was considerably higher than the thermal stability of linear PLA of comparable molecular weight obtained with ROH/Sn(Oct)₂ initiating system. Also hydrolytic stability of cationically prepared PLA was significantly higher than hydrolytic stability of linear PLA. Because thermal or hydrolytic degradation of PLA starting from end‐groups is considerably faster than random chain scission, both thermal and hydrolytic stability depend on molecular weight of the polymer. High thermal and hydrolytic stability, in spite of moderate molecular weight of cationically prepared PLA, indicate that the fraction of end‐groups is considerably lower than in linear PLA of comparable molecular weight. According to proposed mechanism of cationic LA polymerization growing macromolecules are fitted with terminal ? OH and ? C(O)OSO₂CF₃ end‐groups. The presence of those groups allows efficient end‐to‐end cyclization. Cyclic nature of resulting PLA explains its higher thermal and hydrolytic stability as compared with linear PLA. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2650–2658, 2010     

Gas-phase proton-transport self-catalysed isomerisation of glutamine radical cation: The important role of the side-chain

The gas-phase isomerisation reaction of glutamine radical cation from [NH₂CH (CH₂CH₂CONH₂) COOH ]^+• to [ NH₂C (CH₂CH₂CONH₂) C (OH)₂]^+• has been studied theoretically using the MPWB1K functional approach. The [ NH₂ C (CH₂CH₂CONH₂) C (OH)₂]^+• diol species has been found to be the most stable isomer for glutamine radical cation. Moreover, it has been observed that glutamine has a long enough side-chain with basic groups that acts as a solvent molecule favouring the proton-transfer from C _α to COOH group. This fact reduces dramatically the isomerisation energy barriers compared to the same process for glycine radical cation in gas phase. Thus, this reaction can be considered as an example of gas-phase proton-transport catalysed reaction in which the proton-transport is carried out by the reactant molecule itself instead of any solvent. Contribution to the Serafin Fraga Memorial Issue.     

Structural irregularities in poly(vinyl alcohol)

Poly(vinyl alcohols) derived from the product of polymerization of vinyl acetate in methanol have been characterized by various physical and chemical methods before and after NaIO₄ cleavage. The 220-MHz ¹H-NMR spectra confirm the reliability of NaIO₄ titrimetry for estimating 1,2-glycol content and help explain the tendency for viscometry to grossly underestimate the 1,2-glycol content for low molecular weight polymers. The spectra and related chemical evidence indicate that the major endgroups are HOCH₂CH₂? and CH₃CH(OH)CH(OH)CH₂? . ß-Hydroxyethyl groups also occur as short chain branches, mainly attached to α carbon atoms in the normal head-to-tail polymer chain sequence. The concentrations of the branch and endgroups depend on polymerization conditions and help explain polymerization “solvent” effects on physical properties.     

A theoretical approach to substituent effects: Interaction between directly bonded groups in the isoelectronic series XNH3+, XCH3, and XBH3−

Ab initio molecular orbital theory including full geometry optimization at the 4-31G level is used to examine the interactions between substitutents X(X = Li, BeH, BH₂, CH₃, NH₂, OH and F) and substrates Y(Y = NH₃⁺, CH₃, BH₃^?) in the isoelectronic series XNH₃⁺, XCH₃ and XBH₃^?. The results indicate that the interaction energies are dominated by σ-effects. NH₃⁺ is found to interact favorably with the σ-donors (e.g. Li, BeH and BH₂) and unfavorably with the σ-acceptors (e.g. F, OH, NH₂). The reverse pattern a observed for XBH₃^?. The range of interaction energies for XCH₃ is considerably smaller than for XNH₃⁺ and XBH₃^?.     

Synthesis,characterization, and properties of linear poly(ether sulfone)s with dendritic terminal groups

Hybrid linear‐dendritic ABA polymers, where A and B are dendritic and linear polymers, respectively, were synthesized in a single step via step‐growth polymerization of 4,4′‐difluorodiphenylsulfone and bisphenol A using arylether ketone dendrons of first and second generations (G1‐OH and G2‐OH) as monofunctional end‐cappers. These G1 and G2‐terminated poly(ether sulfone)s (G1‐PESs and G2‐PESs) were characterized by ¹H NMR, SEC, DSC, TGA, melt rheology, and tensile tests. The comparison of the glass transition temperatures (T_gs) of these polymers with those of t‐butylphenoxy‐terminated polysulfones reveal that the G1‐ and G2‐PESs have lower T_gs at all molecular weights investigated. However, a plot of T_g versus 1/M_n shows that the difference between the three series becomes negligible at infinite molecular weight and agrees to the chain end free volume theory. The melt viscosities of G1‐PES and G2‐PES with high molecular weights do not show a Newtonian region and, in the high frequency region, their viscosities are lower than that of the control while the stress–strain properties are comparable to those of the control, suggesting that it is possible to reduce the high shear melt viscosity of a PES without affecting the stress–strain properties by introducing bulky dendritic terminal groups. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 958–969, 2008