C1‐Symmetric Heterocyclic Zirconocenes as Catalysts for Propylene Polymerization, 2

Summary: Sixteen C₁‐symmetric zirconocene and one hafnocene complexes bearing the 2,5‐R₂‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand (R = H, Me, Et, Ph) linked to a substituted indenyl ligand have been synthesized and tested in propylene polymerization. Most of the C₁‐symmetric zirconocenes of this type are highly active in propylene polymerization at low MAO/Zr ratios and produce poly(propylene)s (PP) in a broad range of isotacticity and melting points. The molecular weight and crystallinity of the PPs are strongly dependent upon the type of substituents on the indenyl moiety: PPs with T_m between 75 and 156 °C and viscosity average molecular weights between 100 000 and 400 000 have been obtained at 50–70 °C in liquid propylene.

General formula of the indenyl‐based zirconocenes.     

C1‐Symmetric Heterocyclic Zirconocenes as Catalysts for Propylene Polymerization, 1

Summary: The synthesis of C₁‐symmetric zirconocene complexes bearing the 2,5‐dimethyl‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand ( S₂‐3 ) linked to substituted cyclopentadienes is described. Different syntheses of S₂‐3 , the common intermediate for the preparation of these complexes, are discussed. Many of these complexes have been found to be highly active in propylene polymerization, to require very low amounts of methylalumoxane to be activated, and to produce poly(propylene)s of low isotacticity and melting points. ¹³C NMR analysis shows that the poly(propylene)s are fully regioregular and that the stereoerrors are randomly distributed, as shown by the enantiomorphic‐site triad test E ≈ 1. The experimental pentad distribution was fitted using a two‐site model with different probability parameters for each site. The probability of chain back‐skip was also taken into account. The molecular weight and crystallinity of the poly(propylene)s are dependent upon the type of substituents on the cyclopentadienyl ring, and the correlation between mmmm content and melting point of the PP confirms the random distribution of stereoerrors.

Correlation between % mmmm pentad and melting point.     

Metallocenic Isotactic Poly(propylene) and its Copolymers with 1‐Hexene and Ethylene

A comprehensive study of the structure and properties has been performed for copolymers of propylene‐1‐hexene, CiPH, and propylene‐ethylene, CiPE, synthesized by an isotactic metallocene catalyst system. The comonomer content constitutes the most important factor affecting the structure and properties of these CiPH and CiPE copolymers, although the length of the comonomer is also very important. Thus, a considerable decrease in crystallinity is observed in the two kinds of copolymers as the comonomer content increases. The structure in the CiPH copolymers evolves, however, from the typical, monoclinic crystal lattice to mesomorphic‐like, ordered entities for the highest 1‐hexene molar fraction, whereas in the CiPE copolymers the structural evolution with molar fraction goes from a monoclinic lattice to an almost amorphous material. All of these variations in crystal structure significantly influence the viscoelastic and mechanical behavior of these CiPH and CiPE copolymers. Consequently, the location and intensity of the different relaxation mechanisms, as well as the rigidity parameters (storage and Young's moduli and microhardness) and deformation mechanism are strongly dependent upon composition.

     

Propylene Polymerization with rac‐SiMe2(2‐Me‐4‐PhInd)2ZrMe2/MAO: Polymer Characterization and Kinetic Models

Poly(propylene)s were prepared with metallocene catalyst rac‐SiMe₂(2‐Me‐4‐PhInd)₂ZrMe₂/MAO (rac‐dimethylsilylbis(2‐methyl‐4‐phenylindenyl)dimethylzirconium/methylaluminoxane) in heptane solution at temperatures from 50 to 80 °C with varying concentrations of monomer, hydrogen, triisobutylaluminium (TIBA) and MAO. Polymer molar mass depended on the monomer, MAO, TIBA, and hydrogen concentrations and on polymerization temperature. The isotacticity was very high (mmmm > 95%), and only a slight decrease was detected at high temperatures. Regio selectivity was also high; the total amount of 2,1‐ and 3,1‐insertions was less than 0.4 mol‐%. Lowering the monomer concentration and raising the temperature increased the amount of 3,1 defects over the amount of 2,1 defects. End‐group analysis by ¹³C NMR spectroscopy revealed isobutyl and allyl end‐groups. Chain transfer to aluminium and β‐CH₃ elimination were concluded to be the dominating chain‐termination mechanisms. The importance of β‐CH₃ elimination increased with temperature. Hydrogen addition changed both the initiation and termination mechanisms as indicated by the presence of propyl, butyl and 2,3‐dimethylbutyl end‐groups. According to modeling studies, the molar mass follows a first‐order relationship with propylene and hydrogen concentrations, and a half‐order relationship with MAO concentration. Arrhenius‐type activation energy coefficients were 125 kJ · mol^?1 for β‐CH₃ elimination, 66 kJ · mol^?1 for chain transfer to aluminium, and 53 kJ · mol^?1 for chain transfer to hydrogen. A value of 45 kJ · mol^?1 was used for the propagation.

     

Novel High and Ultrahigh Molecular Weight Poly(propylene) Plastomers by Asymmetric Hafnocene Catalysts

Summary: The novel asymmetric ansa‐complexes [1‐(9‐η⁵‐fluorenyl)‐2‐(2,5,7‐trimethyl‐indenyl)ethane]hafnium dichloride ( 7a ) and [1‐(9‐η⁵‐fluorenyl)‐2‐(2,4,6‐trimethyl‐indenyl)ethane]hafnium dichloride ( 7b ) were prepared and used as catalysts for propylene homopolymerization reactions after in situ activation. The synthetic route allows to separate the 4,6‐ and 5,7‐substituted ligand isomers before the complexation step. The orientation of the methyl groups to the “front” (4,6) or to the “back” (5,7) of the tetrahedral hafnocene dichloride species influences their performances in polymerization reactions. Whereas hafnocene ( 7b ) which bears trimethyl substitution at 2,4,6‐positions of the indenyl moiety exhibits only moderate activity, the 2,5,7‐trimethyl substituted structure ( 7a ) produces isotactic poly(propylene)s with high molecular weights (up to = 9.0 × 10⁵ g · mol^?1) and high activities [up to 3.2 × 10⁵ kg of PP (mol Hf × h)^?1]. A comparative analysis of polymerization data and mechanical behavior of 7a , and previously reported 6,7‐indenyl substituted complex 6b are reported.

Typical stress‐strain curves of different types of poly(propylene)s.     

13C NMR Study of the Effect of Coordinating Solvents on Zirconocene‐Catalyzed Propene/1‐Hexene Copolymerization

The solvent effect observed in propene/1‐hexene copolymerizations performed with the isospecific catalyst rac‐Et(Ind)₂ZrCl₂/MAO is studied. A range of solvents with increasing donor character and steric hindrance has been tested, and their effect on copolymer yield, composition, and microstructure has been thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the comonomer reactivities, even though the solvent polarity is not the relevant factor. At the same comonomer compositions in solution, polymerizations carried out in coordinating solvents (e. g., aromatic solvents), lead to the formation of products with considerably decreased content of 1‐hexene. The reduced incorporation of the higher α‐olefin is explained in terms of competition between the nucleophilic medium and the olefin monomer for coordination to the active polymerization site. These results give us valuable information regarding the mechanism of polymerization at the active centers.     

Synthesis and Properties of Syndiotactic Poly(propylene)/Carbon Nanofiber and Nanotube Composites Prepared by in situ Polymerization with Metallocene/MAO Catalysts

Summary: The preparation of syndiotactic poly(propylene) (sPP) nanocomposites with multi‐walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), and carbon black (CB) as fillers was accomplished by the in situ polymerization of propylene with a metallocene/methylalumoxane (MAO) catalyst. Different pre‐treatments were applied to achieve a homogeneous dispersion of the fillers in the matrix. The resulting nanocomposites were investigated with respect to their properties, which were then compared to those of the pure polymer and among each other. The thermal stability of the nanocomposites was slightly enhanced compared to the pure polymer. In addition, the yield strength of the nanocomposites could be slightly raised in comparison to the neat sPP. The most significant influence of the nanofillers was detected on the crystallization behavior. The crystallization temperature was increased with rising filler content in all cases. Moreover, the half‐time of crystallization was significantly reduced in the nanocomposites. The rate constant of crystallization was also greatly enhanced for all nanocomposites as compared to the neat sPP. The nanofillers acted, therefore, as nucleating agents. This effect was most pronounced in the case of MWNTs as fillers.

Influence of the type of filler on the degradation temperature (temperature of maximum weight loss).     

Synthesis of Poly(ester amide)s Derived from Glycolic Acid and the Amino Acids: β‐Alanine or 4‐Aminobutyric Acid

The polymerization of metal salts of N‐chloroacetyl‐β‐alanine and N‐chloroacetyl‐4‐aminobutyric acid was investigated. The former gives a mixture of polymer and a seven‐membered cyclic compound constituted of glycolic and β‐alanine units, and its reaction proceeds in the solid state. However, liquefaction is observed in the second case giving rise to a polymer with a moderate molecular weight. Condensation kinetics of both sodium and silver salts of N‐chloroacetyl‐β‐alanine have been studied by differential scanning calorimetry. Copolymers of glycolic acid and β‐alanine with a molar ratio of glycolic acid/β‐alanine varying from 0.5 to 1.0 have been synthesized by thermal reaction of co‐precipitated crystals of the sodium salts of chloroacetic acid and N‐chloroacetyl‐β‐alanine. NMR spectroscopy indicates that copolymers tend to have a random distribution. The resulting new poly(ester amide)s have been characterized by spectroscopy and thermal analysis.

DSC heating runs corresponding to different mixtures of the sodium salts of chloroacetic acid and chloroacetyl‐β‐alanine.     

Synthesis and Characterization of Poly(glycolic acid‐alt‐6‐aminohexanoic acid) and Poly(glycolic acid‐alt‐11‐aminoundecanoic acid)

Summary: Poly(ester amide)s derived from glycolic acid and ω‐amino acid units, such as aminohexanoic or aminoundecanoic acids, are synthesized by a thermal polycondensation reaction that involves the formation of metal halide salts. Polymerization kinetics of different metal salts are studied by isothermal and nonisothermal methods and the corresponding parameters compared. The condensation reaction begins in the solid state for the aminohexanoic derivatives, although a rapid liquefaction is observed. On the other hand, the melting temperatures of the sodium and the potassium chloroacetylaminoundecanoate salts are lower than the reaction temperatures, and consequently polycondensation proceeds fully in the liquefied state. These polymers are characterized by an alternate disposition of ester and amide groups and can be obtained with high molecular weights and short polymerization times. Thermal properties (glass transition and melting temperatures) of the two new polymers are determined and compared. Thermal stability is also investigated; the results indicated that decomposition temperatures were always far from both reaction and polymer fusion temperatures.

DSC heating scans performed at different rates for potassium chloroacetylaminoundecanote.     

Sulfonation of Poly(propylene) Films with Fuming Sulfuric Acid

Summary: Sulfonation is one of the most commonly used methods for the surface modification of polymers. In this study, the sulfonation of poly(propylene) (PP) films with fuming sulfuric acid has been investigated with the focus on the surface reaction. Analysis of the sulfonated PP films by X‐ray photoelectron spectroscopy (XPS), attenuated total reflectance (ATR) infrared spectroscopy, and chemical modification demonstrated the formation of C?C double bonds and sulfate groups in the sulfonation process. These results and a comparison with low‐density polyethylene (LDPE) films led us to propose a new sulfonation mechanism involving hydride abstraction and the formation of β‐sultones.

The here proposed mechanism of the sulfonation of poly(propylene) involves the formation of unstable β‐sultones.     

Polymerization of Propene and 1,3‐Butadiene with Vanadyl(V) Monoamidinate Precatalysts and MAO or Dialkylaluminum Chloride Cocatalysts

Summary: Two new vanadyl(V) amidinates have been synthesized and characterized by single crystal X‐ray diffraction and ¹H NMR. The complexes are dimeric with Cl bridges and monodentate coordination of the amidinate ligand to pseudooctahedral vanadium. In the presence of organometallic compounds of aluminum, these vanadyl complexes promote polymerization of propene and 1,3‐butadiene.

     

Poly(propylene)‐Layered Silicate Nanocomposites: Gas Permeation Properties and Clay Exfoliation

To monitor the mineral surface coverage and energy, the inorganic cations of two clays (montmorillonite) with different surface area/cation have been exchanged by alkylammonium ions, carrying alkyl chains of different number and length. The prepared OMs were free of unreacted organic ions. With increasing length and number of the alkyl chains, an increase in the basal‐plane spacing (d‐spacing) of the OM was observed. The d‐spacing also increased with increasing CEC of the clay (decreasing available area/cation). The OMs were compounded with PP and their effect on the crystallinity and gas‐barrier properties of the polymer was investigated. The OM had no influence on the degree of crystallinity of PP under the processing conditions used. Oxygen permeation through the composites decreased, depending on the cross‐sectional area of the exchanged organic cation and the CEC of the clay. These parameters control the mineral surface coverage (consequently the surface energy) as well as the tilt angle of the alkyl chains to the mineral surface, and hence the d‐spacing. Increasing the length of the alkyl chains and their number per cation enhanced the d‐spacing, clay exfoliation, and the gas‐barrier properties of the composites. A mixed morphology, consisting of delaminated aluminosilicate layers and OM tactoids of varying thickness was observed but no intercalation took place. The oxygen permeation coefficient of the nanocomposites was found to be a non‐linear function of the volume fraction of the inorganic part of the OM.

     

Role of tert‐Butyl Methyl Ether (TBME) as an External Donor in Propene Polymerization with Dibutyl Phthalate (DBP)‐Containing MgCl2‐Supported Ti Catalysts Activated with Al(i‐C4H9)3

Summary: Stopped‐flow polymerization of propene was first conducted at 40 °C using the TiCl₄/DBP/MgCl₂ catalyst (DBP = dibutyl phthalate) combined with Al(C₂H₅)₃. An induction period was observed at the beginning of polymerization, and the resulted polymer contained a considerable amount of the isolated ethylene unit. The formation of such unusual structures was concluded to be the result of copolymerization with ethylene originating from the ethylated Ti⁴⁺ species. Catalyst washing with toluene and stopped‐flow polymerization at 70 °C brought about a drastic decrease in the contents of both Ti and DBP as well as a disappearance of both the induction period and isolated ethylene unit. The microstructures of polymers revealed that the highly stereoregular polymers are produced at the initial stage of polymerization. Changes in yield and molecular weight of polymers with polymerization time showed that the addition of tert‐butyl methyl ether (TBME) brought about an increase in the concentration of active sites, but did not affect the propagation rate of propene polymerization. Such an increase in the active site concentration caused by an external donor has hardly been reported so far in kinetic studies using the stopped‐flow method; thus, the present result is believed to be a unique example.

Kinetic curves of propene polymerization with the Cat. IV‐Al(i‐C₄H₉)₃ catalyst. (?): with TBME/Al = 0.1 and hydrogen, (?): with TBME/Al = 0.1 and without hydrogen, (○): without TBME and with hydrogen, (?): without TBME and hydrogen.     

Modification of Chlorinated Poly(propylene) via Atom Transfer Radical Graft Copolymerization of 2‐Ethylhexyl Acrylate: A Brush‐like Graft Copolymer

The modification of chlorinated poly(propylene) (CPP) via graft copolymerization of EHA using ATRgP is reported. The kinetic plot of monomer conversion versus reaction time was found to be linear, which is the typical characteristic of a living controlled polymerization. The grafted copolymers were characterized by means of FT‐IR and ¹H NMR spectroscopy, GPC, DSC, and TGA. Mechanical properties were also studied by means of UTM and DMA. The stress/strain plot and the tension set properties indicate that the brush‐type graft copolymer (CPP‐g‐PEHA) behaves as a thermoplastic elastomer.

     

Highly Branched Poly(arylene ether)s via Oligomeric A2 + B3 Strategies

Summary: Branched poly(arylene ether)s were prepared in an oligomeric A₂ + B₃ polymerization of phenol endcapped telechelic poly(arylene ether sulfone) oligomers as A₂ and TFPPO as trifunctional monomer B₃. The molar mass of the A₂ oligomer significantly influenced the onset of gelation and the DB. A high level of cyclization during polymerization of low molar mass A₂ oligomers (U₃ = 660 and U₆ = 1 200 g · mol^?1) led to a high conversion of functional groups in the absence of gelation, and the level of cyclization reactions in the polymerization decreased as the molar mass of the A₂ oligomer was increased. The pronounced steric effect in the polymerization of higher molar mass A₂ oligomers (U₈ = 1 800 and U₁₆ = 3 400 g · mol^?1) resulted in low reactivity of the third aryl fluoride in the B₃ monomer. As a result, only slightly branched (U₈ = 1 800 g · mol^?1) or nearly linear (U₁₆ = 3 400 g · mol^?1) high molar mass products were obtained with higher molar mass A₂ oligomers. The branched polymers exhibited lower Mark‐Houwink exponents and [η] relative to linear analogs, and differences between the branched polymers and linear analogs were less significant as the molar mass of the A₂ oligomers was increased due to a decrease in the overall DB.

     

Formation of Poly(propylene)‐Based Biocomposite Films and Their Use in the Attachment of Methylene Blue

Biocomposite PP‐g‐PAAc‐CS films based on PP were generated and utilized as support of methylene blue, a thiazidic dye. Using a photograft polymerization of acrylic acid, the PP film was functionalized with carboxyl groups (PP‐g‐PAAc), which attached chitosan by electrostatic bond. A longer poly(acrylic acid) chain or a higher CS immobilization temperature led to a higher chain interpenetration and crosslinking reaction. Immobilized MB confirmed to possess redox activity from its reaction with ascorbic acid, where the dye decomposition rate ( ) increases together with the chain interpenetration, then decreasing with the increase in the crosslinking degree.

     

Effect of Et3Al and 9,9‐Bis(methoxymethyl)fluorine on Propylene Polymerization at High Temperature with TiCl4/MgCl2 Catalysts

Summary: A study of the effect of Et₃Al and a diether, 9,9‐bis(methoxymethyl)fluorine (BMMF), on propylene polymerization at high temperature with the use of MgCl₂‐supported catalysts is reported. BMMF could be extracted from a TiCl₄/MgCl₂/BMMF catalyst by Et₃Al at 100 °C, and Et₃Al is an efficient chain‐transfer agent in the presence of BMMF at 100 °C. The results obtained from differential scanning calorimetry (DSC) and ¹³C NMR spectroscopy showed that the isotactic poly(propylene) (iPPs) produced by the catalysts containing BMMF as an internal or external donor, contained ethylene units. This phenomenon was not found in the iPP chains obtained with donor‐free TiCl₄/MgCl₂ catalyst in the absence of any external donor at 100 °C. It is suggested that the decomposition of Et₃Al did not take place in the absence of any donor and took place in the presence of BMMF at 100 °C.

¹³C NMR spectrum of an iPP sample.     

Synthesis of Elastomeric Poly(propylene) (ELPP) Using the Highly Active TiCl4/Dibutyl phthalate (DBP)/MgCl2‐Al(i‐C4H9)3/1‐Allyl‐3,4‐dimethoxybenzene (ADMB) Catalyst

Propene polymerization was first carried out at 70°C using the TiCl₄/DBP/MgCl₂ catalyst in combination with a mixture of ADMB and various alkylaluminium compounds. Both the activity and the molecular weight of the polymer were strongly dependent on the kinds of alkylaluminiums. However, the isotactic index (I.I.) of the polymer was hardly affected by them. In order to control the I.I. value, the propene polymerization was then conducted with the TiCl₄/DBP/MgCl₂‐Al(i‐C₄H₉)₃/ADMB catalyst by changing the polymerization temperature. A suitable selection of polymerization temperatures gave a higher molecular weight PP containing various amounts of APP, i. e., I.I. 4–59%. Mechanical testing indicated that the obtained polymers exhibited a wide range of physical properties from a low modulus thermoplastic elastomer to flexible plastics depending on their stereoregularities. The improved elastic recovery of the obtained PP in the present study could be simply interpreted by the decrease in the [mmmm] or crystallinity of the polymer. Thus, the polymer having the lowest such values was found to have an excellent elastic recovery, which was comparable to that of ELPP as previously reported. The present result is the first example of the synthesis of an elastomeric poly(propylene) using a highly active MgCl₂‐supported Ti catalyst.     

Melt Structure and its Transformation by Sequential Crystallization of the Two Blocks within Poly(L‐lactide)‐block‐Poly(ε‐caprolactone) Double Crystalline Diblock Copolymers

Summary: Sequential crystallization of poly(L ‐lactide) (PLLA) followed by poly(ε‐caprolactone) (PCL) in double crystalline PLLA‐b‐PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.‐% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 °C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 °C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 °C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre‐crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers.

Polarized optical micrographs during isothermal crystallization of (a) homo‐PLLA, (b) homo‐PCL, (c) and (d) block copolymer after 30 min at 122 °C and after 15 min at 42 °C.     

Long‐Chain Branched Poly(Lactide)s Based on Polycondensation of AB2‐type Macromonomers

A series of long‐chain branched poly(d‐/l ‐lactide)s is synthesized in a two‐step protocol by (1) ring‐opening polymerization of lactide and (2) subsequent condensation of the preformed AB₂ macromonomers promoted by different coupling reagents. The linear AB₂ macromonomers are prepared by Sn(Oct)₂‐catalyzed ROP of D ‐ and L ‐lactide with 2,2‐bis(hydroxymethyl)butyric acid (BHB) as an initiator. Optimization of the polymerization conditions allows for the preparation of well‐defined macromonomers (M _w/M _n = 1.09–1.30) with adjustable molecular weights (760–7200 g mol^?1). The two‐step approach of the synthesis comprises as well the coupling of these AB₂ macromonomers and hence allows precise control over the lactide chain length between the branching units in contrast to a random polycondensation.