Comonomer Length Influence on the Structure and Mechanical Response of Metallocenic Polypropylenic Materials

The structure and properties have been comprehensively studied for metallocene copolymers of propylene‐1‐hexene (CiPH) and propylene‐1‐octadecene (CiPOD). The comonomer content constitutes the most important factor affecting both structure and properties in these CiPH and CiPOD copolymers, although the length of the comonomer is also very important. Thus, a considerable decrease in crystallinity and an easier obtainment of mesomorphic‐like ordered entities are observed in the two kinds of copolymers as comonomer content increases. The variations in crystalline structure significantly influence the viscoelastic and mechanical behaviors of these CiPH and CiPOD copolymers. Consequently, the location and intensity of the different relaxation mechanisms as well as stiffness parameters (storage, Young's moduli, and microhardness) and deformation mechanism are strongly dependent upon composition.

     

FTIR Microanalysis and Phase Behaviour of Ethylene/1‐Hexene Random Copolymers

Ethylene/1‐hexene random copolymers with 1‐hexene content in the range of 1–5 mol‐%, synthesised in the presence of new heterogeneous catalyst systems based on bis‐carboxylato and ‐bis‐chloro‐carboxylato titanium chelate complexes, have been characterised by FTIR microspectroscopy (FTIR‐M), DSC calorimetry and X‐ray scattering. The co‐monomer content and sequence distribution in the various samples were determined by means of both FTIR‐M and ¹³C NMR spectroscopy. The deformation bands of methyl groups in the region of 1 400–1 330 cm^?1 were used for the structural analysis of these copolymers. The effect of composition on the crystallinity and phase transitions of copolymers was analysed both in 1 500–1 300 and 760–690 cm^?1 frequency ranges as a function of the annealing temperature. A neat variation of the absorbance ratio of methyl band at 1 378 cm^?1 was recorded between 110 and 130 °C corresponding to the melting range of the copolymer crystals. The crystallisation behaviour of the copolymers was examined by DSC in dynamic and isothermal conditions; the isothermal kinetics were analysed according to the Avrami model. A marked decrease in the bulk crystallisation rate, accompanied by changes in the nucleation and growth of crystals, was found with an increase in the co‐monomer content. The melting behaviour of isothermally crystallised samples was also investigated and the melting temperatures of the copolymers at equilibrium conditions were related to the composition; the experimental data were consistent with the Flory exclusion model of side branches from the crystalline phase. The lowering of crystal growth rate in the copolymers has been accounted for by an increase in the free energy of formation of critical size nuclei due to the effect of the side branches.

     

Synthesis and Characterization of Comb‐Like Copolymers Based on Poly(ε‐caprolactone) and Poly(α‐olefin)

Comb‐like copolymers based on a polyolefin backbone of poly(10‐undecene‐1‐ol) (PUol) with poly(ε‐caprolactone) (PCL) side chains are synthesized in two steps. After synthesis of PUol by metallocene‐catalyzed polymerization, the side‐chain hydroxyl functionalities of this polar polyolefin are used as an initiator for the ring‐opening polymerization (ROP) of ε‐caprolactone (CL). In this context, copolymers with different lengths of PCL grafts are prepared. The chemical structure and the composition of the synthesized copolymers are characterized by ¹H and ¹³C NMR spectroscopy. It is shown that the hydroxyl end groups of PUol act effectively as initiating sites for the CL ROP. Size‐exclusion chromatography (SEC) measurements confirm the absence of non‐attached PCL and the expected increase in molar mass after grafting. The thermal and decomposition behaviors are investigated by DSC and thermogravimetric analysis (TGA). The effect of the length of the PCL grafts on the crystallization behavior of the comb‐like copolymers is investigated by DSC and wide‐angle X‐ray scattering (WAXS).

     

Study of Uniaxially Stretched Isotactic Poly(propylene) by 1H Solid‐State NMR and IR Spectroscopy

Changes in the amount of the rigid, semi‐rigid and soft fractions, molecular mobility and domain thickness of uniaxially stretched iPP were investigated as a function of temperature, draw ratio, drawing temperature and drawing rate. Correlations are established between the thicknesses of rigid domains and the amount of the rigid fractions of uniaxially stretched iPP. Also established are correlations between the thickness of rigid domains and the molecular mobility of the rigid fraction of uniaxially stretched iPP. The drawing temperature has an important effect on the strain‐induced transformation of the spherulitic morphology of iPP to a fibrillar one.

     

Thermoplastic Vulcanizates Based on Highly Compatible Blends of Isotactic Poly(propylene) and Copolymers of Atactic Poly(propylene) and 5‐Ethylidene‐2‐norbornene

Highly compatible thermoplastic/elastomer blends were used to prepare thermoplastic vulcanizates (TPVs) with refined morphologies and improved mechanical properties. Copolymers of atactic poly(propylene) (aPP) and 5‐ethylidene‐2‐norbornene (ENB) (aPP‐co‐ENB) were prepared via metallocene catalysis. Blends of isotactic poly(propylene) (iPP) and the aPP‐co‐ENB rubbers are immiscible in the melt, but the very high compatibility leads to a refinement of the morphology in comparison to blends based on iPP and ethylene‐propylene‐ diene (EPDM) rubber. The aPP‐co‐ENB‐based blends show improved tensile properties, while the relatively high T_g of the rubber phase retards the elastic recovery at room temperature. Dilution of the TPVs with oil broadens the temperature window for applications by a signification reduction of the T_g and improves the elasticity of the TPVs. This study demonstrates that the lower limit of the rubber particle size of TPVs that is attainable via dynamic vulcanization of immiscible blends is in the order of 300–500 nm.

     

Detailed Kinetics of 1‐Hexene Oligomerization Reaction with (n‐Bu‐Cp)2ZrCl2–MAO Catalyst

The paper describes a kinetic study of 1‐hexene oligomerization reaction with the (n‐Bu‐Cp)₂ZrCl₂–MAO catalyst system at 70 °C. GC analysis of the monomer/oligomer mixtures at different reaction times in the course of a 5‐h reaction, provided information on the formation rates of individual oligomer molecules, from the 1‐hexene dimer, C₁₂H₂₄, to the tetradecamer, C₈₄H₁₆₈. Kinetic modeling of the oligomerization kinetics strongly supports the principal premise of polymerization kinetics, that the value of the propagation rate constant of the chain growth reaction does not depend on the number of monomer units in the growing polymer chain. The only exception from this rule is the insertion of a 1‐hexene molecule into the Zr⁺? C bond in the active center bearing a single monomer unit, (n‐Bu‐Cp)₂Zr⁺? CH₂? CH₂? C₄H₉. The rate constant of this reaction in one order of magnitude is higher than the rate constant of 1‐hexene insertion into active centers which are β‐branched with respect to the Zr atom, (n‐Bu‐Cp)₂Zr⁺? [CH₂? CH(C₄H₉)]_n? CH₂? CH₂? C₄H₉.

     

Metallocene Catalysts for 1‐Butene Polymerization

Summary: Isotactic polybutenes of variable isotacticity and melting points of form I in the range 100–125 °C have been prepared with both C₂‐ and C₁‐symmetric zirconocenes. The C₁‐symmetric zirconocenes bearing the bilateral symmetric 2,5‐dimethyl‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand connected by a dimethylsilandiyl bridge to a substituted indenyl ligand produce iPB with higher molecular mass, up to 400 000 at polymerization temperature of 70 °C in liquid butene. The degree of isotacticity depends on the substitution pattern of the indenyl ligand. The correlations between microstructure and melting points of the crystalline forms I and II of iPB have been defined. Some relevant differences in catalyst selectivity between propylene and 1‐butene polymerizations have been identified.

Linear correlation of melting points of form I and form II in isotactic poly(1‐butene)s of different chain regularities.     

Synthesis and Characterization of Block Copolymers of ε‐Caprolactone and DL‐Lactide Initiated by Ethylene Glycol or Poly(ethylene glycol)

Biodegradable copolymers were prepared by ring‐opening polymerization of sequentially added ε‐caprolactone and DL ‐lactide in the presence of ethylene glycol or poly(ethylene glycol), using zinc metal as catalyst. Polymerization was performed in bulk and yielded block copolymers with predetermined PEG/PCL/PLA segments. The obtained polymers were characterized by ¹H NMR, SEC, IR, DSC, TGA, and X‐ray diffraction. Data showed that the copolymers preserved the excellent thermal behavior inherent to PCL. The crystallinity of PLA‐containing copolymers was reduced with respect to PCL homopolymer. The presence of both hydrophilic PEG and fast degrading PLA blocks should improve the biocompatibility and biodegradability of the materials, which are of interest for applications as substrate in drug delivery or as scaffolding in tissue engineering.

Block copolymerization of ε‐caprolactone and DL ‐lactide initiated by dihydroxyl PEG.     

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.     

A Convenient Method for the Synthesis of the Amphiphilic Triblock Copolymer Poly(L‐lactic acid)‐block‐Poly(L‐lysine)‐block‐Poly(ethylene glycol) Monomethyl Ether

The amphiphilic triblock copolymer PLA‐b‐PLL‐b‐MPEG is prepared in three steps through acylation coupling between the terminal amino groups of PLA‐b‐PZLL‐NH₂ and carboxyl‐terminal MPEG, followed by the deprotection of amines. The block copolymers are characterized via FT‐IR, ¹H NMR, DSC, GPC, and TEM. TEM analysis shows that the triblock polymers can form polymeric micelles in aqueous solution with a homogeneous spherical morphology. The cytotoxicity assay indicates that the final triblock polymer micelles after deprotection show low cytotoxicity against Bel7402 human hepatoma cells. MPEG and PLL were introduced into the main chain of PLA affording a kind of ideal bioabsorbable polymer materials, which is expected to be useful in drug and gene delivery.

     

Isotactic Poly(propylene) Crystallization: Role of Small Fractions of High or Low Molecular Weight Polymer

Summary: Two well‐characterized metallocene isotactic poly(propylene) (iPP) samples, one of high and one of low molecular weight, were blended together for studying the effects of a second component on quiescent and shear‐induced crystallization. A small amount of added high molecular weight (HMW) (up to 10 wt.‐%) polymer increases the quiescent crystallization rate. This was observed as a decrease in characteristic halftime of transmitted light intensity. The crystallization halftime increases again when adding more than 10 wt.‐% of HMW polymer. The crystallization halftime of pre‐sheared samples decreases with increasing HMW fraction and is lowest for the HMW sample by itself. For the specific shearing conditions (γ = 600, T_x = 145 °C), wide‐angle X‐ray diffraction (WAXD) images show the presence of the gamma (γ) crystalline phase of iPP for samples containing 25 wt.‐% of HMW and higher. DSC thermograms demonstrated higher crystalline fractions with increasing HMW fraction in pre‐sheared samples.

Optical micrograph of an iPP sample after quiescent crystallization at 145 °C.     

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.     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

Synthesis and Characterization of Defined Branched Poly(propylene)s with Different Microstructures by Copolymerization of Propylene and Linear Ethylene Oligomers (Cn = 26–28) with Metallocenes/MAO Catalysts

Summary: Copolymers of propylene and hexacosene (C_n = 26–28) were synthesized in the presence of three different metallocene catalysts activated by methylaluminoxane. The poly(propylene) copolymers were prepared with iso‐, syndio‐, and atactic backbone microstructures by using different symmetric metallocenes such as rac‐{Me₂Si[2‐Me‐4‐(1‐Naph)Ind]₂}ZrCl₂ ( 1 ), [Ph₂C(Cp)(Flu)]ZrCl₂ ( 2 ), and [(H₃C)₂Si(9‐Flu)₂]ZrCl₂ ( 3 ) and up to 46.6 mol‐% comonomer content in the feed. The influence of the incorporated linear, ethylene‐based side chains into the poly(propylene) backbone were investigated by DSC, GPC, and ¹³C NMR. Generally, a decreasing content of comonomer in the feed enhances the activity of metallocene based catalysts. The determination of the branched microstructure by ¹³C NMR of the copolymers allows a smart identification of the amount of inserted hexacosene because of the separated backbone and side chain signals. Moreover, the relationship between the population of the side chains and the melting behavior of resulting copolymers were discussed. The melting point of the syndiotactic and isotactic poly(propylene) backbone decreases with increasing hexacosene content. When the inserted comonomer content exceeds 2 mol‐%, a second melting point of the crystallized ethylene based side chains can be observed which increases with an increasing amount of hexacosene.

Thermal behavior of isospecific hexacosene/propylene copolymers in dependence on the incorporation of hexacosene.     

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.     

Segmented Block Copolymers with Terephthalic‐Extended Poly(ethylene oxide) Segments

Segmented block copolymers comprised of flexible PEO segments and monodisperse crystallizable bisestertetraamide segments have been synthesized. The influence of the terephthalic units in the soft phase on the transitions and the thermal mechanical and elastic properties are studied. The presence of terephthalic units in the copolymer increases the glass transition temperature of the soft phase by ≈5 °C. The low‐temperature flexibility of the copolymers is improved because of the lower crystallinity and melting temperature of PEO. With the use of terephthalic‐extended PEO segments, segmented block copolymers with low moduli (G' < 15 MPa) and good elastic properties could be obtained.

     

Controlled Synthesis and Characterization of Poly[ethylene‐block‐(L,L‐lactide)]s by Combining Catalytic Ethylene Oligomerization with “Coordination‐Insertion” Ring‐Opening Polymerization

Model poly[ethylene‐block‐(L ,L ‐lactide)] (PE‐block‐PLA) block copolymers were successfully synthesized by combining metallocene catalyzed ethylene oligomerization with ring‐opening polymerization (ROP) of L ,L ‐lactide (LA). Hydroxy‐terminated polyethylene (PE‐OH) macroinitiator was prepared by means of ethylene oligomerization on rac‐dimethyl‐silylen‐bis(2‐methyl‐benz[e]indenyl)‐zirconium(IV)‐dichloride/methylaluminoxane (rac‐MBI/MAO) in presence of diethyl zinc as a chain transfer agent, and subsequent in situ oxidation with synthetic air. Poly[ethylene‐block‐(L ,L ‐lactide)] block copolymers were obtained via ring‐opening polymerization of LA initiated by PE‐OH in toluene at 100 °C mediated by tin octoate. The formation of block copolymers was confirmed by ¹H NMR spectroscopy, fractionation experiments, thermal behavior, and morphological characterization using AFM and light microscopy techniques.

     

Copolymerization of Ethylene and 1‐Hexene with Ansa‐Dimethylsilylene(fluorenyl) (t‐butylamido)Dimethyltitanium Complexes Activated by Modified Methylaluminoxane

ansa‐Dimethylsilylene(fluorenyl)(t‐butylamido)dimethyltitanium complex, Me₂Si(η³‐C₁₃H₈)(η¹‐N^tBu)TiMe₂ ( 1 ) , and its tetraalkylsubsutituted‐fluorenyl derivative, Me₂Si(η³‐C₂₉H₃₆)(η¹‐N^tBu)TiMe₂ (2) , are used as catalysts for ethylene/1‐hexene copolymerization activated by modified methylalminoxane. 2 is found to show remarkable activity and affords high‐molecular‐weight polymer compared with 1 . The monomer reactivity ratios, r_E and r_H, are determined to be r_E = 3.0 ± 0.15, r_H = 0.19 ± 0.06 for 1 , and r_E = 2.9 ± 0.15, r_H = 0.27 ± 0.02 for 2 , indicating that 2 incorporates 1‐hexene slightly more than 1 .

     

Fractionation and Characterisation of Propylene‐Ethylene Random Copolymers: Effect of the Comonomer on Crystallisation of Poly(propylene) in the γ‐Phase

Summary: Three propylene‐ethylene random copolymers, of varying ethylene content, were fractionated using preparative TREF. The TREF fractions were subsequently analysed offline by CRYSTAF, DSC, ¹³C NMR spectroscopy, HT‐GPC, and WAXD. The effect of the ethylene comonomer on the crystallisability of the propylene was investigated, along with the effect of the comonomer on the type of crystal phase formed during the crystallisation. The results show that the comonomer inhibits the crystallisation of the copolymer and that as the ethylene content increases, the crystallisation and melting points decrease. It was also shown that the higher the ethylene content, the more of the γ‐phase crystal type is formed. The inter‐molecular distribution of the ethylene comonomer and stereo errors vary between the samples analysed, and this accounts for the relative differences in crystallisation behaviour of the samples.

¹³C NMR of propylene‐ethylene random copolymer.     

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.