Control of molecular weight distribution of petroleum pitches via multistage supercritical extraction

Packed-column supercritical extraction (SCE), followed by low-pressure gas stripping, was used to produce a dimer-rich pitch fraction from an oligomeric petroleum pitch, Marathon M-50, of broad molecular weight distribution (MWD). Both solvent-to-pitch ratios (S/P) >5 and a positive retrograde temperature gradient of 380-330 °C at 70 bar were found to reduce significantly the amount of trimer+ oligomers in the overhead product from the SCE column. This monomer- and dimer-rich overhead was subsequently stripped of monomer with gaseous toluene in a second packed column at 380 °C and 1.5 bar to obtain an 80+ mol% dimer product with an overall yield, based on the original feed, of 30%. To our knowledge, this is the first reported fractionation of a dimer-rich cut from a petroleum pitch with a demonstrably low level of both lower and higher mol wt impurities.     

Supercritical CO2-assisted preparation of ibuprofen-loaded PEG-PVP complexes

Stoichiometric ratios of poly(ethylene glycol) (PEG, M_w = 400) with poly(vinylpyrrolidone) (PVP, M_w = ±3.1 × 10⁴ and M_w = 1.25 × 10⁶ M_w) were prepared from ethanol cast solutions and in supercritical CO₂. The complex formation was studied via glass transition (T_g) analysis obtained from differential scanning calorimetry (DSC) thermograms. PEG-PVP blends were also loaded with ibuprofen. The molecular dispersion of ibuprofen, mechanism of interaction, the effect of CO₂ pressure and temperature and ageing of blends were also analysed with DSC, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray diffraction (XRD). T_g analysis indicated that supercritical CO₂ can facilitate the formation of stoichiometric PEG-PVP complexes. Processing of PEG-PVP blends with ibuprofen results in the molecular dispersion of ibuprofen mainly bonded to PVP carbonyl groups, without significant disruption of the PEG-PVP complex. Increasing process pressure results in extraction of some PEG fractions. Post-processing ATR-FTIR shifts in ibuprofen-PEG-PVP complexes is greater with supercritical CO₂ processing. These shifts are mainly attributed to atmospheric moisture absorption. Overall it was shown that, ibuprofen-loaded PEG-PVP complexes can be prepared from supercritical CO₂ processing showing similar characteristics to such complexes prepared from solution casting.     

Thermal polymerization of methyl (meth)acrylate via reversible addition-fragmentation chain transfer (RAFT) process

Thermal polymerization of methyl (meth)acrylate (MMA) was carried out using 2-cyanoprop-2-yl-1-dithionaphthalate (CPDN) and cumyl dithionaphthalenoate (CDN) as chain transfer agents. The kinetic study showed the existence of induction period and rate retardation, especially in the CDN mediated systems. The molecular weights of the polymers increased linearly with the monomer conversion, and the molecular weight distributions (M_w/M_ns) of the polymers were relatively narrow up to high conversions. The maximum number-average molecular weights (M_ns) reached to 351?900 g/mol (M_w/M_n = 1.47) and 442?400 g/mol (M_w/M_n = 1.29) in the systems mediated by CPDN and CDN, respectively. Chain-extension reactions were also successfully carried out to obtain higher molecular weight PMMA and PMMA-block-polystyrene (PMMA-b-PSt) copolymer with controlled structure and narrow M_w/M_n. Thermal polymerization of methyl acrylate (MA) in the presence of CPDN, or benzyl (2-phenyl)-1-imidazolecarbodithioate (BPIC) also demonstrated “living”/controlled features with the experimented maximum molecular weight 312?500 g/mol (M_w/M_n = 1.57). The possible initiation mechanism of the thermal polymerization was discussed.     

Investigation of the degree of homogeneity and hydrogen bonding in PEG/PVP blends prepared in supercritical CO2: Comparison with ethanol-cast blends and physical mixtures

The degree of homogeneity and H-bond interaction in blends of low-molecular-mass poly(ethylene glycols) (PEG, M_w = 400, 600, 1000) and poly(vinylpyrrolidone) (PVP, M_w = 9 × 10³) prepared in supercritical CO₂, ethanol and as physical mixtures were studied by differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy and dynamic mechanical analysis (DMA) techniques. Homogeneity of samples prepared in supercritical CO₂ were greater than physically mixed samples, but slightly less than ethanol-cast samples. PEG-PVP H-bond interaction was higher for ethanol-cast blends when compared to blends prepared in supercritical CO₂. This reduced interaction was attributed to a combination of: (1) shielding of PEG-PVP H-bond interactions when CO₂ is dissolved in the blend; (2) rapidly reduced PEG and PVP chain mobility upon CO₂ venting, delaying rearrangement for optimum PEG-PVP H-bond interaction.     

Synthesis and applications of triblock and multiblock copolymers using telechelic oligopropylene

Isotactic polypropylene (iPP)-polystyrene (PS) and iPP-poly(methyl methacrylate) (PMMA) multiblock copolymers were synthesized by atom transfer radical coupling (ATRC) of PS-iPP-PS and PMMA-iPP-PMMA triblock copolymers obtained by atom transfer radical polymerization (ATRP) of styrene (St) and methyl methacrylate (MMA), respectively, using α,ω-dibromoisobutyrateoligopropylene (iPP-Br) as a bifunctional macroinitiator. The iPP-Br was prepared by hydroxylation and subsequent esterification of telechelic oligopropylene having terminal vinylidene double bonds at both ends obtained by controlled thermal degradation of iPP. ATRP of St and (meth) acrylic monomers using iPP-Br formed the corresponding triblock copolymers. It was confirmed that the PMMA-iPP-PMMA triblock copolymer was effective as the compatibilizer for the iPP/PMMA blend. An iPP-PS multiblock copolymer (M_n: 25?000 g/mol and M_w/M_n: 4.1) was prepared by ATRC of PS-iPP-PS triblock copolymer (M_n: 8900 g/mol and M_w/M_n: 1.3). ATRC with St of PMMA-iPP-PMMA triblock copolymer (M_n: 13?000 g/mol and M_w/M_n: 1.4) provided an iPP-PMMA multiblock copolymer containing St chains (M_n: 39?000 g/mol and M_w/M_n: 2.8).     

Direct synthesis of poly(l-lactic acid) in supercritical carbon dioxide with dicyclohexyldimethylcarbodiimide and 4-dimethylaminopyridine

The direct synthesis of poly(l-lactic acid) (PLLA) from an l-lactic acid oligomer has been performed in supercritical carbon dioxide (scCO₂) using an esterification promoting agent, dicyclohexyldimethylcarbodiimide (DCC), and 4-dimethylaminopyridine (DMAP) as a catalyst. PLLA within M_n of 13,500 g/mol was synthesised in 90% yield at 3500 psi and 80 °C after 24 h. The molecular weight distribution of the products was narrower than PLLA prepared with melt-solid phase polymerisation under conventional conditions. Both DCC and DMAP showed high solubility in scCO₂ (DCC: 7.6 wt% (1.63×10⁻² mol/mol CO₂) at 80 °C, 3385 psi, DMAP: 4.5 wt% (1.62×10⁻²mol/mol CO₂) at 80 °C, 3386 psi) and supercritical fluid extraction was found to be effective at removing excess DMAP and DCC after the polymerisation was complete. We show that DCC and DMAP are effective esterification promoting reagents with further applications for condensation polymerisations in scCO₂.     

Half-sandwich samarium (III) diketiminate bromide as a catalyst for methyl methacrylate polymerization

Half-sandwich samarium(III) diketiminate bromide was successfully synthesized and was shown to be active in methyl methacrylate (MMA) polymerization. The effects of temperature, polymerization time and catalyst concentration were studied. Activities of ca. 18 kg of polymethacrylate (PMMA) per mol of samarium per hour were obtained under optimum conditions (0 °C and a MMA/catalyst molar ratio of 100/1), giving a polymer with a molar mass M_n>24,000 g mol⁻¹ and a molar mass distribution (M_w/M_n)<1.4. After 1 h of polymerization, conversions of MMA as high as 96% were observed.     

Polymeric nanoparticles from macroscopic crystalline monomers by facile solid-state polymerization in supercritical CO2

When macroscopic crystalline monomers were polymerized by a free-radical solid-state reaction in the presence of supercritical CO₂ (scCO₂), the resultant products were found to be composed of unexpected nanoparticle morphologies. In particular, the solid-state polymerization (SSP) of amino acid based monomers, acryloyl-β-alanine (ABA) and methacryloyl-β-alanine (MBA), initiated by azobisisobutyronitrile in scCO₂ (at 65 °C and 34.5 MPa), produced corresponding polymers having aggregated spherical architectures. The average diameters of the PABA and PMBA particles were measured to be 94 and 102 nm, respectively. In addition, high molecular weight polymers (PMBA, M_w = 3.8 × 10⁵ g/mol) with a high yield (∼96%) were obtained. The microscopic investigation revealed that a unique particle formation mechanism was involved in the SSP in which large sized crystalline monomers were chipped into small pieces during the initial stage of polymerization and subsequently converted into nanoscale objects after 24 h.     

Lipase-catalyzed synthesis of poly-l-lactide using supercritical carbon dioxide

The present work shows that the enzyme mediated ring-opening polymerization of l-lactide at 65 °C can be achieved using supercritical carbon dioxide. It is reported a biphasic media system where the supercritical phase coexists with a liquid organic phase, which is mainly composed of melted monomer, wherein the growing poly-l-lactide chains are soluble. The immobilized lipase B from Candida antartica was used as the biocatalyst. The results indicated that semi-crystalline polymers with a molecular weight (M_w) up to 12,900 g mol⁻¹ can be attained and that the monomer conversion is related to the biocatalyst concentration and its initial water activity (a_wi). Experiments carried out with denatured enzyme gave no monomer conversion which confirms that the enzymatic mechanism is only involved in our system.     

Small-angle neutron scattering from symmetric blends of poly(dimethylsiloxane) and poly(ethylmethylsiloxane)

We present results of a small-angle neutron scattering (SANS) study of the structure and thermodynamic properties of symmetric blends of deuterated poly(dimethylsiloxane) (d-PDMS) and poly(ethylmethylsiloxane) (PEMS) as a function of temperature (T) (40≤T≤300 °C) and the molecular weight (M_w) (4700≤M_w≤23,200). The radius of gyration (R_g) of d-PDMS was measured using the high-concentration labeling method and revealed unperturbed chain dimensions at all temperatures regardless of the polymer M_w. The random phase approximation (RPA) fits the data for low M_w blends, however it fails to describe the SANS data for M_w>10,000 g/mol. This observation is explained by the fact that for high M_w blends the correlation length of the concentration fluctuations ξ is always large (ξ>R_g), implying that these blends remain microscopically inhomogeneous at all temperatures studied in this work. At the same time, the low M_w blends are randomly mixed (ξ<R_g) at all T and can reach the ‘ideal mixing’ or Θ condition (χ=0).     

Effects of molecular weight on poly(ω-pentadecalactone) mechanical and thermal properties

A series of poly(ω-pentadecalactone) (PPDL) samples, synthesized by lipase catalysis, were prepared by systematic variation of reaction time and water content. These samples possessed weight-average molecular weights (M_w), determined by multi-angle laser light scattering (MALLS), from 2.5 × 10⁴ to 48.1 × 10⁴. Cold-drawing tensile tests at room temperature of PPDL samples with M_w between 4.5 × 10⁴ and 8.1 × 10⁴ showed a brittle-to-ductile transition. For PPDL with M_w of 8.1 × 10⁴, inter-fibrillar slippage dominates during deformation until fracture. Increasing M_w above 18.9 × 10⁴ resulted in enhanced entanglement network strength and strain-hardening. The high M_w samples also exhibited tough properties with elongation at break about 650% and tensile strength about 60.8 MPa, comparable to linear high density polyethylene (HDPE). Relationships among molecular weight, Young's modulus, stress, strain at yield, melting and crystallization enthalpy (by differential scanning calorimetry, DSC) and crystallinity (from wide-angle X-ray diffraction, WAXD) were correlated for PPDL samples. Similarities and differences of linear HDPE and PPDL molecular weight dependence on their mechanical and thermal properties were also compared.     

Plasticization of semicrystalline poly(l-lactide) with poly(propylene glycol)

Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M_w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T_g, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T_g of PLA rich phase of the respective blends, 46.1-47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M_w of 600 g/mol, was also used to plasticize PLA for comparison.     

New insights into the characteristics of early stage crystallization of a polyethylene

Small angle light scattering has been used to probe structure formation during isothermal crystallization of an ethylene-1-hexene copolymer (EH064, M_w = 70,000 g/mol, ρ = 0.900 g/cm³, M_w/M_n ∼ 2, 6.4 mol% hexene). It is shown that clear structural information on size scales ranging from hundreds of nanometers to several micrometers during early stage crystallization can be obtained by this method when crystallizing the polyethylenes at the high temperatures (above the peak melting temperature of a rapidly crystallized polymer sample) required for resolving early stage crystallization without the influence of the crystal growth. The results show that the early stage crystallization is characterized by large scale orientation fluctuations that precede the formation of local crystalline order manifest in X-ray scattering and the initial collapse of these large scale anisotropic/ordered domains. The scattering intensity increases exponentially with time initially, and the wave vector dependence of the growth rate of fluctuations is consistent with predictions for initial stages of a phase transformation process. However, the detailed mechanism cannot be described by existing models. The implications of our results are discussed within the context of proposed models for early stage crystallization.     

Stereocontrolled styrene-isoprene copolymerization and styrene-ethylene-isoprene terpolymerization with a single-component allyl ansa-neodymocene catalyst

The ansa-metallocene complex (CpCMe₂Flu)Nd(C₃H₅)(THF) (1) is an effective single-component catalyst for the production of syndiotactic styrene-rich polymer materials modified by isoprene and/or ethylene. The recovered copolymers have high molecular weights (M_n = 12,000-91,000 g/mol) and unimodal, relatively narrow molecular weight distributions (M_w/M_n = 1.3-2.8). The comonomer feeds can be easily manipulated to tune the respective amounts of monomers incorporated in the copolymer and eventually modify the final properties (T_m, T_g) of the obtained materials.     

Crystallization, structure and properties of plasticized poly(l-lactide)

Poly(l-lactide) (PLA) was plasticized with poly(ethylene glycol)s having M_w of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having M_w of 550 and 750 g/mol, with chains terminated with hydroxyl groups and methyl groups, were used. The effect of different end groups on the plasticization of both amorphous and semicrystalline PLA was studied. The crystallization, structure, thermal and tensile properties of PLA and PLA with 5 and 10 wt% of plasticizers were explored. No marked effect induced by different end groups of plasticizers was found. All the plasticizers used decreased T_g and increased the ability of PLA to cold crystallization. While an amorphous plasticized PLA could be deformed to about 550%, a semicrystalline PLA with the same total plasticizer content exhibited nonuniform plasticization of the amorphous phase and less ability to the plastic deformation. Nevertheless, a 20% elongation at break was achieved for a semicrystalline PLA with 10 wt% of the plasticizer. The plastic deformation of both neat and plasticized PLA was associated with crazing.     

Polymerization of 1,3-butadiene with VO(P204)2 and VO(P507)2 activated by alkylaluminum

The oxovanadium phosphonates (VO(P₂₀₄)₂ and VO(P₅₀₇)₂) activated by various alkylaluminums (AlR₃, R = Et, i-Bu, n-Oct; HAlR₂, R = Et, i-Bu) were examined in butadiene (Bd) polymerization. Both VO(P₂₀₄)₂ and VO(P₅₀₇)₂ showed higher activity than those of classical vanadium-based catalysts (e.g. VOCl₃, V(acac)₃). Among the examined catalysts, the VO(P₂₀₄)₂/Al(Oct)₃ system (I) revealed the highest catalytic activity, giving the poly(Bd) bearing M_n of 3.76 × 10⁴ g/mol, and M_w/M_n ratio of 2.9, when the [Al]/[V] molar ratio was 4.0 at 40 °C. The polymerization rate for I is of the first order with respect to the concentration of monomer. High thermal stability of I was found, since a fairly good catalytic activity was achieved even at 70 °C (polymer yield > 33%); the M_n value and M_w/M_n ratio were independent of polymerization temperature in the range of 40-70 °C. By IR and DSC, the poly(Bd)s obtained had high 1,2-unit content (>65%) with atactic configuration. The 1,2-unit content of the polymers obtained by I was nearly unchanged, regardless of variation of reaction conditions, i.e. [Al]/[V], ageing time, and reaction temperature, indicating the high stability of stereospecificity of the active sites.     

A rheology theory and method on polydispersity and polymer long-chain branching

Model calculations were performed to investigate the sensitivity of zero-shear melt viscosity (η₀ or Eta0) on the molecular weight (MW) polydispersity of linear polymers. Simulated MW distributions (MWD) were generated with the generalized exponential (GEX) distribution function for various levels of polydispersity M_w/M_n and M_z/M_w. For linear entangled polymeric chains in the melt, the linear viscoelastic properties were predicted by using the double reptation blending rule and the so-called BSW relaxation time spectrum, named after the authors: Baumgaertel, Schausberger and Winter [Baumgaertel M, Schausberger A, Winter HH. Rheol Acta 1990;29:400-8]. Published rheological parameters appropriate for polyethylene were used in the calculations. It was found that Eta0 depended mostly on M_w, but it also significantly depended on the extent of high-MW polydispersity M_z/M_w. A revision to the fundamental MW dependency of Eta0 was proposed to compensate for this polydispersity effect. To offset the polymer polydispersity differences, we propose a new MW average (M_HV or M_x with x = 1.5) to replace M_w in the historical rheological power-law equation of Eta0 ∝ M_w^a, where the literature value of exponent “a” ranges from 3.2 to 3.6. The use of M_HV instead of M_w in the power-law equation made the calculated Eta0 independent of the sample high-MW polydispersity. With the removal of the complication from polydispersity effect, the new Eta0 power law can now provide a more robust base for studying polymer long-chain branching (LCB). A new LCB index is thus proposed based on this new melt-viscosity power law. The values of M_HV in the new power law can be calculated for polymer samples from the conventional gel permeation chromatographic (GPC) slice data.     

Affine deformation of single polymer chain in poly(methyl methacrylate) films under uniaxial extension observed by scanning near-field optical microscopy

The conformation of single poly(methyl methacrylate) chain in the uniaxially stretched film was observed by the combination of the fluorescence labeling technique and scanning near-field optical microscopy (SNOM). The dimension of the individual probe chain (M_w = 1.99 × 10⁶) along the extension axis was evaluated from SNOM image. In the high molecular weight matrix (M_w = 1.89 × 10⁶) the average extension ratio at the single chain level coincided with the macroscopic extension ratio. The distribution of the chain conformation was in good agreement with that of the freely jointed chain followed by affine deformation. On the other hand, the probe chain embedded in the low molecular weight matrix (M_w = 1.76 × 10⁵) showed the smaller extension than that expected from the affine deformation. This suggests that the conformation of the probe chain is affected by the relaxation of the short surrounding chains through disentanglement.     

Double propagation based on diepoxide, a facile route to high molecular weight poly(propylene carbonate)

Poly(propylene carbonate) (PPC) with number average molecular weight (M_n) higher than 200 kg/mol was prepared via the terpolymerization of carbon dioxide, propylene oxide and diepoxide using Y(CCl₃OO)₃-ZnEt₂-glycerine coordination catalyst. When equimolar ZnEt₂ and diepoxide were used, double propagation active species were generated in situ by nucleophilic attack of metal alkoxide on diepoxide, leading to PPC of doubled M_n value. The molecular weight of PPC has dramatic influence on its thermal and mechanical performances. PPC with M_n of 227 kg/mol showed modulus of 6900 MPa, while the modulus of PPC with M_n of 109 kg/mol was only 4300 MPa. Moreover, when M_n increased from 109 to 227 kg/mol, a 37 °C increase of the onset degradation temperature was observed.     

Synthesis of polymeric prodrugs of chlorphenesin with saccharide branches by chemo-enzymatic regioselective strategy

A facile and regioselective enzymatic synthesis approach to prepare polymerizable lipophilic chlorphenesin vinyl esters was developed in this research. The influence of different organic solvents, enzyme sources, reaction time and the acylation reagent on the synthesis of chlorphenesin vinyl esters was investigated. Then the polymerizable monomers 1-O-vinylsuccinyl-chlorphenesin (OVSC) and 1-O-vinyladipoyl-chlorphenesin (OVAC) were homopolymerized using AIBN as the initiator. The obtained polymeric prodrugs were characterized with IR, NMR and GPC analyzes. The poly-OVSC has M_n of 1.35 × 10⁴ and M_w/M_n of 1.95, and the poly-OVAC has M_n of 2.37 × 10⁴ and M_w/M_n of 4.30. Moreover, 6-O-vinyladipoyl-d-glucose (OVAG), a biocompatible monomer, was copolymerized with OVSC and OVAC. Polymeric prodrugs of chlorphenesin with saccharide branches were successfully obtained with high molecular weight.