Sara Ronca

Mimicking Ziegler-Natta Catalysts in Homogeneous Phase, 1. C2-Symmetric Octahedral Zr(IV) Complexes with Tetradentate [ONNO]-Type Ligands

Results of propene polymerization in the presence of two known octahedral C2-symmetric Zr complexes bearing tetradentate [ONNO]-type ligands are reported for the first time. Depending on the steric hindrance at the active metal, isotactic site-controlled or weakly syndiotactic chain-end-controlled polymers were obtained, in both cases via highly regioselective 1,2 (primary) monomer insertion. In this respect, the complexes mimic the behavior of the active Ti species on the surface of the heterogeneous Ziegler-Natta catalysts of which they might represent good structural models.

Design of stereoselective Ziegler–Natta propene polymerization catalysts

After five decades of largely serendipitous (albeit formidable) progress, catalyst design in Ziegler–Natta olefin polymerization, i.e., the rational implementation of new active species to target predetermined polyolefin architectures, has ultimately become a realistic ambition, thanks to a much deeper fundamental understanding and major advances in the tools of computational chemistry. In this article, we discuss, as a case history, a unique class of stereorigid C2-symmetric bis(phenoxy-amine)Zr(IV) catalysts with controlled kinetic behavior. A large variety of polypropylene microstructures have been obtained with these catalysts by modulating the steric demand of one key substituent, without altering the nature and symmetry of the ancillary ligand framework, under the guidance of computer modeling. This unusual achievement is relevant per se and for the perspective implications in catalyst discovery.

Electrospun Nanofibres Containing Antimicrobial Plant Extracts

Over the last 10 years great research interest has been directed toward nanofibrous architectures produced by electrospinning bioactive plant extracts. The resulting structures possess antimicrobial, anti-inflammatory, and anti-oxidant activity, which are attractive for biomedical applications and food industry. This review describes the diverse approaches that have been developed to produce electrospun nanofibres that are able to deliver naturally-derived chemical compounds in a controlled way and to prevent their degradation. The efficacy of those composite nanofibres as wound dressings, scaffolds for tissue engineering, and active food packaging systems will be discussed.

A Hemi-metallocene Chromium Catalyst with Trimethylaluminum-Free Methylaluminoxane for the Synthesis of Disentangled Ultra-High Molecular Weight Polyethylene

Recently, it has been shown that by using a single-site catalytic system having titanium as a metallic center, it is possible to tailor the entanglement density in the amorphous region of a semi-crystalline ultra-high molecular weight polyethylene (UHMWPE). This route provides the possibility to make high-modulus, high-strength uniaxially and biaxially drawn tapes and films, without using any solvent during processing. In this publication, it is shown that a single-site catalyst having chromium as metallic center, proposed by Enders and co-workers, can also be tuned to provide control on the entanglement density during synthesis of the UHMWPE. However, to achieve the goal some modifications during the synthesis are required. The synthesized polymers can be processed in the solid state below the equilibrium melting temperature, resulting in uniaxially drawn tapes having tensile strength and modulus greater than 3.5 N/tex and 200 N/tex, respectively. Rheological studies have been performed to follow the increase in entanglement density in melt state with time.

Effect of Ethyl Ester L-Lysine Triisocyanate addition to produce reactive PLA/PCL bio-polyester blends for biomedical applications

We report in this paper the effects of Ethyl Ester L-Lysine Triisocyanate (LTI) on the physical-mechanical properties of Poly(lactide)/Poly(ε-caprolactone) (PLA/PCL) polyesters blends. The PLA/PCL ratios considered were 20/80, 50/50 and 80/20 (wt/wt %) and LTI was added in amounts of 0.0-0.5-1.0 phr. PLA and PCL reacted with LTI during processing in a Brabender twin screw internal mixer to produce block copolymers in-situ. The resulting blends have been characterized by torque measurements, uniaxial tensile tests, Differential Scanning Calorimeter, contact angle measurements with a Phosphate Buffered Saline (PBS) solution, ATR analysis and morphological SEM observations. Experimental results highlighted how LTI enhanced interaction and dispersion of the two components, resulting into a synergic effect in mechanical properties. Mechanical and physical properties can be tailored by changing the blend composition. The most noticeable trend was an increase in ductility of the mixed polymers. Besides, LTI decreased blends wet ability in PBS and lowered the starting of crystalline phase formation for both polymers, confirming an interaction among them. These reactive blends could find use as biomedical materials, e.g. absorbable suture threads or scaffolds for cellular growth.

Synthesis of Disentangled Ultra-High Molecular Weight Polyethylene: Influence of Reaction Medium on Material Properties

The polymerization of ethylene to Ultra-High Molecular Weight Polyethylene (UHMWPE) in certain reaction conditions allows synthesis of nascent powders with a considerably lower amount of entanglements: the material obtained is of great interest from both academic and industrial viewpoints. From an academic point of view, it is interesting to follow the evolution of the metastable melt state with the progressive entanglements formation. Industrially, it is valuable to have a solvent-free processing route for the production of high modulus, high strength tapes. Since the polymer synthesis is performed in the presence of a solvent, it is interesting to investigate the influence that the reaction medium can have on the catalyst activity, resultant molecular characteristics, and polymer morphology at the macroscopic as wells as microscopic level. In this paper, we present the effect that two typical polymerization solvents, toluene and heptane, and mixtures of them, have on the catalytic performance and on the polymer properties. The observations are that an unexpected increase of catalyst activity, accompanied by a significant improvement in mechanical properties, is found when using a carefully chosen mixture of solvents. A tentative explanation is given on the basis of the presented results.

Synthesis and characterization of divinyl-fumarate Poly-ε-caprolactone for scaffolds with controlled architectures

A vinyl‐terminated polycaprolactone has been developed for tissue engineering applications using a one‐step synthesis and functionalization method based on ring opening polymerization (ROP) of ε‐Caprolactone, with hydroxyl ethyl vinyl ether (HEVE) acting both as the initiator of ROP and as photo‐curable functional group. The proposed method employs a catalyst based on aluminium, instead of the most popular Tin(II) 2‐ethylhexanoate, to reduce the cytotoxicity. Following the synthesis of the vinyl‐terminated polycaprolactone, its reaction with fumaryl chloride (FuCl) results in a divinyl‐fumarate polycaprolactone (VPCLF). The polymers obtained were thoroughly characterized using Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) techniques. The polymer has been successfully employed, in combination with N‐vinyl pyrrolidone (NVP), to fabricate films and computer‐designed porous scaffolds by micro‐stereolithography (μ‐SL) with gyroid and diamond architectures. Characterization of the networks indicated the influence of NVP content on the network properties. Human mesenchymal stem cells adhered and spread onto VPCLF/NVP networks showing good biological properties and no cytotoxic effect. Copyright

Laser-flash in-plane thermal analysis: The case of oriented UHMWPE

Laser-flash thermal analysis has been applied to measure the thermal diffusivity of highly oriented samples of Ultra High Molecular Weight Polyethylene. Due to the anisotropy of the sample, in-plane measurements are required instead of through-plane ones.

Relation of Structure to Electrical and Optical Properties

Most plastics materials may be considered as electrical insulators, as they are able to withstand a potential difference between different points of a given piece of material with the passage of only a small electric current and a low dissipation energy. A notable and more recent exception is constituted by intrinsically conductive polymers, which are able to reach conductivities similar to metals. When assessing a polymeric material for its use in an electrical application, information on some properties will be required, such as dielectric constant/power factor, dielectric strength, volume resistivity, surface resistivity, tracking, and arc resistance. The chapter gives an overview of the most important aspects of electrical properties of polymers, including some new applications in the field. The optical properties of plastics are also considered in this chapter. How the structure of a plastic affects transparency and the significance of refractive index and birefringence are discussed. The influences on surface appearances and color are explored. How each of these aspects is quantified is also included.

High-toughness carbon cloth composites for low temperature applications

Carbon Fibre Reinforced Polymers based on a thermoplastic, high performance matrix such as Ultra High Molecular Weight Polyethylene have been produced using two different routes and it was found that in-situ polymerization of the matrix is a possible way forward to achieve a combination of high strength and high toughness in composites.