Rocco Di Girolamo

Copolymerization of CO2 and meso epoxides using enantioselective β-diiminate catalysts: a route to highly isotactic polycarbonates

We report a new class of catalysts for the enantioselective copolymerization of CO2 and alicyclic meso epoxides. C1-Symmetric β-diiminate zinc catalysts were designed on the basis of mechanistic information and subsequently optimized using structure–activity relationships and iterative ligand design. The optimized catalysts yield highly isotactic poly(cyclohexene carbonate) and poly(cyclopentene carbonate) with units of up to 94% ee under mild conditions. A diblock copolymer of CO2/cyclohexene oxide and CO2/cyclopentene oxide was synthesized, demonstrating the controlled behaviour of the catalyst. Solid-state structures and solution-state dynamics of the catalyst were studied to explain trends in enantioselectivities and turnover frequencies. A strong correlation between the isotacticity of the polymer and its melting temperature was observed. X-ray powder diffraction was used to measure crystallinity and study the changes in morphology observed upon annealing.

Enabling Strategies in Organic Electronics Using Ordered Block Copolymer Nanostructures

Today’s lithographic techniques for carving silicon into circuit patterns are unable to achieve the future target of the semiconductor industry of fabricating ultrahigh density memory devices made of memory cells just few tens of nanometers apart. [ 1 ] The primary metric for gauging progress in the various semiconductor integrated circuit technologies is, indeed, the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. The circuit components on today’s silicon chips are more than 100 nm across and modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 μ m 2 . [ 2 ] Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies have no known solution. [ 2 ] Promising ingredients for advances in integrated circuit technology are nanowires, [ 3 ] molecular electronics [ 4 ] and defecttolerant architectures, [ 5 ] as demonstrated by reports of single devices [ 6–8 ] and small circuits. [ 9 , 10 ] Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. The need for very high bit density (the number of memory elements per square centimeter) has pushed the research towards the study of new advanced materials that can overcome these limiting scaling diffi culties and of alternative methods for building memory devices from the bottom up using individual molecules. [ 1 ] These methods start with atoms and molecules and climb up to nanostructures through assembly by various mechanisms of molecular recognition. Self-assembly is emerging as an elegant bottom-up method for fabricating nanostructured materials. [ 11–15 ] Particularly attractive is the self-assembly of organic molecules that, when combined with

Combining polyethylene and polypropylene: Enhanced performance with PE/iPP multiblock polymers

How to make opposites compatible Polyethylene (PE) and isotactic polypropylene (iPP) are the two most widely used commodity plastics and thus make up a large fraction of the waste stream. However, the two plastics will not mix together, which limits options for dealing with mixed waste and decreases the value of recycled products. Eagan et al. report the synthesis of multiblock copolymers of iPP and PE by using a selective polymer initiator (see the Perspective by Creton). The high-molecular-weig ht blocks could be used to reinforce the interface between iPP and PE and allow blending of the two polymers. Science, this issue p. 814; see also p. 797 Polyethylene/isotactic polypropylene multiblock copolymers enable welding of composites of the two immiscible polymers. Polyethylene (PE) and isotactic polypropylene (iPP) constitute nearly two-thirds of the world’s plastic. Despite their similar hydrocarbon makeup, the polymers are immiscible with one another. Thus, common grades of PE and iPP do not adhere or blend, creating challenges for recycling these materials. We synthesized PE/iPP multiblock copolymers using an isoselective alkene polymerization initiator. These polymers can weld common grades of commercial PE and iPP together, depending on the molecular weights and architecture of the block copolymers. Interfacial compatibilization of phase-separated PE and iPP with tetrablock copolymers enables morphological control, transforming brittle materials into mechanically tough blends.

Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water Solutions by Small-Angle Neutron Scattering

Graphical Abstract Aqueous poly(vinyl alcohol) (PVA) solutions subjected to cryogenic treatment form strong physical gels. The cryogenic treatment basically consists of freezing an initially homogeneous polymer solution at low temperatures, storing in the frozen state for a definite time, and defrosting. These gels are of great interest for biotechnology, medicine, the food industry, and many other applications. The outstanding properties of these systems depend on a complex macroporous architecture, whereby PVA chains and water molecules are organized over different hierarchical length scales. The structure and the principal processes subtending the formation of these systems are discussed in the framework of our current understanding of polymer gels. These processes involve formation of ice crystals, PVA crystallization, liquid–liquid phase separation, hydrogen bonding, and entanglements. Small angle neutron scattering is used to follow the cryotropic gelation of PVA/water solutions and detailed information is extracted concerning the gelation mechanism and kinetic parameters related to the formation of these complex systems. Open image in new window

Small Angle X-ray Scattering Investigation of Norbornene-Terminated Syndiotactic Polypropylene and Corresponding Comb-Like Poly(macromonomer)

The structure and crystallization properties of a norbornene end-functionalized syndiotactic polypropylene (sPP) macromonomer (MMsPP) with [rrrr] = 80 mol % and molecular mass Mn = 3600 g/mol and corresponding comb-like poly(macromonomer) (pMMsPP) with Mn = 100000 g/mol have been investigated using wide angle (WAXS) and small angle (SAXS) X-ray scattering, atomic force (AFM), transmission electron (TEM), and optical (OM) microscopy. The analysis reveals the tendency of the macromonomer and poly(macromonomer) to form structures characterized by layers of sPP chains alternated to layers occupied by the cyclic groups (norbornene or polynorbornene), and a small degree of interdigitation of sPP chains belonging to adjacent layers facing each other in an end-to-end arrangement. It is argued that this layering is dictated, upon crystallization, by nanophase separation of the flexible sPP chains in regions apart from those occupied by the cyclic groups coupled to formation of sPP crystals organized in the lamellar morphology. SAXS measurements indicate that in isothermally crystallized samples the layered structures are irregular and include sPP lamellar crystals of uniform thickness separated by amorphous layers whose thickness values have a multimodal distribution. These properties are intrinsic to the chemical structure of our systems and are only minimally influenced by the topological constrains of the covalent bonding of macromonomers in the corresponding comb-polymer.

pH driven fibrillar aggregation of the super-sweet protein Y65R-MNEI: A step-by-step structural analysis

BACKGROUND MNEI and its variant Y65R-MNEI are sweet proteins with potential applications as sweeteners in food industry. Also, they are often used as model systems for folding and aggregation studies. METHODS X-ray crystallography was used to structurally characterize Y65R-MNEI at five different pHs, while circular dichroism and fluorescence spectroscopy were used to study their thermal and chemical stability. ThT assay and AFM were used for studying the kinetics of aggregation and morphology of the aggregates. RESULTS Crystal structures of Y65R-MNEI revealed the existence of a dimer in the asymmetric unit, which, depending on the pH, assumes either an open or a closed conformation. The pH dramatically affects kinetics of formation and morphology of the aggregates: both MNEI and Y65R-MNEI form fibrils at acidic pH while amorphous aggregates are observed at neutral pH. CONCLUSIONS The mutation Y65R induces structural modifications at the C-terminal region of the protein, which account for the decreased stability of the mutant when compared to MNEI. Furthermore, the pH-dependent conformation of the Y65R-MNEI dimer may explain the different type of aggregates formed as a function of pH. GENERAL SIGNIFICANCE The investigation of the structural bases of aggregation gets us closer to the possibility of controlling such process, either by tuning the physicochemical environmental parameters or by site directed mutagenesis. This knowledge is helpful to expand the range of stability of proteins with potential industrial applications, such as MNEI and its mutant Y65R-MNEI, which should ideally preserve their structure and soluble state through a wide array of conditions.

Chirality, entropy and crystallization in polymers: isotactic poly(3-methyl-1-pentene) as an example of influence of chirality and entropy on the crystal structure

We report the synthesis and the crystal structure of isotactic poly((R,S)-3-methyl-1-pentene) (iP(R,S)3MP). A purely random achiral copolymer of two enantiomeric (R) and (S) 3-methyl-1-pentene monomers cannot be obtained by polymerization of the racemic mixture of the (R) and (S) monomers but has been obtained by stereospecific polymerization of 3-methyl-1,3-pentadiene to isotactic 1,2-poly(3-methyl-1,3-pentadiene) (iP3MPD12) and successive hydrogenation. The crystal structures of achiral iP(R,S)3MP and chiral poly((S)-3-methyl-1-pentene) (iP(S)3MP) are different, indicating that crystal structures of polymers may be driven by different entropic effects related to chirality and type of crystal disorder. The conformational disorder of the chiral lateral groups prevails in chiral iP(S)3MP, inducing crystallization of chains of one helical chirality, whereas the entropic effect due to the statistical substitution of chains of different helical chirality in the sites of the lattice prevails in achiral iP(R,S)3MP.

Nanometal Skin of Plasmonic Heterostructures for Highly Efficient Near-Field Scattering Probes.

In this work, atomic force microscopy probes are functionalized by virtue of self-assembling monolayers of block copolymer (BCP) micelles loaded either with clusters of silver nanoparticles or bimetallic heterostructures consisting of mixed species of silver and gold nanoparticles. The resulting self-organized patterns allow coating the tips with a sort of nanometal skin made of geometrically confined nanoislands. This approach favors the reproducible engineering and tuning of the plasmonic properties of the resulting structured tip by varying the nanometal loading of the micelles. The newly conceived tips are applied for experiments of tip-enhanced Raman scattering (TERS) spectroscopy and scattering-type scanning near-field optical microscopy (s-SNOM). TERS and s-SNOM probe characterizations on several standard Raman analytes and patterned nanostructures demonstrate excellent enhancement factor with the possibility of fast scanning and spatial resolution <12 nm. In fact, each metal nanoisland consists of a multiscale heterostructure that favors large scattering and near-field amplification. Then, we verify the tips to allow challenging nongap-TER spectroscopy on thick biosamples. Our approach introduces a synergistic chemical functionalization of the tips for versatile inclusion and delivery of plasmonic nanoparticles at the tip apex, which may promote the tuning of the plasmonic properties, a large enhancement, and the possibility of adding new degrees of freedom for tip functionalization.

Tailoring the properties of polypropylene in the polymerization reactor using polymeric nucleating agents as prepolymers on the Ziegler–Natta catalyst granule

Innovation of the Ziegler–Natta polymerization process for polymerization of propene that allows tailoring of the mechanical properties of isotactic polypropylene (iPP) directly in the polymerization stage is presented. The catalyst is modified by prepolymerization of trimethylallylsilane or vinylcyclohexane so that the catalyst particles are coated by a thin skin of poly(trimethylallylsilane) or poly(vinylcyclohexane) that will act as a nucleating agent for the crystallization of iPP. The presence of the nucleating agent accelerates the crystallization of iPP and affords crystallization of the α form even upon fast crystallization by quenching the melt, a condition that generally produces crystallization of the mesomorphic form. Crystals of the α form so obtained show a nodular morphology and the absence of the spherulitic superstructure. This novel iPP material is characterized by outstanding and unexpected properties of high mechanical strength and modulus and contemporarily high ductility, flexibility and good transparency due to the nodular morphology of the α form.

Relationship Between Molecular Configuration and Stress-Induced Phase Transitions

In semi-crystalline polymers, polymorphic transitions between different crystalline forms can be triggered not only by thermal treatments but also by mechanical deformation. The transformations related to phase changes of the crystals, and those occurring at lamellar length scales by the effect of tensile deformation are studied in detail, focusing on a set of isotactic polypropylene samples, having a molecular mass in the range 100–200 kg/mol, polydispersity index close to two, and different concentration stereodefects along the chains with a uniform distribution. This enables the effect of the microstructure of the chains on the deformation behaviour to be illustrated. The transformations which occur by effect of deformation are followed in real time during stretching through wide and small angle X-ray scattering measurements, made using the high flux of X-rays available at a Synchrotron light sources. The stream of data obtained in the measurements performed in continuum are analysed in the framework of our current understanding of the deformation mechanism of semi-crystalline polymers. This study shows that, during the transformations of a spherulitic morphology into a fibrillar morphology, the stress-induced phase transitions which occur during plastic deformation are regulated by the same factors that govern the textural and morphological changes that is the ability of the entangled amorphous chains to transmit the stress and the intrinsic stability of the lamellar crystals. Since the relative stability of the different polymorphic forms involved in the structural transformations and the intrinsic flexibility of the chains depend on the stereoregularity, we are able to make precise correlations between the stereoregularity of the chains, and the deformation behaviour, paving the way for understanding the material properties at molecular level.