René Rojas

Synthesis and characterization of copolymers of ethylene and 1-octadecene using the rac-Et(Ind)2ZrCl2/MAO catalyst system

The copolymerization of ethylene and 1-octadecene using a bridged metallocene was studied in order to observe the effect of the comonomer on the catalytic activity. A noticeable increase in activity is seen as the concentration of 1-octadecene in the reaction medium increases. 13 C NMR analysis shows 6.4 mol-% incorporation of comonomer at the highest 1-octadecene concentration in the feed used here. The molecular weight of the copolymers shows a drastic decrease that may be attributed to chain termination by transfer or β-elimination of the comonomer. As to the molecular weight distribution, it remains within a narrow range, as expected with metallocene catalysts. The melting temperature and the enthalpy of melting of the copolymers show a decrease with increasing comonomer content. As usual for ethylene copolymers, the X-ray crystallinities are higher than those determined from the enthalpy of melting.

Study of the effect of the monomer pressure on the copolymerization of ethylene with 1‐hexene

The effect of ethylene pressure on the copolymerization of ethylene with 1-hexene was studied. The results show an increasing of productivities (g of polymer/nZr h) with pressure. This tendency was not observed for the activity (g of polymer/nZr h bar) that decreases when pressure is raised. When varing the pressure, the characteristics and properties of the formed copolymers are in accordance with the expectation for changes in the monomer concentration; increasing the pressure causes a decrease in comonomer incorporation. At higher ethylene pressure, the polymer is more crystalline due to less incorporation of 1-hexene and the molecular weight is higher. The density of the copolymers also decreases with comonomer incorporation into the copolymer

The effect of reaction parameters on catalytic activity in the polymerization of ethylene using supported and unsupported metallocene catalysts

Abstract The catalytic activities of two metallocene compounds, EtInd2ZrCl2 and Ind2ZrCl2, in the polymerization of ethylene depend heavily on certain parameters, such as Zr concentration in the reaction, type of the reactor, and stirring speed for homogeneous catalysts. One way or another, all these factors influence the catalytic activity of these systems, but catalyst concentration and stirring speed are the most important ones. In the case of heterogeneous catalysts of the same metallocenes, the most pronounced influence is related to the type of support and its porous nature. The influence of the support on activity and morphology is shown.

Study of metallocene supported on porous and nonporous silica for the polymerization of ethylene

Porous and nonporous silicas are good supports for the synthesis of coordinative heterogeneous catalysts for ethylene polymerization. The catalytic activity of nonporous Aerosil depends critically on the percentage of incorporated zirconium. The calcination of the support also affects the activity, showing the best results with nonporous silica at 1000°C, and with porous silica at 800°C.

Estudio retrospectivo de 232 casos de cáncer y precáncer de labio en pacientes chilenos: Correlación clínico-histológica

Background: The most common type of oral cancer is squamous cell carcinoma, which accounts for approximately 90% of all oral malignancies. Therefore the oral cancer problem primarily concerns the diagnosis, biology and management of squamous cell carcinoma (SCC). However, there are red and white lesions, that are considered premalignant, since they may unpredictably transform into cancer. According to WHO, leukoplakia, eritroplakia and actinic cheilitis are considered precancerous lesions which pathologically may correspond to intraepithelial dysplasia, hyperkeratosis, carcinoma in situ or SCC. Aim: To study the clinical and pathological features of lip cancer and premalignant lip lesions. Patients and methods: A retrospective analysis of 232 cases with lip lesions. The frequency, location, gender, age and pathological diagnosis of the lesions were recorded. Results: One hundred fifty four lesions were observed in men and 78 in women, 92% of lesions were located in the lower lip. Seventy two of the 232 lesions were SCC and 72% of these carcinomas were well differentiated. The age of patients with cancer was higher than that of patients with precancerous lesions. Conclusions: The difference in age between patients with cancer and precancerous lesions, reinforces the existing knowledge that precancerous lesions may transform into cancer if they are not properly diagnosed and treated (Rev Med Chile 2003; 131: 60-6)

Localized states in bistable pattern forming systems

We present a unifying description close to a spatial bifurcation of localized states, appearing as large amplitude peaks nucleating over a pattern of lower amplitude. Localized states are pinned over a lattice spontaneously generated by the system itself. We show that the phenomenon is generic and requires only the coexistence of two spatially periodic states. At the onset of the spatial bifurcation, a forced amplitude equation is derived for the critical modes, which accounts for the appearance of localized peaks.

Chimera-type states induced by local coupling.

Coupled oscillators can exhibit complex self-organization behavior such as phase turbulence, spatiotemporal intermittency, and chimera states. The latter corresponds to a coexistence of coherent and incoherent states apparently promoted by nonlocal or global coupling. Here we investigate the existence, stability properties, and bifurcation diagram of chimera-type states in a system with local coupling without different time scales. Based on a model of a chain of nonlinear oscillators coupled to adjacent neighbors, we identify the required attributes to observe these states: local coupling and bistability between a stationary and an oscillatory state close to a homoclinic bifurcation. The local coupling prevents the incoherent state from invading the coherent one, allowing concurrently the existence of a family of chimera states, which are organized by a homoclinic snaking bifurcation diagram.

Continuous description of lattice discreteness effects in front propagation

Models describing microscopic or mesoscopic phenomena in physics are inherently discrete, where the lattice spacing between fundamental components, such as in the case of atomic sites, is a fundamental physical parameter. The effect of spatial discreteness over front propagation phenomenon in an overdamped one-dimensional periodic lattice is studied. We show here that the study of front propagation leads in a discrete description to different conclusions that in the case of its, respectively, continuous description, and also that the results of the discrete model, can be inferred by effective continuous equations with a supplementary spatially periodic term that we have denominated Peierls–Nabarro drift, which describes the bifurcation diagram of the front speed, the appearance of particle-type solutions and their snaking bifurcation diagram. Numerical simulations of the discrete equation show quite good agreement with the phenomenological description.

Thermal and Mechanical Properties of Polyethylenes Synthesized with Metallocene Catalysts

A series of high density polyethylenes (HDPE) were synthesized via homogeneous polymerization with metallocene catalyst in two different reactors (glass and stainless steel). The thermal and mechanical properties of the polyethylenes, synthesized with two types of reactor and different reaction parameters, are discussed.

Optimization of a relativistic quantum mechanical engine

We present an optimal analysis for a quantum mechanical engine working between two energy baths within the framework of relativistic quantum mechanics, adopting a first-order correction. This quantum mechanical engine, with the direct energy leakage between the energy baths, consists of two adiabatic and two isoenergetic processes and uses a three-level system of two noninteracting fermions as its working substance. Assuming that the potential wall moves at a finite speed, we derive the expression of power output and, in particular, reproduce the expression for the efficiency at maximum power.