UNIVERSIDAD DE CHILE FACULTA FACULTAD D DE CIENCIA CIENCIAS S FÍSICA FÍSICAS S Y MATEM MATEMÁTI ÁTICAS CAS DEPART DEP ARTAME AMENTO NTO DE INGE INGENIE NIERIA RIA QUÍMIC QUÍMICA A Y BIOTEC BIOTECNLO NLOGÍA GÍA
ESTUDI ESTUDIO O DE LA POLI POLIMER MERIZA IZACIÓ CIÓN N DE ETIL ETILENO ENO EN SIST SISTEMA EMAS S HOMOGÉ HOMOGÉNEO NEOS S Y HETEROGÉNEOS
MEMO ME MORI RIA A PARA PARA OPTA OPTAR R AL TÍT TÍTUL ULO O DE INGENI INGENIERO ERO CIVIL CIVIL QUÍMIC QUÍMICO O I
KATHER KATHERINE INE ANDREA ANDREA DELGAD DELGADO O VARGAS VARGAS
PROFESOR GUÍA: DR. RAÚL QUIJADA QUIJADA ABARCA MIEMBR MIEMBROS OS DE LA COMISI COMISIÓN: ÓN: DR. FRANCIS FRANCISCO CO GRACIA GRACIA CAROCA CAROCA DR. HUMBERTO PALZA CORDERO CORDERO
SANTIA SANTIAGO GO DE CHILE CHILE ABRIL 2007
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INDICE DE CONTENIDOS 1
INTRODUCCIÓN ..................................................... ................................................................................................................. ............................................................ 2 1.1 1.2
POLIETILENO .................................................................................................................................4 TIPOS DE POLIETILENO .................................................................................................................4
1.2.1
Procesos de Polimerización Polimerización de Etileno ............................ .......................................... ........................... ........................... ............................ ............................ .................. 6
1.2.1.1 1.2.1.2 1.2.1.3
1.3
SISTEMA CATALÍTICO ...................................................................................................................8
1.3.1 1.3.2 1.3.3 1.3.4 1.3.5
Evolución de los Catalizadores Ziegler-Natta ..................................... ................................................... ............................ ............................ ....................... .........8 Catalizadores Metalocenos ........................... ........................................ ........................... ............................ ............................ ........................... ........................... ..................... .......9 Cocatalizador Metilaluminoxano Metilaluminoxano (MAO) .......................... ........................................ ............................ ............................ ........................... ......................... ............12 Mecanismo de Polimerización Polimerización de Sistemas Metaloceno/MAO........................... ......................................... ............................ ................... ..... 13 Sistemas Catalíticos Heterogéneos (catalizadores metalocenos soportados)............................ ....................................... ............15
1.3.5.1 1.3.5.2
1.4
2
MOTIVACIÓN Y ALCANCES ......................................................................................................... 21 OBJETIVO GENERAL .................................................................................................................... 24 OBJETIVOS ESPECÍFICOS ............................................................................................................. 24
PARTE EXPERIMENTAL ........................................................... ................................................................................................ ..................................... 25 3.1 3.1.1 3.1.2
3.2 3.3 3.3.1 3.3.2 3.3.3
3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5
3.5 3.5.1 3.5.2 3.5.3
3.6 3.6.1 3.6.2
4
Sílica gel como sopo rte ................................................. .............................................................................. .......................................................... ................................................ ................... 16 Modificación del Soporte ........................................................ ..................................................................................... ........................................................... ....................................... ......... 17
OBJETIVOS ........................................................................................................................ 24 2.1 2.2
3
Proceso de Polimerización Polimerización en Fase Slurry .......................................................... ........................................................................................ ......................................... ........... 6 Proceso de Polimerización Polimerización en Fase Gas ......................................................... ....................................................................................... .............................................. ................ 7 Proceso de Polimerización Polimerización en Solución ...................................................... ................................................................................... .................................................. ..................... 8
R EACTIVOS EACTIVOS, MATERIALES Y EQUIPOS ........................................................................................ 25 Reactivos ........................... ......................................... ............................ ........................... ........................... ............................ ............................ ........................... ........................... ................... ..... 25 Materiales y Equipos ........................... ......................................... ............................ ............................ ........................... ........................... ............................ ............................ ..............25
TRATAMIENTO PREVIO DE R EACTIVOS ................. .................................... ............................ ......... 26 EACTIVOS Y MATERIALES.................................. PREPARACIÓN DE LOS SISTEMAS SOPORTADOS.......................................................................... 27 Preparación de la Sílica utilizada como Soporte ............................ .......................................... ........................... ........................... ............................ ..............27 Metaloceno inmovilizado sobre Sílica............................. ........................................... ........................... ........................... ............................ ............................ ..............27 Metaloceno inmovilizado sobre Sílica modificada modificada ........................... ......................................... ............................ ............................ ....................... .........27
MÉTODOS DE POLIMERIZACIÓN .................................................................................................. 28 Pretratamiento Pretratamiento sílicas funcionalizadas.................................... ................................................. ........................... ............................ ............................ ..................... .......29 Polimerización Polimerización Homogénea........................... ........................................ ........................... ............................ ............................ ........................... ........................... ................... ..... 29 Polimerización Polimerización In Situ ............................ .......................................... ........................... ........................... ............................ ............................ ........................... ......................... ............29 Polimerización Polimerización Precontacto ........................... ........................................ ........................... ............................ ............................ ........................... ........................... ................... ..... 30 Polimerización Polimerización Heterogénea .......................... ....................................... ........................... ............................ ............................ ........................... ........................... ................... ..... 30
CARACTERIZACIÓN DE LOS SOPORTES ....................................................................................... 31 Determinación de la superficie del Soporte (BET) ..................... ................................... ............................ ............................ ........................... ................ ... 31 Espectroscopía Infrarroja de Reflectancia Difusa Di fusa (DRIFT)........................... (DRIFT)......................................... ........................... ......................... ............31 Acoplamiento de Plasma Inductivo (ICP) ............................ ......................................... ........................... ............................ ............................ ....................... .........31
CARACTERIZACIÓN DE LOS POLÍMEROS ..................................................................................... 31 Cromatografía de Permeación de Geles (GPC) ...................................... .................................................... ........................... ........................... ................... ..... 31 Calorimetría Calorimetría Diferencia de Barrido (DSC) ............................... ............................................. ........................... ........................... ............................ ................... ..... 32
RESULTADOS Y DISCUSIONES .................................................................................... 33 4.1 4.2
SISTEMAS HOMOGÉNEOS ............................................................................................................ 34 SISTEMAS HETEROGÉNEOS ......................................................................................................... 37
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LISTADO DE ABREVIATURAS Y SIMBOLOS Cp
: ciclopentadienilo (C5H5-)
DRIFT
: espectroscopía infrarroja de reflectancia difusa
DSC
: calorimetría diferencial de barrido
Et
: etil (C2H5)
Flu
: fluorenilo (C13H9-)
GPC
: cromatografía de permeación de geles
HDPE
: polietileno de alta densidad
ICI
: Imperial Chemical Industries
ICP
: acoplamiento de plasma inductivo
Ind
: indenilo (C9H7-)
LDPE
: polietileno de baja densidad
LLDPE
: polietileno lineal de baja densidad
MAO
: metilaluminoxano
Me
: metil (CH3)
Mn
: peso molecular promedio en número
M
: metal
Mw
: peso molecular promedio en peso
P
: presión
PE
: polietileno
PEAD
: polietileno de alta densidad
PEBD
: polietileno de baja densidad
PELBD
: polietileno lineal de baja densidad
PO
: poliolefinas
POSS
: silsesquioxanos oligoméricos poliédricos
PP
: polipropileno
SF
: sílicas funcionalizadas
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1 INTRODUCCIÓN El desarrollo de los materiales ha sido fundamental para la modificación del comportamiento y la cultura humana a través de los tiempos, pudiéndose afirmar que los países más desarrollados son aquellos que han conseguido un mayor conocimiento y dominio de la ciencia y tecnología de los materiales. Actualmente, la complejidad de las necesidades de la sociedad ha llevado a un progresivo abandono de los materiales tradicionales y a una búsqueda de materiales con características inteligentes, así como al desarrollo de procesos de producción que mejoren la calidad, competitividad y compatibilidad con el medio ambiente. Los polímeros son materiales versátiles que combinan excelentes propiedades con ventajas significativas en el peso, procesado y precio; es por eso que los científicos han bautizado el presente siglo como la Edad de los Polímeros. Las poliolefinas (PO) constituyen una importante familia de materiales dentro de los polímeros, siendo el polietileno (PE) y polipropileno (PP) los de mayor consumo mundial, alcanzando más del 65 % de la producción de plásticos en 2005. La producción global de poliolefinas ha sido superior supe rior a 100 millones de toneladas y la demanda de esta clase de polímeros de gran consumo, denominados “commodity”, se espera que siga creciendo en los próximos años[1]. El progresivo aumento en la producción y consumo de poliolefinas se debe fundamentalmente:
A la versatilidad de sus propiedades físicas y mecánicas, y por tanto sus aplicaciones.
Su no-toxicidad y bio-aceptabilidad.
Su favorable desempeño ambiental, tanto en su producción como en su aplicación final.
El bajo costo de materia prima y de producción.
La causa del crecimiento de la industria de las poliolefinas, se basa tanto en el desarrollo de nuevos catalizadores como en los procesos de polimerización. Los grandes cambios han ocurrido tras la consecución de importantes avances científicos y tecnológicos, como se muestra en la Figura 1.
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S O C I T S S A A L P N I E F E D L L O I A L T O O P T E % D L N A E E T M U N L E O R V F
Impacto de los descubrimientos de nuevos catalizadores: 1955 Catalizador Z-N para poliolefinas 1968 Catalizadores de alto rendimiento para PE 1975 Catalizadores de alto rendimiento para PP
Impacto de las nuevas tecnologías:
1977 Proceso LLDE Union Carbide 1982 Proceso Spheripol para PP
su consumo frente al total de los plásticos plásticos [1]. Figura 1. Etapas de crecimiento de PE y PP y su
La expansión comenzó con el descubrimiento por Ziegler y Natta de los catalizadores de metales de transición para la polimerización de etileno y propileno en 1953 y 1954, por lo que ambos recibieron el premio Nobel de la Química en 1963, originando el crecimiento de la industria de las poliolefinas [2].
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1.1 Polietileno El polietileno (PE) es químicamente el polímero más simple. Este polímero se forma a partir del monómero eteno o etileno (C2H4) que por medio de un sistema catalítico en un determinado proceso se transforma en una simple cadena de carbonos e hidrógenos, como se muestra en la Figura 2.
Figura 2. Polimerización de etileno. El polietileno es un material termoplástico, es decir, se ablanda al calentarse y se endurece al enfriarse, y apolar, lo cual le concede una característica hidrofóbica. La mayoría de los polietilenos son parcialmente cristalinos, con regiones re giones cristalinas dispersas dispers as en un material amorfo (no cristalino). Tienen buena resistencia química y procesabilidad, son buenos aislantes eléctricos, y tiene cierto grado de flexibilidad y transparencia [3]. El polietileno tiene una amplia gama de aplicaciones debido a las diferentes propiedades físicas y mecánicas que adquiere dependiendo de las condiciones de operación del proceso, o bien, de los distintos procesos que existen para obtenerlo. ob tenerlo.
1.2 Tipos de Polietileno Las propiedades del polietileno varían ampliamente para diferentes grados de ramificación, se entiende como polímero ramificado aquel que tiene cadenas secundarias unidas a una cadena principal. La densidad del polietileno depende del grado de ramificación y sus propiedades mecánicas dependen del grado y tipo de ramificaciones, la estructura cristalina y el peso molecular [4]. Los tipos de PE más conocidos son:
Poliet Polieti leno leno de B aja Densida Densidad d (PE (P E B D o L D PE ): Este polietileno se caracteriza por presentar un alto grado de ramificaciones lo cual no permite un adecuado ordenamiento de las cadenas en la molécula, molécula, por ello el polietileno de baja densidad es suave y flexible. Sus aplicaciones van desde las bolsas plásticas, hasta el aislamiento aislamiento eléctrico. Se obtiene de polimerización vía radical libre condiciones de alta presión
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aplicaciones donde la rigidez es importante, como en tuberías plásticas. Se sintetiza en procesos de catálisis que operan ope ran a bajas presiones (< 100 atm.) y temperaturas entre 60° C y 70° C. Poliet Polieti leno leno L i nea neal de de B aja Densida Densidad d ( PE L B D o L L D PE ): Este polietileno se produce en procesos similares al polietileno de alta densidad, pero copolimerizando etileno con α-olefinas más largas, especialmente 1-buteno, 1-hexeno y 1-octeno.
La Figura 3 y la Tabla 1 muestran las estructuras de los principales tipos de polietileno y sus características generales, respectivamente.
Figura 3. Diferentes tipos de polietileno.
Tabla 1. Características generales de distintos tipos de polietileno TIPOS DE POLIETILENO CARACTERISTICAS Densidad [Kg/m ]
PEBD
PEAD
PELBD
910-930
940-970
890-930
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1.2.1 Procesos de Polimerización de Etileno En los últimos 20 años los catalizadores heterogéneos han sido claves en el desarrollo de procesos sencillos de polimerización que han dado lugar a la síntesis de poliolefinas para requerimientos específicos. Para la polimerización de etileno a nivel industrial existen distintas tecnologías que requieren catalizadores y condiciones de operación específicas. 1.2.1.1 Proceso de Polimerización en Fase Slurry
La polimerización en fase slurry tiene como características principal tener tres fases que forman una suspensión. El monómero está en fase gas, el solvente y el catalizador (en el caso que no sea soportado) se encuentran en fase líquida y finalmente el polímero a medida que se va formando precipita y se encuentra en fase sólida. El polímero formado es insoluble en el medio de reacción, así, cuando la polimerización procede, se forma una suspensión de polímero. La reacción ocurre dentro de un reactor continuamente agitado, la agitación debe garantizar la dispersión adecuada del monómero, y de las partículas en suspensiones. El proceso opera con temperaturas entre 70° C y 100° C; y las presiones son desorden de 35 atm. El polímero formado puede ser removido por decantación, d ecantación, centrifugación o filtración. En el proceso Solvay ( Figura Ziegler – Natta, Natta, activados con Figura 4 ) se utilizan catalizadores Ziegler compuestos organoaluminio.
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reaccionado se separa, comprime y recicla. Después de separar y secar el polímero, éste se obtiene en forma de polvo. 1.2.1.2 Proceso de Polimerización en Fase Gas
Se utiliza monómero en fase gas con catalizadores Ziegler-Natta, de muy alta eficiencia, en forma de polvo seco. El etileno gaseoso y el catalizador se introducen continuamente en un reactor de lecho fluidizado. En el proceso Union Carbide ( Figura Figura 5) el catalizador es un derivado de cromo, soportado sobre sílica, que en forma de polvo fino se introduce a un reactor de lecho fluidizado junto con el etileno; se puede adicionar comonómero. La reacción ocurre a 20 atm. y a temperatura controlada entre 85 y 100 ºC, dependiendo del producto deseado. Como se requiere gran cantidad de gas para fluidizar las partículas de polietileno sólido en el fondo del reactor, sólo el 2 ó 3% del etileno reacciona por cada pasada. Después de pasar por la zona de reacción, el gas reduce su velocidad pasando a una sección de mayor área, permitiendo la sedimentación de las partículas. El etileno se recicla a través de un compresor.
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1.2.1.3 Proceso de Polimerización en Solución
Además del monómero y el catalizador, se utiliza un solvente común para estos agentes y el polímero, por lo que la reacción ocurre en un medio homogéneo. El catalizador es un óxido de cromo o molibdeno, soportado en sílica o alúmina. La actividad es buena (50 kg polímero/g catalizador) y no es necesario remover los residuos de catalizador. El etileno se introduce al reactor (tipo autoclave) con ciclohexano o benceno. La presión se eleva a unos 10 atm. y se introduce el catalizador en forma de slurry. La reacción ocurre a unos 200 ºC , el polímero se forma en la superficie del catalizador y se disuelve inmediatamente en el solvente. El polímero se precipita por enfriamiento y se seca en un secador rotatorio.
1.3 Sistema Catalítico El sistema catalítico catalítico utilizado para la homopolimerización y copolimerización de etileno etileno con α – olefinas olefinas está constituido por un catalizador y un cocatalizador. La constante búsqueda de nuevos catalizadores que mejoren el proceso de polimerización de olefinas, ha llevado a la generación de diversos sistemas catalíticos. Actualmente los más utilizados utilizados para este este tipo de polimerizaciones son los catalizadores metalocenos junto con el cocatalizador metilaluminoxano (MAO).
1.3.1 Evolución de los Catalizadores Ziegler-Natta En los años 50, Kart Ziegler y Giulio Natta descubrieron un sistema catalítico eficiente para la producción de α-olefinas. Los catalizadores Ziegler-Natta eran compuestos basados en sales de metales de transición como TiCl3 y TiCl4, que en presencia de un cocatalizador de alquilaluminio permiten la síntesis de las poliolefinas en condiciones no extremas de presión y temperatura, además de conseguir un cierto control de la morfología y estructura estereoquímica (estereoregularidad o control estereoespecífico); y un considerable aumento de la actividad.
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Catalizadores de 3 generación (década del 70): Soportados sobre un material inorgánico
(inerte) como sílica, alúmina, oxido de calcio o compuestos de magnesio. Para mejorar la tacticidad fueron usados donores de electrones electrones tales como aminas, fosfinas, ésteres y cetonas. Estos catalizadores presentan una altísima actividad actividad catalítica y un control casi completo de la estereoregularidad y linealidad de PEAD [7]. ta
Catalizadores de 4 generación (década del 70): Soportados sobre partículas esféricas de MgCl2
o SiO2. Por un fenómeno de réplica, las características morfológicas del catalizador son reproducidas permitiendo un control morfológico del polímero formado. De esta forma, la actividad del catalizador aumenta a 500.000 g de polímero/g de catalizador.
A pesar de las ventajas que presentaban los catalizadores Ziegler-Natta, se observó que los centros activos de éstos no eran uniformes, de modo que presentaban distintas reactividades, que impedían la obtención de polímeros con cadenas de longitudes similares. Se obtenían, por tanto, las distribuciones de pesos moleculares variables y anchas , y un limitado control de la microestructura y de las propiedades del polímero [8-10]. En el caso de copolímeros, la presencia de centros con diferente actividad conducía a una mayor concentración de comonómero en pesos moleculares bajos y, por tanto, a materiales muy heterogéneos en cuanto a composición [10,11]. En un intento por resolver los inconvenientes de los catalizadores Ziegler-Natta, se desarrolló en los años 80 una nueva familia de catalizadores Ziegler- Natta solubles denominada metalocénicos (los catalizadores Ziegler-Natta están constituidos por sólidos insolubles como TiCl3, y son por tanto catalizadores heterogéneos). Este impulso se debe a Kaminsky, Sinn y colaboradores [12-14], que descubrieron que la hidrólisis controlada de trimetilaluminio produce un ácido de Lewis denominado metilaluminoxano (MAO), que junto a estos catalizadores genera un sistema catalítico 10 a 100 veces más activo que los tradicionales Ziegler-Natta.
1.3.2 Catalizadores Metalocenos Los catalizadores denominados metalocenos son complejos organometálicos tipo “ sandwich” sandwich” ( Figura Figura 6 ), ), formados por metales de transición del grupo IVB (Ti,Zr,Hf), donde el metal se encuentra normalmente ligado a dos anillos aromáticos, tipo ciclopentadienilo ( C5H5 _ o Cp) y derivados como indenilos (C H o Ind) o fluorenilos(C H o Flu), pudiendo estar estos
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A diferencia de los catalizadores Ziegler-Natta de múltiples sitios activos, este tipo de catalizador presenta una alta eficiencia catalítica con un único tipo de centros activos Figura ( 7 ), ), es decir, todos sus sitios son equivalentes en reactividad, comportándose de modo idéntico durante la polimerización. Este hecho permite obtener distribuciones estrechas de pesos moleculares, un buen control estereoespecífico en las poli(α-olefnas) e incorporaciones homogéneas del comonómero, lo que ha permitido obtener nuevos tipos de materiales que no son accesibles con los catalizadores Ziegler-Natta tradicionales [15,16]. Entre ellos están los homopolímeros y copolímeros de ciclo-olefinas [17] y los copolímeros de etileno y/o propileno con α-olefinas de cadena larga. larga.
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Figura 8. Estructuras típicas de los circonocenos.
Además, los distintos tipos de ligandos, el tamaño de éstos y la existencia de puentes uniendo los dos anillos que constituyen el catalizador establecen diferentes características en el polímero [19]. La capacidad de producir poliolefinas con diferentes tipos de microestructuras, es decir, alta estereoregularidad, proviene principalmente de su simetría, limitación que presentan los catalizadores Ziegler- Natta. En la Figura 9 se relacionan diferentes tipos de catalizadores metalocenos con las estructuras de los polímeros polímeros conseguidos (únicamente polímeros a partir de α-olefinas).
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Estos catalizadores necesitan un cocatalizador que los active, el el metilaluminoxano que produce un sistema más activo y permite alquilar el metaloceno, eliminar impurezas impurez as y reactivar las especies durmientes o inactivas [20].
1.3.3 Cocatalizador Metilaluminoxano (MAO) Los cocatalizadores del tipo aluminoxanos aumentan la eficiencia del sistema catalítico del tipo metaloceno, el metilaluminoxano (MAO) presenta la mayor eficiencia como cocatalizador, siendo el preferido para las polimerizaciones de olefinas [21].Se obtiene por hidrólisis controlada de trimetilaluminio (AlCH3), pero su estructura es compleja [22] y su composición no se conoce muy bien. La estructura aceptada es (CH3AlO)n, en formas lineales o cíclicas [23], con n = 4, 6, 8,12. La Figura 10 muestra algunas de las estructuras propuestas para el MAO.
Figura 10. Estructura del MAO, en sus formas lineal y cíclica.
Para los catalizadores metalocenos homogéneos se requiere una gran cantidad de MAO
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1.3.4 Mecanismo Mecanismo de Polimerización de Sistemas Metaloceno/MAO Aunque se ha establecido un mecanismo de polimerización generalizado para la mayoría de los metalocenos [25], esté no siempre proporciona una explicación global y satisfactoria ya que existen numerosos factores implicados implicados en el proceso de polimerización, como son la solubilidad del metaloceno, la presencia y tipo del cocatalizador [26], el estado físico de los monómeros, y/o el tipo de polimerización (en fase slurry, fase gas o solución) [27]. Durante la polimerización los ligandos ciclopentadienilo permanecen unidos al metal de transición, y definen la estereoselectividad y actividad del catalizador [28]. El mecanismo propuesto de polimerización [25-27,29] se basa, a grandes rasgos, en la formación de un centro activo a través de la reacción entre el catalizador y el cocatalizador, posterior crecimiento de la cadena por complejación e inserción de unidades del monómero y, finamente, terminación terminación de la polimerización por diversos mecanismos de transferencia, especialmente, las reacciones de eliminación β de hidrógeno que interrumpen el crecimiento de la cadena.
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Figura 12. Mecanismo de propagación.
La olefina se coordina en el sitio vacante del metaloceno, generando un estado de transición, el cual es una especie de cuatro centros donde la olefina se comienza a adicionar en la cadena del polímero. En seguida viene la incorporación de la olefina en la cadena y regeneración del sitio vacante [30].
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Figura 14.Transferencia de cadena por monómero; R , R’: grupo alquil o H, P: cadena polimérica.
La transferencia de cadena por monómero sucede cuando la eliminación hidrógeno y la inserción de la olefina en el centro activo ocurren simultáneamente, sin la formación de una ligación M-H. La especia metálica formada puede actuar como un centro activo de una nueva polimerización.
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Los beneficios más significativos que implica el soportar son:
Control de la relación cocatalizador/catalizador y, por ello, un menor consumo del MAO requerido para obtener una máxima actividad, por tanto, menor costo. Impedimento de la adhesión del polímero a las paredes del reactor, y con él un mejor control cinético de la reacción y la difusión.
Obtención de pesos moleculares mayores.
Control de la morfología final del polímero formado.
Sin embrago, los sistemas soportados exhiben algunos inconvenientes, siendo uno de los más importantes la baja actividad que presentan en comparación con los catalizadores nosoportados [34], debido probablemente a la falta de cocatalizador (MAO) que alquile al
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Los grupos silanol que cubren la superficie de la sílica gel (concentración máxima de 8 grupos OH por nm2) [49] podrían reaccionar con los complejos organometálicos Figura (Figura 16 ), ), es decir, con los catalizadores metalocenos formando enlaces covalentes.
Superficie Sílica Figura 16. Interacción circonoceno-sílica
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Los grupos metóxidos (OMe o OCH3) del silano tienen mayor facilidad de hidrolizar en comparación con los grupos etóxidos (OEt o OC2H5) del TEOS, porque el grado grado de hidrólisis y la velocidad del proceso dependen del tamaño de las partículas [63]. Por esta razón, se incorporó la etapa de prehidrolizado del TEOS en la síntesis de SS2. Así, en la etapa de condensación existe una mayor probabilidad que uno de los grupos SiOH que interactúan, para generar el enlace Si-O-Si, provenga del silano hidrolizado y el otro SiOH proceda del TEOS hidrolizado, formando estructuras más homogéneas. Las estructuras de estas sílicas funcionalizadas no están determinadas, pero estudios indican que altas razones molares de agua favorecen la hidrolización, facilitando el entrecruzamiento de cadenas poliméricas y produciendo una red tridimensional de unidades tetraédricas interconectada de siloxanos (-Si-O-Si-), mientras que bajas razones molares de agua agua impiden la hidrolización completa, conduciendo a estructuras oligoméricas [50].Ambos comportamientos son posibles en una síntesis, generando estructuras diferentes en fracciones desiguales.
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El grupo R de las unidades tetraédricas se esquematiza en la Figura 20.
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1.4 Motivación y Alcances Las poliolefinas, en general, y el polietileno en particular, están en continuo desarrollo y su potencial se encuentra aún en expansión debido, entre otras características, a su versatilidad, su biocompatibilidad, ser ambientalmente amigables y su excelente balance b alance costo- producción pro ducción [1]. Por otro lado, los sistemas metaloceno/MAO están comenzando a incrementar su importancia en
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(nBuCp) Zr Cl
(nBuCp) Cl
H
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Se escogió el catalizador (n-BuCp)2ZrCl2, porque ha sido ampliamente utilizado en homopolimerización [52] y copolimerización de etileno [53,54] , reportando alta actividad en comparación con otros catalizadores circonocenos comerciales. Incluso en publicaciones recientes [55,56] se estudia su desempeño como catalizador para polimerización de etileno en diversas condiciones experimentales.
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2 OBJETIVOS
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