Interfering Effects of Growing Chain Epimerization on Metallocene-Catalyzed Isotactic Propene Polymerization

The stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts results from the interplay of two competing reactions, namely isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end; therefore, for this class of homogeneous catalysts, at variance with the “classical” heterogeneous Ziegler−Natta ones, enantioselectivity and stereoselectivity are not (necessarily) coincident. In this paper, possible methods for the separate determination of these two parameters are introduced and applied to propene polymerization in the presence of the prototypical catalyst rac-ethylene−bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2. The results prove that the relatively poor stereoselectivity of this catalyst above room temperature is consequent primarily to chain epimerization; monomer insertion indeed is highly enantioselective up to at least 80 °C. Preliminary evidence for the existence of more than one epimerization mechanism is also presented; this complicates the measurements of enantioselectivity based on 13C NMR characterizations of d-labeled poly(propene)

High-Field 13C NMR Characterization of Ethene-1-13C/Propene Copolymers Prepared with Cs-Symmetric ansa-Metallocene Catalysts: A Deeper Insight into the Regio- and Stereoselectivity of Syndiotactic Propene Polymerization

In this paper, we report the results of a 150 MHz 13C NMR characterization of ethene-1- 13C/propene copolymers at low (<5 mol %) ethene content prepared in the presence of the syndiotacticselective ansa-metallocene catalyst (Me)(Ph)C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (cocatalyst, MAO). In particular, from the fine structure of the resonances of the ethene-1-13C units we conclude that the enantioselectivity of 1,2-propene insertion is substantially lower and the probability of chain back-skip substantially higher after an ethene insertion than after a propene one. Moreover, we find that the regioirregular 2,1-propene units (whose concentration is higher than claimed in the literature) are also substantially stereoirregular

Propene Polymerization Promoted by C2-Symmetric Metallocene Catalysts: From Atactic to Isotactic Polypropene in Consequence of an Isotope Effect

We studied the polymerization of propene-2-d promoted by the prototypical isotactic-specific catalyst system rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2/MAO. In this communication, we report the results of our investigation, documenting a large isotope effect on the balance between polyinsertion and epimerization and, therefore, on the stereospecificity. The mechanistic implications of such results are also discusse

Profile of nurses working in institutions where care to pregnant, parturient and puerperal women was given in the city of Sorocaba -- SP, Brazil (1999)

This study aimed at verifying the institutions where pregnant, parturient and puerperal women were given care. It also aimed at identifying the nurses who participate in such caregiving, the activities performed and the problems faced by them and their level of satisfaction at work. Seventy-four nurses were interviewed in the 35 institutions found in Sorocaba, of whom 27% were specialists and 28% were qualified obstetricians. The others had not been trained in that area. The major problems reported were the lack of skills to perform prenatal nursing consultations, shortage of staff and materials and the accumulation of. Seventy-three percent of the interviewees reported to be satisfied in relation to their jobs.El estudio tuvo como objetivo: enumerar las instituciones que atienden a las embarazadas, parturientas y puerperas; identificar las enfermeras que hacen esa atención, las tareas ejecutadas, los problemas enfrentados y el grado de satisfacción con su trabajo. En 35 instituciones fueron entrevistadas 74 enfermeras: 27% especializadas y 28% habilitadas en Obstetricia; las demás no asistieron a cursos en la especialidad. Los principales problemas encontrados fueron: la ausencia de seguridad para la realización de la consulta prenatal, la restricción de personal y de material, y la acumulación de funciones. 73% refirieron estar satisfechas con su trabajo.O estudo objetivou: alistar as instituições que assistem à gestante, parturiente e puérpera; identificar as enfermeiras que participam dessa assistência, as atividades por elas realizadas, os problemas enfrentados e o grau de satisfação no trabalho. Nas 35 instituições existentes em Sorocaba, foram entrevistadas 74 enfermeiras, sendo 27% especializadas e 28% habilitadas em Obstetrícia; as demais não realizaram nenhum curso na área. Os principais problemas relatados foram falta de segurança para realizar a consulta pré-natal, restrição de pessoal e de material e acúmulo de funções. Setenta e três por cento das entrevistadas referiram estar satisfeitas no trabalho

La carretera de Kabul

Un nuevo estudio de ACNUR investiga las motivaciones y retos asociados en la migración de jóvenes afganos a Europa

The road from Kabul

New UNHCR research investigates the motivations for and challenges associated with the migration of young Afghans to Europe

Highly Regioselective Transition-Metal-Catalyzed 1-Alkene Polymerizations: A Simple Method for the Detection and Precise Determination of Regioirregular Monomer Enchainments

The potentialities of this approach are illustrated for propene polymn. promoted by two well-known ansa-metallocene catalysts (cocatalyst: methylaluminoxane, MAO): the isotactic-specific rac-Me2Si(1-indenyl)2ZrCl2 (I), and the syndiotactic-specific (Me)(Ph)C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (II). A series of copolymn. expts. at variable ethene-1-13C-propene feeding ratios in the presence of I/MAO or II/MAO was carried out to locate the value of ethene incorporation above which "all" 2,1-propene units are followed by an ethene one. The higher regioselectivity and regioirregular sequences of the copolymers were clearly detected by 13C NMR. The 13C NMR spectra of an isotactic polypropylene prepd. with I/MAO at 10°, a copolymer of propene contg. 2.9 mol% of ethene-1-13C prepd. with I/MAO, and an ethene-1-13C-propene copolymer (ethene content 2.5 mol%) prepd. with II/MAO, were presented

Interfering Effects of Growing Chain Epimerization on Metallocene-Catalyzed Isotactic Propene Polymerization

The stereoregularity of polypropylene produced with C2-sym. group 4 ansa-metallocene catalysts results from the interplay of two competing reactions, namely isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end; therefore, for this class of homogeneous catalysts, at variance with the "classical" heterogeneous Ziegler-Natta ones, enantioselectivity and stereoselectivity are not (necessarily) coincident. In this paper, possible methods for the sep. detn. of these two parameters are introduced and applied to propene polymn. in the presence of the prototypical catalyst rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2. The results prove that the relatively poor stereoselectivity of this catalyst above room temp. is consequent primarily to chain epimerization; monomer insertion indeed is highly enantioselective up to at least 80 °C. Preliminary evidence for the existence of more than one epimerization mechanism is also presented; this complicates the measurements of enantioselectivity based on 13C NMR characterizations of d-labeled poly(propene)s