Cp2Zr(13CH3)2 (1) has been used as a probe for the reactivity of metallocene−methylaluminoxane catalysts for olefin polymerization. A 1H and 13C NMR study of the reaction equilibria between Cp2Zr(13CH3)2 and Lewis acids such as AlMe3 (2), B(C6F5)3 (3), and methylaluminoxane (MAO) (4) has been performed. AlMe3 is always present in MAO solutions, and B(C6F5)3 is a relatively strong Lewis acid, which has a capacity to form and stabilize ion pairs comparable to that of MAO. The use of isotopically 13C-enriched dimethylzirconocene has permitted the study of these systems by 13C NMR in conditions as close as possible to usual polymerization conditions, which require large excesses of MAO for reaching high activities. The comparisons of the reactivity of Cp2Zr(13CH3)2 with B(C6F5)3 and with MAO have provided the first direct evidence of the formation in solution of monomeric [Cp2Zr(13CH3)]+[Me·MAO]- (8), of dimeric [Cp2Zr(13CH3)]2(μ-13CH3)+[Me·MAO]- (7), and of the [(Cp2Zr(μ-Me)2AlMe2]+[Me·MAO]- (9) cationic species, having MeMAO- counterions. The influence of temperature, Al/Zr mole ratio, and zirconium concentration on the equilibria of ion pair formation has been elucidated.

Silica-natural rubber composites were prepared by in situ sol-gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler-filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler-rubber interactions. Both filler-rubber and strong filler-filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler-filler interaction, and it is maximum when also the filler-rubber interaction occurs. Strong silica-rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.

13 C NMR spectroscopic studies of the in situ polymerization of 13 C-enriched ethylene ( 13 C2H4) in the presence of Cp2Zr( 13 CH3)2 and methylaluminoxane or B(C6F5)3 as cocatalysts were carried out. The first direct observation of Cp2Zr-polymeryl species was made. The in situ polymerization experiments in the presence of different concentrations of dinuclear [(Cp2ZrMe)2(µ-Me)] + and mononuclear cation species [Cp2ZrMe] + , having [MeB(C6F5)3] -or [MeMAO] -counterions, were performed. These comparisons made it possible to make the assignments of the zirconocene complexes bearing the polymeryl chain as ligands [Cp2Zr 13 CH2P] + [ 13 CH3MAO] -, [Cp2Zr 13 CH2P] + [ 13 CH3B(C6F5)3] -, and Cp2Zr( 13 CH2P)( 13 CH3). They are in equilibrium with each other. Mononuclear Zr-polymeryl ion pairs such as [Cp2Zr 13 CH2P] + [ 13 CH3B(C6F5)3] -and [Cp2Zr 13 CH2P] + [ 13 CH3MAO] -either are the propagating active species or are intermediates, closely related to the active species, in the polymer propagation. IntroductionThe past decades have seen enormous advances in the design and synthesis of "well-defined" group 4 metallocenes for R-olefin polymerization. Metallocenes need to be activated by Lewis acid cocatalysts. 1 Methylaluminoxane (MAO) has been the first and is the most frequently used cocatalyst for these new "single-site" catalysts. 2 These catalytic systems, which are going to be employed in industrial processes, require the use of large excesses of MAO. Due to multiple equilibria present in MAO solutions, metallocene-MAO systems have been considered far too complex to allow identification of the species produced. One of the roles of MAO was presumed to be the formation of 14-electron metal complexes such as [Cp 2 MR] + . This assumption, based on early studies 3 and on the synthesis of stabilized group 4 metallocene ionic complexes, 4 was supported by the direct synthesis of [Cp 2 MMe] + [X] -, in which X -is a "noncoordinating" counterion such as [B(C 6 F 5 ) 4 ] -, 5 and by the observation of the interactions between Cp 2 Zr-( 13 CH 3 ) 2 and solid MAO. 6 Our interest in directly understanding the role of MAO in the polymerization activity in these homogeneous systems prompted us to make efforts to study titanocene-MAO catalysts by 13 C NMR spectroscopy. 7 More recently, we have undertaken the study of zirconocenes-MAO systems which because of their greater stability are more commonly used in this catalysis. Isotopically 13 C-enriched Cp 2 Zr( 13 CH 3 ) 2 (1) has been used as a probe for the reactivity of metallocenes with Lewis acids such as AlMe 3 , B(C 6 F 5 ) 3 , and MAO. 8 Direct evidence of the formation in solution of the mononuclear [Cp 2 ZrMe] + [MeMAO] -(2), the dinuclear [(Cp 2 ZrMe) 2 (µ-Me)] + [MeMAO] -(3), and the [Cp 2 Zr(µ-Me) 2 AlMe 2 ] + [MeMAO] -(4) cationic species, having [MeMAO] -counterions, has been provided. 3 and 4 have been proposed 1d as possible dormant states for the active sites for olefin polymerization and, thus, as possibly responsible for catalyst deactivations according to Scheme...

Silica-rubber nanocomposites were obtained by in situ sol-gel synthesis, using trialkoxysilanes with different functional groups as precursors. The functionalities were selected in order to favor the formation of differently shaped silica particles and/or to modulate the filler-filler and the filler-rubber interactions. The functional groups included (a) alkyl and alkenyl groups: triethoxy(vinyl) (VTEOS), triethoxy(propyl) (PTEOS), triethoxy (octyl) (OCTEOS); (b) N-containing alkyl groups: triethoxy(3-aminopropyl) (APTEOS), triethoxy(3-cyanopropyl) (CPTEOS), triethoxy(3-propylisocyanate) (ICPTEOS); (c) S-containing alkyl groups: trimethoxy(3-mercaptopropyl) (TMSPM), bis(3-triethoxysilylpropyl) disulfide (TESPD), bis(3-triethoxysilylpropyl) tetrasulfide (TESPT); triethoxy(3-octanoylthio-1-propyl) (NXT). Transmission electron microscopy(TEM) investigation suggested a relationship between the morphology of the filler network and the used trialkoxysilanes, as a function of the particle shape and of the interaction of the particle surface groups between them and with the matrix. The dynamic-mechanical properties of nanocomposites, both uncured and vulcanized, were discussed in relation to the network morphology, suggesting a connection between the used silica precursors and the functional properties. The filler-rubber interaction due to substituents which chemically interact with the polymer, promotes the homogeneous distribution of the silica particles in the matrix, while the filler-filler interaction, favored by the shape induced physical interactions or by the chemical interaction among surface groups, mainly contribute to the filler networking and to the dynamicmechanical properties of the composites.

SUMMARY A new family of multi-block copolymers having the structure of poly(ester-carboni*te)s was obtained by a chain-extension reaction involving poly(1actic-glycolic acid) oligomers (PLGA) and o'ligomeric a,o-bishydroxy-terminated poly(ecapro1actone)s (PCDT). The latter were first transformed into a,obis(chloroformate)s, which were subsequently condensed in the presence of amines with both the hydroxylic and the carboxylic end-groups of PLGA oligomers. Several samples differing in the length of the PCDT segments and in the composition of the PLGA segments were prepared and characterized for their physico-chemica1 properties. All of them had high molecular weight, good solubility in organic solvents, and modest swellability in aqueous media. As regards their thermal behaviour, some samples showed evidence of the presence of a crystalline phase. Since these products are potentially useful as bioerodible materials in drug delivery systems, some preliminary results on their degradation behaviour under conditions mimicking those found in biological fluids are reported.