Maria Carmela Sacchi

Vinylic Polymerization of Norbornene by Late Transition Metal‐Based Catalysis

The polymerization of norbornene with two new families of late transition metal-based catalysts derived from: (i) Ni and Pd complexes bearing bulky diammine ligands, with the general formula [ArN=C(R)-C(R)=NAr]MeX 2 and (ii) Fe and Co complexes bearing bulky arylimine ligands with the general formula [(2,6-ArN=C(Me)) 2 C 5 H 3 N]MX 2 is reported. New Co-based complexes have been tested as well. A prevailingly vinylic, amorphous polymer was obtained with all the catalysts tested, whose solubility in organic solvents is substantially dependent on the molucular weight. Polymerization activity greatly varies from one catalyst to another and depends rather on the metal than on the ligand. Solvent polarity and temperature greatly affect the polymerization yields. Besides a dramatic reduction of molecular weights, the addition of small amounts of 1-hexene produces a noticeable increase in the catalytic activity. NMR analysis shows that in all cases a certain percentage of ROMP polymer is present and that, in general, the variations in polymerization conditions, which produce an increase in activity, simultaneously affect a reduction of the ROMP percentage.

Obtaining the γ phase in isotactic polypropylene: effect of catalyst system and crystallization conditions

Abstract Five samples of isotactic polypropylene, iPP, and two copolymers of iPP with 1-hexene, synthesized with different catalyst systems, both heterogeneous and homogeneous, have been studied in order to analyse the effect of the catalyst system, the presence of comonomer units and the crystallization conditions on the phase structure of iPP and, in particular, on the amount of y modification obtained. Minor amounts, if any, of γ modification are present in iPP samples synthesized with highly isospecific Ziegler-Natta catalysts and crystallized from the melt at different cooling rates, ranging from 100 to 3°C/min. On the contrary, considerable amounts of the γ form have been obtained both in samples prepared with catalysts of very low isospecificity or in those prepared with homogeneous metallocene catalysts. It has been shown that the relative proportion of the α and γ modifications can be controlled just by changing the crystallization conditions. A clear influence of the presence of comonomer units on favouring the formation of the γ phase has not been ascertained. Moreover, attempts to deduce the γ content from the d.s.c. melting patterns have been unsuccessful, since the two modifications exhibit very similar melting temperatures.

Cyclic olefin polymerization and relationships between addition and ring opening metathesis polymerization

Abstract Results of our work concerning: (i) the elucidation of the role of the ionic species [Cp2Ti13CH3]+[Cl·MAO]− and of precursors of alkylidene Cp2TiCH2 species in addition alkene polymerization, (ii) the synthetical strategies for converting the living active titanacyclobutanes into σ-titanocene complexes, and (iii) the evidence for the synthesis of polyethylene polynorbornene di-block copolymer are briefly summarized. 13 C NMR spectra of ethylene–norbornene copolymers, prepared with addition catalysts based on ansa-zirconocenes and methylaluminoxane as cocatalyst, and assignments of comonomer pentads in ethylene–norbornene (EN) copolymers are reported.

13C NMR studies of zirconocene-catalyzed propylene/1-hexene copolymers: in-depth investigation of the effect of solvent polarity

Full Paper: Propylene/1-hexene copolymers were prepared with the isospecific Et(Ind) 2 ZrCl 2 /MAO catalyst by using a wide range of solvents. In the order of increasing dielectric constant (e) they are: heptane (1.92), methylcyclohexane (2.02), toulene (2.38), chlorobenzene (CB) (5.68), methylene chloride (8.93) and o-dichlorobenzene (o-DCB) (10.12). The effect of solvent structure and polarity on yield, composition, and microstructure of copolymers was investigated. The data show that solvent polarity, which has an enormous effect on the activity, has a negligible effect on the copolymer composition. Very close 1-hexene contents are obtained with all the solvents used except for toluene, in which the comonomer content is remarkably lower than that in the other solvents for all compositions. Detailed 13 C NMR analysis of the copolymers obtained was performed: the reactivity ratios obtained by statistical elaboration of the experimental values of triad and tetrad comonomer compositions confirm the actual lower accessibility of the active site to 1-hexene in toluene than in the other solvents. This fact can be explained by the competition of the nucleophilic toluene with 1-hexene in the coordination to the active site.

13C NMR Study of the Effect of Coordinating Solvents on Zirconocene-Catalyzed Propene/1-Hexene Copolymerization

The solvent effect observed in propene/1- hexene copolymerizations performed with the isospecific catalyst rac-Et(Ind) 2 ZrCl 2 /MAO is studied, A range of solvents with increasing donor character and steric hindrance has been tested, and their effect on copolymer yield, composition, and microstructure has been thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the comonomer reactivities, even though the solvent polarity is not the relevant factor. At the same comonomer compositions in solution, polymerizations carried out in coordinating solvents (e.g., aromatic solvents), lead to the formation of products with considerably decreased content of 1-hexene. The reduced incorporation of the higher a-olefin is explained in terms of competition between the nucleophilic medium and the olefin monomer for coordination to the active polymerization site. These results give us valuable information regarding the mechanism of polymerization at the active centers.

NMR investigations of the reactivity between zirconocenes and β-alkyl-substituted aluminoxanes

Abstract 1 H and 13 C NMR studies of the reactions between two β-alkyl-substituted aluminoxanes obtained from the in situ reaction between Al( i -Bu) 3 , Al( i -Oct) 3 and H 2 O with two metallocenes Cp 2 ZrCl 2 and Cp 2 *ZrCl 2 (Cp=cyclopentadienyl, and Cp*=pentamethylcyclopentadienyl) have been performed. The β-branched aluminoxanes were obtained from the in situ reaction between Al( i -R) 3 and H 2 O at Al/H 2 O molar ratio of 2 (R= i -Bu (TIBAO), R= i -Oct (TIOAO)). A comparison of the Cp 2 ZrCl 2 and Cp 2 *ZrCl 2 reactivity with MAO, as the reference point for aluminoxanes, as well as with their parent aluminum alkyls AlMe 3 (TMA), Al( i -Bu) 3 (TIBA), and Al( i -Oct) 3 (TIOA) has been made. Cp 2 *ZrCl 2 , because of steric hindrance, is less reactive than Cp 2 ZrCl 2 towards all aluminium compounds considered. The Cp 2 *Zr- i -BuCl and Cp 2 *Zr- i -OctCl produced appear to be more stable than Cp 2 Zr- i -BuCl and Cp 2 Zr- i -OctCl due to the Cp* steric hindrance, which inhibits β-hydrogen eliminations. The lower reactivity of Cp 2 *ZrCl 2 along with the higher stability of the zirconium alkyls produced has allowed us to produce evidence of the greater capacity of TIOAO with respect to TIBAO for yielding alkylated ion pairs. These results explain the polymerization activities yielded by metallocenes when combined with the above aluminoxanes, that is, the Cp 2 ZrCl 2 activity only with MAO, and the high polymerization activities of substituted metallocenes when β-alkyl-substituted aluminoxanes are used as cocatalysts.

Microwave-assisted chemical-free hydrolysis of wool keratin

Wool fibers were submitted to “green hydrolysis” with superheated water in a microwave reactor, in view of the potential exploitation of keratin-based industrial and stock-farming wastes. The liquid fraction was separated by filtration from the solid fraction, which consists mainly of small fragments of wool fibers and other insoluble protein aggregates. The liquid fraction contains free amino acids, peptides and low molecular weight proteins, with a small amount of cystine and lanthionine, and has a different secondary structure when compared with keratins extracted from wool via reductive or oxidative methods. Cleavage of the cystine disulfide bonds without the use of harmful, often toxic, reductive or oxidative agents allows the extraction of protein material from keratin wastes, offering the possibility of larger exploitation and valorization.

Novel aluminum based cocatalysts for metallocene catalyzed olefin polymerization

Abstract The ethylene polymerization activity of rac -Et(indenyl) 2 ZrCl 2 /AlR 3 /modifier was studied. Modifiers were added to β-branched trialkylaluminum Al( i -Bu) 3 (TIBA), and Al( i -Oct) 3 (TIOA), and to their aluminoxanes, TIBAO and TIOAO, respectively, with the aim of obtaining new cocatalysts that operate at low aluminum/transition metal ratios. The modifiers selected contain hetero-atoms and aryl or alkyl groups bearing electron withdrawing substituents or sterically encumbered groups: C 6 F 5 OH, C 6 Cl 5 OH, C 6 F 4 (OH) 2 C 6 F 5 NH 2 , 2,5-(C 6 H 5 ) 2 C 6 H 3 OH, (CF 3 ) 2 CHOH. The in situ reaction between aluminum compounds and modifiers such as C 6 F 5 OH yields new modified aluminum alkyl and aluminoxane cocatalysts bearing one alkoxy or aryloxy group. The simultaneous effect of electron withdrawing substituents on the aryloxy group and of steric hindrance due to β-alkyl substituted alkyls on aluminum leads to good activation of metallocenes at Al/Zr molar ratios as low as 200/1, in conditions in which methylaluminoxane and β-branched alkylaluminoxanes are inactive. NMR studies of the reactions between C 5 Me 5 ZrCl 2 with TIOA, TIOA/C 6 F 5 OH, TIOAO and TIOAO/C 6 F 5 OH demonstrated that in contrast to TIOAO-C 6 F 5 OH, TIOA-C 6 F 5 OH allows the formation of an active alkylated ion pair, thus qualitatively allowing us to understand the polymerization activity of C 5 Me 5 ZrCl 2 based systems.

Ethylene–4-methyl-1-pentene copolymers of complex chain architecture using α-diimine Ni(II) catalysts: synthesis, 13C NMR assignment and understanding the chain-walking mechanism

Ethylene (E) and 4-methyl-1-pentene (4M1P) are copolymerized using an α-diimine Ni(II) catalyst with MAO (methylaluminoxane) or Et2AlCl (diethylaluminium chloride) as the cocatalyst. A series of copolymers with a 4M1P comonomer content ranging from 0.94 to 36.73 mol% are obtained. A detailed 13C NMR assignment is presented and this thorough analysis has opened up the first full description of this interesting family of copolymers. Manifold branched copolymers are obtained with no noticeable differences in the branching distribution by using MAO or Et2AlCl. The resonance assignments are correlated with the chain-walking mechanism: branching analysis shows that the total amount of 2,1 insertion of the comonomer, followed by backward migration of the nickel active species along the polymer chain, is higher than that of 1,2 inserted 4M1P.

Unbridged bicyclic cyclopentadienyl zirconocene complexes: Their possible application as fluxional catalysts in propene polymerization

Three series of novel unbridged bicyclic cyclopentadienyl zirconium complexes, containing six-, seven- and eight-membered saturated rings fused to the cyclopentadienyl unit and bearing different substituents (-H, -CH 3 , -Ph) in position 2, were used in combination with methylalyminoxane (MAO) for the polymerization of propene at different temperatures. All the catalysts have been shown to be active, the lowest activity being observed with the eight-membered ring systems. With most catalysts the polymers obtained are fully atactic when the polymerizations are conducted at 30°C, and become partially isotactic at lower temperatures. The 13 CNMR analysis shows that in one case, that is, when the seven-membered ring is associated with the phenyl substituent in position 2, a preponderance of the isotactic heptad in the methyl region of the 13 C NMR spectrum is observed, which allows us to hypothesize the formation of stereoblocks.