Lido Porri

Conjugated Diene Polymerization

The polymerization of conjugated dialkenes with transition metal catalysts began in 1954, soon after the first results on the polymerization of propylene had been obtained. The first stereoregular dialkene polymer obtained was a polyisoprene with a structure similar to that of natural rubber.1 An isoprene polymer with a structure identical to that of natural gutta percha was obtained soon after,2 along with stereoregular polymers of butadiene and pentadiene.2–4

Diethylbis(2,2'-bipyridine)iron/MAO. A very active and stereospecific catalyst for 1,3-diene polymerization

Diethylbis(2,2′-bipyridine)Fe/MAO is an extremely active catalyst for the polymerization of 1,3-dienes. Polymers with a 1,2 or 3,4 structure are formed from butadiene, isoprene, (E)-1,3-pentadiene and 3-methyl-1,3-pentadiene, while cis-1,4 polymers are derived from 2,3-dimethyl-1,3-butadiene. The 1,2 (3,4) polymers obtained at 25°C are amorphous, while those obtained below 0°C are crystalline, as was determined by means of X-ray diffraction. Mechanistic implications of the results are briefly discussed.

Polymerization of 1,3-dienes with the soluble catalyst system methylaluminoxanes-[CpTiCl3]. Influence of monomer structure on polymerization stereospecificity

Abstract In combination with methylaluminoxanes, [CpTiCl3] gives a soluble catalyst that has remarkable activity for the polymerization of conjugated dialkenes. Polymers with a cis -1,4 structure (> 98%) have been obtained from 2,3-dimethylbutadiene, 2-methylpentadiene and ( Z )-pentadiene. Crystalline polymers with a 1,2 syndiotactic structure have been obtained from 4-methylpentadiene. Other monomers give amorphous polymers with a mixed 1,4/1,2 structure. An interpretation of the mechanism of formation of the polymers is proposed. The mode of orientation of the incoming monomer with respect to the last polymerized unit and the different reactivity of the butenyl groups derived from the various monomers are believed to be the main factors that determine polymer structure.

Molecular structure of 2,2′-thiobis(4-methyl-6-tert-butylphenoxy) titanium diisopropoxy. Influence of titanium-sulfur interaction on coordination geometry

Abstract The molecular structure of 2,2′-thiobis(4-methyl-6-tert-butylphenoxy) titanium diisopropoxy ( 1a ) has been determined. The molecule exists as a dimer, in which the 1,3-dioxadititanacycle is supported by two bridging i PrO ligands. The coordination environment about each Ti center is best described as a distorted octahedron, where the S atom and one i PrO ligand occupy the axial positions, whilst two terminal phenolate and two bridging i PrO ligands occupy the equatorial sites. The eight-membered dioxatitanacycle adopts a symmetric- syn boat conformation, which allows for a significant sulfur-titanium interaction S-Ti distance 2.724(2) A). A comparison between these structural features and those reported for the monomeric tetrahedral complexes 2b and 2d , where the sulfur atom is replaced by a methylene bridge in the bisphenoxy ligand, highlights the influence of the S-Ti interaction on coordination geometry. The structural features suggested for 1a to account for the higher catalytic activity of 1a /MAO in polymerization of α-olefins as compared with 2a /MAO are clearly demonstrated.

Reversible insertion of norbornene into a nickelmethallyl bond

Norbornene reacts at room temperature with π-methallyl nickel chloride giving a red-brown complex in which norbornene is presumably π-bonded to nickel. This complex gives, on reaction with KOOCCH3, an acetato-bridged compound, (C11H17NiOCOCH3)2, in which norbornene is inserted into the nickelmethallyl bond. X-ray structural analysis of the acetato complex indicates an exo-cis insertion of norbornene. The insertion reaction was found to be reversible, depending on the anionic ligand, as shown by the fact that reaction of the acetato compound with HCl, LiBr or KI gives again π-methallyl halide and norbornene.

Polymerization of 1,3-dienes with catalysts based on mono- and bis-cyclopentadienyl derivatives of vanadium

Abstract Butadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene and 4-methyl-1,3-pentadiene have been polymerized with MAO/Cp′VCl 2 ·2PEt 3 and MAO/Cp 2 VCl (Cp′  C 5 H 4 Me, Cp  C 5 H 5 ). Polymers, predominantly cis -1,4, have been obtained from butadiene and pentadiene, a polymer with a mixed cis/trans structure has been obtained from 2-methyl-1,3-pentadiene, while 4-methyl-1,3-pentadiene gave a predominantly 1,2 polymer. Cp′VCl 2 ·2PEt 3 /MAO was found to be particularly active. The different stereospecificities of the systems are discussed with respect to other soluble vanadium systems.

On the mechanism of formation of isotactic and syndiotactic polydiolefins

The mode of formation of isotactic and syndiotactic polymers from 1,3-dienes is examined in the light of the most recent results. An interpretation is given for the formation of trans-1,4 isotactic polymers from CH 2 =CH-CH=CHR (R = Me, Et, Pr, etc.) type monomers with heterogeneous VCl 3 -based catalysts. Evidence is reported showing that stereoregular 1,2 or cis-1,4 polymers derive from a growing polymer chain anti-η 3 -bonded to the transition metal and a cis-η 4 coordinated monomer. The influence on stereoselectivity of the substituents at the central carbon atoms of the monomer is discussed. The peculiar behavior of (Z)-1,3-pentadiene and 4-methyl-1,3-pentadiene, which give 1,2 polymers with catalysts that give 1,4 polymers from other monomers, is attributable to the fact that they can coordinate trans-η 2 , in addition to cis-η 4 .

Polymerization of 4-methyl -1,3-pentad iene with MAO/Ti(OnBu)4. The influence of preparation/ageing temperature upon the stereospecificity of the catalyst

Abstract 4-Methyl-l,3-pentadiene has been polymerized with the system Ti(OnBu) 4 /MAO at different temperatures in the range +20°C to −78°C, using aged and unaged catalysts. The polymers obtained have a 1,2 syndiotactic or a mixed 1,2 syndiotactic/1,2 isotactic structure, depending on the polymerization temperature and on the temperature and time of ageing of the catalyst. The percentage of syndiotactic polymer depends on the polymerization temperature, but also on the time and temperature of ageing of the catalysts. The formation of isotactic polymer together with syndiotactic polymer has been attributed to the presence of two different active centres. The centres that produce the syndiotactic polymer increase with the time and temperature of ageing.

Copolymerization of ethylene with α-olefins and cyclic olefins catalyzed by a Ti(IV) diisopropoxy complex bearing a tridentate [O−,S,O−]-type bis(phenolato) ligand

Ethylene (E) was copolymerized with some α-olefins [1-pentene (PEN), 1-hexene (HEX), and 4-methyl-1-pentene (4M1P)] and cyclic olefins [cyclopentene (CPE), norbornene (NB), and dicyclopentadiene (DCPD)] using the Ti(IV) thiobis(phenolate) complex [2,2′-S(4-Me,6-tBuC6H2O)2]Ti(OiPr)2 in combination with methylaluminoxane (MAO). The catalyst exhibited excellent activities (up to 106 g molTi−1 h−1). Crystalline E/α-olefin copolymers with a strong tendency for comonomer alternation were obtained with good comonomer incorporation (about 15 mol% for [Y]/[E] = 8; Y = comonomer) decreasing in the order HEX > PEN > 4M1P. Random copolymers with NB and DCPD were obtained with efficient comonomer incorporation (from 10 to 40 mol%) even for the Y/E molar ratio = 1 to 2, while the catalyst gave poor CPE incorporation. In order to collect information on the comonomer distribution in the copolymers, boiling solvent extraction was carried out and all the fractions were characterized by DSC, XRD, SEC, and NMR.

Neodymium catalysts for diolefin polymerization influence of the anionic ligand bonded to neodymium on the stereospecificity

Abstract Heterogeneous Neodymium containing systems for the polymerization of diolefins to cis -1,4 polymers are well known. We have focused our attention on homogeneous catalysts prepared from Neodymium complexes and Aluminum tri-alkyls able to polymerize diolefins and have found that the stereospecificity of the catalysts can be modulated through the variation of several parameters such as the nature of the anionic ligand bonded to Neodymium, the Al/ Nd ratio, the concentration and type of Aluminum alkyl in the catalyst. Among these parameters the most efficient in affecting the polymer microstructure is the nature of the anionic ligand bonded to Neodymium. In order to study this effect, the following Nd compounds have been used in combination with Al(iBu) 3 for the preparation of the catalysts: NdCl(OCOCF 3 ) 2 , NdCl(OR) 2 , Nd(OCOCF 3 ) 3 , Nd(acac) 3 , Nd(OR) 3 (R = isopropyl, neopentyl). All these catalysts gave polybutadienes with a predominantly, 1,4-structure (the content of 1,2 units ranged from 3 to 12%) but with strikingly different cis/trans ratios, varying almost continuously from 98.5% cis -1,4 with NdCl(OCOCF 3 ) 2 up to 95% trans -1,4 with Nd(OR) 3 (R = neopentyl). The nature of the anionic ligand was found to remarkably affect also the rate of polymerization: the halogen containing Neodymium compounds gave the catalysts with the highest activity.