Polymerization, activation cationic

Syndiotactic polypropylene (sPP) with a. ..rrrrrmrrrrrmmrrrrr... mixed microstructure is obtained with the iPr[CpFlu]ZrCl2/HAO (MAO methylaluminoxane Cp cyclopentadienyl anion Flu - fluorenyl anion). The structures of the metallocene and the polymers are in accord with chain migratory insertion being the predominant mechanism of chain growth and with stereochemical control being provided by the alternating handedness of polymerization active, cationic Zr monoalkyls. 

Chain-growth polymerization through cationic active species. This is taken up in Sec. 6.11. 

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

Marks TJ, Stern CL, Chen YXC (1997) Very large counteranion modulation of cationic metallocene polymerization activity and stereoregulation by a sterically congested (pefluoroaryl) fluoroaluminate. J Am Chem Soc 119 2582-2583... 

Neutralization reactions in which the charge is destroyed. One version, equation (IX), has been mentioned at the beginning of sub-section 5.1. Whether there are neutralization reactions in which the whole of the anion combines with the active cation is at present not known, and such cases are likely to be rare because the very virtue of an effective polymerization catalyst resides in its having, or generating, an anion of very low nucleophilicity. 

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

The rates of all single-step reactions increase as the temperature increases. This may not be true for multistep reactions such as those involved with multistep polymerizations, here the cationic polymerization. For cationic polymerizations the activation energies are generally of the order > E > E. Remembering that the description of the specific rate constant is... 

Solvent polarity has a significant impact on the nature of ion pairing. The large acceleration of polymerization activity that is sometimes observed in halogenated solvents can be attributed to enhanced ion separation in more polar media, and to possible weak coordination of the solvent to the metal cation. ... 

A novel approach to the production of chiral, polymeric, crown ethers incorporating isomannide was developed by a Japanese group. The optically active divinyl ether 103 was polymerized with cationic catalysts to afford 104, consisting of only cyclic constitutional units.191 In addition, another crown ether (105), containing five ethylenedioxy moieties, was prepared. 

Crivello (lib) has described a series of diaryliodonium (107) and triarylsulfonium salts (108) which function as initiators for ultraviolet activated cationic polymerization. The... 

An ESI mass spectrometer coupled online to a microreactor was used to intercept the catalytically active cationic intermediates of the Ziegler-Natta polymerization of ethylene with the homogeneous catalyst system [Cp2Zr(Me)Cl]-MAO (MAO = methylaluminoxane). For the first time these intermediates were studied directly in the solution and their catalytic activity proved.60... 

With this experimental set-up, highly active, cationic ruthenium-carbene catalysts are used in ring-opening metathesis polymerization (ROMP). Four different structural features of the catalyst [ R2P(CH2) PR2-k2P XRu=CHR]+ (the halogen... 

Presently, the importance of Nd allyl compounds as intermediates in Nd carboxylate- and other Nd-based catalyst systems is widely accepted. As various Nd allyl compounds have been synthesized, characterized and successfully tested as polymerization catalysts this view is supported by solid experimental evidence (Sect. 2.1.1.5 and the references therein). Selected Nd allyl compounds exhibit significant polymerization activities without the addition of cocatalysts. In these cases the active species is neutral. But also cationic active Nd species are taken into consideration (Sect. 2.1.1.5) [288,291]. Cationic species also prevail in the presence of non-coordinating anions. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

Cationic polymerization is initiated by acidic substances. Bronsted acids and particularly Friedel-Crafts halides (viz., their complexes), which are adds in the Lewis sense, are excellent and widelyused initiators for electrophilic polymerizations. The number of cationoid substances which have been described to possess electrophilic polymerization activity is slowly increasing. These materials will be briefly surveyed here. 

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