Monomers insertion

The electron-releasing R group helps stabilize this cation. As with anionic polymerization, the separation of the ions and hence the ease of monomer insertion depends on the reaction medium. The propagation reaction may be written as... 

The propagation center for catalytic polymerization is a chemical compound having an active bond between the catalyst and growing polymer molecule the monomer insertion into this bond occurs as a propagation act. 

The availability of organic ligands in the surface complexes obtained by the reaction of organometallic compounds with supports allows us to consider two possibilities of the initiation process (1) the monomer insertion into the organic ligand-metal bond (bensyl, allyl, etc.) (9a) ... 

The heat of each stage (qi for the w-complex formation and will change to the new values ... 

The characteristics of C NMR spectra for all copolymers were similar. The triad distributions for all copolymo" from NMR monomer insertion are shown in Table 2. Based on the triad distribution of ethylene/l-hex aae copolymers in Table 2, we found that microstructurc of copolymer obtainrai fiom aluminoxane system was slightly different in monomer incorporation, but found significantly when borated system was applied. We suspected that this difference was arising fiom the diffaences in bimetallic complex active species between [aluminoxane] [catalyst] and [Borate] [catalyst] which had the electronic and gMmetric effects fiom the sterric effect of larger molecule of borate compare to the aluminoxane on the behaviors of comonomer insertion in our systems. 

Unlike radical chain polymerisation, initiation in cationic polymerisation uses a true catalyst that is recovered at the end of the polymerisation and is not incorporated at one end of the growing chain. Catalysts for cationic chain polymerisation are molecules able to withdraw electrons, mainly Bronsted (H2SC>4, H3PO4) and Lewis acids (BF3, A1C13, SnCh). The choice of solvent for cationic polymerisation is also important because it plays a major role in the association between cation and counter ion. A too tight association will prevent monomer insertion during the propagation step. However, the use of "stabilized"... 

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

That the sequence shown in Scheme 3 is not the only pathway available for H—NiY formation is indicated by the isolation of 1,3-cyclooctadiene from the reaction products of the dimerization of propene with the n-cyclooctenylnickel system (25) (80) it seems reasonable that the H—NiY species 22 in this case is at least in part formed through direct elimination from 25 without prior monomer insertion into the Ni—C—bond [Eq. (6)] ... 

A prominent feature of this mechanism is that the growing polymer chain alternately swings between two r/.v-disposed coordination sites during each monomer insertion. General mechanistic outlines of this reaction have been extensively examined by large-scale computations and confirmed by experimental means.59 Our present goal is to clarify the localized donor-acceptor-orbital interactions that underlie (4.106), particularly the nature of the alkyl-alkene complex II. 

A necessary (but not sufficient) prerequisite for models of stereospecific catalytic systems is the stereoselectivity of each monomer insertion step. The possible origin of stereoselectivity for models of several kinds of catalytic systems has been investigated through molecular modeling. 

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

Molecular mechanics analyses of the kind described in previous sections are able to rationalize not only the stereoselectivity (and stereospecificity) of regioregular primary insertion steps but also the stereoselectivities relative to occasional secondary monomer insertions as well as relative to primary insertions following these secondary insertions.37d... 

For instance, Figure 1.9a plots as a function of Go the optimized energies for the model complex 5, that is, a ir-ligand with C2 symmetry with (R, R) chirality of coordination of the bridged ir-ligand. Let us recall that, in our framework, energy minima with 0O 0° or 0O 180° can correspond to preinsertion intermediates suitable for primary and secondary monomer insertions, respectively. 

In summary, there is substantial stereoselectivity of this isospecific C2 symmetric catalytic model for the lower energy (and experimentally observed) primary monomer insertion, and the stereoselectivity would also be higher for the higher energy (minor but experimentally detected) secondary monomer insertion. It is worth noting that the stereoselectivity of the isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions,37d... 

Chain-end controlled isospecificity and syndiospecificity for 1-alkene polymerizations at low temperatures with achiral metallocenes have also been reported.2,163 81131135 The polymerization with these catalysts is highly regio-specific in favor of primary monomer insertion. 

Recently, bis(imino)pyridyl Fe(II)-based catalysts have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138 139 Even more recently, catalytic systems based on the octahedral bis(salicylaldiminato)Ti complex have been reported to afford syndiospecific chain-end controlled propene polymerization,140 which possibly occurs through secondary monomer insertion.141... 

Diastereoisomeric transition states calculated for propene insertion in a model for a Brookhart-type Ni(II) catalyst, based on diacetylbis(2,6-diisopro-pylphenylimine)nickel derivative,143,144 are shown in Figure 1.21. Diastereomeric transition states for si (Figure 1.21a) and re (Figures 1.21b,c) monomer insertions into a si chain correspond to like (isotactic) and unlike (syndiotactic) propagations, respectively.144,143... 

However, for the transition states of Figures 1.20 and 1.21, the energy differences between models for si and re monomer insertions, corresponding to... 

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