Ziegler-Natta catalysts insertion polymerisation

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

On the other hand, the polymerisation of trans-1 -(2 H)-propene with heterogeneous Ziegler-Natta catalysts leads to the formation of the respective polymer of threo-diisotactic structure [scheme (49)], which proves the m-insertion of the monomer [26,275] ... 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Allene has been polymerised to high molecular weight linear polymers by various Ziegler-Natta catalysts based on compounds of such transition metals as Ti, V, Cr, Mn, Fe, Co and Ni as precursors and alkylaluminium compounds as activators [439-441], Crystalline and amorphous polymers have been produced, in various proportions, with each of the catalysts used. The crystalline polymers consist predominantly of 1,2-linked (head-to-tail) monomeric units formed by insertion polymerisation as in scheme (68), but some regioirregular-ities resulting from the 2,1-insertion, leading to head-to-head and tail-to-tail arrangements, may be present in the polymer chain ... 

Cycloolefins having rings with more than four carbon atoms do not homo-polymerise in the presence of Ziegler-Natta catalysts based on titanium or vanadium compounds as precursors and alkylaluminium activators. However, these cycloolefins may copolymerise with ethylene via the double bonds while preserving the cycloolefin ring ethylene is able to compensate the steric hindrance at the Ca atom of the growing chain after and before the 1,2-insertion of the cycloolefin [2],... 

In view of the observed inactivity of a, a-disubstituted olefins towards polymerisation with Ziegler-Natta catalysts, it is interesting to note that a, co-diolefins substituted at unsaturated carbon atoms, such as e.g. 2,5-dimethyl-l,5-hexa-diene, also undergo cyclopolymerisation, analogously to unsubstituted parent monomers [2,446], This can be interpreted in terms of a reaction pathway analogous to that shown in scheme (89). The insertions in the cyclopolymerisation appear to be facilitated by the nature of such a process. 

Functionalised a-olefins capable of undergoing insertion polymerisation with Ziegler-Natta catalysts are, in principle, monomers in which the heteroatom (X) does not electronically interact with the double bond to be polymerised in such monomers, the heteroatom is separated from the double bond CH2=CH-(CH2)x X [326,384,518,522-528], Monomers with the heteroatom directly bound to the double bond, i.e. those of the CH2=CH-X type, may also undergo polymerization, but when the heteroatom is silicon or tin (X= Si, Sn) [522-526], Representative examples of the insertion polymerisation of functionalised a-olefins and their copolymerisation with ethylene and a-olefins in the presence of heterogeneous Ziegler-Natta catalysts are shown in Table 3.7 [2,241,326,384,518,522-528],... 

The mechanism of the isospecific polymerisation of styrene and its ring-substituted analogues in the presence of heterogeneous Ziegler-Natta catalysts is the same as for x-olefins. As regards the mode of monomer insertion, it has been reported as primary (1,2) insertion [37,38]. 

The mechanism of polymerisation of alkynes with metathesis catalysts requires that the original triple bond of the acetylenic monomer becomes a single bond in the polymer [scheme (5) of Chapter 2], in contrast to the insertion mechanism of acetylene polymerisation with Ziegler-Natta catalysts, where the triple bond becomes a double bond [scheme (1)]. Ideas about the mechanism of metathesis polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of alkynes, as indeed turned out to be true, as several metal carbenes were found [22-24] to cause alkyne polymerisation. 

The cyclohomopolymerisation of 1,6-heptadiyne by using Shirakawa [85] and related [86] catalysts is a representative insertion polymerisation of acetylenic monomers with Ziegler-Natta catalysts ... 

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. ... 

Similarly to the case of heterogeneous Ziegler Natta olefin polymerisation catalysts, the coordination of the olefin molecule at the cationic metallocene species, due to n bond formation (Figure 2.1), leads to lowering of the energy of the resultant n complex, e.g. the [Cp 2Mt(R)-olefin]1 [Alx(R)x OxX2] complex, which results in activation of the catalyst Mt-C bond and olefin C=C bond for the insertion reaction [136]. 

Regiospecificity of Ziegler-Natta and related coordination catalysts for a-olefin polymerisation implies non-equivalence of the two insertion modes, primary and secondary [schemes (39) and (40) respectively], owing to electronic and/or steric factors. 

As already discussed, most Ziegler-Natta and related coordination catalysts polymerise a-olefins by a 1,2-insertion mechanism, yielding highly regio- and stereoregular polymers. When some nickel-based coordination catalysts are used, however, isomerisation of the active species may accompany the propagation, and polymers containing 2, co-coupled monomeric units are formed [183,191],... 

The polymerisation of olefins by catalysts of the Ziegler-Natta type represents a most important example of the insertion reaction. The Ziegler catalyst formed from TiCl4 and aluminium trialkyl. These component may react in a following way... 

The understanding of direct-initation mechanisms. In this context, tire use of metal salts of strong acids has opened a new area of fundamental studies. Indeed, we believe that the chemical insertion of the olefin between the metal cation and the anion following its adsorption on the salt surface closely resembles recent mechanistic proposals put forward by Zambelli and collaborators to rationalise initiation in stereospecific polymerisation (propene with vanadium-based catalysts). We are inclined to conclude that the cationic nature of stereospecific polymerisations of the Ziegler-Natta type is supported by our findings , viz. electrophilic attack by... 

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