Polymerisation in heterogeneous

The principal characteristics of the different modes of polymerisation in heterogeneous systems are summarised in Table 3.2. 

Polymers can be produced by applying the various types of polymerisation mechanism described above, i.e. all radical polymerisations including controlled polymerisation and ionic polymerisation. In addition to producing polymers with well-defined characteristics, polymerisation in heterogeneous systems can be used to produce well-defined polymer particles including very well-structured composite nanoparticles. For instance, particles with a magnetic core and nanoparticles showing a core-shell-type nanostructure can be... 

Polymerisation in heterogeneous suspension, in which the monomer is mechanically dispersed in a liquid, not a solvent for it and for all species of polymer molecules. 

McLeaty, J.B., Klumperman, B. RAFT mediated polymerisation in heterogeneous media. Soft Matter 2(1), 45-53 (2006)... 

Eree-radical initiation of emulsion copolymers produces a random polymerisation in which the trans/cis ratio caimot be controlled. The nature of ESBR free-radical polymerisation results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstmcture is not amenable to manipulation, although the temperature of the polymerisation affects the ratio of trans to cis somewhat. 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

A review of the application of ESR to the study of free radical polymerisation is given by Yamada and co-workers [146]. A survey of the application ESR spectroscopy spin label/probe methods in heterogeneous polymer systems is provided by Veksli and co-workers [147]. Spin probe methods allow the study of the MD of the polymer, its free volume, phase separation and phase morphology. 

This reaction can be carried out with numerous variations to give a broad range of catalysts. It is a heterogeneous high-surface TiCIs material of which the active sites contain titanium in an unknown valence state. It is quite likely that alkyltitanium groups at the surface are responsible for the co-ordination polymerisation. In more recent catalysts titanium supported on magnesium salts are used [4,5],... 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

Class I catalysts obtained from achirotopic metallocene precursors of the Cp2MtX2 type, which produce atactic polypropylene at elevated temperature in the range of ca 50-70 °C (characteristic of propylene polymerisation in the presence of heterogeneous catalysts), can yield stereoblock isotactic polypropylene at lowered temperature, e.g. — 45 °C (Table 3.1) [22]. 

In the case of olefin polymerisation with homogeneous Ziegler-Natta catalysts, especially with single-site (metallocene) catalysts, the kinetic analysis may become simpler than in the case of polymerisation with heterogeneous catalysts, and in some instances can serve as a very useful tool for uncovering the true polymerisation mechanism [30,243],... 

Termination of the olefin polymerisation with heterogeneous Ziegler-Natta catalysts by the addition of carbon monoxide to the system is often used in the laboratory to determine the active centres of the catalyst. 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

The described chain migratory insertion mechanism, which operates in olefin polymerisation with metallocene-based single-site catalysts, follows that proposed by Cossee [268,277,278] for olefin polymerisation with heterogeneous catalysts there is, however, no back skip of the polymer chain to the previously occupied position prior to the coordination of the next monomer molecule, but rotation of the chain around the axis of the Mt-CH2 bond takes place (Figure 3.19) [358],... 

Activities of a-olefins in insertion polymerisation with coordination catalysts are generally related to the monomer structure both electronic and steric factors influence the activity of a-olefins in stereospecific polymerisation, but the steric factor primarily influences the rate of polymerisation, which has been especially shown for isospecific polymerisation with heterogeneous catalysts [46, 250],... 

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],... 

Subsequent investigations revealed that, in principle, styrene undergoes isospecific polymerisation in the presence of heterogeneous Ziegler Natta catalysts [1-4], Although polystyrene of isotactic structure was also prepared with the use of homogeneous nickel-based coordination catalysts, it appeared to be of low molecular weight [22,23]. 

Table 4.1 Relative reactivity of vinylaromatic monomers in coordination polymerisation with heterogeneous Ziegler-Natta catalyst ...

In view of these results, it was suggested that syndiospecific and isospecific polymerisations with heterogeneous catalysts containing a chlorine atom in the titanium compound or in the support are promoted by homogeneous species formed in the polymerisation system and by heterogeneous species respectively. 

In the case of polymerisation in the presence of heterogeneous catalysts with the transition metal in a low oxidation state, metallacyclobutane species can also be formed by the intentional addition of cyclopropane to the system [117] ... 

The above stereospecific tiirane polymerisations have generally been run in heterogeneous systems. Such conditions essentially make it impossible to determine the detailed structure of active species involved in these polymerisations. Thus, enantiosymmetric and enantioasymmetric polymerisations of propylene sulphide have also been studied in a homogeneous phase by using chiral cadmium thiolates of cysteine esters and chiral cadmium carboxylates of cysteine and methionine [157,160-164]. The most studied is living polymerisation using the cadmium derivative of the isopropyl ester of (.S)-cysteine [160] ... 

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