Polymerisation in heterogeneous systems

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... 

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] ... 

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. 

In coordination polymerisation, the catalyst-monomer complex forms a heterogeneous system in which the metal ion is in the solid phase and the carbanion of the alkyl group is in the solvent phase. The monomer is inserted in between the metal ion and the carbanion and the Polymer chain formed is pushed out from the solid catalyst surface. Because of this coordination polymerisation is also known as insertion polymerisation. 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

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. 

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 field of controlled radical polymerisation (CRP) has seen rapid growth during the past decade, growth that in more recent years has begun to embrace work on CRP in dispersed media. The two-phase nature of heterogeneous polymerisations, however, imposes severe constraints on what can be achieved and restricts the scope for extending the established methods of CRP to heterogeneous systems. 

Nitroxide-mediated CRP has been investigated for use in emulsion polymerisation systems, but with mixed the results. " Atom transfer radical polymerisation (ATRP) offers greater scope than nitroxide-mediated CRP in that it is less discriminating in terms of the monomers that can be used. However, there are few reports of work on adapting ATRP to heterogeneous systems. " Given the established requirements for control of these polymerisations, both nitroxide-mediated CRP and ATRP suffer from problems brought about by... 

Dispersion polymerisation may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerisation. In dispersion and precipitation polymerisation, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerisation the initiator is chosen to be soluble in the disperse phase of the monomer. A comparison of the rates of polymerisation of MMA at 80 C for the three systems was given by Barrett and Thomas [11], as illustrated in Figure 17.10. The rate of dispersion polymerisation is much faster than either precipitation or solution polymerisation. TTie enhancement of the rate in precipitation polymerisation over... 

The system involved in the bulk polymerisation process is the simplest from the point of view of composition and is used for large-scale radical polymerisation. In this process, the initiator is mixed with the monomer, usually under pressure of an inert gas, and the mixture is heated to induce generation of free radicals by thermal decomposition of the initiator. Depending on whether the monomer is a gas or a liquid, the system can be homogenous or heterogeneous... 

Similarly to suspension and emulsion polymerisation, the system used in interfacial polymerisation is heterogeneous, but polymerisation takes place at the interface between both phases. Such a system can be easily illustrated in a practical laboratory course by reaction of a diamine soluble in an aqueous alkaline medium present in the upper part of a beaker, with a diacid chloride soluble in a non-miscible organic solvent such as chloroform present in the... 

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