Heterogeneous systems, polymerisation

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. 

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] ... 

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... 

It was the proof of dispersion [4b], the conductivity jump based on it due to flocculation [17a] and the optimisation thereof [17b,d], that first forced us to develop the new non-equilibrium thermodynamics theory of heterogeneous systems [19,17c,37] that applies to carbon-black compounds and ICP blends. The presumed necessity to achieve good dispersions compelled us to polymerise ICPs of increasingly high purity and to develop, to this end, the analytical techniques that yielded contributions to the understanding of structural aspects (see Sections 3,7.1 and 3.6). 

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... 

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... 

Ruckenstein and Sun [89] have used inverted emulsion polymerisation for the synthesis of PANI rubber composites using an isooctane-toluene mixture and water to form the emulsion and using ammonium persulfate as the oxidant. Inverse emulsion polymerisation consists of an aqueous solution of the monomer, which is emulsified in a non-polar organic solvent and the polymerisation is initiated with an oil-soluble initiator. The reaction is carried out in a heterogeneous system in which the reaction takes place in a large number of reaction loci dispersed in a continuous external phase. 

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

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. 

Although the heterogeneity of the system complicates the kinetics of polymerisation, and the partition of the monomer between the solvent and the precipitated polymer may accelerate the reaction, it seems that these observations do not disprove the reality of helical growth. The existence of this phenomenon is indicated most convincingly by the startling effect of D-isomer on the growth of L-polymer or vice versa. 

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