Homogeneous media polymerization

Polymerization in the crystalline state is, as stated above, thermodynamicaUy different from the same polymerization in the homogeneous medium. Polymerizations, which would not be allowed under homogeneous conditions, may be accomplished in the crystalline state, i.e. monomer may add directly to the active species in the crystal and polymerization is accompanied by simultaneous crystallization. In this process each propagation step is equivalent to an increase of the crystal size. 

However, DMF is a solvent for polyacrylonitrile and the polymerization occurs in a homogeneous medium for solutions containing 30 per cent monomer or less. This reduces the value of these experiments as an argument to show the influence of a matrix effect. Indeed the fact that auto-acceleration disappears when DMF is added to acrylonitrile was considered as a proof for the fact that precipitation of the polymer was the cause of autoacceleration. 

Such a mechanism could also explain the apparent induction period found at the beginning of the polymerizations. With very small particles and large chain transfer, the polymerization could be acting as if it were in a homogeneous medium. This produces very low rates compared with a standard emulsion polymerization. As particle size increases, the rate rises because chain-transferred monomers would not diffuse into the aqueous phase. 

Grafting by chain transfer initiation has been carried out not only in homogenous medium but also by emulsion polymerization techniques, where the monomer and the catalyst are added to a latex containing the original backbone polymer (99). The efficiency of grafting increases with an increase of temperature of polymerization and with an increase of initiator concentration (generally potassium persulfate) these results indicate not only that the chain transfer reaction has a higher activation... 

Similar grafting experiments by the emulsion technique were described (34) in the system vinyl chloride/copolymer butyl methacrylate-methacrylic acid and in the reverse system, and also in the system styrene/polyvinyl chloride. In this last case again as in homogenous medium, the inverse process failed (vinyl chloride on polystyrene). Grafted acrylonitrile copolymers were also prepared in order to improve their dyeability, by polymerizing acrylonitrile in emulsion in the presence of many different polymers as polyvinyl alcohol, polymethacrylamide and polyvinylpyrrolidone (119, 120, 121), polyvinyl acetate and polyacrylic acid (115), wool (224,225), proteins (136), etc. 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

In polymerizations by alkali metals or insoluble organometallics, the initiation step occurs at a phase interface while subsequent propagation reactions may occur in a homogeneous medium. The overall kinetics of such reactions are often very complex and specific to the particular systems. Useful generalizations are more likely to be provided at the present lime by systems in which the initiation and propagation processes are both homogeneous. Such polymerizations are discussed next. 

The overall polymerization reaction in a homogeneous medium involving, for example, an organolithium initiator, may be written as follows ... 

Unlike free-radical and homogeneous anionic polymerizations, cationic polymerizations cannot be described by conventional kinetic schemes involving reactions like initiation, propagation, and so on. This is due to the complexity of the cationic initiation and the variable extents of ion-pair formation at the propagating chain end [cf. Eq. (8.1)]. The possibihty of the reaction being heterogeneous due to the limited solubility of the initiator m the reaction medium further adds to the complexity of the kinetics. 

The composition and the morphology of the membranes are key to effective use of membrane technology. The choice of membranes strongly depends on the types of applications (Koops and Smolders 1991). It is important to know which of the components should be separated from the mixture and whether the component is water or an organic liquid. In general, the component with the smallest weight fraction in the mixture should preferentially be transported across the membrane. The PV membrane can be considered as a dense homogeneous medium in which diffusion of species takes place in the free volume that is present between the macromolecular chains of the polymeric membrane material. 

The second conception follows from the assumption that the polymerization process proceeds mainly in the monomer/polymeric solution, that is in the homogeneous medium and the character of the elementary reactions is determined by diffusion control that is why their kinetic constants depend upon the composition of the monomer/polymeric solution and upon the depth of polymerization, respectively. In this conception of the diffusion-controlled reactions (DCR) a classical kinetic scheme of the polymerization process and, following from this, its kinetic equation are mainly used. However, the parameters of the above-mentioned kinetic equation are not constant and represent a function of the monomer/polymeric solution current state. 

Solution Polymerization. Solution polymerization is used by a few producers in the acrylic fiber industry. The reaction is carried out in a homogeneous medium by using a solvent for the polymer. Suitable solvents are aqueous NaSCN used by Acordis and dimethyl sulfoxide [67-68-5] (DMSO) used by Toray. The homogeneous solution polymerization of acrylonitrile follows the conventional kinetic scheme developed for vinyl monomers (27-29) (see Bulk and Solution Polymerizations Reactors). 

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