Particles of polymerization

Possible structures which may arise dming the phase separation of binary mixtures have been discussed elsewhere. It is important that the microphase particles evolved in this process, which have different composition and size be considered as quasi-particles of polymeric filler and the system as a whole- as a self-reinforced composite material. Thus polymer-polymer systems with incomplete phase separation may be described as systems consisting of polymeric matrix and polymeric filler. The common feature for both such systems and polymers obtained by the introduction of polymeric filler consists of formation of an interphase zone between two separated coexisting phases. 

Rate of polymerization. The rate of polymerization for homogeneous systems closely resembles anionic polymerization. For heterogeneous systems the concentration of alkylated transition metal sites on the surface appears in the rate law. The latter depends on the particle size of the solid catalyst and may be complicated by sites of various degrees of activity. There is sometimes an inverse relationship between the degree of stereoregularity produced by a catalyst and the rate at which polymerization occurs. 

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

Since polymer swelling is poor and the aqueous solubiUty of acrylonitrile is relatively high, the tendency for radical capture is limited. Consequentiy, the rate of particle nucleation is high throughout the course of the polymerization, and particle growth occurs predominantiy by a process of agglomeration of primary particles. Unlike emulsion particles of a readily swollen polymer, such as polystyrene, the acrylonitrile aqueous dispersion polymer particles are massive agglomerates of primary particles which are approximately 100 nm in diameter. 

Eor some uses, higher molecular weight polymer consisting of 150—200 repeat units is required. Such polymer usually is prepared by soHd-state polymerization in which pellets are heated under an inert atmosphere to 200—240°C. The 2G is removed continuously. The rate of polymerization depends on particle size, end group composition, and crystallinity (65). 

A fourth mechanism is called sweep flocculation. It is used primarily in very low soflds systems such as raw water clarification. Addition of an inorganic salt produces a metal hydroxide precipitate which entrains fine particles of other suspended soflds as it settles. A variation of this mechanism is sometimes employed for suspensions that do not respond to polymeric flocculants. A soHd material such as clay is deUberately added to the suspension and then flocculated with a high molecular weight polymer. The original suspended matter is entrained in the clay floes formed by the bridging mechanism and is removed with the clay. 

The term flotoflocculation is used to describe the process of aggregating dispersed oil droplets by the aid of polymeric flocculants (flocculation) then subjecting them to conventional flotation. It is also used, genericaHy, to describe situations where particles are first aggregated then floated. 

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

Stage II Growth in Polymer Particles Saturated With Monomer. Stage II begins once most of the micelles have been converted into polymer particles. At constant particle number the rate of polymerization, as given by Smith-Ewart kinetics is as follows (27) where is the... 

During Stages II and III the average concentration of radicals within the particle determines the rate of polymerization. To solve for n, the fate of a given radical was balanced across the possible adsorption, desorption, and termination events. Initially a solution was provided for three physically limiting cases. Subsequentiy, n was solved for expHcitiy without limitation using a generating function to solve the Smith-Ewart recursion formula (29). This analysis for the case of very slow rates of radical desorption was improved on (30), and later radical readsorption was accounted for and the Smith-Ewart recursion formula solved via the method of continuous fractions (31). 

Polypropylene. One of the most important appHcations of propylene is as a monomer for the production of polypropylene. Propylene is polymerized by Ziegler-Natta coordination catalysts (92,93). Polymerization is carried out either in the Hquid phase where the polymer forms a slurry of particles, or in the gas phase where the polymer forms dry soHd particles. Propylene polymerization is an exothermic reaction (94). 

Most synthetic latices contain 5—10 wt % of nonelastomeric components, of which more than half is an emulsifier or mixture of emulsifiers. One reason for this relatively high emulsifier concentration as compared with natural latex is that emulsifier micelles containing solubiHzed monomer play a principle role in the polymerization process. A high emulsifier concentration is usually necessary to achieve a sufficiently rapid rate of polymerization. Secondly, a considerable fraction of the surface of the polymer particles must be covered by adsorbed soap or equivalent stabilizer to prevent flocculation... 

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

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