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The work of Harkins' and Smith2 first put the mechanism and kinetics of emulsion polymerization on a firm basis. They showed that the equation (where R, is t,he rate of polymerization in mole/liter sec., k , is the propagation constant in liter/mole sec., N is t,he number of particles per liter HzO, No is Avogndro's number and [MI is the monomer concentration in moles/liter in the monomerpolymer particles) agreed well with the observed rates of styrene polymerization for latices with particles less than 0.1 p in diameter. This equation can be derived by assuming initiation in the aqueous phase, the entry of single radicals into the monomer-polymer particle, no transfer of radicals out of the particle, and very rapid termination as soon as a radical enters a particle already containing a polymerizing radical.The last assumption can become invalid for two reasons: the particle may grow to a volume sufficient to allow two radicals to grow simultaneously for an appreciable time, or the termination constant k , could be so low as to prevent rapid termination, even in small particles. The first case, the volume effect, was discussed by Haward3 and recently by Roe and Brass4 and by VanderhofP and co-workers. That k, can decrease during the course of the polymerization was proposed by Norrish and Smith6 and by Trommsdorff7 and is sometimes known as the Trommsdorff or "gel" effect. Gerrens* has shown that the emulsion polymerization of styrene shows a weak gel effect at about 70% conversion and that several radicals per particle are polymerizing simultaneously. Since methyl methacrylate is subject to a strong gel effect in bulk polymerization, we have examined the kinetics of the emulsion polymerization of this monomer and compared them with the theories developed for styrene. EXPERIMENTAL
The work of Harkins' and Smith2 first put the mechanism and kinetics of emulsion polymerization on a firm basis. They showed that the equation (where R, is t,he rate of polymerization in mole/liter sec., k , is the propagation constant in liter/mole sec., N is t,he number of particles per liter HzO, No is Avogndro's number and [MI is the monomer concentration in moles/liter in the monomerpolymer particles) agreed well with the observed rates of styrene polymerization for latices with particles less than 0.1 p in diameter. This equation can be derived by assuming initiation in the aqueous phase, the entry of single radicals into the monomer-polymer particle, no transfer of radicals out of the particle, and very rapid termination as soon as a radical enters a particle already containing a polymerizing radical.The last assumption can become invalid for two reasons: the particle may grow to a volume sufficient to allow two radicals to grow simultaneously for an appreciable time, or the termination constant k , could be so low as to prevent rapid termination, even in small particles. The first case, the volume effect, was discussed by Haward3 and recently by Roe and Brass4 and by VanderhofP and co-workers. That k, can decrease during the course of the polymerization was proposed by Norrish and Smith6 and by Trommsdorff7 and is sometimes known as the Trommsdorff or "gel" effect. Gerrens* has shown that the emulsion polymerization of styrene shows a weak gel effect at about 70% conversion and that several radicals per particle are polymerizing simultaneously. Since methyl methacrylate is subject to a strong gel effect in bulk polymerization, we have examined the kinetics of the emulsion polymerization of this monomer and compared them with the theories developed for styrene. EXPERIMENTAL
synopsisThe relative inhibitory effect of the following compounds on the bulk polymerization of methyl methacrylate were measured: hydroquinone, p-testbutylcatechol, pmethoxyphenol, 2,4-dichloro-6nitrophenol1 n-propyl gallate, di3ertbutyl-p-creso1, 2,2'-methylenebis(Pmethyl-6tertbutylphenol), l-amin0-7-naphtho1, pbenzoquinone, 2,6dichloro-pbenzoquinone, Zamino-1,4-naphthoquinone, three aminoanthraquinones, diphenylamine, p-nitrosodimethylanine, a-and 8-naphthylamine, phenothiazine, N-nitrosodimethylamine, hexamethylphosphoramide, ndodecyl mercaptan, benzenethiol, 2 , s diphenyl-1-picrylhydrazyl, phenyl hydrazine, divinybcetylene, and various antimony and copper salts. Polymerization was carried out in a test tube in B bath at 101.2OC., benzoyl peroxide being used as initiator. Generally, phenols and naphthols were the strongest inhibitors, followed by quinones, aromatic amines, 2,2-diphenyl-l-picryIhydrazyl, antimony pentachloride, phenyl hydrazine, divinylacetylene, and the thiols.
Die Reaktionen der Radikale werden am Beispiel der Emulsionspolymerisation behandelt. Die Polymerisation beginnt mit einer meist kurzen Periode, in der Latexteilchen gebildet werden und läuft dann in den Latexteilchen mit den gleichen Elementarreaktionen (Start, Wachstum, Übertragung, Abbruch) weiter wie in homogener Lösung. Die Radikale treten einzeln von außen in die Latexteilchen ein; dadurch erhält man besonders übersichtliche Verhältnisse. Wachstums‐ und Abbruchskonstanten können, wie am Beispiel des Styrols und des Methylmethacrylats gezeigt wird, verhältnismäßig einfach bestimmt werden. Die Polymerisation verläuft von Anfang an in einer ziemlich konzentrierten Polymerenlösung. Bei fortschreitendem Umsatz wird als schnellste Reaktion zuerst die Abbruchsreaktion durch die Diffusion kontrolliert: die Abbruchsgeschwindigkeit nimmt ab und die Bruttoreaktionsgeschwindigkeit zu (Trommsdorffeffekt). Bei hohen Umsätzen gerät auch die Wachstumsreaktion unter Diffusionskontrolle: jetzt nimmt die Bruttoreaktionsgeschwindigkeit sehr schnell ab. Beim Methylmethacrylat ist der Effekt besonders stark. Aus den diffusionskontrollierten Geschwindigkeitskonstanten der Abbruchs‐ und der Wachstumsreaktion lassen sich nach einer Methode von Schulz die Diffusionskonstanten der Makroradikale und des Monomeren berechnen.
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