Monomer Concentration during Polymerization

We have utilized a variety of techniques for determination of the conversion of monomer to polymer, by measurement of the concentration of C=C bonds at different times during the polymerization of vinyl and allyl monomers. The NMR spectra of samples quenched and dissolved in deuterated solvent showed resonances due to the different H atoms of the monomer and polymer, and NMR was a very good method for determination of conversion. However, 

Fourier transform near-infrared spectroscopy, using the C=CH band at 61S2cm for methacrylates, enabled the conversion to be followed in a single sample throughout the polymerization to high conversion. Parallel experiments were performed for near-infrared spectroscopic determination of monomer concentrations and for ESR measurements of radical concentrations. A further advantage of the near-IR method is its applicability to insoluble crosslinked systems. 

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Various techniques have been studied to increase sohds content. Hydroxy-functional chain-transfer agents, such as 2-mercaptoethanol [60-24-2], C2HgOS, reduce the probabihty of nonfunctional or monofunctional molecules, permitting lower molecular-weight and functional monomer ratios (44). Making low viscosity acryhc resins by free-radical initiated polymerization requires the narrowest possible molecular-weight distribution. This requires carehil control of temperature, initiator concentration, and monomer concentrations during polymerization. 

A range is given for vinyl hexanoate since the low percentage of monomer used makes determination of the point more difficult. From these observations and the shape of the polymerization curves, it seems that vinyl hexanoate behaves similarly to styrene in showing first-order dependence on monomer concentration during polymerization. 

Cationic polymerization of 2-methylpropene at temperatures about 170 K may be almost flash-like the transformation of tetrahydrofuran to an equilibrium polymer-monomer mixture may last tens to hundreds of hours at 260 K. Evidently the overall polymerization rate is a function of many factors which may be interconnected or may act separately. The aim of kinetic measurements is to describe the polymerization, and to find conditions under which it would proceed in the desired manner. This is usually only possible after the various factors and their consequences have been isolated and investigated. The rate of monomer consumption during polymerization mostly depends on the generation rate of active centres, and on their concentration and reactivity. 

Problem 8.11 Polymerization of styrene with sodium naphthalene initiator was performed at 25°C in tetrahydrofuran (THF) using a static technique [8] that is suitable for monitoring fast reactions. The conversion was determined by monitoring the residual styrene monomer spectrophotometrically during polymerization and the concentration of living ends [M ] was determined spectrophotometrically at the end of the experiment. In independent experimental series, the overall rate constant kp was obtained [cf. Eq. (P8.10.2)] both at different concentrations of initiator (and hence [M ]) without addition of electrolyte and at different concentrations of sodium ions from externally added sodium tetraphenyl borate (NaBPh4) salt and constant concentration of initiator. The data are given below ... 

The structure of the hydrophobically modified polymer is set by the number of micelles and the number of hydrophobic monomers present during polymerization. Determination of the structure of the polymer is a problem because of the extremely small concentration of the hydrophobic monomers in the polymer (< 1 mol %). Normal analytical techniques such as NMR are not suflBciently sensitive to determine the number or the sequence length of the hydrophobic regions in such polymers. 

C p maintains its saturation value during intervals I and n provided that the interfacial area of monomer droplets is high enough to allow the monomer transport to the growing particles where it replaces the monomer consumed swelling the polymer formed [146]. However, on the basis of Monte-Carlo simulations, Tauer and Hernandez [147] have claimed that latex particles in emulsion polymerization never experience either a period of saturation with monomer or a constant monomer concentration during interval II, as frequently assumed. 

No more new latex particles can be formed in phase II of the polymerization. On the other hand, diffusion out of the monomer droplets provides a flow of new monomer molecules into the latex particles. The monomer concentration in the latex particles remains constant, and a rate of polymerization results which is zeroth order with respect to the monomer concentration. During this time, the particle surface area increases, since large monomer droplets are being replaced by many latex particles. At the end of this period, the latex particles are only 60% covered with emulsifier. 

Finally study of the variations of DPn versus monomer concentration during the bulk polymerization of styrene initiated by a sodamide S.C.B., showed the living nature of the polymers. This was confirmed by copolymerization performed between styrene and methyl methacrylate. 

Figure 7 Change in relative monomer concentration during BA polymerizations at 40 °C, 1000 bar, and 38% overall monomer conversion at two CO2 contents. From Beuermann, S. Buback, M. Schmaltz, C. Ind. Eng. Chem. Res. 1999, 38, 3338. ...

Figure 9 (a) Film thickness of PMMA monolayers as a function of monomer concentration during the polymerization reaction (temperature 60°C, t = 18 h) all samples have been extracted with toluene after stopping of the polymerization reaction for 20 to 48 hours (b) waveguide spectrum of a 1690 nm thick PMMA layer solid line is a calculation according to Fresnel equations. 

Polymerization Kinetics of Mass and Suspension PVC. The polymerization kinetics of mass and suspension PVC are considered together because a droplet of monomer in suspension polymerization can be considered to be a mass polymerization in a very tiny reactor. During polymerization, the polymer precipitates from the monomer when the chain size reaches 10—20 monomer units. The precipitated polymer remains swollen with monomer, but has a reduced radical termination rate. This leads to a higher concentration of radicals in the polymer gel and an increased polymerization rate at higher polymerization conversion. 

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