Radical Concentration during Polymerization

In favourable circumstances, the radical concentration below the gel point may be obtained by accumulation of spectra in situ. However, the limited time available for accumulation and the changing nature of the spectrum with conversion militate against this procedure. Radical concentrations obtained above the gel point by the quenching and in situ methods (without accumulation) were in good agreement. 

FigiirelO.4 The variation of the radical concentration [R ] with time during the polymerization of methyl methacrylate at 45 °C for different concentrations of AIBN initiator (A) 0.2 M (o) 0.1M ( ) 0.05 M, spectra obtained in situ during polymerization in the ESR spectrometer, (a) 0.05 M AIBN, spectra obtained by quenching to 77 K, enabling accumulation of scans 

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The triazolinyl radical 116 is thermally unstable with a half-life of -20 min at 95 °C. The compound 117 is stable under similar conditions. The decomposition mechanism involves loss of a phenyl radical and formation of a stable aromatic triazene (Scheme 9.26).24 This provides a mechanism for self regulation of the stable radical concentration during polymerization and a supplemental source of initiating radicals. 

Mol. wt. vs. initiator Radical concentration During polymerization At end of reaction... 

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. 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

This reaction is termed chain transfer to initiator and is considered further in Sec. 3-6b. The induced decomposition of initiator does not change the radical concentration during the polymerization, since the newly formed radical (polymer chain. However, the reaction does result in a wastage of initiator. A molecule of initiator is decomposed without an increase in the number of propagating radicals or the amount of monomer being converted to polymer. 

The radical concentration during the polymerization is not changed by reaction (6-49), but the initiator molecule involved has been wasted because its decomposition has not produced a net increase in the conversion of monomer to polymer. Induced decomposition reactions are negligible for azo initiators, but they can be very significant for some peroxides. Peroxydicarbonates, for example, have efficiencies which change greatly with reaction conditions. 

Figure 7. Time evolution of the persistent radical concentration during a polymerization of 0.76 M styrene in fe/t-butylbenzene at 130 °C initiated by the alkoxyamine 6 of Scheme 30 for different alkoxyamine concentrations. The solid lines confirm eq 18.

In polymerization of styrene in benzene at 60°C using 0.1 mol benzoyl peroxide initiator and 1 mol of monomer, is 1.38 x 10, assume steady state and calculate the free-radical concentration during the reaction. If is 176/mol what is the rate of propagation What is the lifetime of a growing radical if k is 7.2 x 10 What is the rate of propagation if the initiator efficiency is 72% ... 

Because the quasi-steady-state approximation for radical concentration during autoacceleration is invalid [35], determination of individual rate coefficient becomes possible under the assumption of an instantaneous quasi-steady state at the point of the irradiation termination. It is assumed that the change in radical concentration during a single measurement at one reaction point is negligible, so the classical expression for the polymerization rate can be used ... 

Electron spin resonance spectroscopy has been employed, together with the cryogenic quenching technique, by the same group [146] to monitor occluded radical concentrations during methyl methacrylate (MMA)/butyl acrylate (BA) core-shell polymerization reactions. 

Cutting GR, Tabner BJ. Radical termination and radical concentrations during the batch emulsion polymerization of methyl methacrylate studied by electron spin resonance spectroscopy. Macromolecules 1993 26 951-955. 

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. 

Most values of / have been measured at zero or low conversions. During polymerization the viscosity of the medium increases and the concentration of monomer decreases dramatically as conversion increases (i.e. as the volume fraction of polymer increases). The value of / is anticipated to drop accordingly. 32, u 9j % For example, with S polymerization in 50% (v/v) toluene at 70 °C initialed by 0.1 M AIBN the instantaneous" / w as determined to vary from 76% at low conversion to <20% at 90-95% conversion (Figure 3.3).32 The assumption that the rate of initiation (kAf) is invariant with conversion (common to most pre 1990s and many recent kinetic studies of radical polymerization) cannot be supported. 

The curve OAE in Fig. 17, calculated according to Eq. (50), shows the course of the rise in the radical concentration following the commencement of illumination when the initial radical concentration is zero, i.e. o = 0. Observation of the rate of polymerization as a function of the time during the interval preceding the steady state (i.e., for t < 2ts) would provide information suitable for the evaluation of r. ... 

The polymerization kinetics have been intensively discussed for the living radical polymerization of St with the nitroxides,but some confusion on the interpretation and understanding of the reaction mechanism and the rate analysis were present [223,225-229]. Recently, Fukuda et al. [230-232] provided a clear answer to the questions of kinetic analysis during the polymerization of St with the poly(St)-TEMPO adduct (Mn=2.5X 103,MW/Mn=1.13) at 125 °C. They determined the TEMPO concentration during the polymerization and estimated the equilibrium constant of the dissociation of the dormant chain end to the radicals. The adduct P-N is in equilibrium to the propagating radical P and the nitroxyl radical N (Eqs. 60 and 61), and their concentrations are represented by Eqs. (62) and (63) in the derivative form. With the steady-state equations with regard to P and N , Eqs. (64) and (65) are introduced, respectively ... 

Equation 3-22 for the polymerization rate is not directly usable because it contains a term for the concentration of radicals. Radical concentrations are difficult to measure quantitatively, since they are very low ( 10-8 M), and it is therefore desirable to eliminate [M- from Eq. 3-22. In order to do this, the steady-state assumption is made that the concentration of radicals increases initially, but almost instantaneously reaches a constant, steady-state value. The rate of change of the concentration of radicals quickly becomes and remains zero during the course of the polymerization. This is equivalent to stating that the rates of initiation Rj and termination R, of radicals are equal or... 

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