PLP-SEC

The PLP-SEC method, like the rotating sector method, involves a non-steady-state photopolymerization [Beuermann, 2002 Beuermann and Buback, 2002 Komherr et al., 2003 Nikitin et al., 2002], Under pulsed laser irradiation, primary radicals are formed in very short times ( 10 ns pulse width) compared to the cycle time ( 1 s). The laser pulse width is also very short compared to both the lifetimes of propagating radicals and the times for conversion of primary radicals to propagating radicals. The PLP-SEC method for measuring kp requires that reaction conditions be chosen so that no significant chain transfer is present. The first laser pulse generates an almost instantaneous burst of primary radicals at high 

L is obtained from the first point of inflection on the low-molecular-weight side of the major peak of the molecular weight distribution plot. In practice this is most easily determined from the maxima in a differential plot of the molecular weight distribution, chi (log M)/d log M versus log M, the dotted plot in Fig. 3-12. For this polymerization it is easy to detect not only L. but also Li, L3, and Lj. The detection of several Lt values acts as an internal check on the PLP-SEC method. For Fig. 3-12, the kp values calculated from L, La, and L are within 2% of each other the value from L4 is within 6-7% of the other values. In practice, closeness between kp values from L and L2 are considered sufficient to validate the PLP-SEC method. 

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Measurements of kp were performed by the rotating sector method and its variations until the late 1980s [Flory, 1953 Odian, 1991 Walling, 1957]. In the late 1980s advances in pulsed laser technology and size exclusion chromatography resulted in the development of a technique called pulsed laser polymerization-size exclusion chromatography (PLP-SEC) [Beuermann and Buback, 2002 Beuermann et al., 1997, 2000 Buback et al., 1986, 1992,... 

A prime purpose of the IUPAC Working Party on Modeling of Kinetics and Processes of Polymerization has been standardization of the experimental conditions and calculation methods for obtaining rate constants and other parameters. Table 3-12 shows the PLP-SEC values of the propagation rate parameters for a number of monomers. For many monomers there is good agreement between the values obtained from the rotating sector and PLP-SEC methods. 

Solvent effects including 2-methyl-l,3-dioxepane (MDOP), as a solvent, on the propagation kinetics of methyl acrylate (MMA) have been investigated using the PLP-SEC technique (PLP = pulse laser polymerization) <2005MI267>, and the composition of dioxolane-dioxepane copolymers has been studied by IR and differential scanning calorimetry (DSC) <2004PB349>. 

For homopolymerizations of methyl methacrylate and of styrene, benchmark value data sets have already been put forward [19,20] and critical data evaluation, by this working party, of kp data for other alkyl methacrylates, functional methacrylates, and alkyl acrylates is underway. Values of for the ethene high-pressure polymerization have not yet been derived from PLP-SEC... 

Figure 4.6-2 Characteristic features of the PLP-SEC measurement (a) the pulsed laser-induced free-radical concentration (cr) versus time profile, where v is the pulse repetition rate (b) the resulting (simulated) polymer molecular weight distribution, wClogio M).

For acrylate PLP-SEC work only limited temperature ranges are available because of the high chain transfer to monomer activity of these substances [55-59]. The pressure dependence has, however, been investigated up to technically relevant conditions (2000 bar) [55]. Experimental results for acrylates and methacrylates are summarized in Figure 4.6-12. The variation of kp with... 

Propagation rate coefficients for styrene, vinyl acetate, and various acrylates and methacrylates were determined by the so-called PLP-SEC technique, which combines pulsed-laser-initiated polymerization (PLP) with subsequent polymer analysis via size-exclusion chromatography (SEC). The PLP-SEC experiment measures the product of propagation rate coefficient, kp, and monomer concentration, Cm ... 

Equation (5), in which Cm and cr are the monomer concentration and the pulse-induced initial radical concentration at t=0, respectively, is used for fitting the experimental monomer concentration vs time traces to yield the parameters (kt)/kp and (kt) Cr. If kp is known from independent PLP-SEC experiments, (kt) is obtained as a function of monomer conversion from successive SP-PLP ex-... 

Since radicals have a certain probability to survive the laser flash and to terminate at a later pulse, the relative concentration of polymer with chain lengths 2DPq, 3DPq,. .., is also increased. As a result, PLP produces a well-structured MWD with peaks at chain length of DPo and its multiples. PLP-SEC has proven to be a robust technique for determining fcp and... 

The PLP-SEC method has been utilized to show that there is little to no solvent influence on propagation kinetics of most monomers as a diffusion-controlled reaction, the same cannot be said for termination [10]. However, the propagation kinetics of water-soluble monomers such as methacrylic and acrylic acid are a strong function of concentration and degree of ionization in the aqueous phase [12]. 

Keywords aqueous-phase polymerization free-radical polymerization methacrylic acid PLP-SEC propagation rate coefficients pulsed-laser initiation water-soluble monomers... 

The PLP-SEC investigations into kp of free-radical polymerization in aqueous phase suggest that kp varies strongly with monomer concentration. For MAA, NIPAm ) and AAm a strong decrease in kp was found upon increasing monomer concentration. The same trend is seen for AA from monomer concentrations of 3 wt.-% on, whereas at very low AA contents kp increases with acrylic acid concentration. Attempts to assign the strong solvent effects to associated struc-tures, ) to dimerization,f l or to local monomer concentrations at the radical site... 

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