Coefficient free radical polymerization

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques. By comparing the infrared absorption spectra of polybutadiene and of the olefins chosen as models whose ethylenic structures correspond to the respective structural units, it has been possible to show that the bands occurring at 910.5, 966.5, and 724 cm. are characteristic of the 1,2, the mns-1,4, and the m-1,4 units, respectively. Moreover, the proportion of each unit may be determined within 1 or 2 percent from measurements of the absorption intensity in each band. The extinction coefficients characteristic of each structure must, of course, be known these may be assigned from intensity measurements on model compounds. Since the proportions of the various units depend on the rates of competitive reactions, their percentages may be expected to vary with the polymerization temperature. The 1,2 unit occurs to the extent of 18 to 22 percent of the total, almost independent of the temperature, in free-radical-polymerized (emulsion or mass) poly butadiene. The ratio of trans-1,4 to cfs-1,4, however,... 

One method is to measure chain-transfer coefficients with low-MW analogues of the polymer. Thus Gilchrist (140) measured the rate at which 14C labelled decane was incorporated into polyethylene in the free-radical polymerization, and hence obtained an estimate of the transfer coefficient with methylene groups this was in fair agreement with another estimate obtained from the effect of the addition of fractions of linear polyethylene on the Mn of the branched polyethylene, which could be separated from linear polymer plus grafted branched polymer by column extraction. Low MW polymer may be used as a transfer agent Schulz and co-workers (189) obtained chain-transfer coefficients in styrene polymerization from the effect of added low MW polymer on Mn. 

As mentioned in Section 9.3, Jackson (141) has obtained estimates of the chain-transfer coefficient of the growing radical with polymer in the free-radical polymerization of ethylene, C,p, by choosing the value so as to fit the MWD. As the polymerization conditions for the polymers mentioned in Table 10.1 are not disclosed, it is necessary to choose typical conditions 220° C and 2000 atm will be selected. Under these conditions Ctp, the ratio of the rate constant for attack on polymer (per monomer unit) to that for propagation, in a homogeneous phase, was found to be about 4.0 x 10 3. This is in good agreement with the known transfer coefficients for the lower alkanes (160), when allowance is made for the differences in pressure and temperature (100). The relation between Ctp and k is ... 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

Once least squares values of the /3 s were obtained, it was desirable to extract from them as much information as possible about the original parameters. To do so, we make one further statement concerning the relations between the rate constants for mutual termination of polymeric radicals of different size. It has been shown (2) that termination rates in free radical polymerizations are determined by diffusion rates rather than chemical factors. The relative displacement of two radicals undergoing Brownian motion with diffusion coefficients D and D" also follows the laws of Brownian diffusion with diffusivity D = D -J- D" (11). It... 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Free-radical polymerization is an inherently statistical process with the probability of the reactions being discussed in the earlier sections being dictated by their rate coefficients. However, during the last 10 yr, a lot of effort has been directed toward controlling the statistical nature of this process. " ... 

Polymers were prepared by AIBN-initiated free-radical polymerization in degassed toluene at 6O C. With the exception of poly(4-aceto-l-naphthyl methacrylate), the polymers precipitated on formation. Polymers were purified by reprecipitation from CHCl 3 into methanol. Copolymer compositions were determined by UV spectroscopy using the extinction coefficients of the corresponding homopolymers and of 2,4-di aceto-l-naphthyl methacrylate monomer as references. Tables I and II list the properties of the homopolymers and copolymers used in the present study. 

From this relation it can be seen that the free-radical polymerization will be of first order in monomer, M, and the effective rate coefficient, will have a temperature dependence that will depend on the activation energies E, Ep and of the elementary reactions ... 

A numerical study of the free-radical polymerization of styrene (Scheme 6.15) compared the behavior of an interdigital micromixer with a T-junction and a straight tube [37, 48], The diffusion coefficient of the reactive species was varied to simulate the viscosity increase during a polymerization. The performance of the polymerization turned out to be largely dependent on the radial Peclet number. This dimensionless number is defined as the ratio of the characteristic time of diffusion in the direction perpendicular to the main flow to the characteristic time of convection in the flow direction (i.e., the mean residence time) and, therefore, is directly proportional to the characteristic length of the reactor. 

The termination reaction of free radical polymerization is a typical example of an intermacromolecular diffusion controlled reaction.3 Photophysical studies carried out in the 1980 s demonstrated for the first time that the reaction is solvent- and molecular weight-dependent. The experiments involved triplet quenching of probes attached to polymer chain ends. A benzil group was linked to the end of one PS sample (PS-B) and an anthryl group was linked to the end of a second PS sample (PS-A). The quenching rate coefficient kq of the benzil phosphorescence by anthryl groups is given by Eq. (3.26), where r0 is the lifetime of benzil phosphorescence in the absence of anthryl and ris the benzil phosphorescence lifetime in the presence of anthryl in concentration [A],... 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

Section 4.6.2 illustrates the experimental procedures that have recently been applied toward the study of high-pressure free-radical polymerization processes. Section 4.6.3 presents results of propagation, termination, chain-transfer (to monomer and to polymer), and P-scission rate coefficients for ethene homopolymerization. Recent results from experiments and modeling investigations into high-pressure copolymerizations (with ethene being one of the monomers) are reported in Section 4.6.4, together with information on homopolymerization rate coefficients of the comonomer species. 

Termination rate coefficients, of several acrylate monomers have been determined via SP-PLP experiments. Figure 4.6-13 shows the dependence of on monomer conversion for free-radical polymerizations of methyl (MA), butyl (BA), and dodecyl acrylate (DA) at 40 °C and 1000 bar. 

Beuermann, S., Buback, M., Davis, T.P., et al., 1997. Critically evaluated rate coefficients for free-radical polymerization 2. Propagation rate coefficients for methyl methacrylate. Macromol. Chem. Phys. 198 (5), 1545-1560. 

Gilbert, R.G., 1996. Critically-evaluated propagation rate coefficients in free radical polymerizations I. Styrene and methyl methacrylate. Pure Appk Chem. 68 (7), 1491-1494. 

M. Ohoka, S. Misnmi, and M. Yamamoto. Determination of propagation rate coefficient of free-radical polymerization of A-vinylcarbazole by pulsed-laser polymerization. Po/ym. /., 31(10) 878-882, 1999. 

Kapfenstein, H.M. and Davies, T.P., Studies in the application of MALDI-TOF MS to the Determination of Chain Transfer Coefficients in Free Radical Polymerization, Macromol. Chem. Phys., 199, 2403 (1998). 

MALDI end-group analysis was investigated for the determination of the chain transfer coefficients (Cs) for free-radical polymerization of PMMA with AIBN as inihator and 2-methyl-2-propanethiol as a chain transfer agent in toluene. Flowever, the values of Cs obtained were not consistent with those obtained with standard methods, and the relahve intensities of the peak series relahve to the oligomers with different end-groups were found to be dependent on the selechon of cahon (Li" or Na - ... 

Zammit, M. D., Davis, T. R, and Haddleton, D. M., Determination of the Propagation Rate Coefficient (kp) and Termination Mode in the Free-Radical Polymerization of Methyl Methacrylate, Employing Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry for Molecular Weight Distribution Anal5 is, Macromolecules, 29, 492, 1996. 

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