Kinetic analysis, of polymerization mechanisms

In this case, a simple kinetic model could not be derived to explain all the experimental observations. Independently prepared hexadecyl trimethyl ammoniurn persulfate was found to be soluble in toluene/AN mixtures and to catalyze polymerization in this homogeneous system at approximately the same rate as that observed in the two phase system. This result implies that anion exchange at the interface (see below) must be essentially complete under these conditions. Factors which complicated further analysis of the mechanism included a) precipitation of poly(acrylonitri 1 e) during the polymerization ... 

Metallic lithium in the form of a suspension has been used to polymerize isoprene (97) but the system is not too suitable for an exact analysis of the mechanism. The conversion-time curves are sigmoidal in shape. Minoux (66) has shown that the overall rate is not very dependent on the amount of lithium dispersion used as expected if the organo-lithium intermediates are highly associated. The molecular weight of the polymer is more dependent on quantity of lithium used. The observed kinetic behaviour is very similar to that shown in lithium alkyl initiation. This suggests that apart from differences in the initiation step, the mechanisms are quite similar. 

Stalagmometric determination of the SDDS adsorption at the aqueous solution-ethyl acrylate interface dependence on the rate of drop formation (volume 0.03 cm ) were carried out in our laboratory by Vasilenko. The measurements showed that establishment of adsorption equilibrium at the CMC occurs at drop formation periods of 15-20 sec, ije., at surface formation rates not exceeding 10 m sec. Adsorption kinetics acquires considerable importance in analysis of the mechanism of particle formation during emulsion polymerization, when tbe rate of organic phase formation and the rate of adsorption layer formation may be commensurate. 

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

Radical initiated polymerization of vinylpyridinium ions has been recognized since 1964 when Shyluk reported 1,2-addition polymerization of 1,2-dimethyl-5-vinylpyridinium methylsulfate initiated by potassium peroxydisulf-ate [39]. Later, Ringsdorf proposed a radical mechanism for the spontaneous polymerization of 1 (R = H) at concentrations greater than 1.0 M in water [37, 42]. The postulated mechanism was supported by experiments in which inhibition of polymerization was demonstrated by radical inhibitors such as dioxygen, copper (II) ions, and 5-butylcatechol. Kinetic analysis of these reactions was also consistent with the proposed radical pathway. 

Kinetic analysis of the polymerization revealed a first-order dependence on monomer, triazole carbene and alcohol when [triazolej/falcohol] = 1, operating by a mechanism comparable to that proposed previously (Scheme 14.9). However, altering the ratio of triazole to alcohol such that [triazole]/[alcohol] > 1 resulted in the observation of a nonlinear dependence of rate on [triazole], and suggesting that a second pathway for lactide enchainment had become significant [28]. The authors proposed that, at high [triazole]/[alcohol] ratios, the direct addition of lactide to the zwitterionic intermediate Z (Scheme 14.9, path D) was able to compete with proton transfer (Scheme 14.9, path B). Further confirmation of the presence of competing mechanisms was obtained by analysis of an unquenched polymerization sample by electrospray ionization mass spectrometry (ESl-MS), in which the primary peaks were attributed to triazole end-capped polymers, while minor f)eaks were observed for both hydroxyl-terminated and macrocyclic PLAs. 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

If chain transfer of the radical center to a previously formed polymer molecule is followed ultimately by termination through coupling with another similarly transferred center, the net result of these two processes is the combination of a pair of previously independent polymer molecules. T. G. Fox (private communication of results as yet unpublished) has suggested this mechanism as one which may give rise to network structures in the polymerization of monovinyl compounds. His preliminary analysis of kinetic data indicates that proliferous polymerization of methyl acrylate may be triggered by networks thus generated. 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

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 ... 

The first part of this report will illustrate how l C-NMR has been utilized in the elucidation of the polymerization mechanisms of cyclic ethers. In the second part, quantitative applications of 13C-NMR for determinations of thermodynamic and kinetic constants will be discussed. The last section deals with possible applications of quantitative 13c-nMR analysis in copolymerization of cyclic ethers. 

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