Linear, irreversible polymerizations

Thermosets are formed by crosslinking (curing) of reactive linear and branched macromolecules and can be manufactured by polycondensation, polymerization and polyaddition. Thermosets can therefore be processed once only with the application of heat and pressure to form semi-finished products or finished articles and cannot be recovered their processing is irreversible. Amongst the most familiar thermosets are the combinations of formaldehyde with phenol, resorcinol etc. (phenolics), urea, aniline, melamine and similar combinations (aminoplastics). 

The absence of EC in the discussions of all these reports " is interesting, since one would suspect that, based on the knowledge about EC reactivity, alkoxide could also attack its cyclic structure and cause irreversible reactions.One possibility would be that, using gas chromatography, the identification of a high-volatility species such as linear carbonate is easy and reliable, while the formation of the polymeric species, which EC would most likely decompose into, is more difficult to detect. 

Since in irreversible linear polymerization the cyclic molecules once formed do not react further, the kinetics equations can be written thus (here Q is the concentration of cycles) ... 

Many reactions familiar to organic chemists may be utilized to carry out step polymerizations. Some examples are given in Table 2.2 for polycondensation and in Table 2.3 for polyaddition reactions. These reactions can proceed reversibly or irreversibly. Those involving carbonyls are the most commonly employed for the synthesis of a large number of commercial linear polymers. Chemistries used for polymer network synthesis will be presented in a different way, based on the type of polymer formed (Tables 2.2 and 2.3). Several different conditions may be chosen for the polymerization in solution, in a dispersed phase, or in bulk. For thermosetting polymers the last is generally preferred. 

In summary, it has to be mentioned that in many studies intrinsic viscosity, inherent viscosity and dilute solution viscosity (DSV) were used in order to monitor the increase of molar mass on monomer conversion. Unfortunately, only a few studies use GPC rather than viscosity measurements. For a few Nd-carboxylate-based catalyst systems linear dependencies of Mn on monomer conversion were established and proof in favor of requirement No. 2 linear increase of Mn with monomer conversion (no irreversible chain transfer) was provided. A more detailed analysis of the data, however, reveals deviations from linearity particularly at low monomer conversions (< 20%). These deviations are particularly pronounced for polymerizations with induction periods. Also the extrapolation of the straight lines to zero monomer conversion reveals intercepts on the Mn-axis. 

If requirement No. 2 linear increase of Mn with monomer conversion (no irreversible chain transfer) is substituted by the more stringent requirement linear dependence of Mn on monomer conversion which passes through the origin it has to be concluded that the compliance of Nd-catalyzed polymerizations with requirement No. 2 is rather an exception than a general rule. 

Ethenyl acetate (vinyl acetate, Vac) is polymerizable only by radical species. Until recently, the polymerization of any monomer was out of control because of the unavoidable occurrence of irreversible termination reactions. In 1995, Matyjaszewski and Sawamoto and coworkers reported that the deleterious impact of these irreversible reactions could be minimized by acting on the kinetics of both the propagation and the termination reactions. Indeed, a decrease in the instantaneous concentration of radicals ([M ]) decreases much more importantly the termination rate (proportional to [M ] ) than the propagation rate (proportional to [M ]). A scheme proposed consists in converting reversibly radicals into unstable covalent species ( dormant species). The last radically polymerizable monomer to fall under this type of kinetic control was vinyl acetate. Indeed, very recently Debuigne and coworkers proposed to polymerize Vac by 2.2 -azobis-(4-methoxy-2,4-dimethyl)valeronitrile (V-70) in the presence of cobalt(II) acetyl acetonate [Co(acac)2]. Under these conditions, a linear relationship is observed between... 

In the cationic polymerization of heterocycles, a similar phenomenon was observed by Goethals in the polymerization of propylene sulfide and trans 2,3-dimethyl-thiirane. The latter monomer polymerizes rapidly and quantitatively to a linear polymer which is then relatively slowly converted into 3,4,6,7-tetramethyl-l, 2,5-tri-thiepane (J67a). In this particular process, the macroring formation is a practically irreversible reaction and differs in this sense from the equilibrium processes discussed so far. The irreversibility is due to the formation of one molecule of cis-butene per one molecule of a cyclic trithiepane ... 

It is worth emphasizing that the linear — ln(l — C) vs time plots starting at the origin prove the absence of irreversible termination in these polymerizations (see also Fig. 42B) [1, 21]. 

The linear — ln(l — Q vs time plots prove the absence of irreversible termination (see Sect. 4.2.23.2). The concentration of MtX affects both IB and St polymerization. At higher [TiCl4], kc and k are apparently higher, presumably because of the higher concentration of growing sites and lower concentration of dormant species (see Scheme 1, equilibria on the left in both sides, and also Sect. 4.1.1.1.3). Increasing [TiCl4] increases the relative rate of initiation (see Sect. 4.1.1.2.3) and the rate of polymerization (see Sect. 4.1.1.4.4). Contrary... 

The MALDI-TOF-MS analysis of the polyMMA, obtained in linear mode shows only one series of peaks, whose interval was regular, ca. 100, the molar mass of MMA unit. It indicates the absence of irreversible chain termination processes via recombination or disproportionation. According the proposed mechanism of polymerization the absolute masses of the peaks should be equal... 

The linear increase of PVAc molecular weight with monomer conversion indicates that the polymerization was a controlled/ living process. But the high polydispersity of PVAc and the nonlinearity of the semilogarithmic plot of ln[M] t suggests notable radical terminations due to the irreversible release of the growing radical chains from the cavity in the dendrimer. 

C. S. Marvel, J. Dec, and H. G. Cooke, Jr. [/ Am. Chem. Soc., 62, 3499 (1940)] employed optical rotation measurements to study the kinetics of polymerization of vinyl-1-phenylbutyrate. In dioxane solution the specific rotation angle represents a linear combination of contributions from the monomer proper and those of the polymerized monomer units. The contribution of the polymerized units can be viewed as independent of chain length. The reaction takes place in a constant-volume system and may be viewed as irreversible. The stoichiometry of the reaction may be viewed as A -I-Pjj Pn+i where A represents the monomer and P the polymer. The following data are characteristic of this reaction. 

The Choice of a Matrix Resin. As was mentioned earlier, the common matrix resins for today s lithography are phenolic resins such as Novolac and poly(4-hydroxystyrene). Though some of our early work had involved simple water soluble alcohols such as poly(vinyl alcohol), schemes for their reversible in situ insolubilization were sometimes complicated by irreversible processes or side-reactions. As a result we chose to test the water-soluble linear polymer that is obtained by free-radical polymerization of 2-isopropenyl-2-oxazoline, 1. Monomer 1 can be polymerized through a variety of techniques, as shown in Scheme 1 (6,7). Both radical or anionic polymerization conditions lead to a polymer containing pendant oxazoline rings, while a more complex structure is obtained under cationic conditions as both the vinyl and the oxazoline moieties are reactive. 

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