Aldehydes polymerization

A characteristic of aldehyde polymerization is the precipitation, often with crystallization, of the polymer during polymerization. Depending on the solvent used, polymerization rate, state of agitation, and other reaction conditions, the polymerization can slow down or even stop because of occlusion of the propagating centers in the precipitated polymer. The physical state and surface area of the precipitated polymer influence polymerization by their effect on the availability of propagating centers and the diffusion of monomer to those centers. 

Vogl, O., Kinetics of Aldehyde Polymerization, Chap. 5 in Comprehensive Chemical Kinetics, Vol. 15,... 

The submitters report that /3-methylglutaraldehyde may be isolated at this point from an analogous hydrolysis. The hydrolysis is carried out with 196 g. of 3,4-dihydro-2-methoxy-4-methyl-2H-pyran in 650 ml. of water and 15 ml. of concentrated hydrochloric acid for 3 hours. After neutralization with sodium bicarbonate, the solution is saturated with sodium chloride and extracted continuously with ether for 20 hours. The ether is removed by distillation, and the product is dried thoroughly by azeotropic distillation using a benzene-hexane mixture. Distillation affords /3-methylglutaraldehyde, b.p. 85-86°/15 mm., 1.4307-1.4351. Yields up to 90% have been secured. The aldehyde polymerizes on standing but is stable as a 50% solution in water or ether. The monomer may be recovered by careful destructive distillation of the polymer. 

ArCHO — ArCOOH. Aromatic aldehydes can be oxidized to the corresponding acid by this reagent (excess) at 25-40° in CH,C12 yields 30-92%. Aliphatic aldehydes polymerize under these conditions. 

Polymerization of aldehydes can be accomplished only at low temperatures. The influence of temperature on aldehyde polymerization initiated by potassium triphenylmethoxide is shown in Figure 3 with n-butyraldehyde as the example. At -78° C. the polymerization goes to high conversions (95%) the conversion decreases substantially with increasing temperature. No polymerization is observed above -15° C. 

Rate of aldehyde polymerization was studied at -75° C. Figure 3 shows that polymerization commences rapidly and is two-thirds complete in 5 minutes in 10 minutes equilibrium is reached at around 95-97% conversion. 

In this review the polymerization of formaldehyde, h her aliphatic aldehydes and haloaldehydes will be discussed with particular emphasis on the kinetics of the polymerization. As will be apparent the kinetics of aldehyde polymerization have not been studied as extensively as the kinetics of more conventional polymerizations, for example, the free radical bond opening polymerizations of styrene, vinyl chloride or methylmethacrylate or the ring opening polymerizations of tetrahydro-furan or ethylene oxide. One reason is that polyoxymethylene is the only polyaldehyde produced commercially and much of our knowledge on formaldehyde polymerization is proprietary information. Another is that the polymerization systems are very complex and the polymers precipitate during polymerization. 

Aldehyde polymerizations are carried out in aprotic anhydrous media. Even small amounts of protonic impurities cause efficient chain transfer reactions and low molecular weight polymer is formed. 

This brings us to an important point in aldehyde polymerization, the problem of precipitation or crystallization of the polymer during polymerization. In all cases of aldehyde polymerization where crystalline polymers are formed, in formaldehyde polymerization and higher aldehyde polymerization to isotactic polymers, precipitation occurs during the polymerization. 

Most aldehyde polymerizations are carried out in solvents of low dielectric constant. The solubility of aldehydes in low dielectric constant solvent is limited. At room temperature, formaldehyde is soluble in pentane only to the extent of 0.5% and in toluene of 2%. Nevertheless the polymerization often proceeds as fast as the monomer can be supplied. Acetaldehyde is miscible with pentane in all proportions above —30°C and... 

Nothing is known about the actual initiators of aldehyde polymerization using Lewis acids. It is, however, believed that a stable counter-ion is always essential for the formation of high molecular weight polymers. An... 

We have presented all these points in order to make the reader realize why relatively few papers in the literature are concerned with the kinetics of aldehyde polymerizations. It is almost impossible to take into consideration all the facts that have been discussed in this introduction in each experiment. Consequently, most authors report simply the time versus conversion curve of the polymerization without a detailed scrutiny of the individual factors. In addition, aldehyde polymerizations are fast, in some cases almost explosive with poor temperature control, and many aldehyde polymerizations are carried out in a semibatch process with continuous addition of monomers, although we know commercial processes are carried out in continuous reaction. 

The only useful conversion—time curve for an anionic aldehyde polymerization has been obtained for 1 n-butyraldehyde (25% solution in pentane) at —78°C with potassium triphenyl methoxide (Fig. 23) [59]. The conversion was determined from polymer weight and monomer remaining in solution. It was shown that the polymerization is very rapid and an 80% conversion is reached in 5 min. 

The literature of aldehyde polymerization, particularly acetaldehyde polymerization, with modified aluminium alkyls as initiators is voluminous. The earliest studies were done by Ishida [60]. He found that there were remarkable differences in polymerization rate behaviour when the... 

It should be noted that some authors [198, 199] connected polymer dyeing at thermal destruction with aldehyde polymerization ... 

Thus the value of Tc really revolves around the actual values of AH° and AS° for polymerization. Table 6.13 lists AH , A5 and calculated values of AG at 25°C for several olefin and aldehyde polymerization systems. The values of Tc calculated from Eq. (6.197) using AH and AS values at 25°C, with the assumption that standard enthalpy and entropy changes have no significant temperature dependence, are also shown in Table 6.13. It is... 

Thiols are oxidized by NH2CI to disulfides (Ingols et al., 1953). Monochloramine reacts with aldehydes (although in many cases the reaction is slow Hauser and Hauser, 1930 Conyers and Scully, 1993) to form N-chloroimines (R-CH = NC1) which then undergo further reactions (elimination of HCl to the nitrile hydrolysis to aldehydes polymerization etc.)... 

Aldehyde polymers were probably known well over 100 years ago. In spite of that, polyoxymethylene is the only product from aldehyde polymerization that is produced in large commercial quantities. Formaldehyde polymerizes by both cationic and anionic mechanisms. An oxonium ion acts as the propagating species in cationic polymerizationsIn the anionic ones, the propagation is via an alkoxide ion. 

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