Polymerization formaldehyde

Industrial phenol-formaldehyde polymerization is a complex process, but the following reactions suggest the successive stages and the possible linkages involved ... 

Formaldehyde polymerizes even without added catalyst, but it is possible that traces of formic acid act as adventitious catalysts in this system. 

Formaldehyde polymers have been known for some time (1) and early investigations of formaldehyde polymerization contributed significantly to the development of several basic concepts of polymer science (2). Polymers of higher aUphatic homologues of formaldehyde are also well known (3) and frequently referred to as aldehyde polymers (4). Some have curious properties, but none are commercially important. 

The tetramethylol derivative of DABT, prepared by reaction of DABT with alkaline aqueous formaldehyde, polymerized readily on cotton. It imparted excellent flame retardancy, very durable to laundering with carbonate- or phosphate-based detergents as well as to hypochlorite bleach. This was accomphshed at low add-on without use of phosphoms compounds or antimony(III) oxide (75—77). 

A plant had a runaway reaction with a phenol-formaldehyde polymerization reaction. The result was one fatality and seven injuries and environmental damage. The runaway reaction was triggered when, contrary to standard operating procedures, all the raw materials and... 

Properties and handling. Formaldehyde is a colorless, toxic gas at room temperature, with a pungent, irritating odor. It is flammable and explosive in presence of air. Both gaseous and liquid forms of formaldehyde polymerize at room temperature, and because of this, it can only be maintained in the pure state for a very short period. Because of these unhandy conditions, there are two ways formaldehyde gets into commerce, as a water solution called formalin and as a solid called paraformaldehyde or trioxane. 

Melamine and formaldehyde, similar to urea-formaldehyde polymerization... 

Both liquid and gaseous formaldehyde polymerize readily at low temperatures and can be kept in the pure monomeric state only for a limited time. Because of these facts, formaldehyde is sold and transferred either in solution or in polymerized form, such as paraformaldehyde and trioxane, described here under Formaldehyde polymers Commercial, 37% solution of formaldehyde (So-called Methanol-free)... 

If the formaldehyde polymerization is exothermic, you may have a problem in interpretation. That is, dynamite probably works quite well when immersed in liquid helium in the presence of an appropriate spark. 

The experiments on the radiation-induced solid-state formaldehyde polymerization at 140 to 4°K were the first to demonstrate the molecular tunneling (i.e., the tunneling of whole molecules and/or molecular groups). 

Formaldehyde polymerizes by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerization and anionic polymerization is initiated by relatively weak bases (e.g., pyridine). Boron trifluoride (BF3) or other Lewis acids are used to promote polymerization where trioxane is the raw material. 

Formaldehyde polymerizes because the two resulting C-O o bonds are very slightly more stable than its C=0 k bond, but the balance is quite fine. Alkenes are different two C-C o bonds are always considerably more stable than an alkene, so thermodynamics is very much on the side of alkene polymerization. However, there is a kinetic problem. Formaldehyde polymerizes without our intervention, but alkenes do not. We will discuss four quite distinct mechanisms by which alkene polymerization can be initiated—two ionic, one organometallic, and one radical. 

U. Why does polymerization occur only at relatively low temperatures often below 200 °C What occurs at higher temperatures Formaldehyde polymerizes only below about 100°C but ethylene still polymerizes up to about 500 °C. Why the difference ... 

The authors of the three publications, so far described, could not find any evidence for N-zwitterion formation. Kiinzel, Giefer, and Kem41) believe they have. This group measured the effectiveness of a wide range of covalent bases as initiators of formaldehyde polymerization. 

The covalent initiators employed are almost always tertiary amines or phosphines. However, Kunzel, Giefer, and Kern43), who studied formaldehyde polymerization, compared triphenylamine, phosphine, stilbene, and arsine. They found a rough correlation... 

Secondly, the nature of the interaction between the monomer-initiator adduct and solvent will be of importance. Co-ordination of a donor solvent to the ammonium ion might be expected to reduce its deactivating inductive influence on the anion. This is analogous to the frequently observed difference in reactivity between free (solvated) and paired ions. Bulky substituents on nitrogen would prevent the formation of a tight solvation shell around it. In this connection two observations, made by Kern et al.43) on formaldehyde polymerization, are very relevant. 

Monodispersely-sized submircon zirconia colloids are useful starting material for ceramics, catalysts, and chromatographic stationary phases. One such process (polymerization-induced colloid aggregation or PICA process) requires entirely reproducible aqueous zirconia sols with no surfactants [1,2]. In the PICA process developed by Her and McQueston [3], the concentrated ( 20 wt %) 100 nm zirconia colloids are aggregated by urea-formaldehyde polymerization reaction to produce the porous zirconia particles in the size range of 4 - 6 pm [1,2],... 

Figure 4. Examples of low-temperature limit of rate constant of solid-state chamical reactions obtained in different laboratories of the USSR, United States, Canada, and Japan (1) formaldehyde polymerization chain growth (USSR, 1973 [56]) (2) reduction of coordination Fe-CO bond in heme group of mioglobin broken by laser pulse (United States, 1975 [65]) (3) H-atom transfer between neighboring radical pairs in y-irradiated dimethylglyoxime crystal (Japan, 1977, [72], (4, 5) H-atom abstraction by methyl radicals from neighboring molecules of glassy methanol matrix (4) and ethanol matrix (5) (Canada, United States, 1977 [11, 78]) (6) H-atom transfer under sterically hampered isomerization of aryl radicals (United States, 1978 [73]) (7) C-C bond formation in cyclopentadienyl biradicals (United States, 1979 [111]) (8) chain hydrobromination of ethylene (USSR, 1978 [119]) (9) chain chlorination of ethylene (USSR, 1986 [122]) (10) organic radical chlorination by molecular chlorine (USSR, 1980 [124,125]) (11) photochemical transfer of H atoms in doped monocrystals of fluorene (B. Prass, Y. P. Colpa, and D. Stehlik, J. Chem. Phys., in press.).

After the formaldehyde polymerization described in the beginning of this section, the next, example of a low-temperature kinetic plateau in reactions related to heavy molecular fragment transfer was observed by Buchwalter and Closs [111] in the photoisomerization of a 1,3-cyclopentadiyl (CPDY) biradical isomerized into a bicyclo-(2,10)-pentane (BCP) ... 

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. 

In some cases, with stannous acylates as initiators, formaldehyde polymerization [10] can be carried out to very high molecular weight polyethymethylene even in the presence of 2 mole % of water. Unlike most polyoxymethylenes made under scrupulously anhydrous conditions with common cationic or anionic initiators, which have a most probable molecular weight distribution, polyoxymethylenes made with stannous acrylates as the initiators have a very broad molecular weight distribution. 

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. 

In solvents of high dielectric constant, (e.g. dimethylformamide), formaldehyde polymerized sluggishly and polymers were formed in low yield. In similar solvents, aliphatic aldehydes could not be polymerized. Precipitated aldehyde polymers have the tendency to absorb monomers. The monomer concentration near the propagating site may be much hi er than that in the surrounding solution. This occurs with n-butyraldehyde polymerizations in pentane [5] the polymer precipitated during the polymerization is highly swollen by the monomer. 

Induction periods of various lengths have been reported for anionic and cationic polymerizations of formaldehyde. It is apparent that the compound that is added as initiator is rarely the actual initiator. Tetraalkyl ammonium acetate used for the potymerization of formaldehyde is a slow initiator but capable of initiating formaldehyde polymerization. Methanol, other alcohols or water, always present in the polymerization mixture, are responsible for the high rates of polymerization. They act as efficient chain transfer agents and alkoxide or hydroxide ions are the actual initiators they initiate by a factor of several powers of ten more efficiently than acetate. 

Very little is known about the effect of this interaction and how important this equilibrium is for the cationic polymerization, especially in solid/liquid interface reactions. Triethyloxonium fluoroborate, an excellent initiator for formaldehyde polymerization, can be visualized as an ethylcarbonium ion solvated by one mole of diethylether. 

Formaldehyde polymerization has been studied in the liquid state, in solution of protic or aprotic solvents and in the gaseous state where gaseous formaldehyde forms directly crystalline polymer. It has been studied with anionic and cationic initiators and by high energy radiations. Although there are more than 100 MM lbs. of poly form aldehyde produced per year, very few papers have been published that are actually concerned with the kinetics of formaldehyde polymerizations. The reason for this lack of detail is understandable when one realizes how difficult it is to obtain pure formaldehyde (with impurities of less than 100 p.p.m). Even pure formaldehyde undergoes side reactions and self condensation which cause new introduction of impurities. 

In the 1950 s and 1960 s a great number of patents [14] were issued suggesting hundreds of initiators for the polymerization of formaldehyde. No actual kinetic data were published, however, and even now the kinetic understanding of formaldehyde polymerization is limited. 

A more difficult question is the initiation of formaldehyde with amines, notably tertiary amines. Kem et al. [15, 16] recently discussed the initiation of formaldehyde with Lewis bases. They favour initiation of formaldehyde polymerization by direct addition to the nucleophilic end of the amine... 

The authors studied the polymerization of formaldehyde with amines including tertiary amines at —78°C in various solvents (Table 1), and determined the conversion after 15 min reaction time. Tertiary amines are highly reactive initiators for formaldehyde polymerizations even at the level of 10 mole T per mole 1 of formaldehyde. The reactivity of the amine is related to its pXg value but also to the branching of the aliphatic side chains of the substituents on the nitrogen atom. Branched amines, especially when the branching is on the a-carbon atom as in the case of a tertiary butyl group, are less effective initiators than tertiary amines with n-alkyl chains. The pX a of the amine is not the essential feature for an efficient tertiary amine initiator, because pyridine was almost as effective as tri-n-butylamine but quinoline, with a similar pK g as pyridine, is almost inactive (Table 1). 

Primary and secondary amines have been found to be much less active as initiators for formaldehyde polymerization, presumably because these compounds react with formaldehyde to form methylol compounds of the amines which are much less basic. 

It is interesting to note that triphenylphosphine is more effective as initiator than tertiary amines. In general it can be stated that the initial rate of formaldehyde polymerization with tri-n-butylamine is proportional to the amine concentration and the initial rate is also proportional to the monomer concentration (Figs. 1 and 2). The temperature dependence of... 

Machacek et al. [18—22] and Vesely and Mejzlik [23, 24] studied the kinetics of formaldehyde polymerization in ether dilatometrically at —58°C. They took into account the effect of impurities, chiefly water and formic acid. The rate of polymerization was chosen by varying the concentration of initiator and monomer so that the initial over-all rate did not exceed 5% conversion per min. Monomer concentrations in diethyl-ether ranged from 2.7 x 10 to 25 x 10 mole 1 . ... 

It was also observed that at constant n-butylamine concentration the over-all rate of polymerization increased with increasing water concentration. In the experiments illustrated in Fig. 3, the water concentration was 2.5x10 mole 1", about 1000 times higher than the initiator concentration. From extrapolation of the rate data to zero, it was noticed that some impurities were present in the polymerization mixture which used up part of the initiator (Fig. 4). Tlie two sets of data were thought by the authors to be due to the fact that they had to prepare a new batch of monomer for each series of experiments. Formic acid or CO2 were inhibitors for anionic formaldehyde polymerization. 

In another set of experiments the authors showed that the initial rate of formaldehyde polymerization was not affected when the water concentration was varied from 2.5 x 10 to 104 x 10 mole 1 at constant... 

In further work, formic acid was added up to 8 x 10 mole 1", while the monomer concentration (4.7 mole r )> initiator concentration (dibutylamine, 8 X 10 mole T ) and water (2.5 x 10 mole 1" ) were held constant. With increasing formic acid concentration the initial rate of formaldehyde polymerization decreased and the molecular weight of the polymer also decreased. 

The kinetics of formaldehyde polymerization in toluene solutions (80%) in the presence of tetrabutylammonium laurate, triethyl amine and calcium stearate were also studied. The initiator activity of these compounds decreased in the order tetrabutylammonium laurate> triethyl amine > calcium stearate. It was found that with triethyl amine no spontaneous polymerization was observed. Spontaneous polymerization was apparently an anionic polymerization and was inhibited by CO or formic acid. In our opinion this is an indication that tertiary amines need a co-initiator for formaldehyde polymerization. In the case of water as the co-initiator HO was the initiating anion which was inhibited by CO 2. 

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