Anionic polymerization, formaldehyde

Polyoxymethylene, also referred to as acetal resin or POM, is obtained either by anionic polymerization of formaldehyde or cationic ring-opening copolymerization of trioxane with a small amount of a cyclic ether or acetal (e.g., ethylene oxide or 1,3-dioxolane) [Cherdron et al., 1988 Dolce and Grates, 1985 Yamasaki et al., 2001]. The properties and uses of POM have been discussed in Sec. 5-6d. 

The industrial synthesis of polyformaldehyde [poly(oxymethylene)] occurs by anionic polymerization of formaldehyde in suspension. For this the purification and handling of monomeric formaldehyde is of special importance since it tends to form solid paraformaldehyde. After the polymerization the semiacetal end groups have to be protected in order to avoid thermal depolymerization (Example 5-13). This is achieved by esterfication with acetic anhydride (see Example 5-7). As in the case of trioxane copolymers (see Sect. 3.2.3.2) the homopolymers of formaldehyde find application as engineering plastics. 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

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. 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

Cyclic acetals polymerize exclusively by cationic mechanism it should be noted, however, that polyoxymethylene may be obtained also by anionic polymerization of formaldehyde. Both processes are used in industry. 

Cationic polymerization of 1,3,5-trioxane provides one of a few examples of industrial application of cationic ring-opening polymerization. Polymerization leads to polyoxymethylene (polyformaldehyde, polyacetal), important engineering plastic. Polyformaldehyde may also be obtained by anionic polymerization of formaldehyde and this process is also used in industry. 

The polyoxymethylenes are presently widely used in different areas. Approximatively one-third of the market is represented by homopolymers and two-thirds by copolymers. Homopolymers are produced by anionic polymerization of formaldehyde using amines, alkoxides, and other types of anionic initiators. The details of these polymerizations will not be discussed in this paper, although some of their properties will be compared to those of copolymers which are obtained by cationic copolymerization of trioxane with cyclic ethers or cyclic esters. Comprehensive reviews on general aspects of synthesis and properties of acetal resins are available [158-162],... 

Anionic polymerization of anhydrous formaldehyde has been disclosed in numerous patents, including the use of amines, phosphines, onium compounds, etc., as initiators. Mechanistically the propagation of the anionic polymerization of formaldehyde proceeds via the polymeric alkoxide ion,... 

Fig. 1. Conversion (in %) of anionic polymerization of formaldehyde in diethyl ether at —78°C as a function of the tri-n-butylamine concentration. Polymerization time 15 min ( ) 3.52 mole 1 CH2O (o) 3.64 mole 1" CH2O.

Fig. 2. Anionic polymerization of formaldehyde in diethyl ether at —78°C with tri-n-butylamine,...

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. 

Formaldehyde polymerization can also be carried out in anhydrous media with cationic initiators. This polymerization is much more complicated than anionic polymerization and may be characterized by the following basic reactions. The electrophilic initiator adds to the carbonyl oxygen with the formation of an oxonium ion... 

The anionic polymerization is mechanistically analogous to the anhydrous formaldehyde polymerization, viz. 

Scheme 1.25. Elementary reactions in the anionic polymerization of formaldehyde to form polyacetal.

By the anionic polymerization of butadiene and the functionalization of the active ends of the polymer by formaldehyde or ethylene oxide ... 

Zwitter-ionic character of anionic polymerization of formaldehyde is claimed by Buehlke et al.243> but details of that study are not available. 

Polymerization of 1,3,5-trioxane (TXN) gives linear polyoxymethylene (POM), a homopolymer of formaldehyde 39). This is the only polyacetal made on the technical scale. Two methods are used for the industrial production of stable, high-molecular-weight POMs. This is either the anionic polymerization of formaldehyde or the cationic copolymerization of the cyclic trimer of formaldehyde TXN with ethylene oxide or 1,3-dioxolane (DXL) ... 

PREPARATIVE TECHNIQUES The homopolymer is prepared by anionic polymerization of purified formaldehyde with the addition of an initiator such as an amine, phosphine, or metal alcohol. The copolymers are manufactured commercially by copolymerization of trioxane, the cyclic trimer of formaldehyde, with small amounts of a comonomer. T5qjically, acetal copolymer reisns have 95% or more oxymethylene units. 

The polymerization of formaldehyde can proceed by a cationic, anionic, or insertion mechanism. The anionic polymerization is started by amines, amidines, amides, ammonium salts, phosphines, etc., and is propagated by alkoxy ions ... 

Example 3.22 Anionic Polymerization of Formaldehyde in Solution (Precipitation Polymerization)... 

Acetal polymers are formed from the polymerization of formaldehyde. They are also given the name polyoxymethylenes (POMs). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s, when DuPont developed Dehin. Hompolymers are prepared from very pure formaldehyde by anionic polymerization as shown in Fig. 2.1. Amines and the soluble salts of alkali metals catalyze the reaction. The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldehyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stabihty is copolymerization with a second monomer, such as ethylene oxide. The copolymer is prepared by cationic methods developed by Celanese and mar-... 

Anionic Polymerization. Anionic polymerizations (Eqs. 8-9) are industrially employed for the production of thermoplastics, most notably that of formaldehyde and -caprolactam and block copolymers (e.g., thermoplastic elastomers of the styrene-butadiene-styrene type). Initiation by alkali metals is achieved by adding across the double bond in anionic polymerization. 

This unzipping depolymerization occurs during polymerization, but it may also occur under thermal stress with the neutralized polymer, unzipping then starts from neutral but unstable end groups such as -OH or -CHO or from statistical chain sdssion. Unzipping will stop at comonomer units such as those from ethylene oxide, dioxepane, and similar monomers, which are not able to depolymerize, and a then stable copolymer will result. Homopolymers, which are usually polymerized anionically from formaldehyde (see Section 7.2.3) will not be stable unless unstable end groups are transformed to stable ones. 

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