Chapter 2

Polymerization and Manufacture of Polyoxymethylene

Johannes Karl Fink

University of Leoben, Leoben, Austria

Abstract

This chapter describes the monomers that are used for polyoxymethylene and the methods of polymerization. The basic monomer is formaldehyde, but in addition the properties of the polymer can be tuned by using other monomers such as trioxane. As comonomers, ethylene oxide, tetrahydrofuran, dioxanes and dioxalones are used. Comonomers may also act as branching agents and chain transfer agents. The polymerization proceeds both as a cationic and anionic process. The process of polymerization is a ring-opening reaction. The polymerization process is an equilibrium process. As polymerization catalysts, various types can be used, such as Lewis acids, super strong acids, organometallic compounds, onium and boron compounds, as well as quaternary ammonium salts and various complexes. In the course of polymerization, undesired side reactions may occur. For this reason, the abovementioned commoners are introduced that can both hinder these side reactions and improve the thermal stability of the polymer. There are basically three poly(oxymethylene) types, i.e., the homopolymer, copolymers and block copolymers.

Keywords: Polyoxymethylene, monomers for poly(oxymethylene), methods of polymerization, catalysts for polymerization, industrial fabrication, special additives

2.1 Introduction

Formaldehyde itself was first prepared by Butlerov in 1859, who also noticed the polymerization [1-3]. Polyoxymethylene (POM) was first studied in detail by Staudinger in the 1920s [4, 5]. Remarkable enough, POM has been suspected to occur in comets as a source for the occurrence of formaldehyde [6, 7]. POM is also addressed as polyformaldehyde, since formaldehyde is the corresponding monomer. Further, POM is a polymeric acetal. Initially, problems with thermal stability, resulting from the end groups, were encountered, which delayed the commercialization of this polymer. The commercialization of POM started with the research by Starr and others [8, 9].

2.2 Monomers

The essential monomer is formaldehyde, however copolymers are known, which are cyclic ethers [10]. Monomers are listed in Table 2.1 and in Figure 2.1.

Table 2.1 Monomers for POM.

Figure 2.1 Monomers and comonomers used for POM.

2.2.1 Formaldehyde

Formaldehyde is a gas at room temperature. It is highly poisonous. In addition to POM it is used for phenol resins and urea resins. It is a basic chemical for synthesis in chemical industries.

Formaldehyde can be synthesized by the oxidation of methanol:

(2.1)

Using high temperatures of 650 °C, formaldehyde can be prepared by the dehydrogenation reaction of methanol:

(2.2)

In order to achieve an ecologically and economically interesting industrial process for the dehydrogenation of methanol, the following prerequisites have to be met [12]:

Examples of specific catalysts include sodium or sodium compounds, aluminum oxide, aluminates, or silver oxide [12]. In addition, catalysts based on zinc, indium, silver, copper and other group III and group IV metals have been described [12–15].

Recently a process for the synthesis of formaldehyde has been described that starts from methane by the oxychlorination of methane to produce methylene chloride [16]. The hydrolysis of methylene chloride then yields formaldehyde. Eventually, the gaseous formaldehyde is condensed for shipment. The byproducts chloroform and carbon tetrachloride are recovered and hydrogenated to provide additional methylene chloride.

The reaction is shown in Figure 2.2.

Figure 2.2 Synthesis of formaldehyde from methane [16].

The first reaction step, i.e., the oxychlorination, achieves the chlorination of methane and recycled methyl chloride used as the source of chlorine. A catalyst is needed and can comprise a copper salt. Because of the poor reactivity of methane, a sufficiently high reaction temperature is required, usually in excess of 375 °C.

This method is claimed to be a comprehensive solution to the manufacture of formaldehyde from methane. The only required raw materials are methane, hydrogen and oxygen or air. There are no byproducts or waste streams that must be handled. In this balanced operation, a maximum efficiency is attained. The chemistry is straightforward and easy to scale up [16].

2.2.2 Trioxane

Trioxane is a trimer of formaldehyde and is also addressed as metaformal-dehyde. In contrast to formaldehyde, it is a solid at room temperature. At 150–200 °C it depolymerizes again to formaldehyde.

Trioxane is prepared by the trimerization of formaldehyde, both in liquid phase and in gas phase [17]. The gas phase process is preferred. For the process in gas phase, as catalysts, vanadyl hydrogenphosphate hemihydrate, (VO)HPO4 x 1/2H2O and 11-molybdo-1-vanadophosphoric acid, H4PVMo11O40 can be used [18, 19]. When cyclic ethers are used as comonomers, there is a risk that these contain peroxides, in particular when they have been stored for a relatively long time before use. Peroxides firstly lengthen the induction time of the polymerization and secondly reduce the thermal stability of the POM formed owing to their oxidative effect [20].

2.3 Comonomers

As comonomers, cyclic ethers such as ethylene oxide, propylene oxide, butylene oxide and styrene oxide have been described. Of these, ethylene oxide is especially preferred.

Also, cyclic formals can be used, such as ethylene glycol formal (1,3-dioxolane), diethylene glycol formal, 1,3-propanediol formal, 1,4-butanediol formal, 1,5-pentanediol formal and 1,6-hexanediol formal. Of these, ethylene glycol formal (1,3-dioxolane) and 1,4-butanediol formal are preferred [21]. The formation of a cyclic formal is shown in Figure 2.3.

Figure 2.3 Formation of a cyclic formal.

2.3.1 Ethylene Oxide

Ethylene oxide is produced by the direct oxidation of ethylene with oxygen or air [22]. Ethylene and oxygen are passed over a silver catalyst, typically at pressures of 10–30 bar and temperatures of 200–300 °C. The reaction is exothermic and a typical reactor consists of large bundles of several thousand tubes that are packed with catalyst. A coolant surrounds the reactor tubes, removing the reaction heat and permitting temperature control.

Ethylene oxide is used as a chemical intermediate, primarily for the production of ethylene glycols but also for the production of ethoxylates, ethanol amines, and glycol ethers [22].

2.3.2 Propylene Oxide

Conventionally, propylene is prepared via the conversion of propylene to chloropropanol and subsequent dehydrochlorination [23], as shown in Figure 2.4. Propylene oxide is considered as a potential human carcinogen [23].

Figure 2.4 Synthesis of propylene oxide.

Another process for preparing propylene oxide has been described that consists of the reaction of propylene with thallium triacetate in the presence of water and an alkanoic acid, e.g., 2-ethyl hexanoic acid [24]. Propylene oxide can also be obtained from propylene using cumene hydroperoxide obtained from cumene as an oxygen carrier [25]. The reaction may be conducted in a liquid phase using a solvent, such as benzene, toluene, or octane, decane, and dodecane. The cumene can be used repeatedly. The oxidation of cumene is affected by autoxidation with an oxygen containing gas such as air, or oxygen-enriched air. The oxidation reaction may be carried out without any additive, such as an alkali. In this process the production of unnecessary organic acids and peroxides can be suppressed. The mechanism of formation is shown in Figure 2.5.

Figure 2.5 Synthesis of propylene oxide via cumene.

Other similar methods of synthesis give propylene oxide in combination with other valuable products [26]. Thes methods are suitable for the synthesis of related epoxides when other alkenes are used instead of propene, e.g., 1-butene, 2-butene, 1-pentene, 1-octene, or 1-dodecene.

2.3.3 Tetrahydrofuran

Tetrahydrofuran can be prepared from 1,4-butanediol using...