Chain transfer trioxane

The enthalpy of the copolymerization of trioxane is such that bulk polymerization is feasible. For production, molten trioxane, initiator, and comonomer are fed to the reactor a chain-transfer agent is in eluded if desired. Polymerization proceeds in bulk with precipitation of polymer and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. The mixing requirements for the bulk polymerization of trioxane have been reviewed (22). Raw copolymer is obtained as fine emmb or flake containing imbibed formaldehyde and trioxane which are substantially removed in subsequent treatments which may be combined with removal of unstable end groups. 

An additional termination in the trioxane polymerization is chain transfer to monomer hy hydride ion transfer, which results in terminating the propagating chain with a methoxyl group while carbocation XXII reinitiates polymerization [Kern et al., 1966 Weissermel et al., 1967]. 

This method is not affected by the side reactions described above. Elimination and addition of formaldehyde as well as cleavage of oxacyclic compounds and chain transfer by polymer regenerate predominantly Pi+, the desired active center. As noted above, fi is the more important reactivity ratio in copolymerizations of dioxolane with a large excess of trioxane. The value of can be determined conveniently by the de-... 

The occurrence and a limited importance of chain transfer by transacetalization cannot be doubted. We proposed this type of reaction for trioxane polymerization as early as 1959 (6) and assumed that intramolecular transacetalization produces some thermally stable macrocyclic polyoxymethylene (10). We have utilized bimolecular chain transfer by polymers to produce thermally stable block copolymers at temperatures over 100°C. [—e.g., with polyesters, polypropylene oxide, or with polyvinyl butyral)] (12). 

In contrast to the previously discussed case of THF polymerization, where chain transfer to polymer is slow as compared to propagation, in the polymerization of cyclic acetals, chain transfer to polymer is fast as compared to propagation and the polymerization is dominated by reactions involving polymer chains. Polymerization of the two best studied monomers of this group, 1,3-dioxolane and 1,3,5-trioxane, shows certain specific features. Thus both systems will be discussed separately in the following sections, with special emphasis on the consequences of the chain transfer to polymer. 

The copolymerization between trioxane and suitable comonomers (ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butane diol formal in amounts of 2-5% by weight) is performed using cationic initiators. The cationic initiators could be Lewis acids, such as BF3 or its etherate BF3Bu20 which was used, for example by Celanese (the mechanism of this reaction was studied in detail [163,164]) or protic acids such as perchloric acid, perfluoroalkane sulfonic acids and their esters and anhydrides. Heteropoly acids were used and also a series of carbenium, oxocarbenium salts, onium compounds, and metal chelates. To regulate the molecular weight chain-transfer agents, such as methylal and butylal, are added. 

The cations formed start the polymerization of trioxane. Thus, good stabilizers against alkali attack are also good chain transfer agents up to 40 polymer molecules are formed per chain transfer molecule. Since both end groups must be sealed, alcohols and esters are not suitable stabilizers against alkali attack because they only form one stable end group. 

Typically in the commercial manufacture of polyacetal copolymer molten trioxane, comonomer, chain transfer agent and catalyst are fed into a reactor capable of providing shear and mixing action and with the means to remove heat generated by the exothermic polymerization reaction. The solid raw polymer is removed, quenched to neutralize any traces of catalyst and washed free of unreacted monomer. The unstable chain ends are usually removed hydrolytically in a basic media. Subsequently, the polymer is washed, dried and compounded with stabilizers and other ingredients and packaged for shipment. 

Poly-1,3,5-trioxan is known to contain hemiacetal -OH groups some of these are formed because of the chain transfer to water 32, Acetylation of the -OH groups greatly enhances tKermal stability of the poly-oxymethylene polymers,and is at the base of the com -mercialization of the first polyacetal (a homopolymer of CH-O) 132a I known, in fact many years ago from the classical works of Staudinger and Kern,... 

NMR and stability studies, that the comonomers are randomly distributed along the chains [237, 238]. This is attributed to the fact that intermolecular chain transfer (transacetalization), contrary to THF polymerization, proceeds on a time scale similar to propagation, and the same holds for the intramolecular chain transfer leading to cycUc polymer [239, 240]. Cationic polymerization of trioxane is usually a precipitation polymerization leading to crystalline polymers, and the size of the macrocydes is determined by the size of the crystalline lamdlae [240]. 

The cationic copolymerization of trioxane with ethylene oxide, 1,3-dioxolane, and suchlike is initiated either with strong protonic acids or Lewis acids, for example BF3. Molecular weight is controlled by the catalyst concentration and monomer purity, and also by chain transfer agents such as methylal [248], which may lead to more stable end groups. Most processes are run below the melting temperature of the polymer (164-167°C) in precipitating agents or in bulk, and are carried out in kneaders or double-screw reactors [249, 250], but there are also some descriptions of melt processes [251]. 

Polymerization of trioxane with 1,3-dioxolane utilizing a BF3 initiator and a methylal chain transfer agent-designated POM. 

Side Reactions 2 and 3 may have similar effects (9). Tetroxane was found to be produced by a fast backbiting reaction during homo- and copolymerizations of trioxane (9). At 30°C. tetroxane reaches an equilibrium concentration of 0.1M. Furthermore, in the copolymerization of trioxane with dioxolane chain ends of the type P2 + cleave off 1,3,5-trioxe-pane (1,3,5-trioxacycloheptane) (18) to yield Pi+ (Reaction 2). Transformation of P2 + into Pi+ without monomer participation can also occur by transfer by polymer (transacetalization as in Reaction 3) ... 

In the homopolymerization of dioxolane below 30°C. tertiary oxonium ions exist exclusively (2, 5). Otherwise hydride transfer would occur (carbonium ions abstract hydride from monomeric cyclic formats) (II, 16). In trioxane polymerization, however, at least some of the active chain ends are carbonium ions they cause hydride transfer and elimination of formaldehyde (9, II, 13). Thus, in copolymerization we must expect two different kinds of structures for cationic chains with terminal trioxane unit. Oxonium ions (I) and carbonium ions (II) may have different reactivity ratios in the copolymerization, but hopefully this does not cause severe disturbance since I and II seem to be in a fast kinetic equilibrium with each other (3). Hence, we expect [I]/[II] to be constant under similar reaction conditions. 

The BF3/l,3j5-trioxane system is one of the few so far discovered in which there is a possibility that monomeric units add at the cationic end of a macrozwitterion. Fortunately, the cation seems to be stable in the presence of its counter anion. As a simple model system with which to study cationic propagation through zwitterion intermediates it is marred by its equilibrium nature and the insolubility of the polymer. Whilst kinetic termination seems to be absent, the authors report transfer to the solvent methylene chloride. Such a reaction would introduce non-zwitterionic chains. 

As the result of hydride transfer from the monomer to the macrocation, the polymer would acquire a —OCH3 end group and the l,3,5-trioxane-2-ylium cation formed would initiate a new chain ... 

It was concluded that the isolated B unit which results from copolymerization of trioxane with a low concentration (ca. 3-5 wt.%) of dioxepane can be inserted in the crystal lattice and/or at its immediate interface. If an active center located on the crystal adds several successive units of DXP, the sequence cannot enter the crystal, and the active chain grows outside until either the formation of a TOX sequence long enough to allow a further crystalline accommodation or the occurrence of a transfer event. These results are in general agreement with the conclusions of the chemical approach of the induction period... 

---