Cychc trimers

Polymerization. Paraldehyde, 2,4,6-trimethyl-1,3-5-trioxane [123-63-7] a cycHc trimer of acetaldehyde, is formed when a mineral acid, such as sulfuric, phosphoric, or hydrochloric acid, is added to acetaldehyde (45). Paraldehyde can also be formed continuously by feeding Hquid acetaldehyde at 15—20°C over an acid ion-exchange resin (46). Depolymerization of paraldehyde occurs in the presence of acid catalysts (47) after neutralization with sodium acetate, acetaldehyde and paraldehyde are recovered by distillation. Paraldehyde is a colorless Hquid, boiling at 125.35°C at 101 kPa (1 atm). 

The term "acetal resins" commonly denotes the family of homopolymers and copolymers whose main chains are completely or essentially composed of repeating oxymethylene units (—CH2—O—). The polymers are derived chiefly from formaldehyde or methanal [50-00-00] either directly or through its cychc trimer, trioxane or 1,3,5-trioxacyclohexane [110-88-3]. 

Nickel halide complexes with amines give mixtures of linear polymer and cychc trimers (30). Nickel chelates give up to 40% of linear polymer (31). When heated with ammonia over cadmium calcium phosphate catalysts, propargyl alcohol gives a mixture of pyridines (32). 

Synthesis. The synthesis of poly(dichlotophosphazene) [25034-79-17, (N=PCl2) (4), the patent polymer to over 300 macromolecules of types (1) and (2), is carried out via controlled, ring-opening polymerization of the corresponding cycHc trimer, (N=PCl2)3 [940-71 -6]. 

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

The ratio of cycHc to linear oligomers, as well as the chain length of the linear sdoxanes, is controlled by the conditions of hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents (60,61). Commercially, hydrolysis of dim ethyl dichi oro sil a n e is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyl dichi orosilane is mixed with 22% a2eotropic aqueous hydrochloric acid in a continuous reactor. The mixture of hydrolysate and 32% concentrated acid is separated in a decanter. After separation, the anhydrous hydrogen chloride is converted to methyl chloride, which is then reused in the direct process. The hydrolysate is washed for removal of residual acid, neutralized, dried, and filtered (63). The typical yield of cycHc oligomers is between 35 and 50%. The mixture of cycHc oligomers consists mainly of tetramer and pentamer. Only a small amount of cycHc trimer is formed. 

The sulfides are associated like the oxides, but to a lesser degree. They are crystalline, sharp-melting, soluble ia many organic solvents, and resistant to hydrolysis. Most are cycHc trimers (106). 

Alkylation with a vatiety of common alkyl halides oi sulfates gives stable dialkylcyanamides. However, the intermediate monoalkylated compounds usually cannot be isolated and cychc trimers or cotrimers with cyanamide ate obtained (13). The reaction can be carried out efficientiy in water or alcohol. Allyl chloride is an especially useful reagent, producing diallylcyanamide [538-08-9J (4). 

Hexachlorocyclotriphosphazene (cycHc trimer) is a respiratory irritant. Nausea has also been noted on exposure (10). Intravenous and intraperitoneal toxicity measurements were made on mice. The highest nonlethal dose (LDq) was measured as 20 mg/kg (11). Linear chloropolymer is also beUeved to be toxic (10). Upon organic substitution, the high molecular weight linear polymers have been shown to be inert. Rat implants of eight different polyphosphazene homopolymers indicated low levels of tissue toxicity (12). EZ has been found to be reasonably compatible with blood (13), and has lower hpid absorption than fiuorosihcone. 

The gamma modification at ordinary temperatures can exist in sohd or liquid form. In sohd form it is a colloidal ice-hke mass melting at 16.8°C. In the hquid form it has a density of 1.9224 g/mL, boiling at 44.8°C. It has a vapor pressure of 433 torr at 25°C. The gamma phase consists of both cychc trimer and monomer molecules. When solid sulfur trioxide melts, it converts to its gamma phase which on sohdification changes to alpha modification. 

The cychc trimer is an important reagent in S N chemistry as a source of both cychc and acyclic compounds (see Scheme 6). In part, the synthetic utihty of (NSC1)3 stems from the ease with which it dissociates into monomeric NSCl in solution. Thus, the trimer provides a facile source of the NS+ cation (see Section 5.2.1). Monomeric NSCl, generated from (NSC1)3, undergoes a [2 + 4] cycloaddition reaction... 

Investigations of isocyanate interacdiion with aqueous NaOH solutions [178] conducted by us estabhshed that triiscKyamu ates are formed with increase of scxlium hydroxide concentration above 20% and the output of isocyanate cychc trimer grows monotonically with the increase of sodium hydroxide concentration above 20%. There are three ion types in the aqueous alkah solutions, namely, Na, H3O, and OH, but it is reasonable to regard hydroxyl ion as the initiating species in isocyanate polsmierization. Hydroxyl ion in the acjueous alkali solutions is enclosed in a hydration sphere, but despite this it displays the initiating action in isocyanate addition polymerization reactions. 

Acetal homopolymers are polymerized from purified anhydrous formaldehyde. Acetal copolymers are copolymerized from cychc 1,3,5-tri-oxane cychc trimer (CsHeOa) of formaldehyde, tjrpically with a cychc ether comonomer such as ethylene oxide [1]. Ethylene glycol is the product of hydrolysis of ethylene oxide. 

Addition products of aldehydes and ketones with ammonia and amines (hemiaminals) are unstable compounds that are then stabilised by a number of subsequent reactions (Figure 8.28). Dehydration of addition products yields imines. Other possible products are enamines, amines containing a double bond linkage -CH=CH—NH- (Figure 8.29), cycHc trimers (symmetrical 1,3,5-triazines) and various coloured polymers, generally of unknown structures. Reaction of the addition products with another amine molecule gives rise to aminals, reaction with another molecule of the carbonyl compounds yields a,a -dihydroxyamines. For example, in addition to 1,3,5-triazines, it is mostly polymers that are formed from aliphatic aldehydes and ammonia or amines. The reaction of formaldehyde with ammonia yields hexamethylenetetramine. 

Cyclic dimer (2), cychc trimer (0.3), terephthalic acid (0.4), benzoic acid 187... 

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