Trimer Group

Figure 7. The s-Keggin (top, left) and the 5-Keggin isomers (top, right) of the Ain complex in polyhedral representation. The s- and 5-Keggin isomers differ by rotation of a single trimeric group of Al(0)e octahedra by 60°. These combine to form the AI30 structure (bottom), which consists of two 5- isomers linked by four AlOe octahedra.

In kanonerovite, Na3Mn [P30io](H20)i2, three (PO4) tetrahedra link into a [P3O10] fragment. All three (PO4) tetrahedra of this trimeric group share one vertex with the same (Mn [Pg.130]

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

Compounds with active hydrogen add to the carbonyl group of acetone, often followed by the condensation of another molecule of the addend or loss of water. Hydrogen sulfide forms hexamethyl-l,3,5-trithiane probably through the transitory intermediate thioacetone which readily trimerizes. Hydrogen cyanide forms acetone cyanohydrin [75-86-5] (CH2)2C(OH)CN, which is further processed to methacrylates. Ammonia and hydrogen cyanide give (CH2)2C(NH2)CN [19355-69-2] ix.orn. 6<55i the widely used polymerization initiator, azobisisobutyronitrile [78-67-1] is made (4). 

Catalysis is usually accompHshed through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-/m(/V,/V-dimethylaminomethyl)phenol are used in the presence of high levels of cmde MDI to promote trimerization of the isocyanate and thus form isocyanurate ring stmctures. These groups are more thermally stable than the urethane stmcture and hence are desirable for improved flammabiUty resistance (236). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

Urethane network polymers are also formed by trimerization of part of the isocyanate groups. This approach is used in the formation of rigid polyurethane-modified isocyanurate (PUIR) foams (3). 

The formation of isocyanurates in the presence of polyols occurs via intermediate aHophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibiUty is improved, and the friabiUty of the derived... 

AEyl chloride reacts with sodamide in Hquid ammonia to produce benzene when sodamide is in excess, hexadiene dimer is the principal product, with some trimer and tetramer (C24, six double bonds). AEylation at carbon atoms alpha to polar groups is used in the preparation of a-aEyl-substituted ketones and nittiles. Preparation of P-diketone derivatives, methionic acid derivatives, and malonic ester, cyanoacetic ester, and P-keto-ester derivatives, etc, involving substitution on an alpha carbon between two polar carbonyl groups, is particularly facEe. 

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

The cyclic trimer (trioxane) and tetramer are obtained by a trace of sulphuric acid acting on hot formaldehyde vapour (i) Figure 19.1). Linear polymers with degrees of polymerisation of about 50 and a terminal hydroxyl group are obtained by evaporation of aqueous solutions of formaldehyde (ii). In the presence of strong acid the average chain length may be doubled. Evaporation of methanol solution leads to products of type (iii). 

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