1,2-Condensation dimer molecular structure

Figure 5. The molecular structure of the 1,2-condensation dimer of CgHi,(NH)[NP(NEt2)2KPNEt2) (XV), as the trisulfide derivative. Ethyl groups on N5, N6, N7, N8 and N9 omitted for clarity.

Figure 337 Formal dimerization-condensation reaction of two hypothetical clusters [Cui2S6(Me Pr2)8], resulting in the molecular structure of [Cu24Si2(PMeiPr2)i2] (8) (without organic groups). Cu shaded spheres ...

The silanediols 15 proved to be stable compounds. Condensation reactions were not observed and are certainly prevented by the bulkiness of the substituents at the silanediol silicon atom. 15a was chosen for an X-ray structure analysis, the results of which confirmed the proposed molecular structure. Bond lengths and angles are in the expected regions (Fig. 1). The crystal structure is characterized by hydrogen bonds (0-0 distances of 2.187 A) which form a dimer of 15a. 

Because each monomer is potentially tetrafunctional, all sorts of partially hydrolyzed and partially condensed species are possible. For example, 10 distinct dimers may be formed. Many cyclic species can occur. Many of the species in solution are metastable with respect to anhydrous, amorphous Si02, and rearrangements occur with time (2). The complexity of this system means that the structure and growth of these polymers can be described only in a statistical and geometric fashion rather than by a more conventional description in terms of topologies (such as chains and cross-links) and molecular weights. The most useful and intuitive description of the structure of these polymers is in terms of fractal geometry. 

Carothers classification (condensation vs. addition) is primarily based on the composition or structure of polymers. The second classification (chainwise vs. stepwise) was proposed by P. J. Floiy, and is based on the kinetic scheme or mechanism governing the polymerization reactions. Step reactions are those in which the chain growth occurs in a slow, stepwise manner. Two monomer molecules react to form a dimer. The dimer can then react with another monomer to form a trimer, or with another dimer to form tetramer. Thus, the average molecular weight of the system increases slowly over a period of time. This is exemplified by the following polyesterification ... 

---