Monofunctional groups

The degree of polymerization is dictated by the ratio of Hquid resin (cmde DGEBPA) to bisphenol A an excess of the former provides epoxy terminal groups. The actual molecular weights attained depend on the purity of the starting material. Reactive monofunctional groups act as chain terrninators. 

Combination of these various methods has led to a vast number of derivatives in which H is progressively replaced by one or more monofunctional group such as F, Cl, Br, I, CN, R, Ar, OR, SH, SR, NH2, NR2, etc. The cyclic silanes SisHm and Si6Hi2 were prepared in the late 1970s via (SiPh) which were themselves the first known homocyclic silane derivatives (F. S. Kipping, 1921) ... 

Telechelic polymers usually bear monofunctional groups at each of their extremities. However, sometimes each end-group is bifunctional, such as in a, co-bis-unsaturated telechelics, or trifunctional as in a, co-bis(trialkoxysilyl) telechelics wherein they participate in crosslinking by the sol-gel reactions (hydrolysis and condensation of alkoxysilane groups). 

A similar class is die titanates. Three-dimensional Ti—O chains form pigments and pigment binders for paints and water-proofing compounds for use on cloth. Their properties can be modified by substituting monofunctional groups for some of the oxygen, for example, by forming esters to interrupt the chains. 

If the monomers are bifunctional, as in the above example, then a linear polymer is formed. Terminating monofunctional groups will reduce the average degree of polymerisation. Polyfunctional monomers, such as glycerol and phthalic acid, are able to form branching points, which readily leads to irreversible network formation (see Chapter 9). Bakelite, a condensation product of phenol and formaldehyde, is an example of such a space-network polymer. Linear polymers are usually soluble in suitable solvents and are thermoplastic - i.e. they can be softened by heat without decomposition. In contrast, highly condensed network polymers are usually hard, are almost completely insoluble and thermoset - i.e. they cannot be softened by heat without decomposition. 

Let NA = number of difunctional groups Na = number of monofunctional groups Nt = total number of prepolymers... 

Finally, small amounts of monofunctional groups (e.g., acetic acid, CHjCOOH) can be added to a polymerization to control molecular weight, or can be present as an impurity in the monomer feed. ... 

Many derivatives can be made where H is replaced by monofunctional groups such as halide, alkyl, -NH2. Many Si and Ge compounds are similar in structure to those of carbon, but trisilylamine... 

Pathway 6 requires the calculation of an intermediate, x (the molecular connectivity index), which is a topological index. The regression equations that link x to Kow (pathway 7) cover only a relatively small number of monofunctional-group chemical classes. 

In 1972, Dolphin and coworkers reported the first covalently linked porphyrin dimer. Reaction of the porphyrin having monofunctional groups and a bifunctional linker molecule gives the bis-porphyrin. Another approach is the reaction of... 

Functionality is even more apparent in monomers that polymerize through a condensation reaction. Let us consider a condensation reaction of monofunctional groups, like classical esterification ... 

In reality, the highest obtainable MW is around 50,000. In addition to the requirement of a very high conversion (which also calls for a long time) it is crucial to avoid a surplus of any of the reactants or the presence of monofunctional groups and contaminants. All of these may nullify a functional group and just substitute it—thus making this end of the growing chain inactive. This requires strict efforts for equivalent functionality (stochiome-tric ratios). Any side product (mostly water) must be removed, otherwise a reverse reaction may operate (depolymerization). 

Additionally, because of diffusion control, the purity of the monomers need not be very high. Fast-reacting monofunctional groups must of course, be excluded. In contrast to melt polycondensation, heat-sensitive polymers, or those with heat-sensitive groups, may also be produced by interfacial polycondensation. 

Ring-shaped substituents are produced when bifunctional reagents react with neighboring monofunctional groups on polymers. An example... 

Compare the functionality of the diacid in this reaction versus the polyamides or polyesters. Here, the diacid acts as a monofunctional group, while when forming polyamides or polyesters, the diacid reacts separately on both ends. For the formation of a... 

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