Macromolecular substitution

The very high reactivity of the P—Cl bonds in (4) forms the basis for the now well-known macromolecular substitution method, which has been used to synthesize polymers of types (1) and (2) and some polymers that are hybrids of these and (3). The method involves nucleophilic reactions of (4), and to some extent of its difluoro analogue, with alkoxides or amines. 

Molecular structural changes in polyphosphazenes are achieved mainly by macromolecular substitution reactions rather than by variations in monomer types or monomer ratios (1-4). The method makes use of a reactive macromolecular intermediate, poly(dichlorophosphazene) structure (3), that allows the facile replacement of chloro side groups by reactions of this macromolecule with a wide range of chemical reagents. The overall pathway is summarized in Scheme I. 

Three approaches have been developed for the synthesis of polyphosphazenes. These are (1) The macromolecular substitution route (2) The cyclic trlmer or tetramer substitution/polymerization route, and (3) Direct synthesis from organosllylphosphazene monomers. This last method Is described In detail In another Chapter and will not be considered further In this review. 

Macromolecular Substitution Route. The current surge in poly-phosphazene research Is mainly a result of the development in the mid 1960 s (2-4) of a substitutive route to the synthesis of organo phosphazene high polymers. Before that time, only a sporadic interest in the subject existed because the known polymers, cross linked poly(dihalophosphazenes), (1,5) were insoluble and hydrolytically unstable. 

The third possibility for synthesizing polymeric substances is the modification of existant natural or synthetic macromolecules (see Chap. 5). These processes can either be chemical or physical. Chemical modifications are reactions on macromolecules without degradation of the main chain (macromolecular substitution routes, polymer-analogous reactions ) like, for example, hydrolysis. 

Linear poly(organophosphazenes) are now synthesized by six different methods. These are (a) The ring-opening polymerization of (NPC12)3 mentioned above, followed by macromolecular substitution (b) Ring-opening polymerization of organic substituted phosphazene cyclic trimers, usually followed by macromolecular substitution ... 

Inherent in the macromolecular substitution method is the possibilty that two or more different organic groups can be introduced either simultaneously or sequentially to give mixed-substituent polymers. Steric hindrance effects that slow the reaction rate after a bulky side group has been introduced allow the controlled introduction of a... 

However, in spite of these complications, the overall macromolecular substitution route allows access to an enormous range of different polymeric structures. Moreover, the length of the polymer chain is usually unaffected by the replacement of halogen by organic side groups. Thus, it is possible to alter the side-group structure in polyphosp-hazenes without changing any other structural features, and this is of vital importance for the assessment of structure-property relationships. 

Conversely, if the polymer could be made by some other route (for example, by macromolecular substitution), it might be stable at moderate temperatures where the rate of depolymerization is very slow, but would depolymerize to the cyclic trimer or tetramer when heated to higher temperatures. In fact, this behavior is found for uncross-linked polymers such as [NP(OPh)2] , that appear to be kinetically stabilized at moderate temperatures, but are sufficiently destabilized thermodynamically by the bulky aryloxy side groups that they depolymerize when heated above 150-200 °C. 

Because most poly(organophosphazenes) are synthesized by a macromolecular substitution route, the disposition of side groups usually depends on the steric and electron-directing characteristics of the incoming groups and on the side groups already present. Thus, the order in which two or more different side groups are introduced will also affect the outcome. 

The introduction of bulky, electron-rich side groups, such as those depicted in structures 3.102-3.105 requires the use of special experimental conditions. If introduced via an aryloxide ion by the macromolecular substitutive route, these side groups impart and suffer considerable steric hindrance. Replacement of every chlorine atom along the phosphazene chain by organic side groups may be difficult. 

In many respects, the polyphosphazenes are the prototype inorganic backbone polymers, that exemplify the principles of ring-opening and condensation polymerization, macromolecular substitution reactions and their potential for molecular design, and an enormous range of derivatives with the same backbone but different organic side groups. 

As mentioned previously, selective intermacromolecular complexation is realized by the control of the difference of the total bond energy between each pair of polymers. Therefore, if P can interact with P2 more strongly than P3 does, the interchain macromolecular substitution reaction of P3 and P3 takes place on the addition of Pj to the P2-P3 complex solution. In these systems, it is expected that a cooperative interchain macromolecular substitution reaction of the type... 

Optical Materials. The polyphosphazene skeleton is electron-rich, which means that it provides a refractive index increment compared to conventional saturated organic backbones. In addition, the macromolecular substitution synthesis aUows highly unsaturated organic side groups to be linked to the skeleton in ways that allow the refractive index, the color, the liquid crystalline, and nonlinear optical characteristics of the polymer to be finely tuned. Thus, the use of these polymers in opto-electronic (photonic) switches and lens systems is a subject of growing interest. 

This combination of macromolecular substitution and access to mixed-substituent polymers underlies the extraordinary versatility of the polyphosphazene platform. By mid-1997 more than 3000 publications and patents had appeared on this subject, and... 

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