Mixed-substituent polymers

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

The steric bulk of steroid structures prevents their use as the only organic side group present. However, mixed-substituent polymers that contain both steroidal side groups and amino acid ester or other cosubstituent units can be readily synthesized. If a saturated A ring is present in the steroid, linkage to the polymer chain is complicated by side reactions that result from dehydration of the steroid (chlorophosphazenes are powerful dehydrating agents). 

Figure 6. General structure for phosphazenes with mesogenic side groups. Example is a mixed substituent polymer (VII) where R represents the trifluoroethoxy group and the mesogen with flexible spacer is represented by the curlicue and rectangular box.

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... 

A series of cobalt carbonyl complexes of polyphosphazenes have been prepared via arene coordination sites. Examples are shown as 3.65 and 3.66.112 These are synthesized via the reactions of (NPC12) with the sodium salt of the appropriate metal-arene terminated alcohol. Mixed-substituent polymers with trifluoroethoxy or phenoxy cosubstituents have also been prepared. 

The principal polyphosphazenes that have been used in hydrogels are those with linear or branched ethyleneoxy side chains, aryloxy groups with carboxylic acid substituents, or mixed-substituent polymers that bear hydrophilic methylamino side groups plus a hydrophobic cosubstituent such as phenoxy or trifluoroethoxy. Cross-linking is usually accomplished by gamma-ray irradiation or, in the case of the carboxylic acid functional species, by treatment with a di- or tri-valent cation. Here, we will consider another example based on MEEP (3.79), a polymer that is well suited to the clean method of radiation cross-linking. 

Schacht and coworkers in Belgium201-212 used pellets of a polyphosphazene with both ethyl glycinate and ethyl phenylalanate side groups for the controlled release of the antitumor agent, mitomycin-C. A 100% ethyl glycinato polymer released the drug too rapidly, but mixed-substituent polymers with 50-65% phenylalanine ester released the drug at an optimum rate. 

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... 

FIGURE 11.3 An illustration of the macromolecular substitution reactions that allow for a wide variety of organic side groups to be linked to the phosphorus atoms to form single-substituent polymers (simultaneous replacement of chlorine atoms) or mixed-substituent polymers (sequential substitution of chlorine atoms). 

An interesting aspect of the chemistry of [(NPCl2)2(NSCl)]n is the difference of reactivity of the S-Cl and P-Cl bonds. Thus, substitution of S-Cl bonds occurs much faster than the P-Cl bonds. Consequently, mixed substituent polymers can be readily prepared. 

FIGURE 1.5 Substituent exchange as an alternative route to the preparation of mixed-substituent polymers. 

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