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FLUORINATED POLYPHOSPHAZENES

HARRY R. ALLCOCK

The Pennsylvania State University

1.1 BACKGROUND

Polyphosphazenes (Structure 1.1) are a broad class of macromolecules with a backbone of alternating phosphorus and nitrogen atoms and with two side groups (R) attached to each phosphorus atom.

STRUCTURE 1.1  

The skeletal architecture may be linear, branched, star, or dendritic, or it may be part of a di- or triblock copolymer in conjunction with organic macromolecules or poly(organosiloxanes) (silicones). However, it is in the wide variety of side groups that this system differs from many other polymer platforms. More than 250 different organic, organometallic, or inorganic side groups have been utilized in single-substituent arrays or in di-, tri-, or higher mixed-substituent patterns. Thus, hundreds of different polyphosphazenes are known with a corresponding diversity of properties and potential uses [1]. These can be divided into different “families” of polymers such as inert biomaterials, bioerodible polymers, optical materials, membranes, ionic conductors, and so on. One of the most important and most interesting families consists of polyphosphazenes that bear fluorinated organic side groups. Examples of polymers within this group are shown in Figure 1.1.

FIGURE 1.1 Examples of fluorinated organophosphazene polymers.

1.2 SYNTHESIS METHODS AND PROPERTY DEVELOPMENT

A number of different access routes have been developed to poly (organophosphazenes) [1]. We have focused on a two-stage sequence that involves first the preparation of a linear polymeric reaction intermediate, poly(dichlorophosphazene), (NPCl2)n, followed in a second step by replacement of the chlorine atoms in this polymer by organic side groups (Figure 1.2) [2–4]. The reactive intermediate is accessible either by a ring-opening polymerization of a cyclic trimer, (NPCl2)3, or via a living cationic condensation polymerization of a phosphoranimine (Figure 1.2) [5–15]. Another route to poly(dichlorophosphazene) is via the condensation reactions of Cl3PN-POCl2 [16], a method that yields lower molecular weight polymers than the ring-opening route. Replacement of the chlorine atoms in (NPCl2)n is accomplished by reactions with nucleophiles such as alkoxides, aryloxides, amines, or organometallic reagents.

FIGURE 1.2 Two-stage synthesis of poly(organophosphazenes).

This is a very different protocol than that exists for most classical polymers, where the side groups destined for the final polymer must be in place on the monomer before polymerization [17]. Modification of the side groups in conventional macromolecules after polymerization is restricted to simple reactions such as hydrolysis of esters or partial sulfonation. It is the high reactivity of poly(dichlorophosphazene) that allows the broad diversity of structure and properties that are a characteristic of poly(organophosphazenes). Other valuable methods have been developed to prepare poly(organophosphazenes) that involve the condensation reactions of organic-substituted phosphoranimines [18–21], but the range of side groups used in that process is more restricted than in the macromolecular substitution method, and the molecular weights tend to be lower.

Use of these synthetic techniques has led to the development of numerous different classes of phosphazene materials, many of which contain fluorine, but others that do not [1]. For example, a versatile class of hydrogel polyphosphazenes and ion conductive materials possesses nonfluorinated oligoethyleneoxy side chains. Nonfluorinated aryloxy substituents give fire-retardant polymers. Amino acid ester side groups or nonfluorinated alkoxy groups generate bioerodible properties that have been developed extensively for tissue engineering applications. Nevertheless, the presence of fluorine in the side group structure has led to some of the most intriguing developments, and this is the focus of the rest of this article. Using the two-step synthesis protocol, molecular diversity is accomplished in several different ways.

Method 1. Different nucleophiles give polymers with different side groups and diverse properties [1–4]. For example, oligoethyleneoxy side groups give water-soluble, water-stable polymers [22]. Aryloxy side units generate hydrophobic, water-insoluble polymers. Amino acid ester side groups or oligopeptide units linked to the polymer skeleton through the amino terminus generate bioerodible characteristics [23]. Fluoroalkoxy or fluoroaryloxy side groups generate hydrophobic, water- and radiation-stable polymers [24].

Method 2. A second method for structural and property tuning involves the introduction of two or more different side groups along the same polymer chain. For example, amphiphilic character is accessible by the use of fluoroalkoxy groups and oligoethyleneoxy side chains, with the exact properties being controlled by the ratios of the two. Two different fluoroalkoxy side groups on the same chain have a striking effect on the polymer morphology. A polyphosphazene with only trifluoroethoxy side groups is a film- or fiber-forming microcrystalline material, similar to poly(tetrafluoroethylene) in surface properties but, unlike Teflon, soluble in ordinary organic solvents such as acetone or methylethylketone. By contrast, the related polymer with both trifluoroethoxy and longer chain telomer fluoroalkoxy groups is an amorphous elastomer, prized for its low glass transition temperature (approximately −60°C), solvent and oil resistance, and impact-absorbing character.

For polyphosphazene molecules that bear two or more different side groups, serious questions exist about the ratios and pattern of distribution of these groups along the polymer chains. These two factors have a significant influence on the properties of the final polymer. For example, a random distribution often precludes crystallinity and favors elastomeric character. Properties that result from a regular distribution may depend on whether the substituents are geminal or nongeminal, cis or trans, or if block structures are present (Figures 1.3 and 1.4).31P NMR spectra can sometimes provide clues about the distribution.

FIGURE 1.3 Initial possible steps after the replacement of the first chlorine atom per chain by the same organic group (spheres).

FIGURE 1.4 Simplified representation of the side group disposition following the introduction of a second type of side group (black spheres) after the distribution of the first side groups has been established.

Note that the order of introduction of the side groups often plays a major role in controlling the distribution pattern and the properties. The distribution of the first substituent will control the positioning of the second substituent and, in turn, the precise combination will control the properties of the fully substituted polymer. Simultaneous addition of the two nucleophiles often yields results that depend on the electronic and steric characteristics of the reagents and also on the reaction conditions.

Method 3. The third opportunity for structural diversity arises through organic side group exchange chemistry (Figure 1.5). Thus, specific polyphosphazenes in solution can be modified by the replacement of one type of organic side group by exposure to another nucleophile. Fluoroalkoxide A will replace fluoroalkoxy group B. Aryloxy side groups with electron-withdrawing substituents may be replaced by trifluoroethoxy groups. Trichloroethoxy side groups can also be exchanged for trifluoroethoxy side units [25]. This is a useful way to fine-tune polymer properties such as solubility or morphology. It can also be used as a method for cross-linking polyphosphazene chains by the use of a difunctional nucleophile that can exchange with some of the organic side groups. Moreover, it provides an excellent method for modification of the surface of a solid polyphosphazene without affecting the composition of the interior [26,27].

Method 4. Finally, the preparation of polymers with multifunctional organic side groups raises a synthetic challenge. For example, if the objective is to produce a polymer with the side groups connected to the main chain by the reaction of a hydroxyl group with the chlorophosphazene, but the incoming nucleophile also bears an amino unit, a carboxylic acid group, or a second hydroxy group, the di- or tri-functional nucleophile will cross-link the chains, precipitate the polymer before all the chlorine atoms are replaced and prevent complete halogen replacement. Hence, a first step must be the protection of the noncoupling functional groups, and a deprotection of these units once chlorine replacement along the polymer chain is complete. A simple example, is the use of the sodium salt of an ester of p-hydroxybenzoic acid as the nucleophile to link the side group to the chain via the hydroxyl group, followed by hydrolysis of the ester function to generate the free carboxylic acid or carboxylate moiety (Figure 1.6). We have developed more complex protection–deprotection procedures to utilize numerous multifunctional biological side groups [23, 28].

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

FIGURE 1.6 A variety of...