Air Force

Carboxylic acid-terminated hyperbranched polybenzoxazole

Physical properties can be tailored for a wide array of applications

Materials

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Dendrimers and hyperbranched polymers have distinctly different properties from their linear analogs. Both dendrimers and hyperbranched polymers have much lower solution and melt viscosities than their linear analogs of similar molecular weights. They also have a large number of chain-ends whose collective influence dictates their overall physical and chemical behaviors. These features are attractive in terms of processability and offering flexibility in engineering required properties for specific applications. However, there is a practical advantage that hyperbranched polymers have over dendrimers at the raw material level. Although dendrimers have precisely controlled structures (designated as generations), their preparations generally involve tedious, multi-step sequences that are impractical and costly in scale-up production. Synthesis of a hyperbranched polymer, on the other hand, is a one-pot process. Large quantities of hyperbranched polymers can be easily produced from ABx (x≧2) monomers.

Because of their excellent thermal and mechanical properties, as well as their optical and electronic characteristics, aromatic, fused heterocyclic polymers such as poly(benzoxazoles), poly(benzothiazoles) and poly(benzimidazolos) continue to attract considerable attention. However, they have limited processability due to the nature of fused ring systems. Their insolubility and their softening temperatures are generally above their degradation temperatures. Chemical modification on these materials, for example, by the use of solubilizing pendants or flexible units in the main chain, has been successful to improve their processability, allowing the optimization of their properties as a function of processability. Another viable approach to achieving this objective is to incorporate the elements of local rigidity and global randomness into the macromolecular architecture. Local rigidity provides the thermal, electronic and optical characteristics of the aromatic fused systems while global randomness frustrates entanglement of the polymer chains, leading to greater solubility. Dendritic structures clearly embody these qualities. However, as noted above, hyperbranched structures have greater synthetic practicality in terms of the production cost.

With motivation from the above points, Air Force scientists have synthesized, novel carboxylic acid-terminated hyperbranched benzoxazole polymers. The polymers of this invention are prepared by polymerization of the corresponding AB2 monomer shown in Fig., 1 wherein Z is —OH, —SH or —NH2HCl. Polymerization of the AB2 monomer can be conducted in polyphosphoric acid (PPA) at a polymer concentration of about 6 weight percent at a temperature of about 120 degrees to 150 degrees C., or in the melt state. Due to the availability of large numbers of carboxylic acid end groups of the hyperbranched poly(benzoxazoles), star-branched block copolymerization can be utilized to tailor their physical properties for various applications. The number of available reactive carboxylic acid end-groups is equal to the degree of polymerization plus one (D)P+1).

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