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emission maximum is shifted to the red region.12

Fifteen different poly[ (amino acid ester)phosphazenes] were synthesized to study their crystalline character and hydrolysis behavior in the solution and solid states. The polyphosphazenes synthesized were poly [ bis(methy1 glycinat-N-y1)phosphazenel , poly[ bis(ethy1 glycinat-N-y1)phosphazenel , poly [bis(tertbutyl glycinat-N-yl)phosphazene], poly[bis(benzyl glycinat-N-yl)phosphazene], poly[bis(methyl alaninat-N-yl)phosphazene], poly[bis(ethyl alaninat-N-yl)phosphazene], poly[bis(tert-butyl alaninat-N-y1)phosphazene] , poly[ bis( benzyl alaninat-N-yl) phosphazene] , poly [bis(methyl valinat-N-yl)phosphazene] , poly-[bis(ethyl valinat-N-y1)phosphazenel , poly [bis(tert-butyl valinat-N-y1)phosphazenel , poly[bis(benzyl valinat-N-y1)phosphazenel , poly[bis(methyl phenylalaninat-N-y1)phosphazenel , poly[bis(ethyl pheny1alaninat-Ny1)phosphazenel , and poly [bis(tert-butyl phenylalaninat-N-y1)phosphazenel . The fully-substituted polymerswere obtained by treatment of poly(dich1orophosphazene) with a large excess of the appropriate amino acid ester. Several of these polymers were crystalline as measured by differential scanning calorimetry and by polarized optical microscopy. Hydrolysis studies were performed to estimate the rates of decomposition of the polymers and the duration over which the polymers maintained their structural integrity. The polymers are potential biomedical materials.

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A new method for the synthesis of poly(dichlorophosphazene) at ambient temperatures is described. It involves the initiation of Cl3PNSiMe3 with small amounts of PCl5 in CH2Cl2 to yield poly(dichlorophosphazene), (NPCl2) n , with narrow polydispersities. The molecular weight of poly(dichlorophosphazene) was controlled by altering the ratio of monomer to initiator. The polymer chains were found to be active after chain propagation since further addition of monomer resulted in the formation of higher molecular weight polymer. Integration of 1H and 31P NMR spectra of these reactions revealed that the polymerization follows first-order reaction kinetics with respect to monomer concentration. Active polymer chains may be quenched or end-capped by the addition of trace quantities of Me2(CF3CH2O)PNSiMe3 or (CF3CH2O)3PNSiMe3. Furthermore, PBr5, SbCl5, and Ph3C[PF6] were also found to be effective initiators in CH2Cl2 at room temperature.

Gold nanoparticles (AuNP) have been proposed for many applications in medicine. Although large AuNP (>5.5 nm) are desirable for their longer blood circulation and accumulation in diseased tissues, small AuNP (<5.5 nm) are required for excretion via the kidneys. We present a novel platform where small, excretable AuNP are encapsulated into biodegradable poly di(carboxylatophenoxy)phosphazene (PCPP) nanospheres. These larger nanoparticles (Au-PCPP) can perform their function as contrast agents, then subsequently break down into harmless byproducts and release the AuNP for swift excretion. Homogeneous Au-PCPP were synthesized using a microfluidic device. The size of the Au-PCPP can be controlled by the amount of polyethylene glycol-polylysine (PEG-PLL) block co-polymer in the formulation. Synthesis of Au-PCPP nanoparticles and encapsulation of AuNP in PCPP were evaluated using transmission electron microscopy and their biocompatibility and biodegradability confirmed in vitro. The Au-PCPP nanoparticles were found to produce strong computed tomography contrast. The UV-Vis absorption peak of Au-PCPP can be tuned into the near infrared region via inclusion of varying amounts of AuNP and controlling the nanoparticle size. In vitro and in vivo experiments demonstrated the potential of Au-PCPP as contrast agents for photoacoustic imaging. Therefore, Au-PCPP nanoparticles have high potency as contrast agents for two imaging modalities, as well as being biocompatible and biodegradable, and thus represent a platform with potential for translation into the clinic.

An account is presented of the development, evaluation, and current status of an unusual series of polymers optimized specifically for biomedical applications. The polymers are based on the polyphosphazene platform with side groups chosen for their ability to sensitize the polymers to hydrolysis to benign small molecules that can be metabolized or excreted from the body. The largest class of these polymers consists of macromolecules with amino acid ester side groups and these are the main focus of the article. However, a variety of polymers with other side groups also show promise as bioerodible species, and these are mentioned later in the article.