Condensation polymers, biomedical applications

Polyesters can be synthesized either by ring-opening polymerization (ROP) or polycondensation. Both of these approaches have merit in the manipulation of properties of degradable polymers. Commercially, ROP is the most widely used practice for the synthesis of PHAs for consumer applications due to the ease of scale up, acceptable purity, and cost considerations. However, for biomedical applications where cost pressures are low and purity and function are paramount, condensation polymerization can yield superior outcomes [20]. Although the class of degradable polymers is rather large and includes poly(butyrolactones), poly(dioxanone), aliphatic poly(carbonates), poly(anhydrides), and poly(hydroxyalkanoates), the focus of the subsequent sections will be on the PGA, PLA, and poly(caprolactone) (PCL) family of polymers, as these are the most widely used polymers in both medical and consumer products arena. 

Polyacetals that have been examined for biomedical applications are often prepared by step or condensation polymerizations. Utilizing a diol monomer and an aldehyde to prepare a polymer requires removal of 1 equivalent of water per acetal (Figure 13.1). Acetal exchange reactions can be used where the small molecule is an alcohol with a lower boiling point than water... 

The family of polyesters comprises all polymers with ester functional groups in the polymer backbone. Polyesters were the first family of synthetic condensation polymers. Their connecting ester groups can be varied over an immensely broad range, making the polyesters a diverse group, with applications from labile biomedical matrices to liquid crystals, fibres and temperature-resistant performance materials [61, 62]. 

Poly(a-esters) are polymers with hydrolytically liable aliphafic ester bonds in their backbone. They can be easily synthesized via ring opening or condensation polymerization (see Chap. 2 for more details). They are the most commercially available and researched polymers for biomedical applications [4, 5, 9]. 

In addition to the biomedical applications of HMOCs, they were also demonstrated as nanoscale reactors for the preparation of nanoparticles. Pt nanoparticles supported by amine-functionalized condensation polymers were highly dispersed in the micropores of HMOCs via a simple chemical reduction reaction of chloroplatinic acid (Figure 4.16). The microporous structure ensured the high dispersion of Pt active sites. As a result, Pt-modified HMOCs (Pt/m-HMOCs) exhibited good activity (97%) and selectivity (99%) even after ten runs for the reduction of nitrobenzene to aniline under mild conditions. 

Poly(a-ester)s, the most expansively studied class of biodegradable polymer, contain aliphatic ester linkages in their backbone which can be cleaved hydrolytically. It is reported that mere aliphatic polyesters with practically small aliphatic chains between ester bonds can decompose over the time needed for the majority of the biomedical applications. Poly(a-ester)s demonstrate enormous diversity and synthetic flexibility and, depending on the monomeric units, can be synthesized from a variety of monomers via condensation polymerization and ring-opening routes [19]. Poly(glycolic acid) and the stereoisomers of poly(lactic acid) are the most expansively investigated poly(a-ester)s polymers. 

Hydroxyl functionalized five-membered cyclic phosphoester monomers, namely 2-(2-hydroxyethoxy)ethoxy-2-oxo-1,3,2-dioxaphospholane (HEEP), have been prepared for the synthesis of a water-soluble hyperbranched polyphosphate (HPHEEP) (13). The polymer was synthesized through a so-called self-condensation ROP (SCROP) in bulk without the addition of any catalyst. The terminal hydroxyl groups potentially provide a unique opportunity for further modification and functionalization in biomedical applications. ... 

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