Hard segment chain extender

Prosthetic heart valves were fabricated from a PU containing a 4, 4 -diphenylmethane diisocyanate hard segment, chain-extended with butanediol and with a polyether soft segment. The rate of calcification of these PU heart valves was much slower in a dynamic in vitro... 

Polyurethanes. The hard portion of polyurethane consists of a chain extender and polyisocyanate. The soft component is composed of polyol segments. 

Step-Growth Gopolymerization. A sample of a block copolymer prepared by condensation polymerisation is shown in equation 30 (37). In this process, a prepolymer diol (HO—Z—OH) is capped with isocyanate end groups and chain extended with a low molecular-weight diol (HO—E—OH) to give a so-called segmented block copolymer, containing polyurethane hard blocks and O—Z—O soft blocks. 

Chain extenders are usually low molecular weight symmetrical diols or diamines. Chain extenders react with isocyanates in the same way as polyols do, but because they are low molecular weight, a high concentration of hydrogen-bonded molecules can associate and phase out of the polyol to form plastic-like domains called hard segments . Hard segments will be discussed in Section 4. Some of the more common diol and diamine chain extenders are shown in Table 3. 

The hard segments may be formed by the reaction of a diisocyanate and a chain extender . Early in the history of polyurethanes, high molecular weight urethanes were formed in two steps. The prepolymer step was the reaction between an excess... 

Siloxane-urea copolymers were synthesized by the reaction of the aminopropyl terminated PDMS oligomers with MDI or HMDI with no chain extenders (Reaction Scheme XI and Tables 14 and 15). Therefore, in these copolymers the hard segments consist of the aminopropyl end groups on the siloxane oligomers and MDI or HMDI backbones as shown below. The soft segment is pure polydimethylsiloxane. 

Gogolewski et al. " combined Pluronic F-68(poly(ethylene-propylene-ethylene oxide) with PCL within the same soft segment. BD or 2-amino-l-butanol was used as the chain extender and HDI as the hard segment. They observed that all the materials calcified and calcification increased with material hydrophilicity. The stmcture and composition of the calcium crystals formed on the materials depended on the PU chemistry. 

Cooper et al. [21, 22] reported in detail the results of their laborious work on the adsorption of four proteins human serum albumin (HSA), fibrinogen (FGN), fibronectin (FN), and vitronectin (VN), on five biomaterials polyethylene (PE), silicone rubber (SR), Teflon-FEP (FEP), poly(tetramethylene oxide)-poly-urethane (PTMO-PU), and polyethylene oxide)-polyurethane(PEO-PU). Hard segments of these polyurethanes are composed of a methylene-bis(p-phenylisocyanate) (MDI) chain extended wih 1,4-butanediol. 

In biomedical applications, a typical TPU consists of PTMO as soft segment and diisocyanatodiphenyl methane as a hard segment. Further, 1,4-butanediol is used as a chain extender (37). 

---