**2. Preparations of polymer materials**

#### **2.1. Syntheses of monomers containing PC group**

In order to prepare polyamides, polyimides and polyurethanes, we have investigated the syntheses of diamine and diol monomers containing PC moiety. At first, the synthesis of 2- (3,5-diaminophenylcarbonyloxy)ethyl phosphorylcholine (DAPC) was carried out to prepare PC-containing polyamide [16]. Then, copolyamides were prepared by the polycondensation of DAPC with isophthaloyl chloride and another diamine comonomer. It was revealed that the obtained copolyamide films exhibited the excellent blood compatibility. These results would be due to the PC unit located at the surface of the polymer film, where the surface is covered with PC unit, and the interaction between the polymer surface and blood ingredients such as cells and platelets is very weak. However, the molecular weight and the PC content of copolyamides from DAPC were not enough to produce a self-standing film and to exhibit the higher biocompatibility, respectively, which would be due to the low reactivity and also the highly hygroscopic property of DAPC. Thus, we have developed a new structure of high molecular weight polymer in order to create the practical biomaterials for several applications, which exhibit the excellent biocompatibility in addition to the processability, the durability to solvents, the thermal stability and the mechanical strength [19].

**Sheme 1.** Synthesis of diamine monomer containing PC moiety (BAPPC).

In these years, we have succeeded in the syntheses of novel diamine and diol monomers containing PC moiety for the preparations of polyamides, polyimides, polyesters and polyur‐ ethanes from these monomers [16 – 23]. It was found that the obtained polymers exhibited the excellent biocompatibility derived from PC unit in addition to the processability, the durability to solvents, the thermal stability and the mechanical strength, which were derived from the

By the way, the development of practical biomaterials will desire the collaborations among chemists, biologists, material scientists. We focused in the field of nanotechnology, especially the processing for free-standing ultrathin films consisting of polymers with a thickness less than 100 nm (often called nanosheets), which exhibited the unique properties such as high adhesive strength, flexibility, transparency and smoothness [24-26]. If the nanosheets could be fabricated from such PC-containing polymers, the applications as new biomaterials would be significantly advanced. Then, we attempted to prepare the nanosheets from the obtained copoly(ester-urethane)s and to investigate the physical properties and the biocompatibility of

This chapter covered the subject of our recent study to develop new biomaterials containing a phospholipid moiety. We describe the preparations of aromatic polyimides and segmented polyurethanes containing PC group, which are obtained by polycondensation or polyaddition using PC-containing diamine and diol monomers. In addition, the fabrication of ultra-thin films, so called nanosheets, composed of these PC-containing polymers is described in detail. The obtained nanosheets exhibited the high adhesive strength, indicating that the nanosheets could conform closely to the desired surfaces due to their exquisite flexibility and low roughness. In this chapter, the physical properties such as thermal stability, biological function as blood compatibility, and surface property of the obtained polymers and nanosheets are

In order to prepare polyamides, polyimides and polyurethanes, we have investigated the syntheses of diamine and diol monomers containing PC moiety. At first, the synthesis of 2- (3,5-diaminophenylcarbonyloxy)ethyl phosphorylcholine (DAPC) was carried out to prepare PC-containing polyamide [16]. Then, copolyamides were prepared by the polycondensation of DAPC with isophthaloyl chloride and another diamine comonomer. It was revealed that the obtained copolyamide films exhibited the excellent blood compatibility. These results would be due to the PC unit located at the surface of the polymer film, where the surface is covered with PC unit, and the interaction between the polymer surface and blood ingredients such as cells and platelets is very weak. However, the molecular weight and the PC content of copolyamides from DAPC were not enough to produce a self-standing film and to exhibit the higher biocompatibility, respectively, which would be due to the low reactivity and also the highly hygroscopic property of DAPC. Thus, we have developed a new structure of high

discussed to reveal the possibility of a new biocompatible polymer material.

**2. Preparations of polymer materials**

**2.1. Syntheses of monomers containing PC group**

main chain components.

4 Advances in Bioengineering

the nanosheet surface.

For this purpose, we designed a new diamine monomer containing PC group, 2-[3,5-bis(4 aminophenoxy)phenylcarbonyloxy]ethyl phosphorylcholine (BAPPC). BAPPC is expected to show the higher reactivity in the polymerization than DAPC, which would be due to the relatively higher reactive amino groups on *p-*position of phenoxy groups. The synthetic route of BAPPC is outlined in Scheme 1. The compound **3** was prepared as an intermediate, which were obtained by the esterification of **2** with 2-bromoethanol. Then, the reaction of **3** with 2 chloro-2-oxo-1,3,2-dioxaphospholane (COP) yielded a phospholane compound. The purifica‐ tion of the product by a silica-gel column chromatography was difficult because it was easily hydrolyzed. Therefore, the extraction of the crude products with chloroform followed by washing with distilled water gave the pure phospholane compound. Finally, BAPPC (melting point=109°C) was obtained by opening the cyclic phosphoric ester moiety with trimethyla‐ mine, followed by the reduction of the nitro groups of **4** with H2 catalyzed by Pd. Although the several reaction steps are necessary to prepare this monomer, all of the reaction steps proceeded smoothly in high yields [18].

**Sheme 2.** Synthesis of diol monomer containing PC moiety (BHPC).

On the other hand, the synthesis of diol monomer containing PC unit was investigated to prepare polyurethanes or polyesters [21]. The synthetic route of the desired diol monomer, 2- (3,5-bis(2-hydroxyethoxy)benzoyloxy)ethyl phosphorylcholine (BHPC), is outlined in Scheme 2. The terminal benzyl groups of compounds **5**, **6** and **7** were introduced as a protection group of the diol moiety. The key intermediate, **6**, was synthesized by esterification of **5** with ethylene glycol, and the incorporation of the PC group was achieved by the reaction of **6** with COP, followed by the ring-opening reaction of the cyclic phosphoric ester moiety with trimethyla‐ mine. Finally, deprotection of the benzyl groups of **7** by Pd-catalyzed hydrogen reduction with H2 gas to afford the desired diol monomer, BHPC. This reaction proceeded quantitatively to give the pure product of BHPC as a white solid (melting point=34°C), although it was so hygroscopic that the obtained solid softened when exposed to moisture.

#### **2.2. Syntheses and properties of polyimides**

In these years, we have achieved the syntheses of polyamides, poly(urethane-urea)s and poly(amide-ester)s containing PC moiety by polycondensation or polyaddition using the novel diamine monomer, BAPPC, and investigated the physical and biological properties of the obtained polymers, as described in our literatures [18 – 20]. These aromatic polymers contain‐ ing PC group showed the thermal stability up to *ca.* 250°C, where the thermal degradation of PC component would started over 200°C that was confirmed by the thermogravimetric analysis of polymers. The heat resistance of these PC-containing polymers over 200°C is enough to use for biomaterial devices, for example, for the thermal sterilization process over 150°C. In addition, the tough films could be prepared by solvent casting from poly(urethaneurea)s and poly(amide-ester)s, which were copolymerized with polycarbonate diol as the soft segment, and the elastomeric property was observed in these films [20]. Furthermore, it has been found that these PC-containing aromatic polymers efficiently reduced the adhesion of proteins and platelets, where the number of adhered platelets of PC-containing polymers was reduced in nearly one-tenth amount as compared with that of polymers without PC group. These results indicate that the PC unit plays an important role for the blood compatibility of the polymers. The amount of adhered proteins and platelets decreased as the increase of the content of PC unit in the copolymers, therefore, the composition of the PC unit was a dominant factor in the reduction of the adhesion of proteins and platelets.

In this chapter, we will describe our recent study for the synthesis of polyimide containing PC group as a biocompatible hard material. The desired copolyimides were carried out by the polycondensation of BAPPC and bis(*p*-diaminophenyloxy)benzene (BAPB) with 4,4'-hexa‐ fluoroisopropylidene diphthalic anhydride (6FDA), followed by the chemical imidization with triethylamine and acetic anhydride, as shown in Scheme 3. As the acid anhydride, 6FDA was used to make the polyimide soluble in some solvents. Table 1 summarizes the compositions and molecular weights of the obtained copolyimides. Four kinds of copolyimides with PC content were prepared by changing the ratio of BAPPC and BAPB in the feed of polymeriza‐ tion. The obtained copolyimides showed the number-average molecular weights (*M*n) at around 1 x 104 .

Synthesis of New Biocompatible Polymers and Fabrication of Nanosheets http://dx.doi.org/10.5772/59633 7

**Sheme 3.** Preparation of copolyimides containing PC moiety (PIPC).

On the other hand, the synthesis of diol monomer containing PC unit was investigated to prepare polyurethanes or polyesters [21]. The synthetic route of the desired diol monomer, 2- (3,5-bis(2-hydroxyethoxy)benzoyloxy)ethyl phosphorylcholine (BHPC), is outlined in Scheme 2. The terminal benzyl groups of compounds **5**, **6** and **7** were introduced as a protection group of the diol moiety. The key intermediate, **6**, was synthesized by esterification of **5** with ethylene glycol, and the incorporation of the PC group was achieved by the reaction of **6** with COP, followed by the ring-opening reaction of the cyclic phosphoric ester moiety with trimethyla‐ mine. Finally, deprotection of the benzyl groups of **7** by Pd-catalyzed hydrogen reduction with H2 gas to afford the desired diol monomer, BHPC. This reaction proceeded quantitatively to give the pure product of BHPC as a white solid (melting point=34°C), although it was so

In these years, we have achieved the syntheses of polyamides, poly(urethane-urea)s and poly(amide-ester)s containing PC moiety by polycondensation or polyaddition using the novel diamine monomer, BAPPC, and investigated the physical and biological properties of the obtained polymers, as described in our literatures [18 – 20]. These aromatic polymers contain‐ ing PC group showed the thermal stability up to *ca.* 250°C, where the thermal degradation of PC component would started over 200°C that was confirmed by the thermogravimetric analysis of polymers. The heat resistance of these PC-containing polymers over 200°C is enough to use for biomaterial devices, for example, for the thermal sterilization process over 150°C. In addition, the tough films could be prepared by solvent casting from poly(urethaneurea)s and poly(amide-ester)s, which were copolymerized with polycarbonate diol as the soft segment, and the elastomeric property was observed in these films [20]. Furthermore, it has been found that these PC-containing aromatic polymers efficiently reduced the adhesion of proteins and platelets, where the number of adhered platelets of PC-containing polymers was reduced in nearly one-tenth amount as compared with that of polymers without PC group. These results indicate that the PC unit plays an important role for the blood compatibility of the polymers. The amount of adhered proteins and platelets decreased as the increase of the content of PC unit in the copolymers, therefore, the composition of the PC unit was a dominant

In this chapter, we will describe our recent study for the synthesis of polyimide containing PC group as a biocompatible hard material. The desired copolyimides were carried out by the polycondensation of BAPPC and bis(*p*-diaminophenyloxy)benzene (BAPB) with 4,4'-hexa‐ fluoroisopropylidene diphthalic anhydride (6FDA), followed by the chemical imidization with triethylamine and acetic anhydride, as shown in Scheme 3. As the acid anhydride, 6FDA was used to make the polyimide soluble in some solvents. Table 1 summarizes the compositions and molecular weights of the obtained copolyimides. Four kinds of copolyimides with PC content were prepared by changing the ratio of BAPPC and BAPB in the feed of polymeriza‐ tion. The obtained copolyimides showed the number-average molecular weights (*M*n) at

hygroscopic that the obtained solid softened when exposed to moisture.

factor in the reduction of the adhesion of proteins and platelets.

around 1 x 104

.

**2.2. Syntheses and properties of polyimides**

6 Advances in Bioengineering


a) The molar ratio of monomers, BAPPC and BAPB, in the polymerazition.

b) PC contents in the copolymers were estimated by 1 H-NMR spectra.

c) Number-avarage and weight-avarage molecular weights, *M*n and *M*w, were determinated by GPC using 20mM LiBr solution in DMF as an eluent.

**Table 1.** Composition and molecular weight of polyimides (PIPC).

These copolyimides were soluble in aprotic polar solvents, such as dimethylformamide (DMF), dimethylsulfoxide (DMSO) and *N*-methyl-2-pyrrolidinone (NMP), at room temperature, whereas they were insoluble in water, methanol, ethanol and acetone. This solubility in specific solvents is advantageous in the processing for medical devices, and the insolubility in other solvents enables the material durable to these solvents. For the solubility in some solvents, the solubility of these copolyimides depended on the PC content of polymers. For example, only PIPC-1 in Table 1 was soluble in chloroform and tetrahydrofuran (THF), but PIPC-2, 3 and 4 were insoluble in these low boiling point solvents. By the way, polyimide without PC unit, which was prepared from 6FDA and BAPD, was soluble in chloroform and THF. Therefore, the solubility of these copolyimides decreased with increasing PC content, where the maxi‐ mum PC content that allowed the solubility in chloroform and THF was 15-20 wt. %. It was speculated that polar PC groups in the side chains would have a strong interaction, which would make the polymer insoluble in these solvent.

On the other hand, it was found by the thermogravimetric analysis that the weight loss of the PIPC series started at *ca.* 250°C, similar to polyamide and poly(urethane-urea) containing PC group, whereas the decomposition temperature of the polyimide without PC group was over 400°C. In addition, the hard but brittle films were prepared from these polyimides by solvent casting method.

#### **2.3. Syntheses and properties of segmented polyurethane**

Segmented polyurethanes generally consist of short alternating blocks of soft and hard segments, and exhibit an elastomeric property. The biocompatibility of segmented polyur‐ ethane is thought to arise from the microphase separation of the soft and hard segments. However, the biostability of segmented polyurethane is not suitable for long-term implanta‐ tion. It has been suggested that the biodegradation and cracking of polyurethane that occurred in vivo was due to the adsorption of proteins, adhesion of macrophages and peroxide formation [27 – 29], which resulted in the reduction of the mechanical strength of segmented polyurethane. Moreover, the soft segment of segmented polyurethane was reportedly degraded by oxygen radicals produced by adherent macrophages [30]. Therefore, several studies of surface or chemically modified segmented polyurethanes have been conducted to improve biostability by reducing the adhesion of cells and proteins [31 – 35]. Ishihara *et al.* have also investigated a polymer composite consisting of segmented polyurethane and MPC polymer to reduce protein adsorption to the polymer surface and to improve the biocompat‐ ibility of segmented polyurethane [36 – 41].

In our previous studies, we have prepared segmented poly(urethane-urea)s containing PC group by using the PC-containing diamine monomer, BAPPC, as a coupling reagent in the polyaddition of diols with diisocyanate [19]. The obtained polymers exhibited excellent biocompatibility, the film surface of which efficiently reduced the adhesion of human platelets. In addition, stress-strain measurements revealed that the poly(urethane-urea) films exhibited high elastic mechanical properties, where the Young's modulus increased with increasing PC content. The aim of the next study was to prepare another type of PC-containing polyurethane from diol monomer (BHPC). Cooper and his co-worker have reported that a PC-containing polyurethane could be prepared using glycerophosphorylcholine as a diol monomer [42]. We have designed the BHPC molecule based on the concepts that BHPC would be more hydro‐ phobic than glycerophosphorylcholine and easier to handle as a monomer for polycondensa‐ tion or polyaddition. We expected that both of the primary hydroxyl groups of BHPC would make the polymer have a high molecular weight due to its higher reactivity than glycero‐ phosphorylcholine with its secondary hydroxyl group. Recently, Khan *et al*. have reported a potential application of poly(carbonate-urethane) as a long-term biomedical implant material due to its resistance to biodegradation and its biocompatibility [43, 44]. Therefore, we selected polycarbonate diol (PCD) to construct the soft segment of PC-containing segmented polyur‐ ethane.

As shown in Scheme 4, the syntheses of segmented polyurethanes containing PC group and polycarbonate segment with different contents were carried out by polyaddition of BHPC and PCD with 4,4'-diphenylmethane diisocyanate (MDI). The compositions and the molecular weights of the obtained polymers are summarized in Table 2. The observed PC contents in mol % were determined by 1 H-NMR and were in good agreement with the molar ratio of BHPC and PCD in the feed of polymerizations.

**Sheme 4.** Preparation of segmented polyurethane containing PC moiety (SPUPC).

group, whereas the decomposition temperature of the polyimide without PC group was over 400°C. In addition, the hard but brittle films were prepared from these polyimides by solvent

Segmented polyurethanes generally consist of short alternating blocks of soft and hard segments, and exhibit an elastomeric property. The biocompatibility of segmented polyur‐ ethane is thought to arise from the microphase separation of the soft and hard segments. However, the biostability of segmented polyurethane is not suitable for long-term implanta‐ tion. It has been suggested that the biodegradation and cracking of polyurethane that occurred in vivo was due to the adsorption of proteins, adhesion of macrophages and peroxide formation [27 – 29], which resulted in the reduction of the mechanical strength of segmented polyurethane. Moreover, the soft segment of segmented polyurethane was reportedly degraded by oxygen radicals produced by adherent macrophages [30]. Therefore, several studies of surface or chemically modified segmented polyurethanes have been conducted to improve biostability by reducing the adhesion of cells and proteins [31 – 35]. Ishihara *et al.* have also investigated a polymer composite consisting of segmented polyurethane and MPC polymer to reduce protein adsorption to the polymer surface and to improve the biocompat‐

In our previous studies, we have prepared segmented poly(urethane-urea)s containing PC group by using the PC-containing diamine monomer, BAPPC, as a coupling reagent in the polyaddition of diols with diisocyanate [19]. The obtained polymers exhibited excellent biocompatibility, the film surface of which efficiently reduced the adhesion of human platelets. In addition, stress-strain measurements revealed that the poly(urethane-urea) films exhibited high elastic mechanical properties, where the Young's modulus increased with increasing PC content. The aim of the next study was to prepare another type of PC-containing polyurethane from diol monomer (BHPC). Cooper and his co-worker have reported that a PC-containing polyurethane could be prepared using glycerophosphorylcholine as a diol monomer [42]. We have designed the BHPC molecule based on the concepts that BHPC would be more hydro‐ phobic than glycerophosphorylcholine and easier to handle as a monomer for polycondensa‐ tion or polyaddition. We expected that both of the primary hydroxyl groups of BHPC would make the polymer have a high molecular weight due to its higher reactivity than glycero‐ phosphorylcholine with its secondary hydroxyl group. Recently, Khan *et al*. have reported a potential application of poly(carbonate-urethane) as a long-term biomedical implant material due to its resistance to biodegradation and its biocompatibility [43, 44]. Therefore, we selected polycarbonate diol (PCD) to construct the soft segment of PC-containing segmented polyur‐

As shown in Scheme 4, the syntheses of segmented polyurethanes containing PC group and polycarbonate segment with different contents were carried out by polyaddition of BHPC and PCD with 4,4'-diphenylmethane diisocyanate (MDI). The compositions and the molecular weights of the obtained polymers are summarized in Table 2. The observed PC contents in mol

**2.3. Syntheses and properties of segmented polyurethane**

ibility of segmented polyurethane [36 – 41].

casting method.

8 Advances in Bioengineering

ethane.


a) The molar ratio of monomers, BHPC and PCD, in the polymerization.

b) PC contents in the copolymers were estimated by 1 H-NMR spectra.

c) Number-avarage and weight-avarage molecular weights, *M*n and *M*w, were determinated by GPC using DMF as an eluent.

**Table 2.** Compositions and molecular weights of segmented polyurethanes (SPUPC).

The obtained polyurethanes, SPUPC-1, 2 and 3, exhibited a good solubility in aprotic polar solvents such as DMF, DMSO and NMP at room temperature, whereas SPUPC-4 was insoluble in these solvents. In addition, SPUPC-1 and 2 were soluble in the low boiling solvents, chloroform and THF, but SPUPC-3 and 4 were insoluble in chloroform and THF. Therefore, a trade-off relation was observed between the solubility and the PC content of polyurethanes, which was similar to PIPC series. It would be due to the strong interaction of polar PC group in the side chain and the highly polar urethane bond in the main chain. To obtain a soluble polymer with high PC content, a different concept for the molecular design or the polymeri‐ zation process should be developed the solubility of these PC-containing polymers. Recently, we have developed PC-containing poly(ester-urethane)s, the solubility of which was im‐ proved to some extent by the introduction of ester bond in addition to the highly polar urethane component [22, 23].

The flexible and self-standing films could be prepared from these segmented polyurethanes by a solvent casting method using DMSO as a solvent. Then, the elastic mechanical properties were observed for these segmented polyurethane films, where the Young's modulus increased with increasing PC content. Furthermore, the introduction of such a polar phospholipid group was effective in improving the resistance to protein and platelet adhesions on the polymer film, which was the result of surface properties derived from the PC moiety [21].
