4. Synthesis of PU using bicyclic monomers derived from 1,4:3, 6-dianhydrohexitols

The 1,4:3,6-dianhydrohexitols (1, 20, 21) have been used as rigid monomers in the synthesis of PU. Polyurethanes containing isosorbide (1) have been prepared by several research groups, and complex polyurethanes with elastomeric character and good mechanical properties gave rise to many patents. They were obtained from isosorbide and diisocyanates in the presence of suitable catalysts (Table 1). Thus, catalytic polymerization of 2-deoxy-1,4:3,6-dianhydro-2-isocianate-L-iditol (50) afforded the corresponding AB-type polyurethane (Figure 4). Bachmann et al. [32] described an alternative synthesis through the 2-azido-5-O-chloroformyl-1,2-dideoxy-1,4:3,6 dianhydro-L-iditol (51) which underwent spontaneous polycondensation, after catalytic hydrogenation, via the 2-amino-5-O-chloroformyl isoidide. The same authors also described the transformation of the 2,5-diamino-dianhydrohexitols 9, 83, and 84 into the corresponding diisocyanates 10, 11, and 13 (Figure 8) which was achieved by reaction with phosgene [9]. A dithiodiisocyanate derivative (12) was prepared from the 2,5-diamino-2,5-dideoxy-dianhydro-

other monosaccharide and disaccharide derivatives such as xylaric acid [37], maltose [36], and glucosides [51] have also been chosen (Table 4). For example, Ates et al. [34, 36] prepared some non-aromatic cross-linked polyurethanes with potential use as surgical tissue adhesives. PEG diol 81 and 4,4'-methylenebis(cyclohexyl isocyanate) (MCHI, 82) were polymerized with a certain amount of a sugar (maltose, sucrose, or xylose) as cross-linker. The reaction was carried out in THF or DMF-THF mixtures, at high temperatures in the absence of catalyst or adding triethylamine (TEA). The material cross-linked with xylose 55 [34] displayed high adhesiveness and biocompatibility properties, making it suitable for being used in the medical field. Depending on the final use of the cross-linked material, the reaction can be conducted in

The use of flexible aliphatic polyether/polyester chains such as PCL 76 [18], PGL 78 [33], PEG 81 [35–37], PPG 79 [38, 39], or oleic ester derivatives [51] (in combination with other diols) can lead to segmented PU as well as carbohydrates-based cross-linked materials with differentiated regions. The synthesis can be carried out "one pot," in which the polyols are incorporated into the polymer feed together with the sugar-based monomers and the other components. For example,the synthesis of highly functionalized low-molecular-weight polyether-polyols initiated by PGL and mixtures of PGL and sucrose has been described [33]. In the "one pot" preparation of rigid polyurethane foams in bulk and using MDI as diisocyanate, the polyether

Even though materials obtained "one pot" displayed good physical and mechanical properties, excellent dimensional stability as well as low friability, an improvement of microphase separation can be accomplished when the polymerization is performed in two stages and the flexible polyol is incorporated in the first step [23, 26, 27]. The last step promotes the formation of urethane or urea linkages, in the so-called hard region with a high density of hydrogen bonds that provides stiffness to that section and elastomeric properties to the final product.

The 1,4:3,6-dianhydrohexitols (1, 20, 21) have been used as rigid monomers in the synthesis of PU. Polyurethanes containing isosorbide (1) have been prepared by several research groups, and complex polyurethanes with elastomeric character and good mechanical properties gave rise to many patents. They were obtained from isosorbide and diisocyanates in the presence of suitable catalysts (Table 1). Thus, catalytic polymerization of 2-deoxy-1,4:3,6-dianhydro-2-isocianate-L-iditol (50) afforded the corresponding AB-type polyurethane (Figure 4). Bachmann et al. [32] described an alternative synthesis through the 2-azido-5-O-chloroformyl-1,2-dideoxy-1,4:3,6 dianhydro-L-iditol (51) which underwent spontaneous polycondensation, after catalytic hydrogenation, via the 2-amino-5-O-chloroformyl isoidide. The same authors also described the transformation of the 2,5-diamino-dianhydrohexitols 9, 83, and 84 into the corresponding diisocyanates 10, 11, and 13 (Figure 8) which was achieved by reaction with phosgene [9]. A dithiodiisocyanate derivative (12) was prepared from the 2,5-diamino-2,5-dideoxy-dianhydro-

4. Synthesis of PU using bicyclic monomers derived from 1,4:3,

bulk [33, 38, 40, 50].

174 Aspects of Polyurethanes

6-dianhydrohexitols

and/or polyols constituted the soft segment.

Figure 8. Diisocyanates obtained from 2,5-diamino-dianhydrohexitols.

L-iodide (83) and thiophosgene, which yielded poly(thio)urethanes and poly(thio)ureas by reaction with the diamino-monomers (9, 83, 84).

Beldi et al. [13] studied the polymerization of 1,6-hexamethylene diisocyanate (3, HDI) and 4,4' methylenediphenyl diisocyanate (23, MDI) with the 1:4,3:6-dianhydrohexitols (1, 20, 21) as well as another novel isosorbide-based ether-diol in DMAc. They established the nature of the end groups and the fraction of cyclic and noncyclic polyurethanes by nuclear magnetic resonance (NMR) and Matrix-Assisted Laser Desorption/Ionization-Time-Of-Flight Mass Spectrometry (MALDI-TOF MS), and demonstrated that the two combined techniques provide a robust method for the identification of structures, chain terminations, and by-products derived from side reactions.

Muñoz-Guerra et al. [18] prepared segmented PU from hydroxyl-end-capped polycaprolactone (3000 g/mol) as soft segment, diisocyanates HDI (3) or MDI (23) and 1,4-butanediol, isosorbide (1), and/or 2,4:3,5-di-O-methylene-D-glucitol (33) as extenders. The comparative effect of the preparation method (in solution or in bulk) and the influence of the selected extender (1 or 33), on the properties of the resulting segmented PU, were evaluated. Hydrolytic degradability was significantly increased by the presence of sugar units, although polymer degradation took place fundamentally by hydrolysis of the polyester soft segment. The same authors also carried out a comparative study of non-segmented polyurethanes with the purpose of evaluating the effect of the replacement of 1,4-butanediol by 1. The polymerizations were accomplished by standard methods using HDI and MDI as isocyanates [52]. It was observed that T<sup>g</sup> values of PU increased with the content in isosorbide.

Koning et al. have investigated water-borne polyurethanes based on isosorbide (1) and other renewable building blocks, such as amino acids and fatty acids [26, 27, 53]. The synthesis of waterdispersible polyurethanes prepolymers was carried out from 1, ethyl ester L-lysine diisocyanate (46), dimethylpropionic acid, and a dimer fatty acid-based diisocyanate [53]. The regioselectivities of the endo- and exo-OH functional groups of 1 and the primary ε-NCO and secondary α-NCO of 46 were found to have only minor consequences for the formation of NCO-terminated PU prepolymers. PU dispersions prepared from these four-component prepolymers showed good storage stability. Fully renewable poly(ester urethane urea)s (PEU) were also synthesized from bio-based starting materials: the renewable polyester diol poly(1,2-dimethylethylene adipate), isoidide diisocyanate (11), and

diaminoisoidide (83) as chain extender. The authors found that the PEU based on the isoidide diisocyanate (11) exhibited satisfactory thermal and mechanical properties [54].

Thermoplastic and cross-linked bio-based PU with tailored properties and high renewable carbon content were synthesized from isosorbide (1), 1,3-propanodiol, and 1,1,1-tris-(hydroxymethyl) propane as the cross-linker reagent, in bulk by varying the molar ratio of the components and without using any chemical catalysts [55].
