2.1. From diol-diisocyanate

Among the different procedures followed for the synthesis of [AABB]-type polyurethanes (PU), the most widely used is the classical isocyanate-based route, a polyaddition reaction of diols (or polyols) and diisocyanates (or polyisocyanates) (Figure 1, Table 1). This is a simple synthetic method conducted under inert atmosphere and in the absence of moisture at room temperature or above (from 25 to 80C), either in solution or in bulk, with excellent conversions within 3–24 h for an extensive group of starting monomers. When the preparation of sugar-based PU is attended, the isocyanate-based method has been the route of choice so far, the sugar moiety being generally incorporated into the diol monomer.

Focusing on the preparation of sugar-based PU by the classical method, the solvents most commonly used are N,N-dimethylformamide (DMF) [11], N,N-dimethylacetamide (DMAc) [13], tetrahydrofurane (THF) [19] and, to a lesser extent, butanone [26], dimethylsulfoxide (DMSO), and hexamethylphosphoramide (HMPA) [31]. The polymerization can be catalyzed by a metal catalyst, the organotin catalyst dibutyltin dilaurate (DBTDL, Figure 2) [32] being the most widely used. Other metal catalysts are the commercial tin mercaptide esters registered as Metatin™ [18] and tetrabutyl titanate (TBT) [6]. Tertiary amines can also accelerate the polymerization process, and among them, the most extensively selected catalysts are 1,4-diazabicyclo[2.2.2]octane (DABCO) [10] and N,N-dimethylcyclohexylamine (DMCHA) [33]. Triethylamine (TEA) was chosen as (co-)catalyst in the preparation of segmented [27] and cross-linked [34] PU.

High temperatures and the incorporation of a liquid diol or polyol in the formulation, namely polyethylene glycol (PEG) [35–37], polyglycerol (PGL) [38], polypropylene glycol (PPG) [38, 39], and polycaprolactone (PCL) [40] are required when the polymerization is performed in bulk.

Figure 1. Most common synthetic routes to polyurethanes.

monomers with free hydroxyl groups [4–6], most syntheses of high-molecular-weight linear

Synthetic polymers obtained from sugar-based monomers are innocuous for human health. Their hydrophilic nature ensures a greater hydrolytic degradability [3], and reduces their environmental impact compared to classic polymers. Thus, the incorporation of sugar-derived units into traditional step-growth polymers constitutes an excellent approach to prepare novel biodegradable and biocompatible materials for application in the biomedical field and other

This chapter describes polyurethanes (PU) having the sugar units incorporated into themain chain. This topic has been partially reviewed before, but during the recent years numerous papers on the subject have been published. Thus, the following sections report on the syntheses, main properties, and applications of this type of sugar-based polymers that have been published mainly during the past decade. Patents have not been included so as to make the reference list more concise.

Among the different procedures followed for the synthesis of [AABB]-type polyurethanes (PU), the most widely used is the classical isocyanate-based route, a polyaddition reaction of diols (or polyols) and diisocyanates (or polyisocyanates) (Figure 1, Table 1). This is a simple synthetic method conducted under inert atmosphere and in the absence of moisture at room temperature or above (from 25 to 80C), either in solution or in bulk, with excellent conversions within 3–24 h for an extensive group of starting monomers. When the preparation of sugar-based PU is attended, the isocyanate-based method has been the route of choice so far,

Focusing on the preparation of sugar-based PU by the classical method, the solvents most commonly used are N,N-dimethylformamide (DMF) [11], N,N-dimethylacetamide (DMAc) [13], tetrahydrofurane (THF) [19] and, to a lesser extent, butanone [26], dimethylsulfoxide (DMSO), and hexamethylphosphoramide (HMPA) [31]. The polymerization can be catalyzed by a metal catalyst, the organotin catalyst dibutyltin dilaurate (DBTDL, Figure 2) [32] being the most widely used. Other metal catalysts are the commercial tin mercaptide esters registered as Metatin™ [18] and tetrabutyl titanate (TBT) [6]. Tertiary amines can also accelerate the polymerization process, and among them, the most extensively selected catalysts are 1,4-diazabicyclo[2.2.2]octane (DABCO) [10] and N,N-dimethylcyclohexylamine (DMCHA) [33]. Triethylamine (TEA) was chosen as (co-)catalyst in the preparation of seg-

High temperatures and the incorporation of a liquid diol or polyol in the formulation, namely polyethylene glycol (PEG) [35–37], polyglycerol (PGL) [38], polypropylene glycol (PPG) [38, 39], and polycaprolactone (PCL) [40] are required when the polymerization is performed in bulk.

2. Synthesis of linear sugar-based polyurethanes (PU)

the sugar moiety being generally incorporated into the diol monomer.

polymers involve derivatives having the hydroxyl groups appropriately blocked [3, 7].

sectors such as foodstuff packaging.

156 Aspects of Polyurethanes

2.1. From diol-diisocyanate

mented [27] and cross-linked [34] PU.

In sugar-based PU, the polyisocyanates and polyisothiocyanates most widely used are displayed in Figure 3, most of them being commercially available.

Other isocyanate-based routes are those in which a rearrangement of several acyl derivatives such as acyl azides (Curtius rearrangement), carboxamides (Hofmann rearrangement), and hydroxamic acid (Lossen rearrangement) conducts to the isocyanate monomer in situ (Figure 1). Thus, an article on the preparation of bio-renewable polyurethanes through Curtius rearrangement has been published. Initially, the synthesis of two new non-hindered diisocyanates based on isosorbide (1) and isomannide (20) without using petroleum-based reagents was attempted. The diisocyanates formation was carried out as follows: first, diols 1 or 20 were esterified with succinic anhydride in bulk at 120C; second, the acidic derivatives were transformed into the corresponding diacid chlorides at low temperature, and lastly the diacid chlorides led to the diisocyanate derivatives 47 and 48 via a two-step Curtius rearrangement with overall conversions ranging from 52 to 60% [28]. Both diisocyanates were proved to be useful as starting materials in the preparation of two PU: a stereoregular PU (with D-manno configuration in all the monomers) and a non-stereoregular PU (with D-gluco configuration).

Diol/diamine

Diiso(thio)cyanates

Reaction conditions

DMF, 25C, 72 h, DBTDL –

DMAc, 80C, 24 h,

From 48 to

[9]

112C

DBTDL

DMAc, 80C, 24 h,

[9]

DBTDL

THF, reflux, 60 h, DABCO –

[10]

T<sup>g</sup>

Others

References

[8]

158 Aspects of Polyurethanes


Diol/diamine

Diiso(thio)cyanates

Reaction conditions THF/DMF, 25/40�C, 3/24 h,

DBTDL

DMF, 40�C, 24 h, DBTDL

From 35 to

Desprotection

[15]

126�C

! PHU

Codiol: BD THF/DMAc,

DBTDL

THF/DMF,

DBTDL

Codiol: BD, PD

DMF/bulk,

 25/130�C, 24 h,

From �60

Segmented

 co-PU

 [18]

> to �3�C

DBTDL/METATIN

Codiol:

PCL-3000/BD

 25�C, 3/24 h,

From 21 to

Desprotection

[17]

79�C

! PHU

 25�C, 1/3 h,

36�C,

[16]

108�C

T<sup>g</sup> From 22 to

110�C

Others

References

[14]

160 Aspects of Polyurethanes

Bio‐Based Polyurethanes from Carbohydrate Monomers http://dx.doi.org/10.5772/intechopen.69606 161


Diol/diamine

Diiso(thio)cyanates

Reaction conditions

DMF, 25�C, 5 h, DBTDL

Codiol: DiT

T<sup>g</sup> From �21

Functionalization

 by

[24, 25]

162 Aspects of Polyurethanes

to 91�C

thiol-ene click reaction

! PHU;

! free COOH;

! free NH2

> DMF, 40/60�C, 48 h,

From 50 to

Desprotection

[7]

161�C

! PHU

DBTDL

Butanone, 30/70�C, 5/7 h,

From 18 to

Dispersion

[26, 27]

58�C

DBTDL/TEA

Others

References

Figure 2. Catalysts for the synthesis of PU via diisocyanate-based polymerization.

To conduct the synthesis of linear [AB]-type homopolyurethanes, both the nucleophilic and the electrophilic groups (e.g., hydroxyl and isocyanate groups [32, 41, 42]) need to be present in one sole monomer (Table 2).

Thiem and coworkers were the first to attempt this approach with success in order to synthesize sugar-based PU. They published the preparation of 2-deoxy-1,4:3,6-dianhydro-2-isocyanato-Liditol (50) from isosorbide (Figure 4), and the difunctional monomer proved to be suitable to polymerize in the presence of a catalyst [32].

The synthesis of a galactitol-based PU from theα,ω-hydroxylisocyanatemonomer52was carried out similarly to the abovementioned iditol-based PU. The preparation of the monomer 1-deoxy-1-isocyanato-2,3:4,5-di-O-isopropylidene-D-galactitol (52) was conducted from D-galactono-1,4-lactone by a four-step route. The monomer was then polymerized in THF in the presence of zirconium(IV) acetylacetonate [Zr(acac)4] as catalyst. The removal of the acetal-protecting groups led to a galactitol-based polyhydroxyurethane (PHU) [41]. In addition, Kolender et al. synthesized a glucitol-based PU by polymerization of anotherα,ω-hydroxylisocyanatemonomer, the 1 deoxy-1-isocyanato-2,3,4,5-tri-O-methyl-D-glucitol (53) [42].

### 2.2. Eco-friendly methods (isocyanate or/and stannous free) to prepare PU

Although polymerization reactions of diisocyanates with diols are the main method to synthesize linear PUs, in the last few years and due to the toxicity of stannous catalysts and common aromatic diisocyanates such as 4,4'-methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), a large number of works have been reported in literature on the synthesis of isocyanate-free PU, also called non-isocyanate PU (NIPU) (Figure 1, Table 3). Two interesting reviews have been recently published in which alternative pathways for PU syntheses are studied in detail [43, 44].

Bio‐Based Polyurethanes from Carbohydrate Monomers http://dx.doi.org/10.5772/intechopen.69606 165

Figure 3. Chemical structure of the most common polyisocyanates and polyisothiocyanates used in the synthesis of sugar-based PU.

### 2.2.1. Dicarbamate and diols

To conduct the synthesis of linear [AB]-type homopolyurethanes, both the nucleophilic and the electrophilic groups (e.g., hydroxyl and isocyanate groups [32, 41, 42]) need to be present in

Thiem and coworkers were the first to attempt this approach with success in order to synthesize sugar-based PU. They published the preparation of 2-deoxy-1,4:3,6-dianhydro-2-isocyanato-Liditol (50) from isosorbide (Figure 4), and the difunctional monomer proved to be suitable to

The synthesis of a galactitol-based PU from theα,ω-hydroxylisocyanatemonomer52was carried out similarly to the abovementioned iditol-based PU. The preparation of the monomer 1-deoxy-1-isocyanato-2,3:4,5-di-O-isopropylidene-D-galactitol (52) was conducted from D-galactono-1,4-lactone by a four-step route. The monomer was then polymerized in THF in the presence of zirconium(IV) acetylacetonate [Zr(acac)4] as catalyst. The removal of the acetal-protecting groups led to a galactitol-based polyhydroxyurethane (PHU) [41]. In addition, Kolender et al. synthesized a glucitol-based PU by polymerization of anotherα,ω-hydroxylisocyanatemonomer, the 1-

Although polymerization reactions of diisocyanates with diols are the main method to synthesize linear PUs, in the last few years and due to the toxicity of stannous catalysts and common aromatic diisocyanates such as 4,4'-methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), a large number of works have been reported in literature on the synthesis of isocyanate-free PU, also called non-isocyanate PU (NIPU) (Figure 1, Table 3). Two interesting reviews have been recently published in which alternative pathways for PU

one sole monomer (Table 2).

164 Aspects of Polyurethanes

polymerize in the presence of a catalyst [32].

syntheses are studied in detail [43, 44].

deoxy-1-isocyanato-2,3,4,5-tri-O-methyl-D-glucitol (53) [42].

Figure 2. Catalysts for the synthesis of PU via diisocyanate-based polymerization.

2.2. Eco-friendly methods (isocyanate or/and stannous free) to prepare PU

One of the methods to use is the transurethanization polymerization between a dicarbamate and a diol in which the side product is an alcohol, usually of low-molecular weight. For example, linear polyurethanes with free hydroxyl groups have been successfully prepared by Galbis et al. [6] from xylitol (55) and the aliphatic carbamates dimethyl hexamethylenedicarbamate (HDC, 56) or di-tert-butyl-4,4'-diphenyl methyl dicarbamate (MDC, 59), to render NIPU with enhanced hydrophilic character.

### 2.2.2. Bis(chloroformate) and diamines

To avoid the use of diisocyanates, a variety of new materials can also be obtained by polycondensation of diamines with freshly prepared bis(chloroformate)s. For example, two reactive bis

Table 2. Selected examples of the use of a sole carbohydrate-based monomer for the preparation of PU.

Figure 4. PU based on 2 deoxy-1,4:3,6-dianhydro-2-isocyanato-L-iditol (50).

(chloroformates) were formed from methyl ether diols of L-arabinitol and xylitol (73, 74, Figure 5). They were polymerized with commercial diamines (cystamine and cystine dimethyl ester) by interfacial polycondensation using sodium lauryl sulfate as surfactant [48]. The new

(chloroformates) were formed from methyl ether diols of L-arabinitol and xylitol (73, 74, Figure 5). They were polymerized with commercial diamines (cystamine and cystine dimethyl ester) by interfacial polycondensation using sodium lauryl sulfate as surfactant [48]. The new

Figure 4. PU based on 2 deoxy-1,4:3,6-dianhydro-2-isocyanato-L-iditol (50).

Table 2. Selected examples of the use of a sole carbohydrate-based monomer for the preparation of PU.

Monomer Solvent T<sup>g</sup> Others References

166 Aspects of Polyurethanes

DBTDL

DMAc/MeOH, 80�C,

THF/DMF, 40�C, 16/72 h,

Zr(acac)4

THF, 60�C, 48 h, Zr(acac)4 – Desprotection

– [32]

! PHU

– [42]

[41]

Bio‐Based Polyurethanes from Carbohydrate Monomers http://dx.doi.org/10.5772/intechopen.69606 167

Figure 5. Reduction-sensitive homopolyurethanes from diamines and bis(chloroformate)s.

NIPU, bearing the labile disulfide bond in their structure, were degradable under reductive environments.

It was demonstrated that the presence of methoxycarbonyl side groups in the PU makes those materials more degradable, not only under hydrolytic conditions but also under reductive environments, probably due to their lower crystallinity and a better water/glutathione penetration in their structure. Moreover, differential scanning calorimetry (DSC) studies showed that the incorporation of the abovementioned pendant methoxycarbonyl groups into the homopolymers resulted in a more rigid material.

This synthetic method provides a useful tool toward the synthesis of chemical diverse NIPU because of the large set of commercial diamines available and the excellent on-hand chemical procedures for the synthesis of amines.

### 2.2.3. Dialkyl(or aryl)carbonate and diamines

Diol/diamine

Dielectrophile

Reaction conditions DMF, 25C, 12 h, cat. (not disclosed)

Toluene-water,

lauryl sulfate

> Table 3.

Summarized

 literature about linear sugar-based

 PU prepared by eco-friendly

 alternative

 pathways.

 25C, 0.5 h, sodium

From 35

Interfacial

[48]

polycondensation

to 63C

T<sup>g</sup>

 From 8 to

59C

Others

References

[47]

168 Aspects of Polyurethanes

The polycondensation reaction between dialkyl- and diarylcarbonates with diamines is another alternative method for the preparation of NIPU (Figure 1). Varela et al. reported the preparation of the diarylcarbonate 1,6-di-O-phenyloxycarbonyl-2,3,4,5-tetra-O-methyl-D-mannitol (65) and its use as comonomer in the preparation of some NIPU at 85C [46].

The reaction of an α,ω-amino-arylcarbonate monomer, 1-amino-1-deoxy-2,3,4,5-tetra-O-methyl-6-O-phenyloxycarbonyl-D-glucitol hydrochloride (54), was tested for the preparation of sugarbased [AB]-polyurethane (Figure 6). Thus, the free hydroxyl group from the starting material was activated by means of the preparation of the phenylcarbonate derivative and further selfpolymerized in THF in the presence of diisopropylethylamine (DIPEA), rending a D-glucitolbased PU with low-molecular weight [42].

Figure 6. Synthesis of [AB]-polyurethane from 1-amino-1-deoxy-D-glucitol.

### 2.2.4. Dicyclocarbonates and diamines

Among the numerous pathways leading to NIPU, the polyaddition of cyclic carbonates with amines seems to be the most interesting route, and it is being widely investigated by numerous research groups (Figure 1). This method has been tested in sugar derivatives by Prömpers et al. They reported the preparation of PHU from D-mannitol-1,2:5,6-dicyclocarbonate 62 and its 3,4-O-isopropylidenene derivative 61 with hexamethylenediamine (60) [45].

Similarly, Besse et al. reported the preparation of linear and branched isosorbide-based polyhydroxyurethanes, with low T<sup>g</sup> values (from 8 to 59C) [47]. The isosorbide-based dicyclocarbonate monomer 70 (Figure 7) was prepared from a diepoxide according to the method previously described by Brocas et al. [49].

However, this route displays two major drawbacks: the low reactivity between cyclic carbonates and amines, and a limited degree of advancement of reaction during the room-temperature polymerization that leads to low-molecular-weight PHU. Consequently, highly polydisperse, low-molecular-weight materials were isolated.

Figure 7. Isosorbide-based dicyclocarbonate monomer.

### 3. Synthesis of cross-linked sugar-based PU and segmented PU

For the preparation of cross-linked sugar-based PU, a monomer with functionality above two is required. Just one work was found in which the cross-linker was a triisocyanate derived from L-lysine derivative [35]. In general, the cross-linking is accomplished by the use of a mono- or a disaccharide with all its hydroxyl groups unprotected. Thus, glucose and sucrose are widely used in the formulations of carbohydrates-based networks [38–40, 50, 51] although

2.2.4. Dicyclocarbonates and diamines

170 Aspects of Polyurethanes

described by Brocas et al. [49].

low-molecular-weight materials were isolated.

Figure 7. Isosorbide-based dicyclocarbonate monomer.

Among the numerous pathways leading to NIPU, the polyaddition of cyclic carbonates with amines seems to be the most interesting route, and it is being widely investigated by numerous research groups (Figure 1). This method has been tested in sugar derivatives by Prömpers et al. They reported the preparation of PHU from D-mannitol-1,2:5,6-dicyclocarbonate 62 and its

Similarly, Besse et al. reported the preparation of linear and branched isosorbide-based polyhydroxyurethanes, with low T<sup>g</sup> values (from 8 to 59C) [47]. The isosorbide-based dicyclocarbonate monomer 70 (Figure 7) was prepared from a diepoxide according to the method previously

However, this route displays two major drawbacks: the low reactivity between cyclic carbonates and amines, and a limited degree of advancement of reaction during the room-temperature polymerization that leads to low-molecular-weight PHU. Consequently, highly polydisperse,

3,4-O-isopropylidenene derivative 61 with hexamethylenediamine (60) [45].

Figure 6. Synthesis of [AB]-polyurethane from 1-amino-1-deoxy-D-glucitol.

3. Synthesis of cross-linked sugar-based PU and segmented PU

For the preparation of cross-linked sugar-based PU, a monomer with functionality above two is required. Just one work was found in which the cross-linker was a triisocyanate derived from L-lysine derivative [35]. In general, the cross-linking is accomplished by the use of a mono- or a disaccharide with all its hydroxyl groups unprotected. Thus, glucose and sucrose are widely used in the formulations of carbohydrates-based networks [38–40, 50, 51] although

Bio‐Based Polyurethanes from Carbohydrate Monomers http://dx.doi.org/10.5772/intechopen.69606 171

Cross‐linker

Dinucleophile

Dielectrophile

Reaction

T<sup>g</sup>

Others

References

conditions

THF/DMF,

2/20 h, DBTDL

THF, 90°C, 5 h,

From −38

Adhesives

[36]

to −8°C

no cat.

DMF, 20/40/60°C,

From 95

Film

[37]

to 132°C

25 h, DBTDL

Codiol: PEG‐1500

 60°C,

39°C, 111°

Linear or cross‐linked

[51]

172 Aspects of Polyurethanes

depending

 on the solvent

C

Table 4. Selected examples of the use of monosaccharides and disaccharides for the preparation of cross‐linked PU.

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 bulk [33, 38, 40, 50].

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 and/or polyols constituted the soft segment.

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.
