Polyol structure

Leighton and coworkers [217] have also used this approach to develop efficient strategies for the synthesis of polyketide-derived natural products [218]. A main motif of these compounds is a skipped polyol structure, as in 6/2-94 this can easily be prepared by a novel Rh-catalyzed domino reaction of a diallylsilyl ether in the presence of CO, followed by a Tamao oxidation [219]. Thus, reaction of, for example, the silane 6/2-93, which is readily prepared from the corresponding ho-... 

The remaining group of sea star compounds are the polyhydroxysteroids (Table 7.10). The aforementioned Echinasteridae family178 elaborates the polyhydroxysteroids shown in Structures 7.145-7.157, one of which (Structure 7.154)191 was previously reported from a Mediterranean sea star Sphaerodiscus placenta. The Antarctic Acodontaster conspicuus produces polyols (Structures 7.157—7.162) 57 hexol (Structure 7.159) can be found in the Mediterranean Coscinaste-rias tenuispina.192 Polyhydroxysteroid (Structure 7.152) has modest cytotoxicity and the polyols shown in Structures 7.158, 7.161, and 7.162 were inhibitory toward sympatric microorganisms none of the others tested displayed notable bioactivity. 

Smith et al. have developed a very elegant route to complex polyol structures by sequential dithiane-epoxide coupling reactions (Scheme 7) [16]. Following the work of Tietze [17], 2-silyl-1,3-dithianes 42 are deprotonated with /BuLi in ether and converted into the stable lithium alk-oxides 43 with enantiomerically pure epoxides. A fast 1,4-Brook rearrangement occurs only after the addition of 0.3 equivalents of hexamethyl-phosphoramide (HMPA) or 1,3-dimethylhexahy-dro-2-pyrimidone (DMPU) to the reaction mixture. A new lithiated dithiane 44 that can undergo... 

Although the absolute configuration has only been established for a few polyene macrolide antibiotics, the search for new, efficient, and selective strategies for the synthesis of their polyol structures is in full swing. A number of the synthetic procedures presented here will certainly be used in future syntheses of this class of natural products [19]. 

The fifth factor important in obtaining high-flame-endurance foams is the choice of polyol structure. Since the beginning of the isocyanurate-foam industry, the major polyols used for modification have been polyether polyols (65, 66), which include polyols having a variety of functionalities (e.g., 2 to 8) and varying equivalent ratios. 

Formulations for producing polyurethanes (PUs) by reaction injection moulding (RIM) usually contain mixtures of polyols and diols in order to achieve the desired properties in the moulded part. The present work forms part (1) of a systematic investigation into the effects of polyol blends and glass fibres on the physical properties of unfilled and filled PUs formed by RIM. In the case of unfilled PUs, by using a multi-component polyol mixture, it is possible to investigate the effects on properties of (a) polyol structure, molar mass and functionality, (b) the relative proportions of diol-based hard blocks and triol-based soft blocks and (c) polyol blend compatibility. The... 

The catalyst selection for blocked isocyanates depends on the nature of the blocking agent, the desired cure temperature, the polyol structure, and the application. Mono-, di- and trialkyltin compounds are effective catalysts for many blocked isocyanate reactions with hydroxyl compounds. In most solvent-borne coatings, DBTDL is used as a catalyst. 

Vogtle s early work on cascade molecules was followed by the synthesis of highly branched polyols in the laboratory of Newcome (Fig. 3) [62], Since the first polyol structures reminded him of small trees, they were named arboroles (lat. arbor - tree). However, it has been the Greek-derived term dendrimer (8ev5pov - tree, pspcx - particle), which has become the most popular term for highly branched monodisperse molecules with fractal character, as first used in a patent... 

In order to understand the effect of polyol structure on the properties of polyurethanes a minimum amount of information about the structure and reactivity of isocyanates is... 

All the previous information regarding the general chemistry of polyurethanes and the structure of isocyanates have a role in the better understanding of how the oligo-polyols get chemically inserted in the high MW polyurethane structure and to understand the role played by the polyol structure in the properties of the resulting polyurethanes. 

Other oligo-polyols 7. New oligo-polyol structures... 

One observes that the tertiary nitrogen atoms in the polyol structure shown in Figure 4.39 have bulky substituents (the polyoxyalkylene chains with equivalent weight of around 1700-2000 daltons). This fact explains the modest catalytic activity in the foaming process. 

By using the raw materials presented diols, triols and dicarboxylic acids, it is possible to obtain a large variety of polyester polyol structures. For example to use one type of glycol and one type of dicarboxylic acid, but many other combinations are possible, such as using one dicarboxylic acid and two types of glycol or to use a glycol together with a small quantity of triol, to obtain a branched polyester polyol. 

Representative PCL are the diols of MW of 2000-4000 daltons, used in hydrolytically stable PU elastomers. The diols used as starters are DEG, 1,4 butanediol and NPG. The melting point of PCL, of MW of 2000 daltons, is in the range of 40-60 °C and of MW of 1000 daltons in the range of 30-40 °C. If a polyfunctional polyol is used as a starter, polyfunctional PCL polyols are obtained. Thus, by polymerisation of CPL initiated by trimethylolpropane (reaction 8.32) a polyester triol is obtained and initiated by pentaerythritol, a polyester tetraol is formed. It is interesting that some low MW PCL triols with a MW of 300-900 daltons are liquid at room temperature (melting points in the range of 6-16 °C). The viscosities of PCL polyols, at 60 °C, are 40-1600 mPa-s, depending on the polyol structure. 

The very low glass transition temperature (Tg) of polysiloxane chains (Tg = -123 °C) is a very attractive property for using these kinds of polymeric chains to build an oligo-polyol structure with terminal hydroxyl groups [1]. The resulting structure called a polysiloxane polyol gives, after reaction with diisocyanates, polyurethane (PU) elastomers which conserve their high elasticity at very low temperatures [1]. 

Aminic polyols (aliphatic or aromatic) are a group of very reactive polyols with the structure of alkanolamines. The high reactivity is conferred by the self catalytic effect of tertiary nitrogen from the aminic polyol structure, in the reaction of hydroxyl groups with the -NCO groups (see Chapter 14). 

The resulting amino polyols (structure 13.2) do not have a polyetheric structure, but if the addition of alkylene oxide continues by extension of the chains derived from hydroxyl groups, real structures of polyether polyols are formed (reaction 13.3). 

A variant of the previously mentioned application is to make the neutralisation of the crude alkaline, rigid polyether polyol with a cyclic anhydride (for example with succinic or maleic anhydride). A structure is obtained in which the potassium salt is chemically linked to a rigid polyol structure (reaction 13.18) and which enters the polyurethane network during the foaming process. Unfortunately, phthalic anhydride, an easily available and cheap cyclic anhydride, leads to partially insoluble potassium salts [36]. 

The DETA-based aminic polyols are synthesised by first adding 4 mols of PO and, in the second step, adding several units of EO without any other catalyst (self catalysis). The addition of EO to the hydroxyl groups formed, being catalysed by the tertiary aminic nitrogen from the aminic polyol structure resulting from propoxylation (reactions 14.6) [11, 12]. 

Of course the thermal stability and char yield depend on the polyol structure too and the aromatic polyols are superior to aliphatic polyols from this point of view. This is the reason for the extremely rapid growth of aromatic polyester polyols, of low functionality, low viscosity and low cost. 

In this section, several methods for rigid polyester polyols synthesis, of minor industrial importance at this moment, but which present a real potential for developing new polyol structures will be presented. 

By using similar chemistry, aromatic polyester polyol structures are obtained by alkoxylation of the phthalic anhydride reaction product with glycerol (reaction 16.10). By the propoxylation of the reaction product of pyromellitic anhydride with DEG, tetrafunctional, highly viscous aromatic polyester polyols (16.11) are obtained. 

Highly hyperbranched polyolic structures are obtained by the polyaddition of an hydroxy epoxidic compound, such as glycidol, to a polyol in cationic [1] or anionic catalysis [11-... 

Relationships Between the Oligo-Polyol Structure and Polyurethane Properties... 

Several general properties, characteristic to classical macromolecular chemistry, are strongly linked to the polyurethane structure, as a direct consequence of the oligo-polyol structure - these are [1, 2, 5, 9, 11] ... 

For crosslinked polymers (in this category they are the majority of polyurethanes, for example flexible, semiflexible and rigid PU foams, etc.), which have a MW that is practically infinite [12], the molecular weight between the branching points (Mc) is considered. The value of Mc depends strongly on the oligo-polyol structure. 

Table 21.3 shows the Tg of some important polymers for oligo-polyols structure. 

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