Polyol Molecular weight distribution

Polyol Molecular Weight Distribution Effect on Mechanical and Dynamic Properties of Polyurethanes... 

Similar materials, hyperbranched polyesters based on bis-MPA and a polyol are now commercially available [11] from Perstorp AB under the trade name Boltorn [12], Figure 8.1. The average number of hydroxyl groups per molecule can be tailored between 8 and 64 and molecular weight can be varied between c. 2000 and 11 000. The co-polymerization of bis-MPA and a polyol core keeps the molecular weight distribution fairly low, typically below 2. 

In this variation of high performance liquid chromatography (HPLC), the columns are packed with material that will hold back molecules dependent on the molecular size. Using the correct solvent system and column types, the molecular weight and distribution of the polyols can be determined. Standard samples are needed to calibrate the system. The molecular weight distribution of prepolymers can also be determined. This will enable an estimation to be made of the number of soft segment chains there are. 

Step-growth copolymerization involves the use of three or more monomers which do not ordinarily all react with each other. Examples include mixtures of acids and polyols in the synthesis of alkyds, as illustrated in the recipes in Table 5-1. Such polymers will contain a random distribution of monomer residues if they are synthesized under conditions in which the polymerization is reversible and the molecular weight distribution is random. Polymers like alkyds are intended to be homogeneous products with properties which represent an average of those of all the component monomers. The copolymerization of linolcic acid in the recipe in Table 5-1 would confer air-drying properties on all the macromolccules in which it is incorporated. 

To conclude, the common physico-chemical characteristics of oligo-polyols for polyurethanes determined by standard analytical methods are hydroxyl number, hydroxyl percentage, primary hydroxyl content, molecular weight, equivalent weight, molecular weight distribution, viscosity, specific gravity, acidity and colour (See Chapters 3.1-3.11). 

The molecular weight distribution (MWD) of polyester polyols is broader than that of polyether polyols. The polydispersity index (PDI) of high MW polyester polyols (e.g., 2000 daltons) is around 2.8. 

The molecular weights (MW) of the commercial polyols are about 2000-5000 daltons and the molecular weight distribution is relatively narrow (MW/Mn =1.35). The hydroxyl number varies from 21-51 mg KOH/g... 

Calculation according to the Eq. (1) gives for A. = 2.648 (T = 443 K) the value = 0.918 and for = 2.693 (T = 473 K) = 0.954. These both values correspond well to the estimation according to the Eq. (1) of Chapter 5 (the discrepancy makes up less than 8%). This circumstance confirms, that in melts of synthesized polyester polyols macromolecnlar coils of different sizes are formed, i.e., certain molecular weight distribution takes place. 

The potassium hydroxide-initiated polymerization of ethylene oxide in alcoholic solvents such as diethylene glycol produces low molecular weight polyols (Mw 600-700) with broad molecular weight distributions because of chain-transfer reactions with alcohol that occur throughout the polymerization, as shown below (98) ... 

Most hydroxy-terminated polyesters are made by coesterifying two polyols (a diol and a triol) and two diacids (an aliphatic dibasic acid and an aromatic dicarboxylic acid or its anhydride). The ratio of moles of dibasic acid to polyol must be less than 1 so as to give terminal hydroxyl groups and avoid gelation. Molecular weight is controlled by this ratio the smaller the ratio, the lower the molecular weight. The molecular weight distribution Mn, and /n are all controlled by the diol-to-triol ratio. The ratio of aromatic to aliphatic dibasic acids controls Tg of the resin. 

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