Excess of glycol

Derive a plausible kinetic model for this reaction. Be sure your model reflects the need for the large excess of glycol. This need is inherent in the chemistry if you wish to avoid by-products. 

To avoid this problem, isophthalic acid is reacted first with a large excess of glycol to a diol intermediate consisting mainly of the glycol-acid (2 1) adduct,... 

These are usually prepared by direct esterification of the ethylene glycol with whichever fatty acid is required. Equimolar concentrations yield a mixture of mono and diesters and the reaction is usually carried out at 170-200°C. A multitude of base and acid catalysts can be used but, generally, sodium hydroxide or a metallic soap is used commercially. To obtain a high monoester content a 3-mol excess of glycol to fatty acid will give up to 70% yields of monoester. 

In commercial practice today reactions of the second type frequently utilize an excess of glycol, increasing the rate of esterification. Later stages of such polycondensations, as in the preparation of polyethylene terephthalate, take place by a transesterification mechanism with liberation of glycol. The direct esterification reaction may be catalysed by a second molecule of the carboxylic acid (self-catalysis) or by an independent acidic catalyst (catalysed esterification). 

The transesterification of a diester and a glycol is customarily carried out using an excess of glycol to drive the equilibrium... 

The polyether diols derived from polyesters seem to represent a compromise between cost and their contribution to superior elongation and tensile properties as compared to plain polyether diols polyether diols from polyester account for about 10% of the flexible PUR production. Examples of useful products formed from hydroxy-terminated esters of terephthalic acid (TPA)— formed, in turn, by the reaction of an excess of glycols with dimethyl terephthalate (DMT) or TPA—and TDI or MDI-PMDI are flexible ski clothing, gaskets, rollers for printing presses, etc. [85],... 

Most of the commercially available poly(ester-imide)s are products made from a large excess of glycol, thus having an OH-number between 100 and 300 mg KOH/g resin, depending on the product, and a molecular weight which is usually adjusted to be below 5000. 

In order to generate terminal hydroxyl groups an excess of glycol is currently used. The reaction takes place in uncatalysed reaction conditions (self catalysis by the acidic carboxyl groups) but the best perfomances (low reaction time, low final acidity) are obtained in the presence of specific catalysts, such as p-toluene sulfonic acid, tin compounds (stannous octoate), antimony, titanium (tetrabutyltitanate), zinc (zinc acetate), manganese (manganese acetate) or lead compounds and more recently enzymic catalysts (lipases) [1, 25]. 

The process takes place at higher temperatures, up to 200 °C, and the equilibrium is pushed to the formation of polycarbonate, by vacuum distillation of phenol. Of course, like the synthesis of all polyesters, in order to generate hydroxyl groups an excess of glycol is needed. The reaction with diarylcarbonates (for example diphenylcarbonate) takes place without catalysts (reactions 8.34 and 8.35). 

Glycolysis is the simplest and oldest method of PET depolymerization. The first patents on PET glycolysis were filed more than 30 years ago.3-9 The method involves the reaction of PET, under pressure and at temperatures in the range 180-240 °C, with an excess of glycol, usually ethylene glycol, which promotes the formation of BHET.10-13 This monomer has to be purified, normally by melt filtration under pressure, prior to its use in the production of new PET polymer. Colours present in the starting PET wastes are not usually removed by glycolysis. The depolymerization is carried out in the presence of a transesterification catalyst, usually zinc or lithium acetate. 

A variety of processes for polyurethane degradation by reaction with different glycols has been described in the literature during the last 30 years.76-83 Polyurethane glycolysis is usually carried out with an excess of glycols at temperatures around 200 °C and in many cases working at atmospheric pressure. After several hours of reaction, the polyurethane is completely liquefied and depolymerized, and catalysts are not necessary. 

An excess of glycol is used to cause complete alcohol removal and form chains with hydroxyl end groups. High molecular weights are accomplished by self-alcoholysis of these molecules to remove the excess glycol and approach a glycol/diacid ratio of 1 in the final polymer. 

The reaction product of adipic acid and propylene glycol prepared with a slight excess of glycol is given by the following formula ... 

The molar ratio of the ingredients shown above is 1.2 0.67 0.33 the excess of glycol is to allow for loss during the reaction and to restrict the molecular weight of the polymer. The mixture is heated at 150—200°C for 6—16 hours and water is continuously distilled from the reactor. Sometimes xylene is added to the reaction mixture to assist in the removal of water by azeotropic distillation and sometimes a catalyst such as p-toluenesulphonic acid is added to reduce the... 

The polyester polyols are conventionally made by simple polyesterification of a glycol and a dibasic acid using a 5-20% excess of glycol to ensure hydroxyl end-groups. An alternative method is provided by the rearrangement (ring-opening, addition) polymerization of e-caprolactone in the presence of an initiator ... 

In the first stage the acid or acids are reacted with an excess of glycols at temperatures between 200°C and 240°C, generally in the presence of a catalyst, to produce a low molecular weight prepolymer plus water or methyl alcohol byproduct. This reaction can be carried at atmospheric pressure or under pressure. An esterification catalyst such as zinc acetate is chosen when acids are used to make the prepolymer, and a transesterification catalyst such as tetraisopropyl titanate when dimethyl esters are employed. 

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