Polyesters reaction mechanisms

Direct oxidation yields biacetyl (2,3-butanedione), a flavorant, or methyl ethyl ketone peroxide, an initiator used in polyester production. Ma.nufa.cture. MEK is predominandy produced by the dehydrogenation of 2-butanol. The reaction mechanism (11—13) and reaction equihbtium (14) have been reported, and the process is in many ways analogous to the production of acetone (qv) from isopropyl alcohol. 

Figure 5.1 Reaction mechanism for the synthesis of polyesters (e.g. poly (ethylene terephthalate)) via molten (melt)-state polycondensation... 

In the system epoxide (epoxy resin) — anhydride, we can thus expect the presence of anhydride, epoxy- and proton donor groups. In their study of the reaction mechanism, Fisch and Hofmann 20 22-24) proposed a sequence of reactions leading to the crosslinking of epoxy resins or to the formation of linear polyesters. The first step is the reaction of the anhydride with hydroxyl groups giving a monoester (Eq.(l))... 

The determination of the evolution of concentrations of different species and functional groups enables one to discern different paths present in the reaction mechanism. For example, Fig. 5.13 shows that as the molar ratio of styrene to polyester C=C double bonds (MR) increases from 1/1 to 4/1, the curves tend to shift downward. For MR = 4/1 there is a very low styrene consumption until the polyester double bonds are converted to 40%. On the other hand, SEM (scanning electron microscopy) shows phase separation of a UP-rich phase in the early stages of the polymerization. Most radicals are probably trapped in this phase, which explains the higher initial conversion of the UP double bonds than styrene double bonds. A kinetic model would have to take this observation into account. 

The last case concerns inhomogeneous networks often produced by chain polymerization (acrylate networks, unsaturated polyesters), where a gradient of crosslink densities is the result of the reaction mechanism and, in some cases, thermodynamic effects (Chapter 7). 

The tensile properties of polyester-based thermoplastic polyurethanes were studied as a function of the time of exposure of the plastics to water, methanol, methanol-water, methanol-Isooctane, and methanol-water-lsooctane. The resulting decrease In the tensile properties of the plastics was attributed to reaction of the plastics with water and methanol. As Indicated by the decrease In properties, reaction with methanol Is Initially faster, but the reaction rate with water Increases with time — presumably because of the autocatalytic nature of the reaction. Nuclear magnetic resonance spectroscopy Indicated that the reaction mechanisms with methanol and water were transesterification and hydrolysis, respectively. 

This equation has been tested (Manaresi and Munari, 1989) for simple self-catalysed aliphatic polyester formation and marked deviations have been found at low and high conversions. This is attributed to the differences in polarity of the medium that may result in different reaction mechanisms since these are also seen for non-polymerizing systems. At high conversions there has been the removal of significant amounts of water and there has been a change in the reaction volume together with a large increase in viscosity. The ability for the reaction to proceed to completion depends on precise stoichiometry since side reactions and loss of volatiles may result in an apparent decrease in reaction rate. 

Figure 1.8 Equations for preparation and curing of an unsaturated polyester resin. The presence of ethylenic unsaturation provides sites for cross-linlang by a chain reaction mechanism in the presence of styrene. Phthalic anhydride increases flexibility by increasing spacing of cross-links.

Although there are notable exceptions as given below, the most common lipase-catalyst used for polyester synthesis is Candida antarctica lipase B (CALB) (please refer to Chapter 14 for more information on the structure and reaction mechanisms of CALB). The immobilized CALB catalyst that has been primarily used is Novozym 435, manufactured by Novozymes (Bagsvaerd, Denmark). Novozym 435 consists of CALB physically adsorbed within the macroporous resin Lewatit VPOC 1600 (poly[methyl methacrylate-co-butyl methacrylate], supplied by Bayer) (please refer to Chapter 3 for more information on Novozym 435). 

The corporate effort behind the development of polyester films such as Cronar photographic film base (1951) and Mylar film (1952) typifies procedures applied to hundreds of Du Pont developments. Sixteen years of development work were required before the fundamental chemistry was translated into commercial success. Along the way numerous chemical engineers and other technical experts were needed to complete studies on such tangential subjects as the crystallization and orientation of polymers, chemical kinetics, and reaction mechanisms. The process of manufacturing the polymer continuously, rather than by separate batches, required new methods of control. New processes had to be developed to manufacture the chemical intermediates. 

Since the cross-linking of polyester-styrene system occurs by a free-radical chain-reaction mechanism across the double bonds in the polyesters with styrene providing the cross-links, the curing reaction does not give rise to volatile by-products (unlike phenolic and amino resins) and it is thus possible to cure without applying pressure. This fact as well as that room temperature cures are also possible makes unsaturated polyesters most useful in the manufacture of large structures such as boats and car bodies. 

Using compressed gas instead of chemicals to foam polyester resins can reduce control problems and heat reactions. Mechanically blended foam processing systems use a non-reacting additive pre-mixed into the resin... 

Recently, typical step-reaction polymerizations, as in polyesters, polyethers, and polyamides, have been forced into chain-reaction mechanisms by designing complex chain ends that react fast with the monomer only. Under the proper conditions, the step reaction can be suppressed almost completely. Such chain-growth polycondensation may even yield living polymers with narrow molar-mass distribution. A link to the initial literature is given in the General References for this section. 

For thermosets with conqilicated reaction mechanisms or where the conqiosition and functionalities of the molecules in the resin mixture are unknown, an enqiiricd ai oach is necessary. In order to describe the viscosity of polyester matrices Kenny et al. (3) adopted a model similar to file one originally used by Castro and Macosko for polyurethanes viscosity (50) ... 

Simple chemical hydrolysis of the hydrolytically unstable backbone is the prevailing mechanism for polyester degradation, which depends on the pH of the solution and may be catalysed by an acid or a base.The acid-catalysed reaction mechanism is given in Fig. 5.2." The reaction is reversible and is displaced towards hydrolysis by an excess of H2O. This mechanism is characterised by acyl-oxygen cleavage, and the removal of the alcohol is the slow stage of the reaction. 

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