Лако-красочные материалы — производство

The theoretical analysis of the kinetic data for bulk polyesterification reactions is difficult because of the high concentrations of reactive end groups at the beginning of the reaction and because of the changes in dielectric constant of the medium during the reaction [3]. According to Flory [4], only the experimental results obtained for extents of reaction above 0.8 should be considered, that is when the polarity no longer changes and when the reactive groups form a dilute solution in the polyester. Within these limits, experi­mental data show that both mono — and polyesterifications are third-order reactions [4], second order in acid and first order in alcohol. A reasonable mechanism involves non — dissociated ion pairs and can be described, and with a protonic catalyst [2], as in the two following schemes:

К

2RC02H « » rc(oh)2+ rco2-

k 0H +

RC(0H) + RC09- + R’OH——————— ► R—lb—0—R‘ RCO, T ——————- ► products

*• alow і і л fast r

OH H

R, R’ = polyester chains — d[RC02H]/dt =* kK[RC02H]^[R‘0H]

According to classic organic chemistry, direct esterification can be catalyzed by either acidic or basic compounds, and Ingold [4] has proposed eight different mechanisms, four for acid-catalyzed and four for base-catalyzed processes. Basic compounds are seldom used as catalysts for polyesterification, and among the acid-catalyzed mechanisms, Aac2 is by far the most frequently observed. Hundreds of compounds have been claimed as effective catalysts in the patent literature; strong protonic acids (H2SO4, benzene-, naphthalene-, and p-toluenesulfonic acids are the most popular), oxides or salts of heavy metal ions (acetates are often preferred for their higher solubility), and organome — tallic compounds of titanium, tin, zirconium, and lead are the catalysts most frequently reported.

The mechanisms proposed for direct esterification of low-molecular-weight esters have been investigated in detail by many workers and have been discussed in detail in a review by Bender [4]. According to these investigations, the following scheme is generally accepted for proton-catalyst reactions.

+

ОН OH OH — u „

7{R-І—ОН] ~н ■ [R—OH] . ‘- [R—i—OH] . 2 —

h<5>r’ iff iff

(2,

, • [RCfOHjOffJ* ■ RC02R’

In this scheme, the reaction of the protonated form (1) of the carboxylic acid with the hydroxy compound to give the addition intermediate (2) is usually taken as the rate­controlling step. This mechanism is usually extrapolated to proton-catalyzed direct poly­esterification.

Owing to the low basicity of substrates such as carboxylic acids, the concentration of protonated species (1) can be extremely low, and alternative mechanisms, involving a nucleophilic attack assisted by compounds able to form hydrogen bonds in cyclic transition states such as (3) or (4), have also been considered [4].