Лако-красочные материалы — производство

The ratio Гі can be considered as the tendency of monomer M; to react with itself (i. e. form a homopolymer) or to react with another monomer type present (i. e. to form a copolymer).

The value of Г! and r2 indicates the composition and structural type (i. e. random or block) of the copolymer being produced.

A value of r, = 1 indicates that ku and k12 are of similar magnitude and there is an even chance of M] or M2 adding to the propagating chain if the propagating radical is M, .

If Гі = r2 or Г] г2 = 1 then both ~Mj and ~M2 radical types show a similar preference for adding Mj and M2 monomer units.

In this case the resulting copolymer will be of a random distribution and the copolymer composition will directly reflect the starting monomer composition.

A value of Г! >1 indicates that the propagating species will only add to its own monomer unit. This will result in homopolymerisation particularly if r2 approaches 0.

An example of this type of system is a styrene, vinyl acetate mixture where r, = 55 and r2 = 0.01 for styrene and vinyl acetate respectively. As a result very little copolymerisation takes place between these monomers.

A value of rj <1 indicates that the propagating species prefers to add a monomer unit of the other species, i. e. Mi will add M2 and Mi will add M,. The result of this is an alternating copolymer.

An example of this type of system is styrene, methyl methacrylate; r, and r2 are 0.51 and

0. 46 respectively. This means that there is twice the probability of Mj adding to Mi and M2 adding to Mi than M, and M2 adding to radicals of their own monomer type.

If the product of Г] and r2 approaches zero or r, = r2 = 0, then the radical will not add monomer of the same type.

In this case, an alternating copolymer is formed.

An example of the use of the reactivity ratio to predict the copolymer composition is illustrated below.

Starting with equimolar concentrations of methyl methacrylate and styrene where r, =

0.

46 and r2 = 0.52, the copolymer composition can be calculated as follows.

The final copolymer will have a composition of 48 mole % methyl methacrylate and 52 mole % styrene, starting from equimolar concentrations of the monomers.

As polymerisation proceeds, the unreacted monomer ratio changes in accordance with the reactivity ratio, and this will cause the copolymer composition to vary from instant to instant, throughout the polymerisation.

It is possible to calculate feed ratios of monomers to obtain a given copolymer composition provided Г! and r2 are known.

Where F! is the mole fraction of monomer M, in the increment of polymer formed at a given stage in the cop. vmerisation and fi is the mole fraction of unreacted monomer M, in the feed and

fi = (1 — fa)

As a general rule?! does not equal f, and both change as polymerisation proceeds.

The polymer obtained over a finite range of conversion will consist of the summation of increments of polymer differing progressively in mole fraction F!

When

1

П = —

Г2

the equation is simplified to

Fi = r-|fi (nfi +І2)

The composition of the polymer at any instant can be approximated by the following expression

mi _ d [Mi] _ [Mi] ( n [Mi] + [М2]) m2 d [М2] [М2] (гг [М2] + [Mi])

Where m, and m2 are the proportions of monomer Mi and M2 in the polymer.

The reactivity ratios are essentially independent of temperature, solvents and other factors which have an effect on the overall reaction rate. This is due to the fact that all four constants (ku, k1>2, k2 , and k22) are similarly affected.

However, the method of initiation has a pronounced effect on the rate constants. There are large differences in reactivity ratio’s between the same monomers polymerised using free radical initiation and those where the polymerisation is conducted using ionic initiation.