190 Chapter 8
The acetylation of cellulose to cellulose acetate involves three stages, pre-
treatment of the cellulose, acetylation and hydrolysis. The aim of pre-treatment is
to purify the raw material and open the fibrous cellulosic structure to facilitate
substitution. Exposure to glacial acetic acid is the usual pre-treatment. The pre-
pared cellulose is mixed with a slurry of glacial acetic acid, acetic anhydride and
sulfuric acid for 5–6 h, depending on the desired degree of substitution. Since the
acetylation reaction is exothermic, the acids must be pre-cooled to limit the tem-
perature rise. As the reaction proceeds, the fibres of cellulose, insoluble in glacial
acetic acid, form soluble cellulose acetate. Just as for cellulose nitrate, various
degrees of substitution of hydroxyl groups are possible. Cellulose diacetate com-
prises a degree of substitution of 2.2–2.3 or 36–38% acetyl (COCH 3 ) and is
suitable for moulding, while cellulose triacetate has a degree of substitution
2.8–3.0 (42–44%) acetyl and is used for photographic film and fibres.
Preparation of synthetic polymers. The polymerisation process (chemical
joining of monomers) generally occurs by means of one of the three major
mechanisms, namely, addition polymerisation, condensation polymerisation
and rearrangement polymerisation.
In addition polymerisation, monomers are converted into polymers without
the formation of side products. Polymers formed by addition polymerisation
include polyethylene, polypropylene, poly(methyl methacrylate), polystyrene
and PVC. Reaction Scheme 2 shows that for addition polymerisation to take
place, the monomer must contain at least one double bond. Double bonds are
broken in the first stage of the process, known as initiation, and the released
valences join to other monomer molecules. A catalyst may be necessary to
initiate or hasten the process (such materials include benzoyl peroxide and
azodi-isobutyronitrile) which is encouraged to decompose into reactive free
radicals by exposure to light or heat. The catalyst’s radicals react with
monomer molecules by addition, generating even more radicals which further
react with monomer molecules. In the second stage or propagation, monomers
continue to add together, forming increasingly longer chains. During the final
stage, known as termination, all the monomer molecules have reacted and the
reaction ceases. The reaction may also be terminated rapidly by quenching with
water. The degree of polymerisation or number of repeat units in the molecular
chain, n, ranges between 500 and 1500; this corresponds to a theoretical
molecular weight average of 100,000–200,000. In practice, all batches con-
tain molecules with a range of chain length.
nCH 2 = CHCl (-CH 2 - CHC 1 -)n
vinyl chloride poly(vinyl chloride)⎯⎯→Reaction Scheme 2Poly(vinyl chloride) is formed by addition polymerisation of vinyl
chloride