Arrangement of Monomer Units

In chain reactions the addition of vinyl monomers to a free radical can occur in one of two ways (Reaction 2.8). 

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In the Type II case, the copolymerization tends toward an alternating arrangement of monomer units. Curve II of Figure 1 shows an example of an alternating copolymer that has an azeotropic copolymer composition, ie, a copolymer composition equal to the monomer feed at a single monomer feed composition. This case is analogous to a constant Foiling mixture ia vapor—Hquid equihbria.T) III... 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

For the case of copolymers, it is also possible to control the arrangement of monomer units in the chain. 

The polymer is based on a simple head-to-tail arrangement of monomer units and is amorphous, since the specific position of the benzene ring is somewhat variable and hence inhibits crystallisation. Despite its generally desirable properties, for many applications it is considered too brittle. Because of this, a number of approaches have been made to modify the mechanical properties of poly (styrene). The most successful of these have been (i) copolymerisation and (ii) the addition of rubbery fillers. 

If each successive addition of monomer molecules to the propagating radical occurs in the same manner as Eq. 3-9 or Eq. 3-8, the final polymer product will have an arrangement of monomer units in which the substituents are on alternate carbon atoms ... 

STRUCTURAL ARRANGEMENT OF MONOMER UNITS 3-2a Possible Modes of Propagation... 

Polymers can be considered on a range of size scales, from that of the molecules to that of the final products (Fig. 2.1). This chapter considers the molecular level the regularity of the arrangement of monomer units in polymer chains, the characterisation of that arrangement, and the effects on the polymer crystallinity. 

The arrangement of monomer units along the polymer chain is quite different for these three types of polymerizations. The monomer units incorporate randomly along the chain since the growing radical has no preference for either monomer. The incompatible monomer pair incorporates in blocks of the more reactive monomer, particularly when the more reactive monomer is present in excess. And, as might be expected, the monomer units alternate in the third case, except when one of the monomers is in great excess. 

In general, there are four kinds of copolymers random, alternating, block and graft. In the random copolymer the arrangement of monomer units is in a statistically random placement along a chain, e.g. 

The polymerization of cyclotrisiloxane with mixed siloxane units was expected to allow for a synthesis of siloxane-siloxane copolymers with a uniform distribution of units. These copolymers are not accessible by the kinetically controlled copolymerization of two cyclic trisiloxane comonomers, which leads to a microsequential order of siloxane units and a gradient arrangement of monomer units along the chain 16J7), So far the sequencing in poly siloxane, obtained from cyclotrisiloxane having two kinds of siloxane units, have been studied only for the polymerization in the presence of anionic initiators (16,18,19). The use of cationic routes for a controlled synthesis of copolymers with a uniform distribution of units is, therefore, of interest. 

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