Antepenultimate unit

This situation can be generalized. If the ratios do not become constant until the ratio of pentads to tetrads is considered, then the unit before the next to last-called the antepenultimate unit-plays a role in the addition. This situation has been observed for propylene oxide-maleic anhydride copolymers. 

The simple copolymer model is a first-order Markov chain in which the probability of reaction of a given monomer and a macroradical depends only on the terminal unit in the radical. This involves consideration of four propagation rate constants in binary copolymerizations, Eqs. (7-2)-(7-4). The mechanism can be extended by including a penultimate unit effect in the macroradical. This involves eight rate constants. A third-order case includes antepenultimate units and 16 rate coefficients. A true test of this model is not provided by fitting experimental and predicted copolymer compositions, since a match must be obtained sooner or later if the number of data points is not saturated by the adjustable reactivity ratios. 

In this system the thermodynamics of any of the four steps from Eq. 2-50 depends on the structure of the penultimate unit in propagation and the antepenultimate unit in depropagation. Such systems will be described in more detail in Chapter 15. 

The situation is more complicated in the case of intramolecular transfer, which occurs through the formation of a six-membered ring. In the case of acrylate (1)/ methacrylate (2), it can be assumed that the methacrylate radical is not reactive enough to back-bite and that the acrylate radical can only abstract hydrogen if the antepenultimate unit on the chain is also an acrylate unit. Thus back-biting can occur only for two monomer sequences (Mi Mi Mi and Mi M2 Mi) at the radical end, as shown in Scheme 4.16 [70]. The overall back-biting rate must be corrected for the sequence probabilities [Eqs. (40)-(43)] at the chain end, according to Eq. (59). 

Where the nature of the preceding diad is important in determining the configuration of the new chiral center (Scheme 26), first-order Markov statistics apply. Propagation is subject to a penpenultimate unit effert (also called an antepenultimate unit effect). Two parameters are required to specify the stereochemistry, P(m r) [=1-P(m]m)] and P(r]r) [=1 - P(r]m)], where P(t[j) is the conditional probability that given a j diad, the next unit in the chain will be an i diad. (In some texts, for example, those by Bovey and Tonelli, P(i]j) is written P(j/t.) It can be shown that... 

The mechanism of spontaneous termination occurring in the polymerization of (meth)acrylic monomers is completely different the termination affecting MMA ,Li+ active chains is mainly an addition reaction onto the carbonyl groups of the antepenultimate units by the growing enolates, followed by an elimination reaction ... 

Multiplicity of active centers may also be due to die influence of penultimate (antepenultimate, etc.) units. This influence has been studied for free radical polymerization and the required equations are available 7). In ring-opening copolymerizations, due to the onium structure of the growing species, this influence should be more... 

In the case of conjugated dienes the free radical is subject to resonance and can thus react through the last carbon atom of the chain or the antepenultimate one monomeric units of respectively 1,4- or 1,2- (or 3,4-, depending on the regiose-lectivity in the case of monosubstituted dienes) type are formed. The free radical polymerization of butadiene leads to approximately 80% of 1,4-type units. 

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