Pentad sequences copolymerization

The sequence length distribution for an ideal copolymerization with r = 5, r2 = 0.2 for an equimolar feed composition is shown in Fig. 6-9. This copolymerization has pn — p2 0.8333 andp 2— p22 — 0.1667. Both M and M propagating centers have a 5 1 tendency to add Mi over M2, but Mi pentad sequences are not the most plentiful, although they are among the most plentiful. The most plentiful sequence is Mi at 16.7% with 14.0%, 11.6%, 9.7%, 8.1%, 6.7%, 5.6%, and 4.7%, respectively, of dyad, triad, tetrad, pentad, hexad, heptad, and octad Mi sequences. There are smaller amounts of longer sequences 3.2% of... 

From an NMR experiment it was determined that in a copolymerization of monomer A with monomer B the number fraction ofBAAAB pentad sequences was 0.04, while the number of AB diad sequences was 0.32. Calculate the number fraction of sequences of three As (AAA), A/3). 

Figure 2. TOX/DOP copolymerization kinetic profiles of pentad sequences and central carbon of CH2CH2CH2CH2O counit, B2. Molar ratio M/B= 9 BFsOEt2/M =30 ppm...

The events occurring during the homogeneous period of the bulk copolymerization of trioxane with DXL have been recently reexamined in conditions as above but using the pairs of labeled comonomers, TOX-dg/DXL and TOX/DXL-In both cases, NMR allowed to track the pathways of the -(CH2-O)- oxymethylene (M) and -(CH2-CH2-O)- oxyethylene (E) units originating from the comonomers, and the fact that the -(CD2-O)- (M ) units were undetectable drastically improved the spectral resolution. Detailed analyses of the evolution of the complex sets of pentad sequences involving E, M, and M rmits confirmed the previous general observations of Lu et but the successive... 

Figure 7 TOX/DXP bulk copolymerization at 70 °C kinetic profiles of pentad sequences and central carbon of -CH2CH2CH2CH2O- unit, B2. Molar ratio M/B = 9 (i.e., DXP -27 mol.%), [BF3 0Et2] = 30 ppm. Reproduced with permission from Cui, M.-H. Zang, Y. Werner, M. etal. ACSSymp. Ser. 2003,834, 228, figure 2.

These observations suggest how the terminal mechanism can be proved to apply to a copolymerization reaction if experiments exist which permit the number of sequences of a particular length to be determined. If this is possible, we should count the number of Mi s (this is given by the copolymer composition) and the number of Mi Mi and Mi Mi Mi sequences. Specified sequences, of any definite composition, of two units are called dyads those of three units, triads those of four units, tetrads those of five units, pentads and so on. Next we examine the ratio NmjMi/Nmi nd NmjMiMi/NmiMi If these are the same, then the mechanism is shown to have terminal control if not, it may be penultimate control. To prove the penultimate model it would also be necessary to count the number of Mi tetrads. If the tetrad/triad ratio were the same as the triad/dyad ratio, the penultimate model is proved. 

More complex schemes have been proposed, such as second-order Markov chains with four independent parameters (corresponding to a copolymerization with penultimate effect, that is, an effect of the penultimate member of the growing chain), the nonsymmetric Bernoulli or Markov chains, or even non-Maikov models a few of these will be examined in a later section. Verification of these models calls for the knowledge of the distribution of sequences that become longer, the more complex the proposed mechanism. Considering only Bernoulli and Markov processes it may be said that at the dyad level all models fit the experimental data and hence none can be verified at the triad level the Bernoulli process can be verified or rejected, while all Markov processes fit at the tetrad level the validity of a first-order Markov chain can be confirmed, at the pentad level that of a second-order Maikov chain, and so on (10). 

When polymerization of unsymmetrical monomers such as propylene occurs, addition can occur in a head-to-tail or a head-to-head shion to provide a variety of different structures. If two or more monomers are copolymerized, a variety of monomer sequences ((fyads, triads, tetrads, pentads, etc.) are possible as illustrated by the possible pentad monomer sequences for poly(l-chloro-l-fluoroethylene-co-isobulylene), (PCFEI), shown in Table 1. In general, it is possible to form copolymers having blodcs of monomers, a statistically random distribution of two monomers, and all possible compositional variations between these two extremes. In the example of PCFEI, it is possible to form pentads with five sequential E s, five sequential I s, and all permutations of E s and Fs between those extremes. The relative number of different monomer sequences provides information about the blockiness of the polymer, and is useful for determining the polymer s monomer composition. 

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