Triad concentrations

The relative triad concentrations are then given by the following expressions (eq. 5-7)... 

Tacticity is most often determined by NMR analysis and usually by looking at the signals associated with the -CXY- group (refer Figure 4.3). The analysis then provides the triad concentrations (mm, mr and rr) and the value of m or P(m) is given by cq. 10. 

Note that the dyad concentrations can be easily calculated from the triad concentrations (eqs. 47-49). 

Similarly, penultimate model reactivity ratios can be estimated from initial monomer feed composition and triad concentrations using eqs. 50-53. 

Using the observed triad concentrations in Table III, the number average sequence lengths for the VCI2 monomer were calculated5 using the equation ... 

Values for rig can also be obtained from triad concentrations using the CCI2 resonance (1 ). 

Peak intensities (I) were used for triad concentrations since they were more easily and accurately measured than the corresponding areas in this particular case. An equation (Equation 5) analogous to 2 was used for butadiene sequence lengths. 

The most significant difference between the two sets of values is the dramatic increase in Arrr with an increase in temperature. Since Armm, Arrm, Armr and Arrr all have similar and large negative values compared to the other Axyz values at 150°, it is understandable that polystyrene spectra recorded at high temperature can provide reliable measures of rr-triad concentrations. 

If the reaction center adopts a preferred configuration with respect to the configuration of the penultimate unit in the chain (Scheme 4.1 k , k,) then Bernoullian statistics apply. The stereochemistry of the chain is characterized by the single parameter, P m) or P r) [= l-B)/ )]. The w-ad concentrations can be calculated simply by multiplying the concentrations of the component dyads. Thus the relative triad concentrations are given by the following expressions (eq. 1 -3)... 

Four different elementary reactions occur in first-order Markov mechanisms, and their rate constants can be calculated using the experimentally determined diad and triad concentrations. A steady state must exist for each diad type in the case of infinitely long polymer chains. A new type is formed by propagation cross-reaction for every type that disappears, i.e., for the stereocontrol... 

Also, the average sequence lengths of ethylene, n(E), and propylene, n(P), can be calculated from the derived triad concentrations ... 

Calculate the triad concentrations in the SAN copolymer discussed in Example 11.1. There can be six different triads formed in the SAN copolymer. These are AAA, AAS, ASA, SAS, SSA, and SSS, where A represents AN and S represents styrene monomers. 

It can be seen from Equations (11.14) and (11.15) that the P(SAS) = P(ASA). It can be seen from Figure 11.3 that the probability of ASA triads reaches a maximum value at 50% mole fraction of AN in the SAN copolymer. The SSA triads also reach a maxima at an AN composition in the SAN copolymer of 35% fraction. Concave curvature was found in the AAS triad concentration versus AN composition curve. The concave to convex to concave change in curvature was found for SSA triad concentration change with AN copolymer composition. The AAA triads rise rapidly in a monotonic fashion with increase in AN copolymer composition. 

As discussed in Chapter 12, the AMS-AMS homopolymerization reaction is not favored at the CSTR reactor temperature due to the ceiling temperature or unzipping effect Hence, the triad concentrations of P(SSS), P(SSA) can be seen to be 0. 

It can be seen from Hgnie 11.4 that the probability of ASA decreases from a maximum value of 1.0 at smaller compositions of AN in the copolymer. Concave curvature was found in the AAS triad concentration versus AN composition curve. The AAA triads rise rapidly in a monotonic fashion with increase in AN copolymer composition. 

FIGURE 11.2 Triad concentrations in PPM in SAN copolymer versus mole fraction AN in copolymer for AAA, AAS, and ASA triads. 

What is the maxima in a triad concentration of copolymer mean ... 

Is there a point of inflection in the triad concentration curve for copolymers as a function of monomer concentration ... 

The dyad probabilities for copolymer were calculated as a function of reactivity ratios and monomer composition. A first-order Markov model was developed to predict the chain sequence distribution of SAN and AMS-AN copolymers. The six triad concentrations for SAN copolymer were calculated. Nin dyad and 27 triads for random terpolymers were calculated and tabulated in Tables 11.3, 11.5-11.7. 

Figure 5.11 shows the plots of normalised acrylonitrile (A) and butyl centred triad concentrations versus the mole fractions of acrylonitrile (fA) and butyl acrylate (fB) in the copolymers, respectively. The increase in the concentration of acrylonitrile in the copolymers increases the fraction of the AAA triad, whereas it decreases the fraction of the BAB triad. The fraction of the AAB triad initially increases with the increase in concentration of acrylonitrile, passes through a maximum value, and then starts decreasing. The maximum fractions of AAB and BBA triads are obtained at 0.60 and 0.55 mole fractions of the respective monomers. 

Figure 5.11 Normalised acrylonitrile (A) and butyl acrylate (B) centred triad concentration plotted against mole fraction of acrylonitrile (f ) and butyl acrylate (fg). Reproduced with permission from A.S. Brar and A. Sunita, Polymer, 1993,...

If the discoloration chemistry is due mainly to the cydization of AN sequences, the increase in discoloration with increasing AN content should correspond to the increase in AN sequences. The dot of discoloration during molding at 280 °C vs AN triad concentration in the coixdymers as measured by... 

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