Complexation of stereoregular poly(methyl methacrylates). 8. Calorimetric investigations

The results of our calorimetric investigation of the complexation of long isotactic poly(methyl methacrylate) (it-PMMA) “matrices” with much shorter syndiotactic poly(methyl methacrylate) (st-PMMA) “oligomers” in a dilute solution of dimethylformamide (DMF) are presented. The heat of complexation at 30 °C and a total polymer concentration of 2.119 × 10-3 g/g is estimated to be 7.0 kJ/basemole of complex. Interpretation in terms of a simple model, discussed before, shows that complexation becomes more favorable for increasing degrees of occupancy of the matrix due to additional local ordering of occupied sections. Viscosity measurements confirm this conclusion. The critical chain length, defined as the chain length of the oligomers, below which no appreciable complexation takes place, is under the given circumstances shown to be approximately 60 monomer units. A continuous exchange and selection process is shown to be characteristic for the complexation of oligomers with a lower percentage of syndiotactic triads. This is apparent from (i) a shift in the position of the maximum of the heat of complexation to higher amounts of oligomers, (ii) a nonlinear dependence of the free energy of complexation on the degree of occupancy of the matrix, and (iii) an increased width at half-maximum of the calorimetric signal. Less heat per gram of polymer than expected is found for a lower total polymer concentration, especially for an excess of matrices. It is attributed to a suppressed local ordering due to the more swollen structure of the matrix. The reversed system of long syndiotactic matrices and isotactic oligomers as well as the influence of the matrix chain length is considered briefly.