Polymodal molecular weight distribution

The majority of amine-initiated block copolypeptides were often subjected to only limited characterization (e.g., amino acid compositional analysis) and, as such, their structures, and the presence of homopolymer contaminants, were not conclusively determined. Some copolymers, which had been subjected to chromatography, showed polymodal molecular weight distributions containing substantial high- and low-molecular-weight fractions.The compositions of these copolymers were found to be different from the initial monomer feed compositions and varied widely for different molecular weight fractions. It appears that most, if not all, block copolypeptides prepared using amine initiators have structures different than predicted by monomer feed compositions and likely have considerable homopolymer contamination due to the side reactions described above. 

This can result in polymodality of the molecular weight distribution when the exchange between the chain carriers is slow and they lead to the independent growth. Polymodality of the MWD is observed indeed. This phenomenon has been explained by differences in diffusion of the short and long polymer chains, but multiplicity of the chain carriers seems to us to be a more probable explanation. 

Polymerizations often afford polymodal product molecular weight distributions,18 with typically a low molecular weight fraction of several hundred daltons (fraction I), an intermediate fraction of about 4,000 (fraction II) and then a high molecular weight fraction of 10s or 106 (fraction III), though the relative proportions of the fractions depend... 

Figure 6 shows calibration curves for three other two column combinations, each representing 30,000 to 40,000 plates per set. The 10 A° plus 10 A° curve can be interpreted to show a deficiency in relative pore population in the range equivalent to about 50,000 to 600,000 molecular weight. The other two, properly calibrated, can conceivably be used for determination of molecular weight distributions. However, utility for resolution of specific polymodal mixtures is too difficult to assess from calibration curve alone. How much curvature of a calibration curve translates into utility or non-utility Calibration curves indicating pore size populations all have the same shape for given column combinations whether the plate count level is 5000 plates or 20,000 plates or 80,000 plates. 

Gel permeation chromatography indicates a polymodal polydispersity with a broad molecular weight distribution. Currently it is not possible to specify the definite values of the average molecular weights due to a lack of comparable standards. 

Raising the reaction temperature accelerated the polymerization but at the same time also the polymodal character of the molecular weight distribution became much more pronounced. Dilution of the monomer slowed down the rate of reaction and the molecular weight built up. 

Figures 3.33 and 3.34 demonstrate the kinetic activity distribution functions obtained from monomodal MWD curves for PrCl3-3(tributylphosphate)-Al(i-C4H9)3 and GdHal3-3(tributylphosphate)-Al(i-C4H9)3 catalytic systems in the isoprene polymerisation process. The functions demonstrated in the fignres are also seen to be polymodal. The initial stage of isoprene polymerisation demonstrates that the activity of AC, formed by both neodyminm and praseodymium catalytic systems, is predominant in the maximum of the low molecular weight area of the /(/w P)-ln M distribution curves (Figure 3.32).

---