Question

The living character of living radical polymerization is limited under certain conditions, such as high monomer conversion, high initiator concentration, and high targeted molecular weight (>100,000). Explain why these conditions result in broadening of PDI and some difficulty in producing block copolymers with well-defined block lengths of different monomers.

Answer 1

Block co-polymers are special type of Polymersin which each molecule consists of two or more different segments of different segments of varying monomers joined in some architecture by a covalent bond. Hence , block Co-polymers can be classified by the number of blocks and their arrangements. Block co-polymers with two, three , and more blocks are called di-blocks, tri-blocks and multi-blocks. Some topology can be linear, in which the blocks are connected end - to - end , and stars, where blocks are connected via one of their ends at a single junction .

Well defined macromolecules can be obtained from living polymerizations in which propagating centers do not undergo either termination or chain transfer. Under such circumstances, the number of propagating species is a constant during the whole polymerization process, so the plot of Mn Vs conversion is linear . The degree of polymerization ' DP ' of the synthesized polymer in a living polymerization can be caliculated from the conversion and initialmolar ratio of monomer [Mo] and initiator [Io] .

   Dp = [Mo] / [ Io ] Conv

The product of an ideal living process is nearly monodisperse polymer, charecterized by a Poisson distribution of molecular weights. The Polydispersity index (PDI ) is determind is determined as

   PDI = Mw / Mn = 1 + Dp / (Dp + 1 ) ^2 approximately equal to = 1 + 1/Dp where Dp >>1