Reaction, chain, copolymer stepwise

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

In 2000, Hadjichristidis et al. reported the first successful synthesis of an exact graft copolymer composed of a PI main chain and two PS branches by a stepwise iterative methodology [229]. This involved the addition of PILi to l,4-bis(phenylethenyl)benzene, in order to introduce the DPE moiety at the chain-end and create a subsequent linking reaction of PSLi with the resulting... 

A straight-forward approach to the synthesis of pure substances, which can be extended to pure copolymers such as the ribonuclease, is the stepwise addition of one monomer, A, to a monomer, B, followed by separation from the excess chemicals. A third step can then add the next monomer. Again, separation from the excess chemicals has to follow this reaction step. This separation becomes more difficult as the chain gets longer, since the differences in properties between the successively made molecules become smaller as one approaches the length of polymer molecules. A yield of 90% for each of the 123 steps needed to make ribonuclease, for example, would produce only 0.002% of the polymer. In order to make a reasonable amount, the enormous, practically impossible yield of 99% must be achieved for each combined reaction and separation step. Only then can one count on converting 29% of the monomers into the proper polymer. 

Schubert and coworkers explored the use of pentafluoroslyrene and its copolymerization with styrene to obtain homo- and copolymer scaffolds for further functionalization with protected thiosugars via a thiol-para-fluoro click reaction (Figure 2.14). The first functionalization of the side chains resulted in about 60% conversion. Subsequent functionalization led to an increase in side chain conversion of up to 90%. The observed increase in functionalization could also be applied in a stepwise fashion to give access to heteromultivalent glycocopolymers. Selected copolymers showed self-assembly into spherical nanoparticles during nanoprecipitation with diameters ranging from 70 to 720 nm. 

---