I have made polymers with molecular weight upto 6000 using RAFT polymerization with polydispersity index of around 1.1 but when i tried to increase the molecular weight further, the control was not there and polydispersity index values were also higher.
With the RAFT process, the Mw target is decided by the initial molar ratio between monomer and chain transfer agent. If the monomer initial concentration is constant, Mw target increase by decreasing the initial concentration of chain transfer agent, the key to control the RAFT process. So the control decreases and the PDi increases.
There is always a limit of Mw target where the control is completely lost ...
Hi Santosh
The observed and calcuated MW were almost similar and the % conversion was also very high. More than 90%
That's because the initiator has been consumed (consider the half life time of AIBN, for example) and realistically speaking, you still need those to generate the chains. One solution will be to terminate the polymerization at a reasonable Mw and PDI, collect it and restart it again as macroinitiator with fresh monomers and initiators. Hope it helps.
It will be good to sepcify your monomers, RAFT agent and reaction conditions instead of a general question. As said above, much depends on the choice of the reagent, monomer and reaction condition and ratio of RAFT to monomer ratio.
One can change the ratio between monomer and CTA to get high molecular weight. Use high ratio between CTA/initiator. In ATRP, Cu(II) plays the role of deactivator which generates during the polymerization after termination between growing chains. To prevent this, the reaction can be started by adding Cu(II). In RAFT, the CTA does the job of only chain transfer and not deactivation as in ATRP. Another important thing in RAFT is, although the polymerization is controlled the initiation is not instantaneous which is the requirement for any living polymerization. In ATRP, almost all the initiator is consumed in the initiation step at the early stage of polymerization (ofcourse, depends on factors like initiator structure, catalyst system). In RAFT, since the initiator used is common free radical initiator (AIBN or BPO) the initiation is not fast enough and the initiator keeps generating radicals during the entire course of polymerization. Probably that is why it is very difficult to get control with HMW polymers. Reinitiation is one option but if any free radical initiators are added for that, then again that will give broad PDI due to the fact that the polymers generated from the initiator are always of low molecular weight compared to already preexisting polymers. Instead if the reinitiation is carried out without using any free radical initiator then the success of getting HMW polymers depends entirely on the ability of the pre-polymers in generating radicals. Based on this to get high molecular weight with RAFT,
proceed with high CTA/initiator ratio with a good CTA structurally suitable for the growing radical (acrylate or methacrylate or styrenic). Probably if the monomer is known you can get suggestion about suitable CTA for that monomer.
RAFT has typically major problems at higher monomer conversions. If you study the literature you can find many papers that clearly show that often the window of control in RAFT is not too broad (for example DOI:10.1017/j.ejps.2010.08.003). Also, you necessarily have rapid initiation vs. propagation, which present a major limitation for controlling the molar mass and the dispersity in particular. As Cristopher and the others pointed out, RAFT often needs fine-tuning to obtain the desired result
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