I am using AIBN as radical initiator and a raft agent
(4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid) for RAFT polymerization. I am wondering how come methacrylate monomers are only growing on the radical fromed from RAFT agent. Because my assumption is that monomers should grow from both AIBN radical initiator as well as the RAFT agent, resulting in polymers with two different end.
This may be because the methacrylate monomers are not very reactive because of the methyl (donor) group, and with the raft agent there is an activating group (z) for which polymerization has been carried out.
Monomers grow on the AIBN initiator as well !! Actually in RAFT polymerization you should observe both end groups (i.e : R RAFT end group and initiator, in your case AIBN). However, to keep your chain alive , it is primordial to reduce as much as possible the amount of initiator (AIBN). Because if you add to much amount of initiator, you will generate PMMA-AIBN deads chains. Normally, it is a common use to employ a ratio of [I]/[CTA] = 0.1. If you have a look on the RAFT polymeriation mechanism, you will see that the first step "initiation" is a reaction between a radical from initiator and the momonmer. This reaction occurs but in low concentration (depending of your temperature, reaction time and half life time of your initiator). So you just don't see your AIBN end group because it's too low comparing to the RAFT end group and PMMA signals, but for sure AIBN end groups are somewhere.
Thank you for your great answers. In this case, RAFT is not the greatest method for making block copolymers as compared to ATRP. Because for instance, if I am polymerizing MMA with a poly(ethylene glycol) RAFT macroinitiator, I will end up with both PEG-PMMA block copolymer as well as some AIBN-PMMA hompolymer.
Indeed, but RAFT is much more convenient and versatile than ATRP. In the case of standart ATRP, termination reactions also occur. It's always a question of balance . In your case if you start from a PEG-RAFT macro-CTA and if you use a relative low ratio of initiator (0.1), and depending of your conditions you can reasonably expect more than 95 % of PEG-PMMA block copolymers and few % of MMA homopolymers.
Marc Guerre sorry , but can you provide reference for your answer on using too much initiator may generate M-I dead chain ? currently, im doing research on the effect of ratio I/CTA agent towards NIPAm polymerization. and i did used ratio 0.1 but the experiment was not achieve.
Hi Dinie durrani Afiqah , it is quite a common knowledge and this is based on the polymerization mechanism used in RAFT. " reducing the initiator concentration in RAFT polymerization is therefore the key to optimize livingness of the system". If you want more information, have a look on this paper.:
Article 50th Anniversary Perspective : RAFT Polymerization—A User Guide
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