If a surface is being grafted by a monomer, usually an active radical initiator is used (chemical or radiation). This would initiate both surface grafting and solution polymerization. Is it possible to change the ratio of grafting and solution polymerization (yield) simply by changing the concentration of the initiator and the monomer, or other measures are also necessary? If the goal is grafting, solution polymerization should be avoided. I have seen papers where the surface radical generation is done in a separate step followed by redox initaition in solution, but that does not prevent solution polymerization.
If you open the recipien and enter the air, or warm a little after open or added a solvent, the reaction will be finished.
Dear Guillermina. Thank you for your comment. Perhaps I was not precise enough. Termination of the reaction is not a problem, but air should be excluded during the reaction, as it deactivates the radicals. My question is how to avoid polymer formation in the solution while the grafting reaction is going on. It would improve the yield of grafting instead on precipitation in the neighboring solution.
Dear all;
I think the question is misdefined. What I understood by solution polymerization is the homopolymerization of the monomer to be grafted. If so, I think if there is a functional group on the substrat ready for oxidation such as in the case of cellulose for example, then an oxidation agent is used to create free radicals sites on the substrat which in turn will starts to graft the monomer on it. Regards
Dear Abdelkader, you are right, the radical source is intended to create reactive sites on the surface to be the initiator of chain growth form the surface. As the radical source is in the homogeneous phase, however, it will create radicals not only on the surface, but also in the solution, therefore the chain growth will proceed not only on the surface, but also in the surrounding solution. This latter I would like to avoid or minimize. One possible solution is dilution.
My Dear Friend Gyorgy,
& Esteemed Colleagues,
From my poor experience, i predict that the source of the uncontrolled polymerization in the solution phase is initiated by the leaving radicals/anions/cations from the initiators bonded to the surface itself.
Since the surface initiation is in inevitable, we should consider different initiation mechanisms other than leaving group including systems. Because during the production of the active initiator fragmented molecule/surface release a complementary active ion/radical which causes initiation in the solution phase where it is rich by monomers. The case also valid for living/controlled polymerization methods.
My first suggestion for a possible solution to that problem would be like this;
Initiation mechanism with formation of free species should be avoided for heterogeneous system like surface coating.
My hypothesis for an alternative system includes a U shaped initiator where it will locate on the surface from the both side and radical forming cleavage should be performed in the middle of the initiator molecule. With this strategy, we will have 2 active initiated sides at ones.
For the sake of this system following modifications can be followed;
1- The bond between the initiator and the surface must be strong. I thing we already have this alternatives from the vast alternatives from the SAM studies.
2- The active radical/ion formation should be started by a sterile mechanism like photo initiation.
3- The mobility of the species in the system is as important as the initiation and bond stability between the initiator and surface. To reduce the solution phase polymerization, the temperature should be kept at the moderate levels for reduced mobility of the active species.
These are my raw thoughts on the problem and a possible solution for now. I hope our experienced colleagues will be helpful by showing/proving my mistakes and misconceptions here to find the better.
Cordially Yours,
Kerem KARAKUŞ.
Dear Kerem, Thank you for your ideas, I will try to utilize them. UV initiation has also been tried with some success (and several version shave been described n the literature) but it has it s disadvantages too. Additionally the polymerization in the homogeneous phase can be observed in this case too. Radical initiation and radical polymerization is , as a rule, much simpler than ionic polymerization which frequently requires very special conditions. UV initiated ionic polymerization is also possible, but (as far as I know) not for unsaturated molecules, but for epoxies and other ring opening polymerization. Anyway I will carefully consider your proposals. best regards Gyorgy
A simple solution would be to use two different initiators, one for propagation and a second for grafting. The second could be a vinyl-functional initiator which activates only at high temperatures. It would be incorporated during the initial solution polymerization, after which the temperature could be raised to initiate a second phase of polymerization for "grafting" purposes. Thus you can tune your two compositions separately.
Regards,
Lou Floyd
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