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ahlok2002
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[*] posted on 16-11-2004 at 06:31
polymerization...


during the emulsion polymerization the monomer migrates from the large monomer droplets to the growing micelles to sustain polymerization. On average, there is one radical per micelle. so why the polymerization not directly occur at the monomer micelle...? is that possible the propagation or polymerization occurs at the monomer micelles?

why must the radical form at the micelle then associate with disolved monomer then only penetrates into empty micelle to propagate...?
can the radical form inside the empty micelle the the propagation start??

when we add the chain transfer agents into a system , why the DP will reduce and the rate of polymerization remain unchanged?
is than any equation to support this?

thanks for help...




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chemoleo
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[*] posted on 16-11-2004 at 06:55


I think you have to be a bit clearer in your post.
What are you polymerising, what is your radical inducing agent, and what is the chemical reaction behind it? How big are the monomers, how viscous is the monomer vs the polymer? Some polymers are liquid, you see, so the reaction rates are greater due to greater diffusion rates.

Anyway - I would suspect that the initial rate of polymerisation during the reaction is exponential - i.e. radicalisation occurs exponentially, until essentially all the radical forming agent is used up. Thereafter, the reaction rate will become linear - i.e. a set amount of radical polymer is present.
However, now you have other terms coming in. I.e. reduced molecular movement (higher viscosity) will lower polymerisation rates. Heat development, on the other hand, will INcrease reaction rates. So the heat capacity of the stuff will matter too, the cooling rates and so on.
Then, at last, the reactant will be used up (monomer), so this will slow down polymerisation too.

I suspect if you plotted reaction rate vs time, you'd get a curve resembling slightly Gaussian shape (with a flat bit in the middle) - the initial lag would correspond to the generation of radicals, then you'd get a steady reaction rate (which increases with temp, however), and this would decrease exponentially again to zero as the monomer is used up, and as viscosity increases.




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ahlok2002
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[*] posted on 16-11-2004 at 07:31


em...

i'm refer to general emulsion polymerization reaction..

i'm trying to understand the explanation from text and paper...that is some fact that that i not really can get..so i'm refer to the mechanism it self rather that external factors that influence the rate of polymerization.

the first question is the when chain transfer agent is added..the degree of polymerization will reduced and the rate of polymerization remain unchange..why??

second is...normally the initiation of the emulsion polymerization take place in the water first then only enter the micelle to propagate...but from the text i get..they say the radical that form sufficient long length (becomes less polar as the chain length increase drive the growing chain penetrate into the empty micelle). so i wonder why not it will enter the monomer micelle, there have more monomers to support the polymerization...can explain?

i hope you all can get me...i'm poor in expressing in words..thanks for help!




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[*] posted on 22-12-2005 at 02:40


As I see it (for free radical polymerisation of methyl methacrylate or styrene) your free radical initiator - something like BPO or AIBN - is actually dissolved in the monomer droplets which are dispersed in the water. Your surfactant which forms a micelle around the droplet (thus stabilising it in the emulsion) plays no part in the reaction. When the temp is raised to the activation temp of the initiator the reaction takes place INSIDE the droplet. The water is only there to shape the droplet into a little sphere.

The whole reaction from initiation through propagation to termination happens inside the droplet for a 'general' emulsion polymerisation such as this. It doesn't start in the water.

Chain transfer is where the propagating radical is transfered to another chain, monomer, solvent or initator molecule. Thus the propagation is terminated for the inital chain and the chain grows no more - therefore no more chain growth and a lower DP. The radical is still active though (unless it met another grwing chain and the radicals terminated each other) and continues propagating on whatever it was transfered to -- thus the rate of polymerisation will remain unchanged.
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