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I'm working with some very basic free-radical initiated polymerization reactions. For the most part, I just follow a recipe I'm given (by a member of my group or the broader literature), it works, and I don't worry too much about the chemistry of why it works. Clearly, that will only work for so long, and I'd like to better understand the chemistry of what I'm doing.

So what general rules can I use to predict what kind of initiator will polymerize a given monomer?

To give a concrete example of what I mean: I know that some persulfate-based initiators will polymerize some acrylamide-based monomers. Can I generalize that to all persulfate initiators and all acrylamide monomers? What about methacrylamides? What about styrenes? Essentially, at what point have I changed the reactants so much that the chemistry doesn't work anymore?

Odian's Principles of Polymerization (4th ed) reviews some things that have been done, but doesn't discuss any general rules. Are there other good sources for this information?

EDIT: I am looking specifically for chemical reasons. Are some initiators better at reacting with some monomers? Are there any issues like side reactions I should take into account?

EDIT(2): from @Ben Norris's answer and the ensuing comments, it sounds like, as long as I can get the initiator to radicalize, and everything is soluble in the solvent, then I will be able to use any initiator with any monomer. Is that correct? I've found another textbook on this, Polymer Chemistry: An Introduction by Malcom Stevens, and that seems to support this.

Colin McFaul
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2 Answers2

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It's been my experience that initiators are chosen for: 1) initiation temperature, 2) solubility, and 3) cost.

1) Initiation temperature - Most radical initiators are stable at room temperature; they need to be heated to dissociate. For example, the half-life of benzoyl peroxide at room temperature is very long, but it is about 30 minutes at 92 °C and 1 minute at 131 °C. However, some monomers boil or decompose at temperatures lower than those temperatures. AIBN (azobisisobutyronitrile) decomposes above 65 °C, so less heat is needed. Both benzoyl peroxide and AIBN are suitable for bulk polymerization of most monomers, including acrylamides, acrylates, styrenes, etc.

2) Solubility - Benzoyl peroxide and AIBN are soluble in organic solvents, which include most monomers for bulk polymerizations. However, if you want to perform your polymerization in water, for example if you are doing an emulsion polymerization, then you need a water-soluble initiator, like a peroxodisulfate salt.

3) Cost - Every initiator has a price, even if you are only using tiny amounts. AIBN may have a lower activation temperature than benzoyl peroxide, but AIBN is also more expensive. There are other more specialized inhibitors that are even more expensive.

Ben Norris
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  • This is very helpful. I've clarified that I'm more interested in chemistry-specific reasons. Are some initiators better at reacting with some monomers? Are some imitators particularly prone to certain side reactions? – Colin McFaul Jun 28 '12 at 15:35
  • Initiators are usually present in such low concentrations that initiator induced side reactions are not serious problems. I'm not sure about incompatibility. All of the initiators I listed work with the monomers you listed. For more information, my reference book of choice is the Handbook of Polymer Synthesis from Wiley. – Ben Norris Jun 28 '12 at 16:44
  • What Ben Norris wrote, is first "chemistry-specific", and second all what can be said. – Georg Jun 28 '12 at 19:19
  • Ben and @Georg, I've added a question to clarify this, but this sounds like any initiator will work with any monomer, as long as they are all soluble and I can get the initiator to produce radicals. Is that correct? – Colin McFaul Jul 03 '12 at 15:46
  • @ColinMcFaul - That's about the size of it. – Ben Norris Jul 03 '12 at 20:26
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Apart from cost, solubility and temperature, the only question is wether the initiator is able to initiate the chain reaction. Because afterwards, it doesn't matter any more, the polymeric radical reacts with more monomer regardless of what the original initiator was.

So the first question is whether your polymer radical is reactive enough to work, but not too reactive, so it would make all kinds of radical (side) reactions instead of just extending the chain.

So, except in the latter case where your monomer was bad at radical polymerisation anyway, the only initiator that wouldn't work is one making very stable radicals. I don't think anyone would advertise those.

There might be a few corner cases, where the addition of the first molecule of a specific monomer takes too long, so the small and highly diffusive initiator radicals have a good chance to recombine before starting a new chain. I don't know of any actual examples for that.

Karl
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