Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: wereworm73 on July 11, 2006, 05:04:30 PM
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If you add a polyene to BF3 and no nucleophiles are present, would any cycloadditions occur along with polymerization reactions?
When the carbocation forms, it could attack a double bond from a neighboring polyene molecule & polymerize. Or...could the carbocationic polyene also react with one of its own double bonds and cause a cycloaddition reaction (especially if the concentration of polyenes is very low)? And if the polyene was highly conjugated, like beta-carotene, could cycloadditions still occur to form three-membered rings?
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In a nutshell, yes to all of the above.
In George Odian's book "Principles of Polymerizaiton", there was a section that dealt with cationic initiators. The jist of some work done by Joe Kennedy was that you can never actually dry solvents well enough to provide a nucleophile-free system. Even in a dry-box, there will always be sufficient water to complex with BF3 and initiate polymerization.
There is a book entitled "Hydrocarbon Resins". The most common monomer used to make hydrocarbon tackifier resins is 1,3-pentadiene (piperylene is a common name). In the "Hydrocarbon Resins" book, the authors speculate that cyclization does occur. Instead of the perhaps expected 1,4-addition reaction, the authors stated that terpene type structures form (ie 6-membered rings). A few years back I recall a Macromolecules journal article by Macedo dealing with quantifying the amount of cyclization by IR. A significant gel content is obtained with many polymerizations of 1,3-pentadiene unless very specific conditions are adhered to (indicating a multitude of side reactions including crosslinking). Some of the side reactions can be reduced by polymerizing at lower temperatures. Not familiar with any work done with beta-carotene. Much of the work on 1,3-pentadiene polymerization was also done with AlCl3 due to cost and toxicity issues.
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Wow, now that's got to be one crazy polymer. With all those different side reactions, you'd probably get something different every time you make it.
I wonder if palladium or platinum absorbing the 1,3-butadiene molecules would orient them in a way that favors one type of reaction over the other. Maybe Pd or Pt plus low temperatures would discourage the side-reactions even further? Or if something very porous was used instead, could it work the other way around, perhaps favoring intra-molecular cycloadditions (because the butadienes can't get to each other as easily since they'd be inside little holes)?