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Photochemical Initiation Of Radical Polymerization Reactions
 

Photochemical Initiation Of Radical Polymerization Reactions

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Organic polymers can be found in a wide variety of household products ranging from plastic cups and bottles, to car tires and fabrics. One method to synthesize polymers is via radical polymerization chemistry.

The radical polymerization reaction uses building blocks, such as alkene monomers, to form a polymer of various lengths and branching pattern.

The reaction consists of initiation, propagation, and termination. One approach to accomplishing radical initiation is by introducing a photoinitiator, which creates a free radical when exposed to UV or visible radiation.

This video will focus on photochemically initiated polymerization and will illustrate the principles of radical polymerization reactions, using the example of a polymerization of styrene with a benzoyl peroxide initiator, and some applications

Development of methods for initiation, propagation, and termination allow chemists to control polymer structure in order to generate polymers with specific targeted applications. This is important, as the properties of the material can be affected by the length of the chain and by chain branching.

In order for radical polymerization chemistry to proceed, a radical initiator is needed. Benzoyl peroxide can serve as a photochemical radical initiator.

Photochemically-promoted homolytic cleavage of the O-O single bond results in two carboxyl radical species, which decompose to form phenyl radicals and CO2.

These phenyl radicals can add to an olefin such as styrene to generate a new C-C bond and a benzylic radical.

The newly-formed benzylic radical then reacts with a second molecule of styrene, propagating a radical chain reaction. Polymerization continues until the reaction terminates, usually via the coupling of two radical species.

In order to photochemically cleave benzoyl peroxide, it must absorb photons to yield a molecular excited state, which then undergoes O-O cleavage. Since benzoyl peroxide absorbs only in the UV portion of the electromagnetic spectrum, a photosensitizer is required to induce radical initiation under visible light irradiation.

Benzophenone, which is a common photosensitizer, absorbs photons in the visible portion of the electromagnetic spectrum to generate a singlet excited state. Intersystem crossing affords the triplet excited state, which is longer-lived than the singlet excited.

The energy from the triplet excited state is then transferred to benzoyl peroxide, causing the cleavage of the O-O bond to generate carboxyl radicals. However, there is also a competing reaction in which the triplet excited state undergoes relaxation back to its singlet ground state.

If relaxation is fast relative to energy transfer, then the sensitization is inefficient. The efficiency of sensitization is measured by quantum yield, which is the number of photoreactions accomplished per photon absorbed.

Now that we have discussed the principles of photochemical initiation in a radical polymerization reaction, let's look at an actual procedure

Add 13 mg of benzoyl peroxide to a 10-mL volumetric flask and fill it to the line with toluene. This is your stock solution. Using a volumetric pipette, transfer 0.5 mL of this solution to a UV-vis cuvette, and dilute with 3.5 mL of toluene.

Prepare a blank cuvette containing only toluene, and measure the absorption spectrum at a range of 300-800 nm using a spectrophotometer. Repeat this step with the cuvette containing benzoyl peroxide and subtract the background spectrum.

Transfer 1 mL of the benzoyl peroxide stock solution to a pre-weighed 25-mL round bottom flask with a stir bar, and dilute with 10 mL of toluene and 3 mL of styrene. Attach a septum and degas the mixture by bubbling nitrogen gas through the solution using a nitrogen filled balloon.

In a fume hood, clamp the reaction flask fitted with the nitrogen filled balloon to a stir plate. Turn on the Hg-arc lamp fitted with a 350 nm long-pass filter. With magnetic stirring, irradiate the solution for 10 minutes.

Then, concentrate the mixture on a rotary evaporator. Weigh the flask to obtain the mass of the remaining non-volatile residue. Lastly, prepare and take an NMR spectrum in CDCl3.

Add 25 mg of benzophenone to a 25 mL volumetric flask and fill it to the line with toluene. This is your stock solution. Using a volumetric pipette, transfer 0.5 mL of this solution to a UV-vis cuvette, and dilute with 3.5 mL of toluene.

Measure the absorption spectrum of benzophenone in toluene at a range of 300-800 nm on a spectrophotometer, and subtract the spectrum of the blank cuvette.

Transfer 1 mL of the benzoyl peroxide and benzophenone stock solution to a tared 25-mL round bottom flask with a stir bar, and dilute with 10 mL of toluene and 3 mL of styrene. Attach a septum and degas the mixture using a nitrogen filled balloon.

In a fume hood, clamp the reaction flask fitted with the nitrogen filled balloon to a stir plate. Turn on the Hg-arc lamp fitted with a 350 nm long-pass filter. With magnetic stirring, irradiate the solution for 10 minutes.

Concentrate the mixture on a rotary evaporator. Measure the weight of the flask to obtain mass of the non-volatile residue, and obtain an NMR spectrum in CDCl3.

Transfer 1 mL of the benzophenone stock solution to a tared 25-mL round bottom flask containing a stir bar, and dilute with 10 mL of toluene and 3 mL of styrene. Attach a septum and degas the mixture using the nitrogen filled balloon method.

Then, repeat the procedure of irradiating, isolating, and analyzing the product as performed in the previous reactions.

The UV-vis measurements of benzoyl peroxide and benzophenone show that the former does not display substantial absorption in the visible region; whereas, the latter absorbs a substantial amount. This is consistent with the theory that a photosensitizer is needed to assist in initiating radical formation.

The reaction in the presence of both photoinitiator and photosensitizer yielded an oily, nonvolatile residue whose NMR spectrum is consistent with the structure of polystyrene. Polystyrene has characteristic peaks of a broad multiplet in the aromatic region between 7.2 and 6.4 ppm, and the multiplet of aliphatic protons between 1.9 and 1.5 ppm, with the integration ratio of 1 to 2. Whereas the control reactions in the absence of photoinitiator or photosensitizer yielded only unreacted starting materials.

Now that we have discussed a procedure for polymer synthesis using photochemical initiation, let's look at a few applications.

When two or more different monomers polymerize together, the result is called a copolymer. Typical copolymers include acrylonitrile-butadiene-styrene and ethylene-vinyl acetate. Photoinduced synthesis of copolymers can be achieved by introducing a second monomer subunit at a critical point during the polymerization reaction.

An example of a block copolymer is Poloxamer 407, which has been used to functionalize carbon nanotubes, which suffer from poor solubility and their tendency to aggregate. To overcome this problem, Poloxamer 407, which consists of a hydrophobic block of polypropylene glycol flanked by two blocks of polyethylene glycol, is used as a nonionic surfactant. By modifying the surface, the carbon nanotubes can disperse in an aqueous solution.

Polymeric three-dimensional structures are often useful in drug delivery or tissue engineering. Patterned devices can be synthesized by placing a patterned mask over a functionalized layer of a polymer, and the unprotected surface is subjected to photoinduced polymerization.

For example, patterned hydrogels can be functionalized with an array of thiol-containing peptides. First, the hydrogel is functionalized with an acrylate, then covered with a photomask, and treated with the peptides, resulting in a thiol-ene "Click"-reaction. These functionalized gels can be used to identify different peptides and their potential to elicit cellular responses.

You've just watched JoVE's introduction to photochemical initiation of radical polymerization reactions. You should now understand its principles, the procedure, and some of its applications. Thanks for watching!

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