급진적 중합 반응의 광화학 개시
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Overview
출처: 데이비드 C. 파워스, 타마라 엠 파워스, 텍사스 A&M
이 비디오에서는 중요한 상품 플라스틱인 폴리스티렌을 생성하기 위해 스티렌의 광화학적으로 시작된 중합화를 실시할 것입니다. 우리는 광화학의 기초를 배우고 간단한 광화학을 사용하여 급진적 인 중합 반응을 시작할 것입니다. 구체적으로, 이 모듈에서는 과산화물의 벤조일 의 광화학과 스티렌 중합 반응의 광시이터로서의 역할을 살펴볼 것입니다. 설명된 실험에서는 광화학 반응의 효율(양자 수율로 측정)에 파장, 광자 흡수 및 흥분 상태 구조의 역할을 조사할 것입니다.
Principles
Procedure
Results
Based on the UV-vis absorption spectra that we collected, benzoyl peroxide does not display substantial absorption in the visible spectrum. The lack of visible-light-absorption is consistent with the lack of polymerization chemistry that is observed when a sample of styrene is photolyzed in the presence of benzoyl peroxide. The residue left behind following evaporation of the photoreaction described in step 2 contains only benzoyl peroxide; no styrene-derived products have been generated.
In contrast to benzoyl peroxide, benzophenone does absorb a substantial amount of visible light (> 300 nm). Photolysis of a mixture of benzophenone, benzoyl peroxide, and styrene results in the formation of some polystyrene. The polymerization results in the formation of an oily, non-volatile residue that remains after evaporation of the photoreaction. In addition, 1H NMR analysis of the residue indicates the presence of polystyrene. Polystyrene is characterized by diagnostic 1H NMR signals: the aromatic protons appear as a broad multiplet from 7.2-6.4 ppm and the aliphatic protons appear as multiplets centered at 1.9 and 1.5 ppm that integrate in a 1:2 ratio.
Finally, note that benzophenone alone is not a competent photoinitiator. Only when the sensitizer, initiator, and substrate were all present did polymerization proceed.
Applications and Summary
In this video, we have seen the impact of structure on the reactivity of radical initiators for olefin polymerization chemistry. We have examined photochemical conditions that: 1) did not contain appropriate absorbers, 2) contained appropriate absorbers but not appropriate initiators, and 3) contained both initiator and absorber molecules. These systems highlight the role of photon absorption and the importance of quantum efficiency on photochemical reactions.
Radical initiators are important tools for the production of polymer materials. Photo-initiated polymerization reactions find applications in a variety of areas. For example, photochemically initiated polymerization chemistry can be used to make designer materials in which the monomer that is incorporated is changed on demand, which allows precise control over the molecular structure of the material that results (Figure 3).
Figure 3. Photochemical control over block copolymer synthesis provides a strategy to making designer materials.
A second application is using photochemically initiated polymerization to generate 3-dimensionally patterned structures from polymers. Typically, such patterning is achieved by generating a mask, which prevents irradiation of areas of a surface that are covered with an appropriate monomer. Photoinduced polymerization is then carried out to generate a structure in which polymerization has been accomplished in the relief of the mask.
In this experiment, we saw the critical impact of photosensitizers on the efficiency of photochemical reactions. The concepts explored here underpin an important area of polymerization chemistry, which endeavours to identify highly efficient sensitizers and initiators. One example which we can understand based on our experiments is the use of unimolecular initiator-sensitizer hybrids.1 By combining the sensitizer, which has strong absorption in the visible spectrum, and the radical initiation benzoyl peroxide in the same molecule, we effectively increase the quantum yield of sensitization and thus more efficiently initiate polymerization. These observations highlight the importance of molecular design in identifying highly efficient polymerization initiators.
References
- Greene, F. D.; Kazan, J. Preparation of Diacyl Peroxides with N,N'-Dicyclohexylcarbodiimide. J Org Chem. 28, 2168-2171 (1963).
Transcript
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!