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Photoinduced Polyaddition for the Synthesis of PUR

Polyurethane polymers (PUR) are build up from organic units linked by urethane units. Figure 1 shows the basic mechanism of the formations of a link in PUR by polyaddition and the resulting urethane group. Chain propagation can be achieved by using polyisocyanates and polyols as monomers. Since a wide range of different monomer units with functional groups and a varying number of end groups can be used for the production of PUR, the range of polyurethane polymers and plastics is similar broad. The production of PUR was reported first by O. Bayer in 1937.[1]

  pureq1

Figure 1: Mechanism for the formation of an urethane unit by polyaddition.

The polyaddition reaction begins with a nucleophilic attack of the alcohol to the carbon atom of the isocyanate. Electron density is shifted towards the isocyanates’ nitrogen atom and a secondary amine is formed with the alcohols’ hydrogen. This reaction is often catalyzed by deprotonation of the alcohol, e.g. by tertiary amines.

A photochemical induced polyaddition of urethanes was reported by MARQUET ET AL. in 2014.[2] The reaction of polyfluorinated alcohols and phenyl isocyanate yields new highly fluorinated urethanes. The proposed reaction mechanism is given in Figure 2. It was also shown, that the reaction with not fluorinated alcohols is not induced by UV-light. In contrast to the mechanism with not fluorinated alcohols, the fluorinated alcohol is much less nucleophilic and more acidic. Hence, the formations of a strong hydrogen-bonding complex between the alcohol and isocyanate is proposed. Radiation then initiates proton shift and nucleophilic addition.

 pur2

Figure 2: Proposed Mechanisms for the Reaction of Phenyl Isocyanate with 2,2,2-Trifluoroethanol or Ethanol.[2]

 Another way to use UV-light for the formation of polyurethanes is the use of photoinitiators. Permissionneeded In contrast to the mechanism shown above, polymerizations initiated with photoinitiators in most cases are free radical polymerizations. For polyurethanes, UV-light is often used for curing or crosslinking of polyurethane coatings. This provides an easy application of the coating, which then can be cured with a controllable process. Highly crosslinked polymers can be produced as shown in Figure 3. The hyperbranched polyester H10 is used as core molecule. By joining polyurethane chains to the core molecule and the dispersing this hyperbranched polymer with photoinitiator in water, a UV-curable waterborne hyperbranched polyurethane dispersion can be produced. This dispersion can for example deposited as a film and cured with UV-Light. Within this process the number of double-bond-groups (C=C) is increased.[3]

pur3 

Figure 3: Synthesis of H10 and UV-curable waterborne hyperbranched polyurethane dispersion (WHPUD).[3]

 

  1. BAYER, O.: Das Di-Isocyanat-Polyadditionsverfahren (Polyurethane), 1947, 59, 257–272, DOI: 1002/ange.19470590901.
  2. SOTO, M.; SEBASTIAN, R. M.; MARQUET, J.: Photochemical activation of extremely weak nucleophiles: highly fluorinated urethanes and polyurethanes from polyfluoro alcohols, 2014, 79, 5019–5027, DOI: 1021/jo5005789.
  3. LIN, X.; ZHANG, S.; QIAN, J.: Synthesis and properties of a novel UVcurable waterborne hyperbranched polyurethane, 2014, 11, 319–328, DOI: 1007/ s11998-013-9520-4.

 

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