Photopolymerization with new LED systems emitting in the near infrared region

The NIR sensitized photopolymerization following a radical and/or cationic polymerization mechanism was investigated using heptamethine cyanine as sensitizer and either iodonium salt or oxime ester as coinitaiting component.  New high-power LED prototypes emitting between 800 nm - 900 nm served as excitation source.  They exhibited an excitation intensity of ≳1 W·cm-2.  These new high-power LED prototypes have unexpectedly brought new impetus into the photochemistry of heptamethine based cyanine operating as sensitizer in a photoinduced electron transfer (PET) reaction.  They even facilitate to overcome internal reaction barriers in the reaction system where low-power NIR LEDs mostly failed.  The strong non-radiative deactivation of the sensitizer can be seen as one source to promote the reaction between excited state of the sensitizer and the co-initiator.  These new light sources enabled much more heptamethine cyanines as sensitizer in the PET.  Overall, these NIR photoinitiator systems initiated well radical polymerization of acrylate monomers such as TPGDA, TPMTA,and PEGMA.

Particularly heptamethine cyanines carrying a cyclopentene moiety in the central point of the cyanine chain and a diphenylamino group at the meso-position of the methine chain were found as the first time in this work to initiate cationic polymerization of epoxides such as Epikote 357 or Epikote 828 in combination with iodonium salts.  The selective oxidation of the cyclopentene resulting in a fulvene moiety enabled to work cationic polymerization.  The possibility to initiate cationic polymerization was extended to oxetanes and vinyl ethers resulting in high conversions as well.  The vinyl ether derivatives exhibited the best performance in cationic polymerization followed by oxetanes and oxiranes.  These studies also included a variation of anions comprised in both the cationic cyanine sensitizers and the iodonium salts selected from those derived from weakly coordinating anions (WACs) such as aluminates ([Al(O-t-C4F9)4)4], [Al(O-(i-C3F7)CH3)4]), fluorinated phosphates ([PF6], [PF3(C2F5)3], [PF3(n-C4F9)3]), and methides ([C(O-SO2CF3)3]). [Al(O-t-C4F9)4)4] showed better performance compared to [Al(O-(i-C3F7)CH3)4] while [PF3(C2F5)3]did not exhibit the expected performance compared with [Al(O-t-C4F9)4)4].

These initiating systems can also initiate the hybrid polymerization based on free radical and cationic polymerization resulting in formation of interpenetrating polymer networks (IPNs).  Dynamic mechanical analysis (DMA) proved formation of IPNs comprising epoxy and acrylate monomers while the use of the same monomer system failed in a typical UV initiating system where thioxanthone served as photosensitizer.

In addition, oxime esters also served as alternative coinitiators together with heptamethine based cyanines applying NIR excitation.  Selected heptamethine sensitisers carrying as terminal group either benzo[e]- or benzo-[c,d]indolium substituents facilitated initiation of free radical photopolymerization where several oxime esters served as co-initiators applying different high-power NIR-LEDs emitting either at 805 nm, 860 nm, or 870 nm.  The huge amount of heat released by non-radiative deactivation of heptamethine also enabled this system to overcome an internal activation barrier appearing in the PET reaction proceeding in the excited state.  Moreover, this heat promoted thus photopolymerization while the application of thermal treatment failed to initiate polymerization.  This will bring new directions for development of technologies requesting internal barriers.



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