Luigi Angiolini

Synthesis and photoinitiation activity of radical polymeric photoinitiators bearing side‐chain camphorquinone moieties

Novel polymethacrylic photoinitiators bearing in the side chain the photosensitive camphorquinone moiety, suitable to the radical polymerization and crosslinking of multifunctional (meth)acrylic monomers under irradiation with visible light, have been synthesized starting from the corresponding methacrylic monomer. The monomer has also been copolymerized with a methacrylic comonomer bearing a pendant tertiary amine group acting as coinitiator, in order to check the effect on the photoinitiation activity of a close vicinity between camphorquinone and amine moieties disposed along the polymer chain. Low molecular weight compounds resembling the repeating units of the polymeric derivatives have also been prepared as reference models with the aim of establishing the influence of the macromolecular chain on the photoinitiating activity. The activity of the above mentioned low- and high molecular weight photoinitiating systems, variously combined, has been checked by microwave dielectrometry in the photocuring at room temperature of a mixture of mono- and difunctional acrylic monomers by irradiation at a wavelength above 400 nm. The resulting kinetic parameters have been related to the structural features of the systems investigated.

Mannich bases in polymer chemistry

Mannich bases, the characteristic structure of which is X sbnd;CH2N< (X = ketoalkyl, phenol, amide, phosphonic group, etc.), are widely employed as polymers or monomeric precursors of polymers in the production of substances with a large number of different applications in industry. This review covers the existing literature on Mannich bases involved in polymer chemistry on the basis of a classification of the polymeric structures which have been synthesized and describes their applications.

Photomodulation of the chiroptical properties of new chiral methacrylic polymers with side chain azobenzene moieties.

We have investigated the photoinduced optical properties of a new class of chiral methacrylic polymers characterised by the presence in the side chain of an optically active pyrrolidinyl ring linked to a trans-azoaromatic system. The homopolymers are enantiomerically pure and their strong optical activity indicates that the macromolecules assume, both in solution and in solid thin films, highly homogeneous conformations with a prevailing chirality. As expected, the studied polymers exhibit reversible linear dichroism and birefringence when irradiated with linearly polarised light. By irradiating with circularly polarised light, we have discovered that it is possible to photomodulate the chiroptical properties of the polymer films. After irradiation with L-polarised light, the CD spectra of the films show a net inversion of their relative sign. The effect is reversible and the original shape of the CD spectra can be restored by pumping with R-polarised radiation. This unexpected new phenomenon can be explained in terms of the ability of the L-polarised radiation to invert the prevailing helicity of the polymeric chains. The observed effect seems to open new possibilities for the use of azobenzene-containing materials as chiroptical switches.

Polymeric photoinitiators having benzoin methylether moieties connected to the main chain through the benzyl aromatic ring and their activity for ultraviolet-curable coatings

Abstract New polymeric UV photoinitiators, bearing the benzoin methyl ether moiety linked to the side chain through the 4′ position, were synthetized by radical homopolymerization of the corresponding monomers, 4′-methacryloyloxy benzoin methyl ether and 4′-methacryloyloxy α-methyl benzoin methyl ether, prepared in turn after a thorough investigation of synthetic methods. For comparison, low molecular weight structural models of the repeating co-units of the polymers having the pivaloyloxy group linked to the 4′ position of the benzoin methyl ether moiety were also prepared. All polymeric and model compounds were fully characterized and employed in the photoinitiated polymerization and crosslinking of a standard acrylic formulation for clear UV curable coatings. The photoinitiating activity of these systems, measured by microwave dielectrometry, indicates a higher cure fastness for the model compounds with respect to the corresponding polymeric derivatives, and such a behaviour is tentatively interpreted in terms of the fragmentation mechanism of the benzyl methyl ether radicals formed on α-photocleavage of the benzoin moiety.

Excited-State Properties of Camphorquinone Based Monomeric and Polymeric Photoinitiators

The excited-state properties of a new polymeric photoinitiator-bearing camphorquinone or/and amine moieties were studied and compared to the behaviour of the precursor molecules. The triplet state of the polymeric system was extremely short-lived, due to the close vicinity of the amino group. In addition, the singlet state also reacted with the amino group. A study on camphorquinone and methyldiethanolamine (=2,2′-(methylimino)bis[ethanol]) revealed that both these pathways led to the formation of a ketyl radical and an amine-derived radical. Therefore, high efficiency of the radical generation was expected. However, the radical photopolymerization of a polyfunctional mixture of acrylic monomers with various combinations of monomeric and polymeric photoinitiators gave evidence that the polymeric structure of the photoinitiating system may differently affect the overall cure rate of the formulation.

Synthesis, chiroptical properties and photoresponsive behaviour of optically active poly[(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene]

Abstract A novel optically active monomer containing a photochromic chromophore, such as trans-(S)-4-(2-methacryloyloxypropanoyloxy)azobenzene and its low molecular weight model compound trans-(S)-4-(2-pivaloyloxypropanoyloxy)azobenzene were prepared. Free radically initiated homopolymerization of the methacrylic monomer afforded in fairly high yield a rather high molecular weight polymer which was molecularly and structurally characterized. The molecular mobility of the polymer in all trans configuration, as well as chiroptical properties, photoisomerization kinetics and photochromic behaviour were investigated and compared with those of the low and high molecular weight related compounds. The polymer photoresponsive properties were evaluated by circular dichroism measurements at various extents of photoisomerization. All the results are discussed in terms of structural requirements of the macromolecules.

Methacrylic polymers bearing side-chain permanent dipole azobenzene chromophores spaced from the main chain by chiral moieties: synthesis and characterization

Abstract Novel optically active polymethacrylates bearing in the side chain a chiral group of one prevailing absolute configuration linked to the trans -azoaromatic chromophore, have been synthesized by radical homopolymerization of the corresponding optically active monomers, prepared in turn from optically active precursors by synthetic methods avoiding any racemization of the chiral center. These polymeric derivatives, namely poly[( S )-3-methacryloyloxy-1-(4′-X-4-azobenzene) pyrrolidine], with X=CN, CHO, CHC(SO 2 CH 3 )(CN), CHC(CN) 2 , are characterized by the presence of the 3-hydroxy-pyrrolidinyl ring as the chiral moiety and the azoaromatic donor-acceptor conjugated system as the moiety with permanent dipole moment. The polymers display optical activity in solution much higher than that afforded by the corresponding low molecular weight models, representative of the monomeric repeating unit, purposely synthesized. Owing to the substantially stereoirregular structure of the main chain, this suggests that the overall optical activity is mainly due to conformational dissymmetry of the macromolecules. Spectroscopic evidence suggests the presence in the above mentioned polymeric derivatives of dipole–dipole interactions between the side-chain azoaromatic chromophores, occurring as a consequence of their anchorage to the polymer backbone, which favors their aggregation.

Recent advances on photosensitive polymers: Polymeric photoinitiators

This overview deals with recent developments on structure—activity relationships in polymeric photoinitiators for UV curable coatings. In particular, polymeric systems bearing side-chain benzoin methyl ether moieties are examined and the effect on the photoinitiation activity discussed in connection with the different anchorage positions of the photosensitive group to the polymer backbone. In addition, novel polymeric photoinitiators based on acyldiphenylphosphinoxide moieties are discribed and their photoinitiation activity related to the nature of the acyl group in the photoreactive molecule. Finally, polymeric systems having pendant thioxanthone and α-morpholino acetophenone moieties in the same macromolecule that are able to behave as photosensitizing and photoinitiating species, respectively, are reported. The much higher photoinitiation activity found for the above copolymers, compared with the mixtures of the corresponding low molecular weight analogs, is interpreted in terms of excitation energy transfer from the side-chain thioxanthone to α-morpholino acetophenone moieties, favored by their forced proximity along the polymer chain.

Synthesis and chiroptical properties of optically active photochromic methacrylic polymers bearing in the side chain the (S)‐3‐hydroxypyrrolidinyl group conjugated with the trans‐azoaromatic chromophore

Novel optically active polymethacrylates, namely poly[(S)-3-methacryloyloxy-1-(4-azobenzene)pyrrolidine] and poly[(S)-3-methacryloyloxy-1-(4′-nitro-4-azobenzene)pyrrolidine], have been synthesized by radical polymerization of the corresponding monomers, prepared in turn through a synthetic route preserving the asymmetric center by any racemization reaction. These homopolymers are characterized by the presence in the side chain of an optically active pyrrolidinyl ring linked to the trans-azoaromatic system through the nitrogen atom. The optical activity of the polymers in solution appears much higher than that observed with the low molecular weight models, purposely synthesized for comparison. Circular dichroism spectra of the synthesized products demonstrated that, in solution, the macromolecules assume highly homogeneous conformations with a prevailing chirality to a larger extent with respect to analogous systems previously investigated.

Photoinduced chiroptical bistability in new chiral methacrylic azobenzene-containing polymers

The photoinduced chiroptical properties of an optically active methacrylic polymer bearing in the side-chain a conjugated trans-azoaromatic chromophore linked to the main chain through a chiral cyclic pyrrolidine moiety of one prevailing absolute configuration have been investigated. The polymeric film is characterized by a strong optical activity that can be amplified and reversibly modulated by irradiation with circularly polarized (CP) light at 488 nm. A remarkable modification of the shape of the circular dichroism (CD) spectrum after the initial irradiation steps is observed, with an amplification of the CD signal upon increasing the total irradiation fluence. The most important results are connected with the handedness of the CP radiation: left circularly polarized (CP-L) light is able to reverse the relative sign of the CD spectra, whereas by pumping with right circularly polarized (CP-R) radiation, it is possible to recover the original shape of the signal. This well reproducible and reversible effect suggests the possibility to use the investigated material for chiroptical switching.