Daniele Caretti

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.

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.

Synthesis and chiroptical properties of optically active methacrylic polymers bearing the (S)‐ and/or (R)‐2‐hydroxysuccinimide moiety linked to the trans‐azobenzene group in the side chain

The synthesis of novel optically active monomers, namely (S)-(+)- and (R)-(-)-2 methacryloyloxy-N-(4-azobenzene) succinimide, is described. The monomers have been radically polymerized to afford the corresponding optically active homopolymers, characterized by the presence of an optically active rigid succinimide ring in the side chain linked to the trans-azoaromatic system through the imide nitrogen atom. A series of copolymers containing both the enantiomeric monomers in different molar amounts, have also been prepared in order to observe possible effects on the optical activity of the resulting deritatives. The chiroptical and conformational properties of the polymers in solution have been investigated by circular dichroism measurements and compared with those of low molecular weight model compounds, purposely synthesized, with the aim to verify the existence of cooperative interactions between chromophoric groups disposed along the macromolecular chain according to dissymmetric conformations.

Optically active polymethacrylates with side-chain L-lactic acid residues connected to push-pull azobenzene chromophores

Abstract Novel optically active methacrylic homopolymers with side-chain l -lactic acid moieties interposed through ester bonds between the main chain and trans -4-azobenzene chromophores bearing in the 4′-position electron withdrawing groups, such as the cyano and nitro residues, have been synthesized and structurally characterized. Investigation by circular dichroism of the chiroptical and conformational properties of the polymers in solution indicates a much higher optical activity of these latter with respect to that of the corresponding low molecular weight model compounds, purposely prepared, due to the presence of dissymmetric conformations of one prevailing handedness. The trans to cis photoisomerization kinetics of the azoaromatic double bond has been measured in solution for polymers and models and the resulting data compared with those of previously reported similar photochromic derivatives. The influence of the nature of the electron withdrawing group in the 4′-position of the azobenzene chromophore on the chiroptical properties of the polymers is also discussed.

Synthesis and characterization of polystyrenes containing side-chain tributyltin carboxylate moieties linked to the aromatic ring through a 1,2-ethylene spacer

A new triorganotin monomer, tributyltin 3-(p-styryl)-propionate (TBTSP), has been synthesized and submitted to radical homo- and copolymerization with styrene thus affording the corresponding polymeric derivatives having variable content of triorganotin moieties. The low molecular weight compounds, tributyltin 3-(p-ethylphenyl)-propionate and tributyltin 3-( p-isopropylphenyl)-propionate, have also been prepared as structural models of the organotin repeating co-units of the polymers. Characterizations in solution and solid state by IR, NMR and thermal techniques of all the polymers and low molecular weight compounds synthesized allow to conclude that the tin atoms are tetraand/or penta-coordinate depending on structure, physical state and tin content of the polymers. q 2000 Elsevier Science Ltd. All rights reserved.