Zi-Chen Li

Vinyl monomers bearing chromophore moieties and their polymers. III. Synthesis and photochemical behavior of acrylic monomers having phenothiazine moieties and their polymers

Four acrylic monomers bearing phenothiazine moieties, i.e., N-acrylyl-phenothiazine (APT), N-acrylyl-2-chlorophenothiazine (ACPT), N-acrylyl-2-acetylphenothiazine (AAPT), and 10-acrylyl-1-azaphenothiazine (AAzPT) were synthesized by dehydrohalogenation of the corresponding N-(β-chloropropionyl)-substituted phenothiazine derivatives with 1,8-diazabicyclo[5.4.0]undec-5-ene (DBU). These monomers could easily be polymerized by initiation with AIBN. The emission fluorescence spectra of the monomers and their polymers were recorded, which showed that the polymers displayed much stronger fluorescence than their corresponding monomers at the same chromophore concentrations. This phenomenon, as termed as “structural self-quenching effect,” was commonly observed for acrylic monomers bearing chromophore moieties and ascribed to the coexistence of the electron-donating chromophore and the electron-accepting double bond in the same molecule. Because of the formation of exciplex, the monomer APT, as well as ACPT, AAPT, AAzPT, and their polymers, could initiate the photopolymerization of AN. The charge transfer phenomenon between P(APT), P(ACPT), and C60 was also explored.

Vinyl monomers bearing chromophore moieties and their polymers. II. Fluorescence and initiation behavior of N-(4-N',N'-dimethylaminophenyl)maleimide and its polymer

A maleimide bearing electron-donating chromophore, N-(4-N′,N′-dimethylaminophenyl)-maleimide (DMAPMI) was synthesized from N, N-dimethylaminoaniline and maleic anhydride in the presence of acetic anhydride and sodium acetate. DMAPMI can be easily copolymerized with vinyl acetate (VAc). In addition, it can be easily homopolymerized by UV light irradiation or by using AIBN or BPO as an initiator. The fluorescence spectra of DMAPMI and its polymer or copolymer were recorded and compared at the same chromophore concentrations. It was observed that the fluorescence emission intensity of DMAPMI was much lower than those of its polymers. This may be due to the occurrence of intermolecular charge transfer interaction between the electron-donating dimethylaminophenyl moiety and acrylic electron-accepting carbon-carbon double bond in the monomer. The model compound, N-(4-N′, N′-dimethylaminophenyl)succinimide (DMAPSI), which has no carbon-carbon double bond, displayed the same fluorescence behavior as DMAPMI polymers. The fluorescence of DMAPMI polymers and DMAPSI can be quenched by electron-deficient compounds such as AN, TCNE, MMA, etc. All these results supported the above conclusion. This is another example of the “fluorescence structural self-quenching effect” termed by us previously and demonstrates again that this phenomenon is not an accidental but a general one for acrylic monomers bearing electron-donating chromophores. Study of the initiation behavior of DMAPMI and its polymer showed that they could initiate the photopolymerization of AN, by combination with BPO, they could also initiate the thermopolymerization of vinyl monomers such as MMA.

Vinyl monomers bearing chromophore moieties and their polymers. V. Synthesis and fluorescence behavior of a vinyloxy monomer bearing electron-accepting chromophore moiety, N-(2-(vinyloxy)ethyl)-1,8-naphthalimide, and its polymer

A vinyloxy monomer bearing electron-accepting chromophore, N-(2-(vinyl-oxylethyl)-1,8-naphthalimide (VOENI), was synthesized by reaction of potassium 1,8-naphthalimide with 2-chloroethyl vinyl ether. VOENI can be homopolymerized by cationic initiation and copolymerized with maleic anhydride (MAn) under radical initiation. The fluorescence behaviors of VOENI and its polymers were investigated. It has been found that the fluorescence intensity of the VOENI monomer is much lower than that of its polymers at the same chromophore concentration. This means that a structural self-quenching effect (SSQE) has been also observed in the vinyloxy monomer consisting of an electron-accepting chromophore, which has opposite electronic structure in comparison with acrylates bearing electron-donating chromophores as we have reported previously. The SSQE is attributed to the charge-transfer interaction between the electron-accepting chromophore and the electron-donating double bond in the same molecule. The fluorescence quenching of 1,8-naphthalic anhydride and P(VOENI-co-MAn) by ethyl vinyl ether (EVE), dihydrofuran, triethylamine (TEA), etc. evidences that the electron-rich vinyloxy group does act as an important role in the SSQE of VOENI. C 60 can also quench the fluorescence of the polymers, and an upward deviation from the linearity of the Stern-Volmer plot was observed showing that C 60 acted as a powerful electron donor to quench the fluorescence of the copolymer.

Synthesis and ring-opening polymerization of α-chloromethyl-α-methyl-β-propiolactone

α-Chloromethyl-α-methyl-β-propiolactone (CMMPL) was synthesized by dehydrohalogenation of α,α-dichloromethyl-β-propionic acid which was obtained by chlorination of α,α-hydroxymethyl-β-propionic acid (DMPA). Due to high strain of the four-numbered ring, CMMPL can be polymerized by ring-opening with or without an initiator. Both electrophiles like trifluoroacetic acid (TFAA) and nucleophiles like triethylamine (TEA) and pyridine, as well as organometallic compounds such as stannous octoate [Sn(Oct) 2 )], aluminium triisopropoxide [Al(O 1 Pr) 3 and tetrabutyl orthotitanate [Ti(OC 4 H 9 ) 4 ], were found to be effective initiators. The polymerization can be conducted by either solution or bulk polymerization. P(CMMPL) is insoluble in almost all organic solvents at room temperature. An endothermic peak (ca. 214 ∼ 250°C) attributed to the melting transition of P(CMMPL) was observed in DSC curves. P(CMMPL) tends to have high crystallinity (40% ∼ 60%) as demonstrated by its X-ray diffraction patterns, and the crystallinity was found to vary with the types of initiator used.

Vinyl monomers bearing chromophore moieties and their polymers. VIII. Synthesis and fluorescence behavior of a vinyloxy monomer having an electron-accepting chromophore moiety, p-((vinyloxy)methyl)benzonitrile and its polymers

A vinyloxy monomer having an electron-accepting chromophore moiety, p-((vinyloxy)methyl)benzonitrile (VOMBN), was synthesized by reaction of p-(hydroxymethyl)benzonitrile with ethyl vinyl ether (EVE) in the presence of mercuric acetate. VOMBN can easily be cationically homopolymerized and copolymerized with EVE by using Lewis acids such as boron trifluoride etherate (BF 3 . OEt 2 ) as a catalyst and radically copolymerized with maleic anhydride (MAn) using AIBN as an initiator. The fluorescence behaviors of VOMBN, its copolymer P(VOMBN-co-MAn), and its saturated model compound p-(ethoxymethyl)benzonitrile (EOMBN) were investigated in acetonitrile. It has been found that the fluorescence intensity ofVOMBN is much lower than its copolymer and EOMBN at the same chromophore concentration. A fluorescence structural self-quenching effect (SSQE) is also observed for VOMBN as we have reported previously [Li, F. M.; Chen, S. J.; Li, Z. C.; Qiu, J. J Polym Sci Polym Chem 1996, 34, 1881]. This phenomenon has been attributed to the inter- and intramolecular charge transfer interaction between the electron-accepting cyanophenyl chromophore and the electron-donating vinyloxy group in the same molecule. The dependence of the fluorescence intensity of VOMBN on solvents of different viscosities is evidence that the SSQE of VOMBN mainly occurs intramolecularly. The fluorescence of EOMBN and P(VOMBN-co-MAn) was quenched by a series of electron-rich vinyloxy compounds which do not have chromophore moieties, such as dihydrofuran (2H-furan), dihydropyran (2H-pyran), furan, and EVE. It is observed that the higher the electron-donating ability of the quenchers, the greater the quenching efficiency. P(VOMBN) and the random copolymers of VOMBN with EVE show broader fluorescence spectra as compared to the alternating copolymer P(VOMBN-co-MAn). This indicates that there is a mutual interaction between the adjacent cyanophenyl groups in P(VOMBN) and P(VOMBN-co-EVE), whereas such an interaction does not exist for P(VOMBN-co-MAn) in which the cyanophenyl groups are isolated by the rigid succinic anhydride rings.

Vinyl monomers bearing chromophore moieties and their polymers. IV. Fluorescence and photosensitizing behavior of 8-acryloyloxyquinoline and its polymer

Acrylic monomers bearing electron-donating quinolyl moiety, i.e., 8-acryloyl-oxyquinoline (AQ) was prepared and polymerized. It was found that the fluorescence intensity of AQ was much lower than that of P(AQ) at the same chromophore concentration. The fluorescence of P(AQ) could be quenched by electron-deficient vinyl monomers, such as acrylonitrile (AN) and methyl methacrylate (MMA). This is another example of the fluorescence structural self-quenching effect termed by us previously, and demonstrates again that this phenomenon is not an accidental but a general one for acrylic monomers bearing electron-donating chromophores. The photopolymerization of acrylonitrile (AN) sensitized by AQ and P(AQ) as well as combining with carbon tetrabromide (CBr 4 ) was studied kinetically. From the rates of the polymerization (Rp) and overall activation energies obtained for these four systems, it was found that Rp sensitized by the binary systems was much higher than by AQ or P(AQ) alone, while the molecular weights of the resulting P(AN) were lower. The fluorescent analysis of the resulting P(AN)in solution showed that the sensitizers also entered into the P(AN) chains. A mechanism of charge transfer complex (CTC) formation was tentatively suggested for the photopolymerization of AN initiated by these four systems.

Ring‐opening copolymerization of α‐chloromethyl‐α‐methyl‐β‐propionolactone with ε‐caprolactone

α-Chloromethyl-α-methyl-β-propionolactone (CMMPL) has been copolymerized with e-caprolactone (CL) using a wide range of feed compositions and aluminium triisopropoxide [Al(OiPr)3] as an initiator. Random copolymers of CMMPL with CL were obtained. The pendant chloromethyl groups of the copolymer were converted to quaternary ammonium salts by reaction with pyridine, resulting in an increased hydrophilicity of the copolymers.

Vinyl monomers bearing chromophore moieties and their polymers. VI. Synthesis and photochemical behavior of acrylic monomer bearing phenoxazine moiety and its polymer

An acrylic monomer having phenoxazine moiety, i.e., N-acryloylphenoxazine ( APO ), has been synthesized by dehydrochlorination of N-(3-chloropropionyl) phenoxazine with 1,5-diazabicyclo [5.4.0]undec-5-ene in dimethyl sulfoxide. The monomer can be polymerized with AIBN as an initiator. The photochemical behavior, including the fluorescence and photosensitizing properties of this monomer and its polymer, has been studied. It has been recorded that the absorption spectrum of polymer P(APO) displays a few blue shifts compared with its monomer APO. It has also been observed that the fluorescence emission intensity of the monomer is dramatically lower than that of its polymer at the same chromophore concentration. This may be ascribed to the charge transfer interacting between the coexisting electron-accepting acrylic carbon-carbon double bond and the electron-donation phenoxazine moiety in APO, intramolecularly or intermolecularly on excitation. The fluorescence of the APO polymer, which does not have carbon-carbon double bond, can be quenched by electron-deficient unsaturated nitriles and esters, clarifying that the electron-deficient carbon-carbon double bond does play an important role for the fluorescence quenching of the monomer. Thus, we term such phenomena as structural self-quenching effect, differing from the concentrational self-quenching effect, which is caused mainly by concentrational factors. The fluorescence quenching of P(APO) by C 60 has also been demonstrated. The formation of the charge transfer complex of P(APO) with C 60 in the ground state is revealed by the upward deviation from the linearity of the Stern-Volmer plot. APO can act as a photoinitiator to sensitize the photopolymerization of vinyl monomers such as acrylonitrile in dimethyl formamide and pursued kinetically. From the ultraviolet analysis of the PAN sensitized by APO, it is proved that APO not only sensitizes the photopolymerization of AN, but also incorporates in the PAN chain.

Vinyl monomers bearing chromophore moieties and their polymers. VII. Synthesis, photochemical, and initiation behavior of acrylic monomers bearing phenothiazine oxide moieties and their polymers

Four acrylic monomers bearing phenothiazine oxide moieties, that is, N-acryloyl-phenothiazine-5-oxide (APTO), N-acryloyl-2-chlorophenothiazine-5-oxide (ACPTO), N-acryloyl-phenothiazine-5,5-dioxide (APTDO), and N-acryloyl-2-chlorophenothiazine-5,5-dioxide (ACPTDO) were synthesized by oxidation of corresponding N-acryloyl-phenothiazine (APT) and N-acryloyl-2-chlorophenothiazine (ACPT) using sodium perborate as an oxidant. These monomers could easily be polymerized by initiation of AIBN. The emission fluorescence spectra of the monomers and their polymers were recorded, and the results indicated that these 4 new monomers possess a fluorescence structural self-quenching effect (SSQE), as we have reported previously. Moreover, with the change of the electronic structure of sulfur atom in the phenothiazine chromophore, that is, from sulfide to sulfoxide and sulfone groups, the tendency of SSQE of these monomers is in the order of APT > APTO > APTDO. This would be ascribed mainly to the decrease of electron-donating abilities of monomers in a sequence of sulfide, sulfoxide, and sulfone groups; that is, at the sulfur atom of these monomers, APT has 2 lone-pair electrons, APTO has 1 lone-pair electrons, and APTDO completely loses its lone-pair electrons. Based on the exciplex formation, the monomers APTO, APTDO, ACPO, and ACPTDO could act as sensitizers for the photopolymerization of acrylonitrile (AN). The combination of APTO or ACPTO with organic peroxides such as BPO could also initiate the polymerization of vinyl monomers, such as AN, by redox nature.

Vinyl monomers bearing chromophore moieties and their polymers. IX. Initiation and photochemical behavior of water and liposoluble acrylic monomers having pyrimidinyl moiety and their polymers

Two acrylic monomers bearing a pyrimidinyl moiety, N-acryloyl-N′-2-pyrimidinylpiperazine (APMP) and N-methacryloyl-N′-2-pyrimidinylpiperazine (MPMP), are prepared by reactions of N-2-pyrimidinylpiperazine with corresponding acryloyl chlorides in the presence of triethylamine. APMP and MPMP can be polymerized either by using radical initiators such as azobisisobutylonitrile or potassium persulfate (KPS) or by UV light irradiation without any sensitizer. APMP, MPMP, and their polymers are water soluble and liposoluble. They can act as sensitizers to initiate the photopolymerizations of acrylonitrile (AN) in DMF and acrylamide (AAm) or N-acryloylmorpholine (AMPL) in an aqueous medium. They can also act as one component of a redox initiation system by combining with KPS to initiate the polymerization of AAm in an aqueous medium, and a superhigh molecular weight up to 106–107 for P(AAm) or 105–106 for P(AMPL) is obtained. The above polymerizations are pursued kinetically. The mechanism of the photopolymerizations initiated by MPMP or P(MPMP) are confirmed by an electron spin resonance study. By the fluorescent analysis of PAN and P(AAm) initiated by MPMP, APMP, or their polymers we confirm that they not only initiate the polymerization but also enter the polymer chains. The fluorescence spectra of MPMP, APMP, and their polymers are recorded. A fluorescence structural self-quenching effect is also observed. The fluorescence of P(MPMP) can be quenched by adding electron-deficient unsaturated compounds such as methacrylonitrile, AN, fumaronitrile, tetracyanoethylene, methyl acrylate, and methyl methacrylate and the correlation between the Stern–Volmer constants and the electron deficiency of the quenchers is described. The fluorescence quenching of P(MPMP) by a water-soluble C60 derivative is also demonstrated.