View PDF Version

 

Precision synthesis of diblock copolymers via free radical photopolymerization†

Jingfang Li*^a, Qilu Deng^a, Zhihao Xing^b, Jiaxin Yu^b, Wenjie Li^b, Xianju Zhou^a, Xiaoqun Zhu^bc and Jun Nie^bc
aSchool of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, China. E-mail: lijf@cqupt.edu.cn
^bState Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing, 100029, China
^cJiangsu Jicui Photosensitive Electronic Materials Research Institute limited company, Jiangsu, 214201, China

First published on 23rd August 2024

The primary methods for synthesizing block copolymers include atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer polymerization (RAFT), both of which are types of controlled radical polymerization (CRP).^10–12 To data, numerous advanced techniques, such as photo-ATRP,¹³ photoinduced electron transfer RAFT (PET-RAFT),^14–16 electrochemical stimulus ATRP (e-ATRP),¹⁷ and acid-triggered RAFT,¹⁸ have been developed, significantly advancing the field of block copolymer preparation. However, these methods generally require expensive catalysts (trithiocarbonate derivatives,^19,20 metal complexes) and the use of solvents, and exhibit relatively low efficiency.

Photo-induced free radical polymerization, also known as free radical photopolymerization (FRP), offers advantageous reaction conditions including low energy requirements, a solvent-free environment, high polymerization rates, and spatiotemporal control.¹⁴ It is considered the most mature method extensively used in industrial applications such as coatings, paints, and adhesives.^21–23 Briefly, FRP provides a direct and rapid pathway for synthesizing polymers. However, most FRP studies have concentrated on the production of random copolymers.²⁴ Block copolymers synthesized via FRP generally require a bifunctional or multifunctional photoinitiator containing chromophores that can be selectively activated by specific irradiation wavelengths.²⁵

Since 1999, Yagci has dedicated efforts to designing bifunctional photoinitiators for the synthesis of diblock copolymers. Bisacylphosphane oxide (BAPO) was initially proposed,²⁶ with its initiating mechanism as follows: BAPO can undergo two sequential P–C α-bond cleavages upon exposure to 420 nm and 385 nm light, respectively, facilitating the growth of two different polymer chains from the monoacylphosphine oxide terminal group. Nonetheless, BAPO exhibited low efficiency in producing diblock copolymers under dual-wavelength excitation due to the poor photo-activity of α-benzoyl phosphonyl radicals. Moreover, the highly photo-active other benzoyl radicals, produced simultaneously with α-benzoyl phosphonyl radicals by BAPO, participated in the polymerization, significantly disrupting the purity of the diblock copolymer. To address these challenges, Yagci proposed a modified bifunctional photoinitiator, dibenzoyldiethylgermane (DBDEG), which includes germanium in 2009.²⁷ Although germyl radicals demonstrated higher initiating efficiency compared to that of α-benzoyl phosphonyl radicals, unwanted benzoyl radicals were still produced (Scheme 1(a)).

	Scheme 1 Synthetic route of diblock copolymers via bifunctional (a) photoinitiators BAPO (previous work reported by Yagci) and (b) PAA-2959/CQ.

In this study, a novel bifunctional photoinitiator, 2-(4-(2-hydroxy-2-methylpropanoyl)phenoxy)ethyl(E)-3-(1-methyl-1H-pyrrol-2-yl)acrylate (PAA-2959), featuring both pyrrole ethyl acrylate and α-hydroxy ketone chromophoric moieties, was developed to synthesize diblock copolymers, as depicted in Scheme 1(b). Hydrogen donors on the N-methylpyrrole unit of PAA-2959 were abstracted in the presence of the highly biocompatible photoinitiator camphorquinone (CQ) under LED@465 nm excitation. This process initiated the generation of aminoalkyl radicals, leading to the polymerization of the first series of monomers and ultimately producing macromolecular photoinitiators with α-hydroxy ketone groups. These groups generate benzoyl radicals through C–C α-bond cleavages under LED@275 nm irradiation, facilitating the polymerization of the second series of monomers. Additionally, other reactive species, including ketyl radicals produced by both CQ and PAA-2959, exhibit relatively low initiating activity but eventually convert into acetone compounds, ensuring the precise synthesis of diblock copolymers.

PAA-2959 was synthesized through a three-step reaction. The synthetic route and chemical structure of PAA-2959 are detailed in the ESI.† To validate the concept described above, the photochemical properties of PAA-2959 were initially investigated. The maximum absorption peak of PAA-2959, located around 338 nm, was clearly observed from the UV-vis absorption spectra (Fig. 1(a)), attributable to the π–π* transition from the highest occupied molecular orbital (HOMO) to the lowest occupied molecular orbital (LUMO) in the pyrrole ethyl acrylate group. Moreover, another absorption peak at 275 nm is likely associated with the α-hydroxy ketone group of PAA-2959, similar to the UV absorption spectrum of Irgacure 2959, which has a similar α-hydroxy ketone structure. A steady-state photolysis experiment was conducted to assess the photochemical reactions of PAA-2959 under continuous LED@325 nm irradiation in ACN at room temperature. The absorbance at the 338 nm peak decreased rapidly with increasing irradiation time. Moreover, the absorbance at the 275 nm peak initially decreased, then increased, exhibiting a blue shift to 251 nm, closely aligning with the photolysis behavior of Irgacure 2959 under LED@275 nm irradiation (Fig. S9, ESI†). To explore the photochemical reactions indicated by the degradation of the 338 nm peak, real-time ¹H-NMR spectra of PAA-2959 before and after irradiation were obtained. As shown in Fig. 1(d), double peak hydrogen signals at 7.58 ppm and 7.55 ppm (H1), and 6.27 ppm and 6.23 ppm (H2) with coupling constants of 16 Hz, were detected, associated with the –CHCH– bond adjacent to the N-methylpyrrole unit of trans-PAA-2959. Post-irradiation, H1 and H2 signals persisted, but new hydrogen signals at 6.97 ppm and 6.94 ppm (H1′), and 5.62 ppm and 5.59 ppm (H2′) with coupling constants of 12 Hz appeared.

	Fig. 1 (a) UV-vis absorption spectrum and HOMO–LUMO orbitals of PAA-2959 (2.5 × 10−3 mol L−1) in acetonitrile(ACN); (b) further chemical structure information related to trans/cis-PAA-2959; (c) steady state photolysis of PAA-2959 (2.5 × 10−3 mol L−1) under LED@325 nm irradiation; (d) 1H-NMR spectra of trans-PAA-2959 and that irradiated by an LED@325 nm.

NMR results confirmed a significant trans → cis isomerization of PAA-2959 under light irradiation. NMR integration indicated that approximately 30% of PAA-2959 (20 mg/0.6 ml) converted from trans- to cis-isomer in d₆-DMSO within 5 minutes of LED@325 nm irradiation (70 mW cm⁻²). Additionally, the optimized chemical structures of trans/cis-PAA-2959, displayed in Fig. 1(c), revealed that trans-PAA-2959 maintained an almost planar conformation, while cis-PAA-2959 adopted a twisted conformation due to an intramolecular hydrogen bond formed, as evidenced by the hydrogen signal at 3.70 ppm (H3) from the N-methylpyrrolidone unit splitting. This trans → cis photo-isomerization altered the chemical structure of PAA-2959, resulting in a twisted conformation which might lead to other photobleaching reactions like dimerization and a decrease in absorbance of the 338 nm peak during the photolysis experiment.²⁸

To further identify the photochemistry properties of PAA-2959 irradiated by dual-wavelength illumination, steady state photolysis of PAA-2959 in the presence of CQ was firstly investigated. As we can see, the intensities of the absorption band ranging from 300 nm to 500 nm gradually decreased as the solvent of CQ/PAA-2959 was photobleached under the irradiation of LED@465 nm (Fig. 2(a)). Importantly, α-aminoalkyl radical signals with a set of hyperfine coupling cleavage constants (a_N = 15.60 G, a_H = 3.6 G) were trapped by N-tert-butyl-α-phenylnitrone (PBN) in the solution of CQ/PAA-2959 after being photolyzed via electron paramagnetic resonance (EPR) test (Fig. 2(c)). This demonstrated that PAA-2959 really played the role of a co-initiator and hydrogen on the N-methylpyrrolidone unit might be abstracted by CQ for free radical photopolymerization. The photopolymerization kinetics of CQ/PAA-2959 (1 wt%/2 wt%) as an initiating system and HDDA as a monomer under the illumination of LED@465 nm was carried out by FT-IR. Fig. 2(b) reveals that the final double conversion of HDDA induced by CQ/PAA-2959 was nearly 92% and the kinetic curve reaches a plateau within 50 s, indicating a high initiating level of CQ/PAA-2959. Furthermore, the absorption curves around 275 nm of CQ/PAA-2959 irradiated by LED@465 nm could still decline rapidly upon LED@275 nm irradiation (Fig. 2(d)). EPR test results also revealed that both benzoyl radical (a_N = 15.8 G, a_H = 14.1 G) and ketyl radical (a_N = 24.1 G, a_H = 14.9 G) signals were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the solvent of PAA-2959 irradiated by LED@275 nm, indicating that PAA-2959 undergoes a C–C α-bond cleavage for free radical photopolymerization as Irgacure 2959 does. In addition, PAA-2959 (1 wt%) exhibits a relatively high initiating efficiency for the polymerization of monomer HDDA, of which the final double conversion reached 80% within 300 s of irradiation time.

	Fig. 2 (a) Steady state photolysis of PAA-2959 (6.8 × 10−3 mol L−1) in ACN under LED@465 nm irradiation within 10 s; (b) photopolymerization kinetics of the initiating system CQ/PAA-2959 (1 wt%/2 wt%)/HDDA under LED@465 nm irradiation; (c) ESR spectra obtained after LED@465 nm irradiation of CQ/PAA-2959 dissolved in ACN/PBN; (d) steady state photolysis of PAA-2959 irradiated by LED@465 nm irradiation under continued LED@275 nm irradiation within 40 s; (e) photopolymerization kinetic of PAA-2959 (1 wt%) under LED@275 nm irradiation; (f) ESR spectra after LED@275 nm irradiation of PAA-2959 dissolved in ACN/DMPO.

As the photo-initiating mechanism of PAA-2959 was confirmed, diblock copolymers were synthesized, as illustrated in Fig. 3(a). Samples of CQ (10 mg) and PAA-2959 (20 mg) were prepared in 0.8 ml of phenyl methacrylate (PMA) and irradiated under LED@465 nm for 5 minutes during the first polymerization step. The obtained compound PPMA-2959 was isolated by precipitation in cyclohexane, and for purification and removal of residual photoinitiator CQ/PAA-2959, it was reprecipitated from THF several times. In the second step, 50 mg of PPMA-2959 was dissolved in 0.6 ml of butyl methacrylate (BMA) and irradiated under LED@275 nm for 5 minutes. Diblock copolymers PPMA-b-PBMA were then precipitated in methanol. Notably, gel permeation chromatography (GPC) tests (Fig. 3(b)) showed that the diblock copolymer PPMA-b-PBMA had a number average molar mass (M_n) of 46280 and a weight average molar mass (M_w) of 173043, obtained using the initiating system CQ/PAA-2959. Compared with PPMA-2959 (M_n = 40711, M_w = 138189) prepared during the first photopolymerization step, PPMA-b-PBMA exhibited an increase in M_n and M_w, indicating successful integration of the PBMA polymer chains into PPMA. Furthermore, ¹H-NMR results revealed that hydrogen signals H1 (6.28 ppm), H2 (5.84 ppm) and H1′ (6.01 ppm), H2′ (5.62 ppm) on the CH₂C– double bond of both monomers PMA and BMA shifted to the alkyl region between 1.5 and 1.0 ppm in the ¹H-NMR spectrum of PPMA-b-PBMA, demonstrating the high purity of these diblock copolymers resulting from complete reaction of monomers PMA and BMA.

	Fig. 3 (a) Diblock copolymer PPMA-b-PBMA prepared by using initiating system CQ/PAA-2959 through dual wavelength excitation; (b) Mn, MW and GPC curves of PPMA and PPMA-b-PBMA prepared by PAA-2959; (c) 1H-NMR spectra of monomer PMA, BMA and diblock copolymer PPMA-b-PBMA in the solvent of DMSO-d6.

In conclusion, a novel bifunctional photoinitiator PAA-2959, featuring two photoactive chromophores, can be excited by dual-wavelength polymerization using selected LED@465 nm and LED@275 nm irradiation. Following this initiating mechanism, PAA-2959 enables the efficient and convenient production of diblock copolymers with CQ via free radical photopolymerization. Importantly, compared to the strategy of using the photoinitiator BAPO previously reported, the initiating system CQ/PAA-2959 does not generate excess radicals with high photo-activity, resulting in high purity and precise molecular weight control of the diblock copolymers. Undoubtedly, PAA-2959 exhibits potential value for enhancing the sequence and performance of photocurable materials.

Jingfang Li: conceptualization, methodology, software, data curation, writing – original draft, founding acquisition; Qilu Deng: methodology, investigation; Zhihao Xing: methodology, investigation; Jiaxin Yu: methodology, investigation; Wenjie Li: methodology, investigation; Xianju Zhou: data curation; Xiaoqun Zhu: supervision; Jun Nie: supervision.

This work was supported by the Science and Technology Research Program of the Chongqing Municipal Education Commission (Grant No. KJQN202200631). The authors also appreciate the support by Jiangsu Jicui Photosensitive Electronic Materials Research Institute limited company and the help provided by Nuoyan Li in scientific drawing.

Data availability

Conflicts of interest

1. M.-N. Antonopoulou, R. Whitfield, N. P. Truong, D. Wyers, S. Harrisson, T. Junkers and A. Anastasaki, Nat. Chem., 2022, 14, 304–312 CrossRef CAS PubMed .
2. R. Ye, X. Sun, X. Mao, F. S. Alfonso, S. Baral, C. Liu, G. W. Coates and P. Chen, Nat. Chem., 2024, 16, 210–217 CrossRef CAS PubMed .
3. M. H. Stenzel, Chem. Commun., 2008, 3486 RSC .
4. C. Diaz and P. Mehrkhodavandi, Polym. Chem., 2021, 12, 783–806 RSC .
5. C. Miglani, M. Banoo, D. Nath, J. Ralhan, S. Sil, J. P. Joseph, S. K. Pal, U. Gautam and A. Pal, Chem. Commun., 2023, 59, 13195–13198 RSC .
6. E. R. L. Brisson, M. J. H. Worthington, S. Kerai and M. Müllner, Chem. Soc. Rev., 2024, 53, 1984–2021 RSC .
7. C. Cai, L. Wang and J. Lin, Chem. Commun., 2011, 47, 11189 RSC .
8. H. Xu, J. Bijleveld, M. Hedge and T. Dingemans, Polym. Chem., 2019, 10, 5052–5069 RSC .
9. L. Yu, Y. Zhang, X. Dai, Q. Xu, L. Zhang and J. Tan, Chem. Commun., 2019, 55, 11920–11923 RSC .
10. N. P. Truong, G. R. Jones, K. G. E. Bradford, D. Konkolewicz and A. Anastasaki, Nat. Rev. Chem., 2021, 5, 859–869 CrossRef CAS PubMed .
11. K. Parkatzidis, H. S. Wang, N. P. Truong and A. Anastasaki, Chem, 2020, 6, 1575–1588 CAS .
12. K. G. E. Bradford, R. D. Gilbert, M. A. S. N. Weerasinghe, S. Harrisson and D. Konkolewicz, Macromolecules, 2023, 56, 8784–8795 CrossRef CAS .
13. M. Rolland, R. Whitfield, D. Messmer, K. Parkatzidis, N. P. Truong and A. Anastasaki, ACS Macro Lett., 2019, 8, 1546–1551 CrossRef CAS PubMed .
14. N. Corrigan, J. Yeow, P. Judzewitsch, J. Xu and C. Boyer, Angew. Chem., Int. Ed., 2019, 58, 5170–5189 Search PubMed .
15. J. Phommalysack-Lovan, Y. Chu, C. Boyer and J. Xu, Chem. Commun., 2018, 54, 6591–6606 RSC .
16. J. C. Theriot, G. M. Miyake and C. A. Boyer, ACS Macro Lett., 2018, 7, 662–666 CrossRef CAS PubMed .
17. P. Chmielarz, S. Park, A. Simakova and K. Matyjaszewski, Polymer, 2015, 60, 302–307 CrossRef CAS .
18. M.-N. Antonopoulou, G. R. Jones, A. A. Kroeger, Z. Pei, M. L. Coote, N. P. Truong and A. Anastasaki, Nat. Synth., 2024, 3, 347–356 CrossRef .
19. Q. Zhang, R. Zeng, Y. Zhang, Y. Chen, L. Zhang and J. Tan, Macromolecules, 2020, 53, 8982–8991 CrossRef CAS .
20. J. Tan, D. Liu, Y. Bai, C. Huang, X. Li, J. He, Q. Xu, X. Zhang and L. Zhang, Polym. Chem., 2017, 8, 1315–1327 RSC .
21. K. C. H. Chin, G. Ovsepyan and A. J. Boydston, Nat. Commun., 2024, 15, 3867 CrossRef CAS PubMed .
22. T. Gao, Y. Zhang, F. Morlet-Savary, B. Graff, J. Zhang, P. Xiao, F. Dumur and J. Lalevée, Small, 2024, 2400234 CrossRef CAS PubMed .
23. W. Liao and M. Jin, Prog. Org. Coat., 2024, 189, 108290 CrossRef CAS .
24. J. Li, Y. Hao, M. Zhong, L. Tang, J. Nie and X. Zhu, Dyes Pigm., 2019, 165, 467–473 CrossRef CAS .
25. A. Eibel, D. E. Fast, J. Sattelkow, M. Zalibera, J. Wang, A. Huber, G. Müller, D. Neshchadin, K. Dietliker, H. Plank, H. Grützmacher and G. Gescheidt, Angew. Chem., Int. Ed., 2017, 56, 14306–14309 Search PubMed .
26. E. D. Günersel, Y. Hepuzer and Y. Yağcı, Angew. Makromol. Chem., 1999, 264, 88–91 CrossRef .
27. Y. Y. Durmaz, M. Kukut, N. Moszner and Y. Yagci, J. Polym. Sci. A Polym. Chem., 2009, 47, 4793–4799 CrossRef CAS .
28. L. Deng, L. Tang and J. Qu, Prog. Org. Coat., 2022, 167, 106859 CrossRef CAS .