Photocrosslinkable polymers with degradable properties

ABSTRACT Photocrosslinkable polymers and ultraviolet (UV)-curable resins are significant applications such as coatings, adhesives, photoresists and printing plates. Crosslinked polymers are

insoluble and infusible; therefore, it is very difficult to remove crosslinked materials from substrates. Recently, much attention has been paid to the recovery or recycling of crosslinked

polymers due to environmental regulations. To this end, photocrosslinkable polymers with degradable properties have been studied. Photocrosslinkable polymers and UV-curable resins with

degradable properties are also realized as highly functionalized photopolymers. This article reviews our recent research on photocrosslinkable polymers with degradable properties.

Multifunctional methacrylates and epoxides with degradable properties were prepared, and their UV-curing and degradation profiles were studied. Furthermore, polymers with crosslinkable units

and degradable side chains were prepared and characterized in a similar manner. Some potential applications of these monomers and polymers were also described. SIMILAR CONTENT BEING VIEWED

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MATERIALS BY IMPLANTING VARIOUS TYPES OF DYNAMIC CROSS-LINKS Article 12 March 2021 INTRODUCTION Recently, increasing attention has been paid to the recovery or recycling of polymeric

materials due to environmental regulations. The recycling of thermosets, which are widely used as adhesives, printing plates and matrices for composite materials, is particularly

challenging. The intractability of cured thermosets is based on their highly crosslinked networks. However, if the network bonds are cleaved through chemical reactions or physical

treatments, thermosets become easy to recover or recycle. To this end, thermosets that are thermally or chemically degradable under a given condition have been reported.1, 2, 3, 4, 5, 6, 7,

8, 9 In addition, novel crosslinking–decrosslinking systems based on supramolecular architectures,10 topological gels11, 12 and dynamic covalent bonding have been studied.13, 14

Photo-curable and photocrosslinkable polymers are significant materials due to their wide use in industrial applications such as negative-type photoresists, solder masks, coatings, printing

inks and adhesives.15 Ultraviolet (UV)-curable and photocrosslinkable polymers have been extensively studied. The majority of these polymers were designed to obtain excellent thermal and/or

mechanical properties and enhanced chemical durability. UV-cured and photocrosslinked polymers are insoluble and infusible, which renders them difficult to thoroughly remove from composites

without damaging the underlying materials. However, in some cases, there is a need to remove crosslinked or cured materials from substrates to allow for the repair or reuse of the

substrates. This paper reviews our recent research on UV-curable and photocrosslinkable polymers with degradable properties that serve as novel functional materials. DESIGN CONCEPT

Photo-curable and photocrosslinkable polymers can be obtained via several pathways. Figure 1 shows three types of these systems (1)–(3). In system (1), multifunctional monomers bearing

cleavable linkages are designed. UV curing is performed using a conventional photoinitiator. Methacrylate, acrylate and epoxy units serve as polymerizable units. Acetal, hemiacetal ester,

carboxylate, carbonate and sulfonate units serve as cleavable units. The cleavable units incorporated into the monomers are easily decomposed in the presence of a strong acid at or above

room temperature. A general formulation of a methacrylate-based photo-curing system includes a mixture of a multifunctional monomer, a photoinitiator and a photoacid generator (PAG).

Generally, photo-curing is carried out with 365-nm light, and 254-nm light is utilized for photoinduced acid-catalyzed degradation of the cured polymers. In system (2), novel polymers or

oligomers are designed to have both crosslinkable units, such as an epoxy, and thermally degradable units, such as tertiary esters, in the side chain. This type of polymer or oligomer was

used together with a PAG. Upon irradiation, the photogenerated acid can induce the crosslinking reaction to form networks. The network is degraded upon baking, resulting in the formation of

linear polymeric molecules and small molecules. System (3) is a blend of a base polymer and a thermally degradable crosslinker. The crosslinker contains both reactive sites and cleavable

units. Photocrosslinking occurs via photoinduced reactions between the reactive sites on the crosslinker and on the polymer side chains. Generally, a PAG is used to induce the

photocrosslinking reaction. An example of this system is a blend of a thermally degradable crosslinker with epoxy units and a polymer or oligomer with reactive units, such as phenols,

carboxylic acids or epoxies. Upon heating at relatively low temperatures, the crosslinking reaction occurs, whereas degradation of the crosslinked structure occurs upon heating at relatively

high temperatures. MULTIFUNCTIONAL MONOMERS WITH DEGRADABLE PROPERTIES METHACRYLATES Multifunctional methacrylate monomers containing degradable units were prepared, and their UV-curing and

degradation properties were studied. Acetal, hemiacetal ester and tertiary ester degradable units were incorporated into the monomers. Methacrylate monomers (1–3) with acetal moieties were

prepared from the reaction between the corresponding vinyl ethers and 2-hydroxyethly methacrylate.16 The monomers were purified using column chromatography. The monomer synthesis yields were

62–79 %, depending on the monomer structure. Hemiacetal ester linkage containing methacrylates (4–7) were also synthesized from the reaction between the corresponding vinyl ethers and

methacrylic acid.17 Yields were 51–85 %, depending on the monomer structure. The methacrylate chemical structures are shown in Figure 2. The UV-curing and degradation properties of the

methacrylate monomers were studied in monomer films. Cured sample films (1–3 μm thick) containing 1 wt% 2,2-dimethoxy-2-phenylacetophenone (DMPA) and 1 wt% triphenylsulfonium triflate (TPST)

were prepared on a silicon wafer via irradiation at 365 nm under a nitrogen atmosphere. No curing was observed as a result of irradiation in air because of inhibition by oxygen molecules.

Upon irradiation at 365 nm, DMPA decomposed to initiate the radical polymerization of methacrylate units. However, TPST did not decompose because TPST does not absorb light at 365 nm. The

UV-curing efficiency was measured by determining the insoluble fraction of the irradiated films, and the conversion of methacrylate units upon irradiation was measured using changes in the

peak attributed to C=C bonds at approximately 1640 cm−1. The photoinduced insoluble fraction of methacrylate monomers was ∼100% at the exposure dose of 100–200 mJ cm−2, and methacrylate

conversion was 60–80 %, depending on the monomer structures. For the series of methacrylate monomers containing acetal moieties, the relative UV-curing efficiency decreased in the order

1>2>3. Furthermore, the UV-curing efficiency of the hemiacetal ester-containing methacrylate monomers decreased in the order 7>5≈4>6. The same findings were observed with

photo-DSC measurements. Monomers with a bulky core unit demonstrated reduced UV-curing efficiency. The degradation properties of the UV-cured films were studied by measuring dissolution.

Using the monomers 1–3, UV-cured films (1–3 μm thick) containing 1 wt% DMPA and 1 wt% TPST were prepared via irradiation at 365 nm (200 mJ cm−2) under N2. The UV-cured films were thermally

stable up to 180 °C. The UV-cured films were exposed to 254-nm light (200 mJ cm−2) in air and then baked at various temperatures for 10 min. The dissolved film fractions in methanol when

baked at 100 °C were 90%. The amounts of dissolved fractions were not dependent on monomer structure. The dissolution properties of UV-cured films obtained from 4–7 were similarly studied.

The UV-cured films were stable up to 160 °C in the absence of strong acids. When the UV-cured films were exposed to 254-nm light at room temperature, the dissolved fractions for films of 4,

5, 6 and 7 were 60, 90, 10 and 40%, respectively. However, after baking at 80 °C, 100% dissolution was observed for all the cured films. Monomer 6 demonstrated poor degradability due to its

hydrophobic core unit as the degradation reaction for hemiacetal ester units requires water from the atmosphere. Multifunctional methacrylate monomers (8–11) bearing tertiary ester

degradable units were also prepared.18, 19, 20 Structures of these monomers are shown in Figure 3. UV curing of the monomers was performed via conventional radical polymerization, and

complete insolubilization was observed. The exposure energy required to induce complete insolubilization of the thin film was dependent on the monomer structure. It is well known that

tertiary esters of carboxylic acids thermally decompose to generate carboxylic acid and olefin compounds, and this reaction is widely used for chemically amplified positive-type photoresists

in large scale integration production.21 Although the ester decomposition temperatures (_T_d) were observed to be approximately 250 °C, the _T_d values decreased to 100–150 °C in the

presence of strong acids. Decomposition of tertiary ester units at temperatures >180 °C induces the formation of acid anhydride units, which can cause crosslinking of the obtained linear

polymers. Thus, acid-catalyzed decomposition of tertiary esters is preferable for solubilizing cured resins. A mechanism for UV curing of 7 and the corresponding degradation of the cured

resin is shown in Scheme 1. In this case, the cured sample (∼0.5 μm thick) was prepared by irradiation at 365 nm (200 mJ cm−2). The cured film was irradiated at 254 nm in air and

subsequently baked at 80 °C. Cleavage of the hemiacetal ester units was confirmed using Fourier transform infrared spectroscopy. The peak at 1134 cm−1 due to -O-C-O- bonds disappeared when

the sample was exposed to 254-nm light and subsequently baked. The peak due to the ester carbonyl (1727 cm−1) slightly shifted to 1704 cm−1, and the peak ascribed to hydroxyl groups

(2800–3600 cm−1) appeared, which suggested the formation of carboxylic acid groups. Here, the water needed for hydrolysis of the hemiacetal ester units can be supplied from atmospheric

moisture due to the thinness of the UV-cured film. UV-curable resins based on thiol-ene and thiol-yne systems have been widely studied.22 In general, these systems include a photoradical

generator, multifunctional thiol compounds, and compounds containing C-C double or triple bonds. The thiol-ene and thiol-yne systems have several advantages, including low curing shrinkage

and reduced oxygen inhibition in curing compared with conventional UV-curing systems based on multifunctional acrylates. A film made from a mixture of 8 (R=CH3, _n_=1), pentaerythritol

tetrakis(3-mercaptobutylate), DMAP and di(tert-butylphenyl)iodonium triflate provided a UV-curable material with degradable properties. The cured film obtained by irradiation at 365 nm

became soluble in tetrahydrofuran after irradiation at 254 nm and subsequent baking at 110 °C.23 EPOXIDES Multifunctional epoxides (12–18) bearing tertiary esters as degradable units were

prepared.24, 25, 26, 27 Epoxide structures are shown in Figure 4. The novel epoxy with tertiary ester moieties was prepared from the esterification of α-terpineol with acid chlorides and

subsequent epoxidation of the olefinic double bonds using Oxone, a monopersulfate compound (2KHSO5/KHSO4/K2SO4). Although the epoxides were not cured via UV irradiation in the presence of a

PAG such as TPST, a mixture of these epoxides, poly(vinyl phenol) (PVP) and a PAG could be used as a degradable UV-curable resin. Sample films prepared from a mixture of PVP, epoxide and a

PAG (9-fluorenilideneimino p-toluenesulfonate) became insoluble in organic solvents following UV irradiation in air and subsequent baking at relatively low temperatures (60–120 °C). The

UV-curing efficiency, as determined by photoinduced insolubilization, was high and decreased in the order 17>16>15>14. The exposure dose required for 100% insolubilization of thin

films (∼0.5 μm) was 100–200 mJ cm−2, depending on the epoxide structures. The UV-cured film became soluble in methanol on baking at 140–160 °C. The degradability of the cured films was

determined by dissolution in methanol and was found to decrease in the order 14>15>16>17. Thus, there was a trade-off between curing efficiency and degradability. POLYMERS As shown

in Figure 5, the polymers (19–22) containing both crosslinkable units, such as epoxies or oxetanes, and thermally degradable units, such as tertiary esters of carboxylic acid, sulfonic acid

esters and carbonate esters, in their side chains were prepared.28, 29, 30, 31, 32, 33 Polymers 19–22 were synthesized using conventional radical copolymerization of the corresponding vinyl

monomers. Photocrosslinking of the polymers was studied in combination with a PAG. Sample films (∼1 μm thick) containing a PAG were exposed to UV light, and the resulting insoluble fraction

was measured. Crosslinking occurred by the photochemically generated acid-catalyzed ring-opening reactions of epoxy and oxetane units. Generally, a postexposure bake treatment at 40–80 °C

was required for acceleration of the crosslinking reactions. If the crosslinked polymer films were baked at 100–130 °C, degradation of the tertiary ester units occurred and the films became

soluble. The crosslinked polymer films of 19, 20 and 22 became soluble in water after degradation because the degradation products consisted of a copolymer of methacrylic acid and

styrenesulfonic acid and low-molecular-weight compounds. The polymer obtained by the degradation of the crosslinked film of 21 was poly(vinyl phenol), which was soluble in aqueous alkaline

solutions. Oligomer 23 (_Mn_∼3000) was obtained by the polyaddition of corresponding dicarboxylic acid with divinyl ether.34 The oligomer had two types of degradable units, a hemiacetal

ester unit in the main chain and tertiary ester units in the side chains. Oligomer 23 had methacrylates in the side chains as radically polymerizable units. A thin film of 23 with DMPA and a

PAG (TPST) was photocrosslinked via irradiation at 365 nm under N2 atmosphere. The crosslinked film became soluble in acetone after irradiation at 254-nm light and subsequent baking at

100–120 °C. Decomposed products were poly(methacrylic acid) and low-molecular-weight compounds. APPLICATIONS The mechanical and thermal properties of UV-cured materials are strongly affected

by network structure. Although it is significant to obtain the kinetic chain length of cured materials obtained from photochemical polymerization of multifunctional monomers, analysis of

cured materials is not easy because the cured materials are insoluble and infusible. Some methods, such as solid state 13C NMR,35, 36 viscoelastic property assessment,37, 38 pyrolysis in

combination with GC-MS39, 40 and simulations41 of polymerization kinetics, have been studied to obtain information on the network structure of cured resins. The degradable multifunctional

methacrylates were utilized to obtain the kinetic chain length of their UV-curing reaction.17, 42 The chain length was obtained by measuring the molecular weight of the linear polymer

generated by the decomposition of the UV-cured resins. Thin films of multifunctional methacrylates with a photoradical initiator and a PAG were radically photopolymerized with a conventional

method using 365-nm light. Then, the cured films were exposed to 254-nm light to induce acid-catalyzed decomposition. The decomposition products were low-molecular-weight compounds and

poly(methacrylic acid). The poly(methacrylic acid) was treated with diazomethane to obtain poly(methyl methacrylate), which was subjected to gel permeation chromatography analysis. In this

way, the effects of photoinitiator concentration and strength of the exposed 365-nm light on the kinetic chain length for monomer 7 were studied. In this experiment, thin films consisting of

7, DMPA and di(4-tert-butylphenyl)iodonium triflate were used. UV curing was carried out at 365 nm under a N2 atmosphere, and the conversions of 7 were 75–81 %. The number-average molecular

weight (_Mn_=37 000, _Mw/Mn_=2.5) of the PMMA obtained by exposing the cured resins to higher intensity light (5.5 mW cm−2) was lower than that obtained by curing with lower light intensity

(0.5 mW cm−2) (_Mn_=43 000, _Mw/Mn_=2.9). When the concentration of the photoradical initiator DMPA was increased from 0.5 wt% to 1.5 wt%, the _Mn_ decreased from 46 000 to 26 000. This

result corresponded with simulations of kinetic chain lengths of conventional UV-curing systems.41 When the concentration of DMPA was 2.5 wt%, PMMA with higher and lower molecular weights

co-existed, showing a larger polydispersity index (_Mw/Mn_=6.8). The addition of mono-functional methacrylates to 7 increased the _Mn_ value of PMMA obtained from the UV-cured resin. This

finding suggested that efficient diffusion of monomers in the curing system decreased the probability of termination reactions. Nanoimprint lithography is a cost-effective high-resolution

patterning technology that does not require expensive optical elements, and it has therefore emerged as a promising technology for device manufacturing.43, 44 In particular, UV-nanoimprint

lithography (UV-NIL) does not require heating and cooling steps and is generally operated at lower pressures. The UV-NIL process is as follows: (1) UV-curable resins are placed on substrate,

such as a Si wafer, and a finely patterned quartz mold is placed on the resin. (2) The mold is pressed at ∼1 MPa and irradiated at 365 nm under reduced pressure. (3) The quartz mold is

removed to obtain UV-nanoimprinted patterns of the cured resin on the Si wafer. This process has rapid cycle times and allows for the selection of a number of UV-curable monomers with varied

functional groups. Acrylates and methacrylates are widely used in UV-NIL due to their commercial availability, low viscosity and rapid polymerization rates via radical propagation. Epoxides

and vinyl ethers are also used. One of the problems in UV-NIL is the fouling of the quartz mold by the cured resins. It is difficult to remove the cured resins that remain on the quartz

mold. In the current study, the degradable UV-curable resins that remained in the quartz mold were successfully de-crosslinked and dissolved by washing with organic solvents. Thus, the

degradable resin is an attractive option for use in UV-NIL. Monomers 4–7 were utilized in UV-NIL. Patterns with line widths from 200 nm to 100 μm were prepared. Due to the monomer

viscosities, the nanoimprint pressure used in this study was 0.8–1.0 MPa. This pressure is relatively high for a UV-based nanoimprint. To address this potential issue, the monomers shown

here can be used in combination with low-viscosity mono-functional monomers. UV-curable resins for UV-NIL need to demonstrate low shrinkage during the curing reactions. The monomer

shrinkages were 1–3% with monomer conversions of 60–70% and decreased in the order 5>4>6≈7. The height shrinkage of the UV-nanoimprinted patterns was defined as 100 × (_h_1−_h_2)/_h_1,

where _h_1 is the depth of the mold line patterns and _h_2 is the height of the nanoimprinted line patterns. It is known that the acrylic monomers with high acrylic equivalent values

demonstrate low shrinkage during polymerization. The acrylic equivalent values of the monomers were 185–270 and decreased in the order 7≈6>4>5.45 A replica mold is sometimes used in

the UV-NIL process to prevent the loss of the original mold by serious damage. Replica molds are typically made of organic resins. The degradable UV-curable resins are convenient for precise

fabrication of replica molds according to the process shown in Figure 6. The degradable UV-curable resin with a photoradical initiator and a PAG was applied to the quartz plate and put into

the original quartz mold. The quartz mold was pressed and irradiated at 365 nm to cure the degradable resin. After releasing the quartz mold, the template mold made of degradable resin was

obtained on a quartz plate. Using the template mold, UV-NIL was performed for conventional UV-curable resins. The template mold was dissolved away by irradiation at 254 nm to obtain the

replicated resin mold. In this process, the degradability of the UV-curable monomers is essential because it is otherwise impossible to separate the template mold and the replicated resin

mold. Figure 7 shows micrographs of (a) the quartz mold (line width=20 μm and aspect ratio=1/20), (b) the template mold made by 7 on a quartz plate, (c) the replica mold and (d) the

imprinted patterns obtained using the replica mold. The replica mold was obtained from a mixture of pentaerythritol triacrylate and dipentaerythritol hexaacrylate (molar ratio=2:3). UV-NIL

using the replica mold gave good imprinted images. The shrinkages of the line height of the template and replica molds were ∼1 and ∼3%, respectively, compared with the original quartz

mold.46, 47 CONCLUSIONS Several types of photocrosslinkable polymers and UV-curable multifunctional monomers with degradable properties were designed and prepared. It was shown that these

polymers and monomers were highly photocrosslinkable and UV curable. The thin films of the crosslinked and cured polymers were degraded to allow for dissolution in solvents. Thus, these

polymers and monomers are environment-friendly materials and novel photopolymers that can be utilized in applications, such as negative-type photoresists, adhesives, inks and coatings, and

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Applied Chemistry, Osaka Prefecture University, Osaka, Japan Masamitsu Shirai Authors * Masamitsu Shirai View author publications You can also search for this author inPubMed Google Scholar

CORRESPONDING AUTHOR Correspondence to Masamitsu Shirai. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Shirai, M. Photocrosslinkable polymers with

degradable properties. _Polym J_ 46, 859–865 (2014). https://doi.org/10.1038/pj.2014.79 Download citation * Received: 10 May 2014 * Revised: 06 July 2014 * Accepted: 08 July 2014 *

Published: 03 September 2014 * Issue Date: December 2014 * DOI: https://doi.org/10.1038/pj.2014.79 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this

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