Mehmet Arslan*a,
Ozgur Ceylanb,
Rabia Arslana and
Mehmet Atilla Tasdelena
aDepartment of Polymer Materials Engineering, Faculty of Engineering, Yalova University, 77100 Yalova, Turkey. E-mail: mehmet.arslan@yalova.edu.tr
bCentral Research Laboratory, Yalova University, 77100 Yalova, Turkey
First published on 24th February 2021
The chemical functionalization or modification of polymers to alter or improve the physical and mechanical properties constitutes an important field in macromolecular research. Fabrication of polymeric materials via structural tailoring of commercial or commodity polymers that are produced in vast quantities especially possess unique advantages in material applications. In the present study, we report on benign chemical modification of unsaturated styrene–isoprene–styrene (SIS) copolymer using available backbone alkene groups. Covalent attachment of aldehyde functional substrates onto reactive isoprene double bond residues was conveniently carried out using UV-induced Paterno–Büchi [2 + 2] cycloaddition. Model organic compounds with different structures were utilized in high efficiency chemical modification of parent polymer chains via oxetane ring formation. Functionalization studies were confirmed via 1H NMR, FT-IR and SEC analyses. The methodology was extended to covalent crosslinking of polymer chains to obtain organogels with tailorable crosslinking degrees and physical characteristics. Considering the outstanding elastic properties of unsaturated rubbers and their high commercial availability, abundant reactive double bonds in backbone chains of these polymers offer easy to implement structural modification via proposed Paterno–Büchi photocycloaddition.
Photo-induced methodologies are valuable chemical tools in design and fabrication of precision polymers as well as large volume industrial applications.14 UV- or sunlight activated reactions usually require mild reaction conditions compared to alternative reactions that may necessitate thermal activations or toxic/expensive catalyst usage. In addition, photochemical methods enable spatial and temporal modulations over reactions offering accurate external control in polymer synthesis, modification or crosslinking.15–17 UV-sensitive polymers are used as photoresists in manufacturing of printed circuits.18 Such polymers can be used in UV-patterning and photolithography by employing a photomask to define the UV-exposure area which in return lead to fabrication of biosensors, cell engineering scaffolds and analyte detection tools.19 In conjunction with atom economic addition based chemical transformations; photochemical methods have found diverse applications in modification of industrial unsaturated elastomers.20,21
Photoaddition reactions proceeding on [2 + 2] cycloaddition route between an excited carbonyl and an alkene are referred to as Paterno–Büchi reactions and are important carbon–carbon bond forming synthetic methods in organic chemistry.22,23 Carbonyl compounds employed in these reactions are mainly aldehydes and ketones reacting preferably with electron rich alkenes to give corresponding oxetanes. Despite numerous examples of [2 + 2] cycloaddition based reactions of electron deficient alkenes in synthesis and functionalization of polymeric materials,24–28 excited carbonyl-based photochemical transformations have rarely been applied in polymer modification. In a seminal study, Junkers and coworkers reported facile tailoring of aldehyde functional ATRP polymer end groups with unactivated alkenes via [2 + 2] Paterno–Büchi cycloaddition route.29 Although complete functionalization of aldehyde bearing substrates required large molar excesses of the alkene compounds, the process has shown to be still efficient to achieve close to quantitative functionalization reactions. Same group extended the approach to modification of aldehyde bearing cellulose surfaces with alkene end group functional polymers.30 We envisioned that facile [2 + 2] photocycloaddition reaction can be utilized in grafting of carbonyl containing compounds to unsaturated rubber elastomers which carry abundant alkene groups in polymer backbones. With the possible coalescence of highly practical photocycloaddition with flow chemistry,31 Paterno–Büchi reaction could be a valuable tool for industrial high scale modification of commercial unsaturated polymers.
In this contribution we report on facile UV-induced covalent modification of a commercial unsaturated elastomer, namely styrene–isoprene–styrene copolymer. Efficient and easy to implement methodology is based on [2 + 2] Paterno–Büchi cycloaddition of various aldehyde containing compounds grafted on the available alkene groups of the middle block polyisoprene chain via oxetane formation (Scheme 1). The functionalization of copolymer with various carbonyl substrates was evidenced by 1H NMR, FT-IR and SEC analyses. The approach was extended to crosslinking of polymer chains under UV-illuminated conditions by employing a bisaldehyde functional crosslinker. Analysis of obtained organogels using rheological measurements and crosslinking density calculations via solvent absorption revealed that the properties of the materials can be fine-tuned by changing of the crosslinker stoichiometric ratios. It can be anticipated that reported benign and efficient photocycloaddition modification and crosslinking methodology can be a useful chemical tool in structural tailoring of alkene functional polymers, especially industrial unsaturated elastomers.
Entry | Gel | Crosslinker (%) | Yield (%) | dp (g cm−3) | Vp | Mc (g mol−1) | χ | ν (×10−5) (mol cm−3) |
---|---|---|---|---|---|---|---|---|
1 | Gel (2.5) | 2.5 | 92 | 0.9130 | 0.0486 | 13522 | −0.5168 | 6.75 |
2 | Gel (5.0) | 5.0 | 95 | 0.9134 | 0.0594 | 9514 | −0.5207 | 9.60 |
3 | Gel (7.5) | 7.5 | 97 | 0.9205 | 0.0683 | 7505 | −0.5240 | 12.27 |
4 | Gel (15.0) | 15 | 94 | 0.9560 | 0.0781 | 6126 | −0.5277 | 15.61 |
5 | Gel (20.0) | 20 | 99 | 0.9918 | 0.0860 | 5348 | −0.5307 | 18.55 |
6 | Control | 0 | No gelation | — | — | — | — | — |
(1) |
(2) |
(3) |
The photo-induced modification of alkene groups of SIS copolymer was studied under UV illumination with primarily benzaldehyde derivatives (Scheme 1). To better demonstrate the broad structural modification amenity of alkene groups, benzaldehyde and related compounds containing triethylene glycol and trifluorocarbon groups as well as aliphatic butyraldehyde were employed in functionalization reactions. Due to the relatively high stoichiometric ratio between alkene and aldehyde groups that an efficient Paterno–Büchi post-polymerization modification demands,29 the stoichiometry was adjusted so that 0.2 equiv. of aldehyde groups were coupled with residual alkenes on polyisoprene block of copolymer. A small amount of benzophenone as photosensitizer was also employed in photocycloaddition process. After the reactions, the modified polymers were obtained in their pure form with near quantitative yields by simple methanol precipitation. Initially, the functionalization efficiency of copolymer with triethylene glycol aldehyde (Al-TEG) under addressed conditions was studied by FT-IR, 1H NMR and SEC analyses. After the modification, the resulting copolymer SIS-g-TEG (20) (20% mol eq. aldehyde feed to alkenes) displayed characteristics proton signal at 3.37 ppm belonging to methoxy methyl protons and signals between 4.11–3.54 ppm belonging to ethylene oxide protons (Fig. 1A). Oxetane protons as stereo/regio-isomers give resonances in a relatively wide region between 5.37 ppm to 2.73 ppm.29 The functionalization efficiency was calculated as 94.5% (18.9% grafting to alkenes) based on the integrations of SIS c, c′ and a, b protons to f methoxy methyl protons after grafting (Fig. 1A and B). Considering the irregular arrangement of the structural units of polyisoprene block originated from the cis-1,4-, trans-1,4-, 3,4- and 1,2-polymerization modes of isoprene monomer, one can speculate on the regio- and stereospecific addition route of carbonyl groups. From the mechanistic understanding of Paterno–Büchi reaction, successful cycloaddition is governed by several factors including the electron richness of the alkene, relative stability of the adducts, kinetic as well as steric factors.39 Although it can be anticipated that not all alkene modes in polyisoprene block of SIS copolymer display similar reactivity towards carbonyls, results suggested that overall functionalization is still efficient to graft aldehyde bearing compounds.
Fig. 1 1H NMR spectrum of (A) pristine SIS copolymer, (B) SIS-g-TEG (20) and (C) SIS-g-TEG (30) functionalized copolymers. |
In order to investigate the effect of feed aldehyde concentration to functionalization degree, another copolymer SIS-g-TEG (30) with 30% mol eq. aldehyde to alkenes was also synthesized. 1H NMR analysis revealed 86.1% (25.8% grafting to alkenes) functionalization degree demonstrating the efficiency of the grafting reaction (Fig. 1C). Successful photocycloaddition grafting of aldehyde bearing compound Al-TEG onto residual isoprene double bonds was also studies by FT-IR analysis. As it can be seen in Fig. 2, the modified copolymer SIS-g-TEG (20) displayed characteristic absorption bands at 1247 cm−1 due to the C–O–C stretchings and enhanced bands in the region 1650–1450 cm−1 due to the grafting of aromatic benzaldehyde groups. It is important to note that UV-induced reactions were implemented in oxygen free atmosphere and no unwanted reactions or crosslinking was accounted. SEC analysis of modified polymers revealed unimodal size distributions though slightly higher molecular weight distributions were observed compared to parent SIS copolymer (Fig. 3).
After successful and efficient UV-induced functionalization of SIS copolymer with Al-TEG, we prompted our effort to graft various other functional aldehydes to demonstrate broad structural modification amenity of alkene groups. Compounds Al–CF3, Al–Bu and Al–Ph with 20% feed ratios (20% mol eq. aldehyde feed to alkenes) were employed in [2 + 2] photocycloaddition reactions in above-mentioned reaction conditions (Scheme 1). For all the functional carbonyl compounds employed, grafting was achieved with considerably high functionalization degrees as 16.3%, 14.6% and 15.7% (for aimed 20% grafting to alkenes) for Al–CF3, Al–Bu and Al–Ph, respectively. In 1H NMR spectrum of functionalized copolymer SIS-g-CF3, covalent modification was accounted by the presence of characteristics aliphatic signal at 4.26 ppm and aromatic signals at 7.20 and 6.80 ppm (Fig. 4A). Similarly, appearance of aliphatic signals between 2.46 to 0.98 ppm of SIS-g-Bu after butyraldehyde cycloaddition (Fig. 4B) and phenyl protons at 7.33 ppm of SIS-g-Ph after benzaldehyde cycloaddition (Fig. 4C) demonstrated the successful photo-induced grafting reactions. These functionalization reactions were also followed by FT-IR analysis in which all modified polymers exhibited characteristic absorption bands of both parent SIS copolymer and grafted compounds (Fig. 2).
After demonstrating facile and effective covalent modification of unsaturated elastomer SIS with aldehyde compounds via [2 + 2] cycloaddition, we extended the approach to UV-induced crosslinking of polymer chains with a bisaldehyde containing crosslinker, namely terephthalaldehyde. Unsaturated polymers are indispensable industrial polymers that have important applications in their crosslinked insoluble form. Common drawbacks are the weak mechanical properties and low glass transition temperatures of neat unsaturated elastomers that might impede their practical use. On the other hand, abundant backbone double bonds of such polymers with both natural and synthetic origin provide utility in curing chemistries. However, the complicated curing processes that currently employed and the requirement of toxic additives might obligate more green technologies that have to be pursued. Therefore, UV-promoted, efficient and atom economic Paterno–Büchi cycloaddition could be a suitable alterative chemistry in fabrication of crosslinked rubber elastomers.
In this context, crosslinking of SIS copolymer in solution state with a bisaldehyde crosslinker was studied by employing various crosslinker ratios (Scheme 2 and Table 1). In these relatively low crosslinker ratios obtained materials are organogels with high solvent absorption profiles. After UV-induced cycloaddition crosslinking, unreacted species were removed by washing the gels with toluene. FT-IR was used to monitor the crosslinking reaction of dried gel samples by observing the variation of aromatic peaks in the region 1650–1450 cm−1. As it is seen in Fig. 5, increased crosslinker ratio resulted in increased aromatic bands in this region due to the involvement of terephthalaldehyde groups in the networks.
Scheme 2 UV-induced crosslinking of SIS copolymer with a bisaldehyde crosslinker. Representative picture of transparent Gel (7.5) in toluene swollen state. |
In order to gather further information on the effect of crosslinker feed to the crosslinking efficiencies, the molecular weight between crosslinks (Mc) and the crosslink densities (ν) of gels were calculated from absorption experiments done in toluene. An increased crosslinking density was accounted in case of employing a higher crosslinker feed demonstrating the successful inter-chain photocycloaddition reactions (Table 1). It is important to note that the effect of the crosslinker ratio increase on crosslinking density change is less drastic when a higher crosslinker feed was employed. This can be due to the lowered stoichiometric ratio between alkenes and aldehydes when terephthalaldehyde amount increased, causing less effective cycloaddition crosslinking.29
The gelation process was in situ monitored by rheology measurements of model Gel (5.0) system in time sweep test. As it is seen in Fig. 6A, the intersection point of storage (G′) and loss (G′′) moduli to reflect sol to gel transition was achieved within minutes. In dynamic rheological analysis, gels displayed increased storage modulus values by increasing crosslinker ratio (Fig. 6B). Samples exhibited permanent elastic character in 0.01–100 Hz frequency range. Relatively low oscillation frequency dependencies of G′ suggest uniform crosslinking throughout the gel networks. The damping factors (tanδ, G′′/G′) of organogels were less than one in all crosslinking degrees representing the higher elastic character of gel networks over viscous behavior.40 Morphological characterization of organogels on freeze-dried samples revealed non-porous rubbery structures (Fig. 7).
In the light of obtained results, mild and efficient UV-induced Paterno–Büchi crosslinking reaction was demonstrated as a versatile chemical tool to fabricate crosslinked materials with tailorable properties. This type of organogels could be attractive materials in various applications of industrially important unsaturated elastomers.
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