DEVELOPMENT OF BIPHENYL MONOMERS AND ASSOCIATED CROSSLINKED POLYMERS WITH INTRAMOLECULAR PI-PI INTERACTIONS

Abstract

Monomers containing biphenyl moieties were employed to create two sets of covalently crosslinked polymers that displayed noncovalent interactions in their 3-dimensional network. The biphenyls (precursors) used were 2-phenylphenol, 4-phenylphenol and 4,4’-dihydroxybiphenyl, and their acrylated forms were synthesized and named as 2-phenylphenolmonoacrylate (2PPMA), 4-phenylphenolmonoacrylate (4PPMA), and 4,4’-dihydroxybiphenyldiacrylate (44BDA), respectively. These were characterized by differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) to confirm the successful acrylation reaction. Polymers were synthesized via free radical polymerization reactions with varying crosslinker contents, and their network properties were characterized using swelling studies and compressive modulus tests. Interestingly, swelling studies did not show the expected decreasing swelling ratio with increasing crosslinker content, while compression testing indicated the expected trend of increasing modulus with increasing crosslinking density. The unexpected swelling results are hypothesized to result from the intramolecular interactions between the biphenyl side groups that result in noncovalent crosslinks.

2. Introduction

Crosslinked polymers are a class of polymers with a three-dimensional mesh network that do not dissolve but rather swell when exposed to good solvents.^1–3 These gels can be classified as physically, or chemically crosslinked networks based on the type of connections between their linear chains. Chemically crosslinked polymers have permanent covalent bonds that hold the structure together, whereas physically crosslinked polymers exhibit noncovalent interactions between their linear chains.^4–13 These interactions can include entangled chains, hydrogen bonding, ionic interactions, hydrophobic interactions, and supramolecular chemistry.^7,11–13

Chemically crosslinked polymers are used when an irreversible and stable system is desired, but they lack the ability to adapt their structure upon certain stimuli. Alternatively, physically crosslinked systems can adapt and respond to their environment but are not stable over long periods of time and lack the mechanical integrity of chemically crosslinked polymers. Combining noncovalent interactions with covalent crosslinking can produce a flexible yet stable polymer. Such polymers are called as supramolecular crosslinked polymers and are currently being developed and applied in various fields like shape memory polymers.^14–21

Functionalization of the polymer with side groups that can interact with each other is a crucial part in determining the properties of a physically crosslinked system.^7,22 One such group that can be used is a biphenyl ring. Biphenyl groups are known to be mesogenic moieties that can be used to form liquid crystalline polymers with nematic, smectic and cholesteric phases^23–26, and materials containing biphenyl rings have a wide variety of unique properties such as being antimicrobial and antifungal.^27,28 Additionally, biphenyl moieties can interact with each other through pi-pi stacking interactions. Utilization of these pi-pi stacking interactions has led to wide variety of research in the fields of self-assembled monolayers, shape memory polymers, and molecular recognition.^29–34 For molecular recognition, prior reports suggest that molecules containing biphenyl rings bind to the S2B1 antibody with high affinity due to the presence of aromatic domains that lead to strong pi-pi stacking interactions.^34,35 These interactions can also be observed between biphenyl molecules and humic matter.³⁶ Including biphenyl moieties in a polymer can be used to produce a high affinity adsorbent for binding other molecules that have biphenyl groups such as polychlorinated biphenyls (PCBs). In this work, we report a method for the development of crosslinked polymers containing biphenyl moieties where the smectic interactions of the biphenyl moieties can act as additional crosslinkers and be utilized for aforementioned properties.

Previous reported literature of crosslinked polymers containing biphenyl moieties have studied their crystalline nature, reversible interactions as well as phase change properties.^37–40 Our study utilizes novel biphenyl based monomers to synthesize a crosslinked polymeric network. The monomers used in this study are synthesized in house and are referred to as 2-phenylphenolmonoacrylate (2PPMA), 4-phenylphenolmonoacrylate (4PPMA), and 4,4’-dihydroxybiphenyldiacrylate (44BDA), respectively. The monomer 4PPMA (referred to as 4-biphenylyl acrylate) and crosslinker 44BDA (referred to as 4,4’-bis(acryloyl)biphenyl) have been previously reported in the literature and characterized for their mesomorphic behaviour.^41,42 In this report, these monomers and crosslinker were used to synthesize unique crosslinked polymers, and we studied the resulting properties of the crosslinked network and its associated biphenyl interactions. To synthesize acrylate monomers, we utilized a previously reported method for acrylating phenolic compounds, to form their respective acrylated molecule.^43–46 Here, the biphenyl based precursors used are 2-phenylphenol, 4-phenylphenol, and 4,4’-dihydroxybiphenyl and were converted to 2PPMA, 4PPMA and 44BDA through acrylation process. 2PPMA, 4PPMA were used as the monomers, and 44BDA was used as a crosslinker in the system. These monomers and crosslinker were further characterized by techniques such as differential scanning calorimetry (DSC) to determine their phase transition temperatures and nuclear magnetic resonance (NMR) for structural characterization. Crosslinked polymers were synthesized using these monomers with varying crosslinker contents. The crosslinking density and mechanical integrity of the polymers were characterized using their swelling analysis and compressive modulus testing.

3. Experimental

3.1. Materials

Precursors 4,4’-dihydroxyphenyl, 4-phenylphenol, 2-phenylphenol, initiator ammonium persulfate (APS ≥ 98%) and acryloyl chloride were purchased from Sigma-Aldrich Corporation (St. Louis USA). All organic solvents were purchased from Sigma-Aldrich and Fischer Scientific (Hampton USA). Chemicals were used as received and without any further purification.

3.2. Synthesis of Acrylated Biphenyls

Synthesis of acrylated biphenyl was conducted by a method previously reported in the literature.^43–45 Briefly, biphenyls were functionalized with acrylate groups by performing a reaction with acryloyl chloride. 10 g of biphenyl was dissolved in 200 mL of tetrahydrofuran (THF) to give a final concentration of 50 mg/mL. 6.1 mL (1.5 mole equivalent) of Triethylamine (TEA), which is a tertiary amine and used to abstract the proton from the hydroxyl group of the precursor, was added to the solution. 3.6 ml (1.5 mole equivalent) of acryloyl chloride, which replaces the hydroxyl group with acrylate group, was added dropwise to the solution while stirring the mixture in an ice bath. The amount of TEA and acryloyl chloride added to the solution was 1.5 times the amount of the hydroxyl groups present in the precursor to ensure that the conversion of hydroxyl groups to acrylate groups is almost 100% (Table 1). The reaction was allowed to proceed overnight under ambient conditions. The purification process included numerous steps beginning with removing any TEA-HCl salt, which precipitated during the reaction, with vacuum filtration. Further, excess THF was removed through the use of a rotary evaporator. The solid was then dissolved in dichloromethane (DCM) and washed with 0.1 M hydrochloric acid (HCl) and 0.1 M potassium carbonate (K₂CO₃) to remove excess TEA and excess acryloyl chloride, respectively. Additionally, anhydrous magnesium sulfate (MgSO₄) was added in excess to remove any water present in the solution. Finally, DCM was evaporated using a rotary evaporator.⁴⁴ The final products were obtained as solid for 44BDA and 4PPMA and a viscous liquid for 2PPMA. These products were stored at −20°C until further use. The 44BDA used in this study was synthesized previously by our group. ⁴⁵ The structures of monomers, crosslinker and their respective precursors can be seen in Figure 1.

Table 1:

Composition of biphenyls, acryloyl chloride and TEA in initial reaction mixture.

Biphenyl	Number of hydroxyl groups	Acryloyl chloride (mol)	TEA (mol)
2-Phenylphenol	1	1.5	1.5
4-Phenylphenol	1	1.5	1.5
4,4’-dihydroxybiphenyl	2	3	3

Figure 1:

Structures of precursors (2-phenylphenol, 4-phenylphenol and 4,4’-dihydroxybiphenyl), acrylated monomers (2PPMA, 4PPMA) and acrylated crosslinker (44BDA).

3.3. Characterization of Monomers and Crosslinker

3.3.1. Differential Scanning Calorimetry (DSC)

The melting temperature of monomers (2PPMA, 4PPMA), crosslinker (44BDA), and precursors was measured by DSC (DSC Q200, TA Instruments Inc., New Castle, USA). A known amount of the compound was added to the t-zero pan and the pan was sealed. The temperature was ramped up at 5°C/min and the heat required per gram to increase the temperature was calculated in reference to an empty pan.

3.3.2. Nuclear Magnetic Resonance (NMR)

Proton-NMR spectra of the monomers, crosslinker and precursors were obtained from Varian Gemini NMR 400 MHz spectrometers connected to a Vnmrj software interface. 5 – 7 mg of the sample was weighed and dissolved into 700 μL of deuterated dimethyl sulfoxide (DMSO-d6). The solution was then transferred into NMR sample vials and analyzed for additional structural characterization.

3.3.3. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR)

ATR-FTIR was used to determine the presence of characteristic functional groups in the monomers as well as the precursors using a Varian 7000e FTIR spectrophotometer. The samples were placed on a diamond ATR crystal, covered with a glass coverslip, and the IR spectra was obtained. The spectrum was obtained between 700 and 4000 cm⁻¹ with a spectral resolution of 8 cm⁻¹ and 16 scans coaddition was used.

3.4. Synthesis of Crosslinked Polymers

Three different concentrations of crosslinked polymeric films (2.5, 5 and 7.5 mol % crosslinker, 44BDA) were synthesized through free radical polymerization using either 4PPMA or 2PPMA as the monomer. The reaction was carried out using DMSO to dissolve the monomer and crosslinker. For consistency, it was assumed that total mole to solvent volume ratio was constant at 4 mmol/mL. Feed compositions are given in Table 2.

Table 2:

Feed compositions of the polymeric films.

Name	Monomer	Monomer (mol %)	Crosslinker(44BDA) (mol %)	Total mmoles	Total amount of DMSO (mL)
4PP97.5	4PPMA	97.5	2.5	1	0.25
4PP95.0	4PPMA	95.0	5.0	1	0.25
4PP92.5	4PPMA	92.5	7.5	1	0.25
2PP97.5	2PPMA	97.5	2.5	1	0.25
2PP95.0	2PPMA	95.0	5.0	1	0.25
2PP92.5	2PPMA	92.5	7.5	1	0.25

For a typical synthesis of 4PP97.5, the total number of moles used was 1 mmol. Accordingly, 7.2 mg of 44BDA and 218.6 mg of 4PPMA were dissolved in 250 μL of DMSO in a glass vial. A sonication bath was used to ensure complete dissolution of 4PPMA and 44BDA. Initiator, APS, at a concentration of 0.5 g/mL in water was added to the mixture to comprise 4.0 wt% of the total 44BDA and 4PPMA weight. The reaction mixture was then pipetted between glass slides with 1 mm Teflon spacers and placed in an oven at a temperature of 80 °C for 2 hours. After polymerization, the films were washed with DMSO to remove any unreacted monomer/crosslinker and eventually cut into small discs of 3.54 mm diameter using a cork borer. The discs were washed with acetone to remove DMSO for ease of drying and placed in a vacuum oven at 50 °C and 6 in. Hg pressure overnight. The thickness of the dried discs was measured to be 0.87 ± 0.01 mm.

3.5. Characterization of Polymers

3.5.1. Swelling Studies

Dried films were examined for their swelling properties. The dry mass was measured before immersing them in 5 mL of the desired solvent for at least 24 hours (determined by an initial kinetic study) at 25 °C.⁴⁵ The equilibrium swollen mass was measured after gently wiping the surface DMSO from the film. The swelling ratio (q) is defined as:

$$swellingratio\left(q\right)=\frac{M_{swollen}}{M_{dry}}$$

Where M_swollen is the swollen mass of the film after 24 hours and M_dry is the dry mass of the film after oven drying. Three replicates were completed for each study and the values are reported with standard deviation.

3.5.2. Mechanical Testing

The dry and DMSO-swollen polymer discs were evaluated for their compression modulus with unconfined compression testing using a BOSE ELF 3300 system, without any initial preload. Samples were deformed at a rate of 0.003 mm/sec until the sample fractured or was compressed to 0.4 mm. The compressive moduli correspond to the slope of stress vs. strain curve up to a strain of 0.15.⁴⁷

4. Results and Discussion

4.1. Characterization of Monomers and Crosslinker

4.1.1. Differential Scanning Calorimetry

DSC was used to calculate the melting temperature of the monomers, crosslinker, and precursors. DSC thermograms are shown in Figure 2, and it can be seen that the melting peak of the 4PPMA, 2PPMA and 44BDA shifted approximately 100°C lower from their precursors. This can be attributed to hydrogen bonding occurring in the precursors due to the presence of hydroxyl groups that are no longer present in the acrylated compounds. Also, this change can be explained by the presence of bulkier group in the monomers decreasing the crystal packing. Additionally, it can be seen that the melting peak of 2PPMA, 4PPMA, and 44BDA are −50.0°C, 61.67°C, and 144.65°C indicating that former one is in a liquid phase whereas the latter two are in a solid phase at room temperature.

Figure 2:

Differential scanning calorimetry thermograms showing the melting peak of the precursors, monomers and crosslinker: a) 2PPMA, b) 4PPMA and c) 44BDA.

4.1.2. Nuclear Magnetic Resonance

Proton-NMR was used to confirm the monomer/crosslinker structure and the spectra are shown in Figure 3. DMSO-d6 used contains trimethylsilane (TMS) which was used as a reference in the NMR spectra. As expected, a hydroxyl peak is present in all three precursors. Upon acrylation of the precursors, the hydroxyl peak vanished and the peaks corresponding to alkene hydrogens appeared in monomers/crosslinker spectra. This confirms that the acrylation reactions proceeded to near 100% completion, forming the desired monomers and crosslinker. Finally, area under the curve can be correlated to the number of aromatic hydrogens as well as alkene hydrogen atoms present in the molecule. As an example, the 2PPMA monomer the ratio of number of aromatic hydrogens to the ratio of number of alkene hydrogens through the molecular structure $=\frac{9}{3}=3$, whereas the ratio of number of aromatic hydrogens to the ratio of number of alkene hydrogens through the area under the curve through NMR spectroscopy $=\frac{7.21}{0.78+0.77+0.78}=3.09$. These values are quite similar helping us to confirm the expected structure. ^48–50

Figure 3:

1H-NMR spectrum of precursors, monomers and crosslinker a) 2PPMA b) 4PPMA c) 44BDA. (The values below the peak represent area under the curve for that particular peak, and the values in the bracket represent the hydrogen to which the peak correspond.)

4.1.3. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy

ATR-FTIR analysis was used to determine the functional groups present in the samples. As seen from Figure 4, a peak is observed around ~3400 cm⁻¹ in the precursors which corresponds to the -OH group that is absent in the monomers indicating high conversion of the hydroxyl group. There is also a peak observed at ~1735 cm⁻¹ and at ~1640 cm⁻¹ in the monomers, corresponding to the presence of ester –C=O and alkene functionalities respectively, which is absent in the precursor suggesting the presence of an acrylate group only in the monomers.⁵¹ These results indicate successful conversion of hydroxyl group to the acrylate group.

Figure 4:

Representative ATR-FTIR spectra of the monomers in comparison to the precursors

4.2. Copolymerized Film Synthesis

A schematic of the crosslinked polymer formed upon reaction of 4PPMA with 44BDA is presented in Figure 5. Due to the limited solubility of 44BDA in DMSO, the maximum concentration of crosslinker used was 7.5 mol % because in order to incorporate additional crosslinker (greater than 7.5 mol %), the amount of DMSO required would increase. However, as the total volume of DMSO needs to be consistent across all variations, the total amount of DMSO in films with lower crosslinked density (2.5 mol %) increases which results in a dilution of the solution which prevents film formation for 2.5 mol % crosslinking concentration. Thus, the minimum and maximum amount of crosslinker concentration was maintained at 2.5 mol % and 7.5 mol % respectively.

Figure 5:

Crosslinked network schematics of 4PPMA-co-44BDA polymer.

4.3. Characterization of Polymers

4.3.1. Swelling Studies

The swelling response of 4PPMA and 2PPMA films was analyzed in DMSO at 25°C. The resulting swelling ratios for the films can be seen in Figure 6. It is noticed from the graph that these films do not follow the expected trend of increased crosslinking density leading to decreased swelling. By doing a one-way ANOVA among the swelling ratio values of the 2PPMA system and a one-way ANOVA among the swelling ratio values 4PPMA system, it was shown that there is not a significant difference in the swelling ratios of the both systems at = 0.5. This can be attributed to the restriction of the network swelling due to attraction of the biphenyl side groups induced by pi-pi stacking present between the polymer chains. It is hypothesized that pi-pi stacking interactions between the biphenyl groups provide noncovalent crosslinks, as can be seen in Figure 7. Such layered noncovalent interactions are also sometimes referred to as smectic interactions and have been reported previously.^52–55 These smectic interactions have shown to restrict the motion of the polymer chains in the network, thus minimizing the swelling ratio.^56–58 Since the chain mobility decreased with increasing covalent crosslink density (i.e., increasing 44BDA), the potential for noncovalent crosslinks decreased with increasing 44BDA. As a result, the effect of the covalent crosslinker content is minimized which is evident from Figure 6. It can also be seen from the graph that the 2PPMA films swell more than the 4PPMA films. We postulate this to be a result of steric hinderance within the biphenyl group of 2PPMA molecules due to the presence of a bulky group at the ortho position. This prevents a strong intramolecular interaction to exist within the 2PPMA polymer as compared to 4PPMA, where the biphenyl groups are expected to be present in a coplanar structure leading to stronger intramolecular interactions.

Figure 6:

Swelling studies of 2PPMA and 4PPMA films in DMSO. Temperature: 25°C. Data were plotted as mean ± standard deviation and 3 measurements were taken for each sample.

Figure 7:

Proposed intramolecular interactions between biphenyl side groups

To confirm the presence of the smectic interactions, these systems were swollen in an aromatic solvent (i.e., toluene), and the results are included in Figure 8. For this case, a higher swelling ratio is observed at the 2.5% crosslinked system for both the 2PPMA and 4PPMA system. It is hypothesized that this aromatic solvent is able to disrupt the non-covalent pi-pi stacking interactions between the biphenyl moieties of the polymer. The swelling ratios of 5.0 and 7.5 percent of both the 2PPMA and 4PPMA system are not significantly different, which could result from these systems having less pi-pi stacking interactions to disrupt due to the higher covalent crosslink density.

Figure 8:

Swelling studies of 2PPMA and 4PPMA films in toluene. Temperature: 25°C. Data were plotted as mean ± standard deviation and 3 measurements were taken for each sample

The polymer network with the covalent as well as noncovalent interactions are visualized in Figure 9. When the amount of covalent crosslinker is reduced, the polymer chains have higher mobility and therefore are allowed to interact with each other to a greater extent through noncovalent interactions. Similarly, if the amount of covalent crosslinker is high, the mobility of polymer chains is restricted resulting in fewer noncovalent interactions. Thus, the total effective crosslinks (covalent and noncovalent) were found to be relatively consistent across these systems, which can be seen from the swelling studies.

Figure 9:

Polymer network depicting covalent as well as noncovalent interactions.

4.3.2. Mechanical Testing

The compressive modulus of the dry and swollen films is shown in Figure 10. As crosslinking density increases, there is less free volume available for movement, giving the film higher compressive strength.^11,59–62 A similar trend was also observed in the dry as well as swollen polymeric films from figure 10. The mechanical testing of the swollen film clearly indicated a trend of increasing compressive modulus with increasing crosslink amount, which was not observed from the swelling studies. Previous studies have shown that the layered smectic interactions are affected in the presence of a compressive forces⁶³, and therefore, the results we obtained were attributed to energy applied to the system (in this case, mechanical energy) which is able to overcome the relatively weak smectic pi-pi stacking interactions minimizing the impact of the noncovalent crosslinks. Thus, the extent of covalent crosslinking was found to have the expected impact on the mechanical properties analyzed.

Figure 10:

Mechanical testing of the swollen films a) 2PPMA, b) 4PPMA and dry films c) 2PPMA, d) 4PPMA. Data were plotted as mean ± standard deviation and 6 measurements were taken for each sample.

5. Conclusions

Crosslinked polymers with biphenyl moieties were successfully synthesized by crosslinking monomers (i.e., 2PPMA and 4PPMA) with varying amounts of 44BDA. The successful syntheses of the monomers were confirmed by an observable shift in their melting peak as compared to their respective precursors, as well as NMR results showing conversion of biphenyls to their respective acrylated molecules with near 100% efficiency. Interestingly, swelling studies did not show the expected trend of decreasing swelling ratio with increasing crosslinker content, while compression testing indicated the expected trend of increasing modulus with increasing crosslinking density. The unexpected swelling results are hypothesized to result from the intramolecular interactions between the biphenyl side groups that result in noncovalent crosslinks. These materials and associated unique properties are expected to have useful applications in various fields, such as shape memory polymers, molecular recognition for pollutant removal (e.g., polychlorinated biphenyls) from the environment.