Journal of Organic & Inorganic Chemistry

Keywords

Statistical copolymers; Reactivity ratios; Benzyl methacrylate; Diethylaminoethyl methacrylate; Thermal analysis; Free radical polymerization

Introcuction

Since the rise of the polymer era; copolymerization, simultaneously emerged as a successful method to engage and enhance the properties and application range of derived polymers. The incorporation of two different monomers, with diverse chemical and/or physical properties, on the same structure, can lead to materials with tailor-made properties [1,2]. Numerous monomers have been involved in these copolymerization studies [3-9]. Among them nitrogen containing monomers such as diethylaminoethyl methacrylate (DEAEMA) are of great interest due to their versatile nature. Specifically they can undergo post-polymerization reactions, they have excellent adhesion properties and due to the amine group offers a pH and temperature stimuli responsive character as well as water solubility by quaternization to ammonium salts [10,11]. pH-Responsive polymeric systems provide the possibility of fabricating tailorable “smart” functional materials, which may find numerous potential commercial applications, such as controlled drug delivery, personal care, oil exploration, industrial coatings, viscosity modifier, biological and membrane science, colloidal stabilization, water remedation [12-14], reversible gelation, CO₂ sensitive micellization [15,16] etc. Thermosensitivity was reported for PDEAEMA exhibiting lower critical solution temperature, LCST, meaning that the polymer precipitates from water at elevated temperatures [17,18]. Block copolymers containing PDEAEMA blocks are widely used in the literature as pH - and temperature responsive materials in aqueous solutions, as a consequence of the increase in hydrophobicity upon deprotonation [19,20]. On the other hand benzyl methacrylate (BzMA) is a very interesting monomer, which can be used as a substitute in the methacrylate family of monomers of the very commonly used styrene (St) in copolymerization studies [21]. PBzMA is also hydrophobic and amorphous as PSt but with a much lower glass transition, T_g, value [(T_g)PBzMA=54°C instead of (T_g)PSt=100°C] [22], leading to the formation of more flexible and easily processable copolymeric structures. BzMA has been polymerized by polymerizationinduced self-assembly (PISA) [23,24] using controlled radical polymerization techniques, such as reversible additionfragmentation chain transfer (RAFT) polymerization [25,26]. Potential applications of these structures involve drug delivery, coatings, contact lenses, sterilizable gels and emulsifiers [27,28]. It was found recently that PBzMA exhibits LCST in ionic liquids, i.e., a sharp decrease in solubility upon heating the sample above a critical temperature [29]. In the present study, the synthesis of poly(benzyl methacrylate) (PBzMA) and poly(diethylaminoethyl methacrylate) (PDEAEMA) statistical copolymers via free radical polymerization, is described. The objective of this study was to measure the monomer reactivity ratios and the structural parameters of the copolymers. The thermal properties of the copolymers were also studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Experimental Methods

Materials

All polymerizations were conducted using high-vacuum techniques described elsewhere [30]. BzMA and DEAEMA were purchased from Aldrich and were vacuum-distilled from calcium hydride via the high vacuum line. 2,2’-Azobisisobutyronitrile, AIBN (Aldrich) was recrystallized twice from methanol, filtered and dried under vacuum. Benzene (Aldrich) was vacuum distilled from CaH₂ into a flask containing n-BuLi and finally distilled over the vacuum line.

Copolymerization studies

For the purpose of inquiring/studying the reactivity ratios of these two monomers; a set of five copolymers of BzMA and DEAEMA were prepared. Different feed ratios were involved in each copolymerization (monomer molar ratios: 80/20, 60/40, 50/50, 40/60 and 20/80). Copolymerization reactions were quenched at rather low yields (~ 10% yields). The copolymerization procedure was monitored by size exclusion chromatography, NMR, and UV spectroscopy. The experimental results were processed on the basis of the Fineman-Ross (FR), inverted Fineman-Ross (IFR) and Kelen-Tüdos (KT) equations. The computer program COPOINT was employed as well [31].

Copolymerization of BzMA with DEAEMA

The copolymerization reactions were performed in glass reactors, in benzene solutions. AIBN was employed as initiator at the concentration of 0.1% wt. As an example for sample 20/80 3.06 g (17.586 × 10^-3 moles) BzMA, 12.77 g (69.027 × 10^-3 moles) DEAEMA, 0.016 g AIBN and 150 ml of benzene were added in the reactor. Prior to polymerization, the mixtures were degassed under high vacuum using three freeze-thaw cycles and the reactors were flame-sealed. The copolymerizations were conducted at 60°C for 3 hours. The polymers were precipitated in methanol, the supernatant was decanted and dried. The same procedure was repeated as many times as necessary in order to remove any remaining unreacted monomers. Finally, the copolymer was dried overnight in a vacuum oven.

Characterization techniques

Size Exclusion Chromatography (SEC) experiments were carried out at 40°C using a modular instrument consisting of a Waters model 510 pump, U6K sample injector, 401 refractometer, 486 UV spectrophotometer, and a set of 5 μ-Styragel columns with a continuous porosity range from 500 to 106 Å. The columns were housed in an oven thermostatted at 40°C. THF was the carrier solvent at a flow rate of 1 mL/min. The system was calibrated with nine PS standards having molecular weights in the range of 970–600,000. The glass-transition temperatures were obtained by differential scanning calorimetry (DSC) using a 2910 modulated DSC model from TA instruments. The samples were heated or cooled at a rate of 5°C/min. The second heating results were obtained in all cases. The thermal stability of the copolymers was studied by thermogravimetric analysis (TGA) employing a Q50 TGA model from TA instruments. The samples were heated at a rate of 10°C/min under nitrogen atmosphere. The composition of the copolymers was also determined from ¹H-NMR spectra, which were recorded in chloroform-d at 30°C with a Varian Unity Plus 300/54 NMR spectrometer.

Results and Discussion

Statistical copolymers of BzMA with DEAEMA

The free radical copolymerization of BzMA with DEAEMA was conducted in benzene solutions using AIBN as the initiator at 60°C (Scheme 1). In all cases the conversion was kept at low values, to satisfy the differential copolymerization equation. The copolymers were purified by repeated precipitations in methanol, where both monomers are soluble. The molecular characteristics of the samples are given in Table 1. The samples have typical high molecular weights and molecular weight distributions in the range expected for conventional radical polymerization. Compared with statistical copolymers of BzMA prepared by radical polymerization in bulk [32] or PBzMA and statistical copolymers, prepared by atom transfer radical polymerization (ATRP) [33] it is obvious that in the present study the solution copolymerization leads to rather narrow distributions. Different copolymers are symbolized by the various feed molar ratios of the monomers, e.g., sample 20/80 indicates the copolymer for the synthesis of which 20% BzMA and 80% DEAEMA were employed as the molar feed composition. The molecular weights were measured by SEC using a calibration curve constructed by polystyrene standards. NMR spectroscopy was employed to measure the composition of the copolymers. The analysis was based on the signals at 4.70 and 3.80 ppm which are attributed to the -CH2- groups adjacent to the carboxylate groups of BzMA and DEAEMA monomer units respectively. A characteristic example is given in Scheme 2.

Table 1: Molecular characteristics of the copolymers^a.

Monomer reactivity ratios and statistical analysis of the copolymers

The monomer reactivity ratios were determined using the Fineman-Ross (FR) [34], inverted Fineman-Ross (IFR) [34] and Kelen-Tüdos (KT) [35] graphical methods. A computer program, called COPOINT, was also employed [36-38]. According to the Fineman-Ross method, the monomer reactivity ratios can be obtained by the equation:

G=Hr_BzMA-r_DEAEMA (1)

where the reactivity ratios, r_BzMA and r_DEAEMA correspond to the BzMA and DEAEMA monomers, respectively. The parameters G and H are defined as follows:

G=X(Y-1)/Y and H=X²/Y (2)

with X=M_BzMA/M_DEAEMA and Y=dMBZMA/dM_DEAEMA (3)

M_BzMA and M_DEAEMA are the monomer molar compositions in feed and dM_BzMA and dM_DEAEMA the copolymer molar compositions.

The inverted Fineman-Ross method is based on the equation:

G/H=r_BzMA-(1/H)_DEAEMA (4)

The plots of the G versus H values and the G/H versus 1/H values yield the reactivity ratios r_BzMA and r_DEAEMA from the intercept and the slope of the graphs.

Alternatively, the reactivity ratios can be obtained using the Kelen-Tüdos method which is based on the equation:

η=(r_BzMA+r_DEAEMA/α)ξ-r_DEAEMA/α (5)

where η and ξ are functions of the parameters G and H:

η=G/(α+H) and ξ=H/(α+H) (6)

and α a constant which is equal to (H_maxH_min)1/2, H_max, H_min being the maximum and the minimum H values, respectively from the series of measurements. From the linear plot of η as a function of ξ, the values of η for ξ=0 and ξ=1 are used to calculate the reactivity ratios according to the equations:

ξ=0 ⇒ η=-r_DEAEMA/α and ξ=1 ⇒ η=r_BzMA (7)

The detailed copolymerization data for all systems are provided in Table 2. The graphical plots concerning the methods previously reported are given in Figures 1-3, whereas the reactivity ratios are summarized in Table 3. The computer program COPOINT evaluates the copolymerization parameters using comonomer/ copolymer composition data as obtained from copolymerization experiments with finite monomer conversion. Although the mathematical treatment can be applied up to full monomer conversion, it is recommended not to exceed 20-30 mol%. COPOINT numerically integrates a given copolymerization equation in its differential from. The copolymerization parameters can be obtained by minimizing the sum of square differences between measured and calculated polymer compositions. Errors of the fitted parameters are estimated from the statistical error of the sum of square differences, as well as from a quadratic approximation of this sum in the vicinity of the optimized values of the copolymerization parameters. The plots were linear in all cases and for all graphical methods, thus indicating that these copolymerizations follow conventional copolymerization kinetics and that the reactivity of all polymer radicals is determined only by the terminal monomer unit. It is obvious that all methods provide similar data concerning the reactivity ratios for both monomers. According to the data obtained by COPOINT r_BzMA=0.600 and r_DEAEMA=0.398. This result implies that the homopolymerization of the two monomers is not favored, or in other words that there is a tendency for the production of an alternating copolymer. However, the probability for the incorporation of BzMA units is slightly greater than for the incorporation of DEAEMA units (r_BzMA>r_DEAEMA). This kind of copolymerization is characterized as nonideal azeotropic [39]. The statistical distribution of the dyad monomer sequences M_BzMA-M_BzMA, M_DEAEMA-M_DEAEMA and M_BzMA- M_DEAEMA were calculated using the method proposed by Igarashi [40]:

Where X, Y and Z are the mole fractions of the M_BzMA-M_BzMA, M_DEAEMA-M_DEAEMA and M_BzMA-M_DEAEMA dyads in the copolymer respectively and ?BzMA the BzMA mole fraction in the copolymer. Mean sequence lengths BzMA and μDEAEMA were also calculated using the following equations [41]:

Table 2: Copolymerization data for the statistical copolymers.

Table 3: Reactivity ratios measured by different methods.

The data are summarized in Table 4, and the variation of the dyad fractions with the BzMA mole fraction in the copolymers is displayed in Figure 4. The results confirm the conclusions drawn by the reactivity ratios and show the high content of BzMADEAEMA heterodyads in the copolymer structure.

Table 4: Structural data of the copolymers.

Glass transition temperature of the statistical copolymers

The thermal properties of the copolymers are influenced by their chemical structure and composition and the monomer sequence distributions. Several relationships have been employed to describe the effect of these parameters on the glass transition temperature of the copolymers [42]. The simplest equation describing the effect of composition on T_g is the Gibbs-Di Marzio equation [43]:

T_g=?_BzMATg_MMA + ?_DEAEMATg_DEAEMA (13)

Where, ?_BzMA, ?_DEAEMA are the mole fractions of BzMA and DEAEMA, respectively, in the copolymer and Tg_BzMA, Tg_DEAEMA the glass transition temperatures of the two homopolymers, respectively. Tg_DEAEMA was found to be equal to 293 K, whereas Tg_PBzMA is equal to 327 K.

A similar relationship was introduced by Fox [44]:

Where w_BzMA and w_DEAEMA are the weight fractions of BzMA and DEAEMA in the copolymer.

Where wBzMA and wDEAEMA are the weight fractions of BzMA and DEAEMA in the copolymer.

it is assumed that the BzMA-BzMA, DEAEMA-DEAEMA and DEAEMA-BzMA or BzMA-DEAEMA dyads have their own glass transition temperatures, Tg_BzMA-BzMA, Tg_{DEAEMA-DEAEMA} and Tg_BzMA-DEAEMA, respectively. Tg_BzMA-BzMA and Tg_{DEAEMA-DEAEMA} can be considered as the glass transition temperatures for the respective homopolymers, whereas Tg_BzMA-DEAEMA is the glass transition temperature of the alternating copolymer P(BzMA-alt-DEAEMA). Wi is the weight fraction of the i component and P_BzMA-BzMA, P _{DEAEMA-DEAEMA}, P_BzMA-DEAEMA and PDEAEMA -BzMA are the probabilities of having various linkages. These probabilities can be calculated using the monomer reactivity ratios [46]:

Barton suggested the following equation [27]:

Tg=XTg_BzMA-BzMA+Y_{DEAEMA-DEAEMA}+ZTg_BzMA-DEAEMA (20)

Where X, Y, Z are the monomer dyad fractions (Equations 11-13).

To apply these theories it is necessary to know the glass transition temperature of the respective alternating copolymers. However, these data are not provided in the literature. Therefore, the linearized forms of the Johnston and Barton equations are used to obtain the Tg_BzMA-DEAEMA values. The plots, given in Figures 5 and 6, are straight lines passing through the origin. This is an indication that these theoretical methods can better predict the T_g values of statistical copolymers or, in other words, that the monomer sequence distribution is an important parameter defining the T_g of a statistical copolymer. The Tg_BzMA-DEAEMA values, calculated by the Johnston and Barton equations, are 312.8 and 311.6 K, respectively. It is obvious that the results obtained by the two methods are not identical but in very close agreement. For the statistical copolymers one transition was observed, meaning that the samples are not phase separated, and the T_g values increase upon increasing the BzMA content. The experimental results concerning the T_g of the statistical copolymers along with the predictions of all models are given in Table 5. It is evident that the predicted values from the Gibbs-DiMarzio and Fox equations are in better agreement with the experimental results. It is obvious that the Johnston and Barton equations overestimate the Tg_BzMA-DEAEMA values. Since the copolymers are characterized by high heterodyad contents it is reasonable to expect positive deviations of the predictions made by these methods, compared to the experimental results. However, even the Johnston and Barton methods can efficiently be applied for the prediction of the T_g values of the copolymers. This result can be attributed to the random placement of the comonomer units along the copolymeric chains and to the relatively similar T_g values of the respected homopolymers.

Table 5: Glass transition temperatures of the homo- and copolymers.

Kinetics of the thermal decomposition of the statistical copolymers

TGA measurements were employed to study the kinetics of the thermal decomposition of the statistical copolymers. The activation energy, Ea, of mass loss upon heating was calculated using the isoconversional Ozawa-Flynn-Wall (OFW) [47-49] and Kissinger methods [50]. The OFW approach is a “model free” method, which assumes that the conversion function F(α), where α is the conversion, does not change with the alteration of the heating rate, β, for all values of α. The OFW method involves the measuring of the temperatures corresponding to fixed values of α from experiments at different heating rates β. Therefore, plotting lnβ vs. 1/T in the form of

should give straight lines with slope directly proportional to the activation energy, where T is the absolute temperature, A is the pre-exponential factor (min^-1) and R is the gas constant (8.314 J/K.mol). If the determined activation energy is the same for the various values of α, then the existence of a single-step degradation reaction can be concluded with certainty. The OFW method is the most useful method for the kinetic interpretation of thermogravimetric data, obtained from complex processes like the thermal degradation of polymers. It can be applied without knowing the reaction order. The linearity of the OFW diagrams in a broad range of conversions proves that it is an efficient method for the analysis of the thermal decomposition of the copolymers. A characteristic example is given in Figure 7. However, deviations from the linearity were observed in very low or very high conversions, since these results refer to the decomposition of parts of the macromolecules that are not representative of the overall copolymeric chains, due to the statistical character of the distribution of the different monomer units.

The activation energy E was also calculated from plots of the logarithm of the heating rate versus the inverse of the temperature at the maximum reaction rate in constant heating experiments, according to the Kissinger method. Using the equation:

(22)

where T_p and α_p are the absolute temperature and the conversion at the maximum weight-loss and n is the reaction order. The E values can be calculated from the slope of the plots of ln(β/T_p ²) versus 1/Tp. Characteristic examples are given in Figures 8 and 9. The statistical copolymers were thermally degraded at different heating rates. Examples of the derivative thermogravimetry measurements under different heating rates are displayed in Figures 10-13. The two homopolymers present similar range of decomposition temperatures, showing two maxima of their thermal decomposition. The main decomposition peak is similar for both homopolymers, observed between 330 and 370°C. The second decomposition peak is observed at lower temperatures for PBzMA (285-315°C) compared to PDEAEMA (410-440°C). These results indicate that the decomposition mechanism is rather complex for both monomers. The main decomposition event is probably attributed to the decomposition of the main chain,

which is chemically similar for both homopolymers. The second decomposition peak is probably related with the decomposition of the side groups. It seems that diethylaminoethyl- groups offer greater thermal stability compared to the benzyl groups. In other words, the thermolysis of the side groups of PBzMA is favored. This result is verified by literature reports showing that the general mechanism of the thermal degradation of the polymethacrylates involves the loss of the ester side group to form methacrylic acid followed by cross-linking [51]. In the case of the copolymers the temperature range of the decomposition is much broader covering the ranges of both homopolymers. The samples 80-20 and 20-80 show two decomposition maxima at temperatures close to PBzMA and PDEAEMA respectively, obviously due to the much higher content in BzMA and DEAEMA units. The other samples show three decomposition maxima, combining the thermal degradation behavior of both homopolymers [51]. The activation energies measured by the Kissinger method (Table 6) have lower values than that measured by the OFW method (Table 7), something which is rather common in the literature and reflects the different theoretical background on which the two methods are based. The activation energies from the OFW method change considerably upon changing the decomposition conversion, indicating that the decomposition mechanism is complex during the thermal decomposition for both the homoand the copolymers. PBzMA shows lower Ea values compared to PDEAEMA reflecting its lower thermal stability. It is rather surprising that the copolymers reveal higher Ea values than both homopolymers. This behavior can be attributed to the fact that at temperatures below 300°C the thermolysis of the side groups of BzMA units takes place leading to the formation of moieties that may react with the diethylamine groups of the DEAEMA monomer units leading to temporal crosslinking and therefore to transient networks, which are thermally very stable. Upon increasing the BzMA content of the copolymers the Ea values gradually increase. The thermolysis of the side groups of BzMA facilitates the formation of the transient networks leading to increased thermal stability of the structures, in agreement with literature results.

Table 6: Activation Energies, J, by the Kissinger Method.

Table 7: Activation Energies, J, by the OFW Method.

Conclusion

Free radical polymerization techniques were employed for the synthesis of statistical copolymers of benzyl methacrylate (BzMA) and diethylaminoethyl methacrylate (DEAEMA). The reactivity ratios were measured in the frame of several graphical and non-graphical methods. The computer program COPOINT revealed that r_BzMA=0.600 and r_DEAEMA=0.398, implying that a nonideal azeotropic copolymerization is effective. The homopolymerization of the two monomers is not favored, since both reactivity ratios are lower than unity. Therefore, a tendency for the production of an alternating copolymer prevails with slightly greater probability for the incorporation of BzMA than DEAEMA units (r_BzMA>r_DEAEMA). The dyad monomer distributions and the mean sequence lengths were also measured. The T_g values of the copolymers were measured and were compared with the predictions of several theoretical models. It was found that Fox and Gibbs-Di Marzio equations fit better the experimental data. The kinetics of thermal decomposition of the copolymers was also studied and the results were compared with those of the respective homopolymers. In all cases the mechanism of thermal decomposition is complex. In the case of PBzMA the process involves initially the decomposition of the side groups. The copolymers revealed higher activation energies of thermal decomposition, Ea, values than both homopolymers. This behavior can be attributed to the thermolysis of the side groups of BzMA units leading to the formation of moeities that may react with the diethylamine groups of the DEAEMA monomer units leading to temporal crosslinking and therefore to transient networks, which are thermally very stable.

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