Introduction

Macromolecules of a desired structure and molecular weight can be synthesized by controlled/“living” radical polymerization (CRP) techniques, such as atom transfer radical polymerization (ATRP) [1–5], nitroxide-mediated polymerization [6, 7], and reversible addition fragmentation chain transfer (RAFT) polymerization [8–19]. ATRP has a great number of advantages as compared with other CRPs. It includes a lot of monomers, and offers a general and efficient way to synthesize various (co)polymers [20], and does not require difficult conditions and has tolerance for functional groups and impurities [21, 22].

Block copolymers that have excellent physical properties are one of the most important polymeric materials used in technological applications and theoretical research because of their exceptional properties based on micro-phase separation [23–31]. The viscosity of a star block copolymer is higher than that of linear copolymer having the same molecular weight. Hence, star block copolymer is mostly used as a resistant material. There are a great number of excellent articles published on this subject [32–43].

In recent years, the one-step process has been successfully used for the synthesis of block and graft copolymers using different techniques. The process has more advantages than other popular methods. Due to the applicability of at least two transformation steps simultaneously, side reactions which lead to homopolymer formation are minimized [44–58]. Farah et al. carried out the synthesis of poly(ε-caprolactone-b-styrene) block copolymers through the combination of ATRP and ROP in the presence of 2-bromoisobutyryl bromide and bipyridine using N-methylpyrrolidione as the solvent [59]. Furthermore, various copolymers containing styrene [59–61], n-butyl acrylate [60], methyl methacrylate [61], tert-butyl acrylate [62], benzyl acrylate [62], ε-caprolactone [59, 60], l-lactide [61, 62] monomers were synthesized by a combination of the ATRP and ROP methods.

The present work is an extension of our recent studies involving the one-step synthesis of copolymers through simultaneous RAFT polymerization and ROP processes [17, 18, 63]. In this study, we synthesized poly (MMA-b-CL) triarm block copolymers using 3-cholor-1,2-propiondiol (ATRP-ROP initiator) by the simultaneous ATRP and ROP of the reactants in one-step. Star block copolymers synthesized could be used to prepare with the desired segment ratio by changing the polymerization conditions. The effect of the reactions conditions on the parameters was also investigated.

Experimental

Materials

3-Chloro-1,2-propanediol and copper(I) bromide (CuBr) were received from Aldrich and used as received. Dibutyltindilaurate (DBTDL) and petroleum ether were supplied by Merck and used as received. Benzene, chloroform, and tetrahydrofuran (THF) were received from Sigma-Aldrich and used as received. N,N,N′,N′,N″-Pentamethyldiethylenetriamine (PMDETA) was supplied by Fluka and used as received. Methanol and ethanol were received from Birpa and used as received. ε-Caprolactone (CL) was supplied by Alfa Aesar and used as received. MMA was received from Merck, which was purified as follows: it was washed with a 10 wt% aqueous NaOH solution, dried over anhydrous CaCl2 overnight, and distilled over CaH2 under reduced pressure before use. All other chemicals were reagent grade and used as received.

Instrumentation

The molecular weights and molecular weight distributions were measured with Malvern Viscotek RI-UV-GPC max gel-permeation chromatography (GPC) with THF as the solvent. A calibration curve was generated with four polystyrene standards: 2960, 50,400, and 696,500 Da, of low polydispersity. Fourier-transform infrared (FTIR) spectra were recorded using an Alpha-p Bruker model FTIR spectrometer. 1H-nuclear magnetic resonance (1H-NMR) spectra of the samples in CDCl3 as the solvent, with tetra methylsilane as the internal standard, were recorded using a Bruker Ultra Shield Plus, ultra-long hold time 400 MHz NMR spectrometer. Thermogravimetric analysis (TGA) measurements of the polymers were carried out under nitrogen using a Perkin Elmer Pyris 1 TGA and Spectrum thermal analyzer to determine thermal degradation. Differential scanning calorimetry (DSC) measurements were carried out by using a Perkin Elmer Diamond DSC series thermal analysis system. Dried sample was heated at a rate of 10 °C/min under nitrogen atmosphere.

One-step polymerization

Poly(MMA-b-CL) triarm block copolymers were synthesized using two different monomers in one-step process. Specified amounts of ATRP-ROP initiator, MMA, CL, DBTDL (catalyst for ROP of CL), PMDETA, CuBr, and benzene (as solvent) were charged separately into a Pyrex tube, and subsequently argon was purged into the tube through a needle. The tube was tightly capped with a rubber septum and was dropped into an oil bath thermostated at 110 °C for fixed time. After the polymerization, the reaction mixture was poured into an excess of methanol to separate the block copolymers. The copolymers were dried at 40 °C under vacuum for 3 days. The yield of the polymer was determined gravimetrically.

Fractional precipitations of the polymers

Fractional precipitations (γ) of the polymers were carried out according to the procedure reported in the literature [63–65]. Vacuum-dried polymer sample (approximately 0.5 g) was dissolved in 5 mL of THF. Petroleum ether was added as drop wise to the solution with stirring until turbidity occurs. At this point, 1–2 mL of petroleum ether was added to complete the precipitation. The precipitate was removed by filtration. The solvent was THF and the nonsolvent was petroleum ether. In this solvent–nonsolvent system, the γ values were calculated as the ratios of the total volume of nonsolvent used for the first fraction to the volume of solvent used.

$$\gamma {\text{ value}} = \frac{{\text{Volume}\;\text{of}\;\text{nonsolvent, mL}\;(\text{petroleum}\;\text{ether})}}{{\text{Volume}\;\text{of}\;\text{solvent, mL}\;(\text{THF})}}$$

The nonsolvent addition into the filtrate solution was continued according to the same procedure mentioned above to determine the γ value for the second fraction if there is.

Results and discussion

One-step polymerization for poly(MMA-b-CL) triarm block copolymers

The one-step polymerization of a vinyl monomer and a lactone initiated by ATRP-ROP initiator is shown in Scheme 1. This process creates two new active sites—a site on an equal number of hydroxyl group for ROP reaction and a chloride group for ATRP. During this one-pot synthesis, ATRP of MMA is carried out simultaneously as ROP of CL proceeds, to yield the block copolymer. The effects of polymerization time, initiator concentration, and monomer concentration on the copolymerization in the presence of ATRP-ROP initiator by the application of simultaneous ATRP and ROP processes have been studied. The results of the one-step polymerization of MMA and CL are shown in Tables 1, 2, 3. The monomer conversion was calculated from the weight of recovered polymer. The conversion of monomer was between 13.42 and 47.73 wt%. Increases in the molecular weights of the copolymers as compared with that of the initiator can confirm block copolymer formation.

Scheme 1
scheme 1

Reaction pathways in the synthesis of poly(MMA-b-CL) triarm block copolymers

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Table 1 The effect of the polymerization time on one-step block copolymerization for poly(MMA-b-CL) triarm block copolymers
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Table 2 The effect of the amount of ATRP-ROP initiator on one-step block copolymerization for poly(MMA-b-CL) triarm block copolymers
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Table 3 The effect of the amount of the monomer on one-step block copolymerization for poly(MMA-b-CL) triarm block copolymers
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The FTIR spectrum of 3-chloro-1,2-propanediol in Fig. 1a shows 3321 cm−1 for –OH groups, 2884–2953 cm−1 for aliphatic –CH2 and –CH groups, 1031 cm−1 for –C–O groups, 704 cm−1 for –Cl groups. The FTIR spectrum of the triblock copolymer is shown in Fig. 1b. The signals at 2949–2993 cm−1 for aliphatic –CH2 and –CH3, 1722 cm−1 for –C=O, 1140 cm−1 for –C–O of the copolymer appear in the FTIR spectra. The –OH signal diminishes at the FTIR spectrum of the copolymer (Fig. 1b) according to the –OH signal of the initiator (Fig. 1a). The 1H-NMR spectrum of 3-chloro-1,2-propanediol in Fig. 2a shows the 3.6 ppm for –OH protons, 3.9 and 4.0 ppm for –CH2 and –CH protons, 4.0 and 5.0 ppm for –OCH2 protons. Typical 1H-NMR spectra of the copolymer in Fig. 2b show 0.7 ppm for –CH3 protons of poly-MMA segment, 0.9 ppm for –CH2 protons of poly-MMA segment, 1.1 ppm for –OH protons of poly-CL segment, 1.3 ppm for –CH and –CH2 protons of 3-chloro-1,2-propanediol, 1.8 ppm for –OCH2 protons of poly-CL segment, 3.5 ppm for –OCH3 protons of poly-MMA segment.

Fig. 1
figure 1

FTIR spectrum of 3-chloro-1,2-propanediol trifunctional initiator (a), and poly(MMA-b-CL) triarm block copolymer (b)

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Fig. 2
figure 2

1H-NMR spectra of 3-chloro-1,2-propanediol trifunctional initiator (a), and poly(MMA-b-CL) triarm block copolymer (b)

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The effect of the polymerization time on the one-step block copolymerization is presented in Table 1. Polymerization time dependence of M n on the one-step copolymerization is shown in Fig. 3. First, longer polymerization times cause higher polymer molecular weights. Second, the polymers with lower molecular weights are obtained for polymerizations of longer durations. Longer polymerization times cause higher polymer yields. Higher amounts of ATRP-ROP initiator cause a higher polymer yield (Table 2). Interestingly, the value of M n can only decrease if new chains are generated. However, that is not in accordance with a controlled polymerization. Increased amounts of initiator in the reaction mixture lead to the formation of a higher number of active centers. Consequently, increased numbers of growing radicals are formed in the system. Hence, it may be expected that they have shorter poly-MMA and poly-CL segments, which is confirmed by a decrease in the molecular weights of the block copolymers, as shown in Table 2. The same situation was also observed in our previous articles [16, 17, 63]. Dependence of ATRP-ROP initiator concentration on M n for the one-step copolymerization is shown in Fig. 4. Increasing the amount of monomers also causes an increase in both the yield and the molecular weights of the copolymers as expected (Table 3). Dependence of MMA concentration on M n for the copolymerization is shown in Fig. 5. The M w/M n values of the triarm block copolymers are between 1.98 and 3.23 (Tables 1, 2, 3). Because more than one propagating center initiates the polymerization, the M w/M n values of the block copolymers are relatively higher than expected. Because DBTDL, ROP catalyst of CL, can interfere with the radical polymerization of MMA, the block copolymers with very broad molecular weight distributions can be formed. All GPC chromatograms were unimodal and indicated more the molecular weight values of block copolymers than that of ATRP-ROP initiator. For example, Fig. 6 shows the unimodal GPC curves of the block copolymers (MB-3, MB-4, MB-5, and MB-6 in Table 3). The polymer composition of the copolymers was calculated using the integral ratios of the signals corresponding to the –OCH3 groups of poly-MMA (δ = 3.5 ppm), –OCH2 groups of poly-CL (δ = 1.8 ppm). The poly-MMA content of copolymers was more than the poly-CL content. Generally, the values of polymer composition of the copolymers did not change as shown Tables 1, 2, 3.

Fig. 3
figure 3

Dependence of polymerization time on M n for poly(MMA-b-CL) triarm copolymers

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Fig. 4
figure 4

Dependence of ATRP-ROP initiator on M n for poly(MMA-b-CL) triarm copolymers

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Fig. 5
figure 5

Dependence of MMA on M n for poly(MMA-b-CL) triarm copolymers

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Fig. 6
figure 6

GPC curves of the triarm block copolymers

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Thermal analysis of poly(MMA-b-CL) triarm block copolymers

Thermal analysis of the samples was carried out by taking DSC, and TGA curves. All samples exhibited glass transition temperatures (T g). The reported T g values were obtained from the second heating curves. T g value of the block copolymer (BA-6) was 5 °C (Fig. 7). T g values were reported in the literature for homo poly-CL, and homo poly-MMA as −72 °C [66, 67], and 105 °C [68], respectively. The T g value observed by DSC appears between T g of the poly-MMA homopolymer and T g of the poly-CL homopolymer. The only one T g value for the sample shows the miscible nature of the related homopolymers. The same situation (the observation of only one glass transition) can also be seen in our previous articles [18, 63]. Similarly, TGA showed that in the block copolymers, poly-MMA, and poly-CL blocks did not have individual decomposition temperatures (T d) (Fig. 8). TGA showed interesting properties of the block copolymer indicating continuous weight loss starting from 13 °C to nearly 430 °C with a derivative at 375 °C. The first decomposition observed at about 200 °C may have been caused by the solvent traces. One main individual T d of the block copolymers can be attributed to the high miscibility of the polymerizable methacrylate groups of poly-MMA with poly-CL moieties of the copolymers.

Fig. 7
figure 7

DSC curve of the triarm block copolymer

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Fig. 8
figure 8

TGA curve of the triarm block copolymer

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Fractional precipitation

The fractional precipitation (γ) values of poly(MMA-b-CL) block copolymers were between 0.42 and 0.68. In the solvent–nonsolvent system, γ values were found to be 0.50–0.55 for homo poly-MMA [16], 1.02–1.20 for homo poly-CL [18]. The γ values of the block copolymers were generally between that of homo poly-MMA and that of homo poly-CL. Fractional precipitation behavior can give an evidence for the formation of block copolymer.

Conclusions

One-step synthesis of block copolymer was carried out ATRP of MMA and ROP of CL using 3-cholor-1,2-propiondiol initiator. The initiator has demonstrated the characteristic initiator behavior in the copolymerization of MMA and CL. A set of one-step synthesis, and ATRP and ROP conditions of triarm block copolymers, poly(MMA-b-CL), were evaluated. The block copolymers were relatively obtained in high yield and molar weight. The proposed procedure for the preparation of block copolymers is simple and efficient. Basically, controlling the polymerization parameters such as ATRP-ROP initiator concentration, monomer concentration, and polymerization time, ATRP-ROP initiator can be promising materials in order to obtain block copolymers.