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. 2025 Jul 26;19(1):223. doi: 10.1186/s13065-025-01597-w

Synthesis of ultra-high molecular weight poly(methyl methacrylate) initiated by the combination of copper nanopowder with organic halides

Ming Yuan 1,✉, Xiaofang Han 1, Zekai Zhang 1, Ruijia Wang 1, Shengrong Guo 1,✉
PMCID: PMC12297669  PMID: 40713622

Abstract

In this study, organic halides, such as 2-bromobutane (C4H9Br), ethyl α-bromophenylacetate (BPA), ethyl 2-bromoisobutyrate (EBiB), and ethyl 2-bromopropionate (EBP) are utilized in conjunction with copper nanopowder (Nano-Cu) to initiate the polymerization of methyl methacrylate (MMA). Among these, BPA combined with nano-Cu exhibits the highest reactivity, resulting in the production of poly(methyl methacrylate) (PMMA) with a number-average molecular weight (Mn) of 1.91 × 106 Da, a weight-average molecular weight (Mw) of 3.46 × 106 Da, and a polydispersity index (PDI) of 1.81. A kinetic analysis of the polymerization reveals that the reaction orders for MMA, BPA, and nano-Cu concentration are 0.76, 0.49, and 0.77, respectively. The activation energy of the polymerization of MMA initiated by BPA is calculated to be 59.6 kJ/mol. The molecular weight of PMMA product is determined using gel permeation chromatography (GPC), while the structure of the synthesized PMMA is characterized through proton nuclear magnetic resonance (1H NMR). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) is employed to analyze the precise structure and molecular weight of PMMA. Additionally, the nano-Cu recovered after the polymerization is examined using X-ray photoelectron spectroscopy (XPS). An electron paramagnetic resonance (EPR) spectrometer is employed to detect the reaction intermediate formed during the polymerization. Results from MALDI-TOF, XPS and EPR reveal that the polymerization of MMA proceeds via a radical mechanism, with the combination of nano-Cu and BPA leading to the formation of BPA residual radicals that initiate the polymerization of MMA.

Keywords: Poly(methyl methacrylate), Ultra-high molecular weights, Organic halides, Kinetics, Mechanism

Introduction

Poly(methyl methacrylate) (PMMA) is an important polymer material, typically prepared through radical polymerization [1, 2], ionic polymerization [3, 4], and coordination polymerization [5, 6]. PMMA has garnered significant attention due to its high transparency, low cost, and excellent chemical and mechanical properties. PMMA is widely used in various fields, including optical and electrical materials [7–9], bone cement [10–12], and denture materials [13–15]. The molecular weight significantly affects the physical properties of polymers. For instance, as the molecular weight increases, key characteristics, such as fracture surface energy, tensile strength, and shear modulus, are significantly enhanced [16, 17]. Consequently, developing a simple route for synthesizing ultra-high molecular weight PMMA has garnered considerable attention from scientists.

“Living”/controlled radical polymerization techniques, including reversible addition-fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP), are efficient techniques for producing polymers with tailored structures [18–20]. ATRP and RAFT techniques have been utilized to synthesize ultra-high molecular weight polymers. For instance, PMMA with Mn ranging from 105 to 106 Da was prepared via ATRP and RAFT [21, 22]. However, the synthesis of ultra-high molecular weight PMMA through ATRP and RAFT often necessitates harsh conditions, including high pressure. Additionally, free radical polymerizations such as emulsion polymerization, solution polymerization, and suspension polymerization have been employed to achieve ultra-high molecular weight PMMA. For instance, Ono and colleagues demonstrated an emulsion polymerization approach for preparing PMMA, achieving an Mn of 6.31 × 105 Da [23]. Similarly, Lashgari et al. reported a suspension polymerization method for MMA, yielding PMMA with an Mn of 4.03 × 105 Da [24]. Lyoo et al. reported an Mn of 3.61 × 106 Da for PMMA synthesized through suspension polymerization [25]. These polymerization techniques are effective for producing ultra-high molecular weight PMMA and other polymers. However, the high energy consumption and complex processes required to separate suspending agents or dispersants from the final product remain unavoidable challenges. Improvements in coordination catalysts have enabled the use of various coordination polymerization methods to synthesize high molecular weight PMMA [26–28]. However, these methods typically require being conducted under strict oxygen-free and anhydrous conditions. Moreover, coordination catalysts can be quite costly. Plasma-initiated polymerization has also successfully produced ultra-high molecular weight PMMA [29]. However, the complexity of the mechanism makes it challenging to achieve linear polymers through this approach.

In our previous study, we discovered that Pd nanoparticles (Pd NPs) combined with hydrosilanes can initiate the polymerization of MMA [30]. The activation of Pd NPs facilitates the formation of organosilicon radicals from the hydrosilane, which in turn initiates the polymerization process. The produced PMMA had an Mn of 2.00 × 106 Da. This study reveals that organic halides, such as BPA and EBiB combined with nano-Cu can induce the polymerization of MMA. Similarly to the Pd NPs reaction system, the activation of nano-Cu allows the BPA initiator to generate residual carbon radicals from BPA to initiate the polymerization of MMA. Fortunately, a PMMA with an Mn of 1.91 × 106 Da and an Mw of 3.46 × 106 Da was obtained via this polymerization route. The synthesized PMMA was characterized using 1H NMR and MALDI-TOF. Additionally, X-ray photoelectron spectroscopy (XPS) and electron spin resonance (EPR) were utilized to explore the underlying mechanism.

Experimental section

Materials

Methyl methacrylate (MMA, 99%), ethyl α-bromophenylacetate (BPA, 97%), and nano-Cu (99.9%, 10–30 nm) were purchased from Macklin in China. Ethyl 2-bromopropionate (EBP), ethyl 2-bromoisobutyrate (EBiB) and N-tert-butyl-alpha-phenylnitrone (PBN, > 98%) were supplied by Aladdin in China. 2-bromobutane (C4H9Br, 99%) was acquired from Tansoole (China). The solvents, including methanol (99.5%), tetrahydrofuran (THF, 99%), acetone-d6 (99.9%), and n-hexane (97%), were also provided by Macklin. Before the polymerization, the inhibitor in the monomer was removed by an alumina-packed column.

Analysis and measurements

A gel permeation chromatography (GPC) was equipped with a Waters 1515 isocratic HPLC pump, a Styragel HT5 GPC column, a Styragel HT3 GPC column, and a Waters 2414 refractive index detector to determine the molecular weight of the prepared PMMA. THF was used as the eluent, maintaining a flow rate of 1.0 mL/min and a temperature of 35 °C. A GPC calibration curve was established using PMMA standard polymers with precisely defined molecular weights to calculate the molecular weight of the prepared PMMA. The molecular weight test range of the instrument is approximately 800 Da to 4 × 106 Da. Generally, the PMMA extracted from the reaction tube was first dissolved in tetrahydrofuran. Subsequently, the solution was filtered to remove any particles that could potentially block the chromatographic column. Finally, 40 µL of the filtered solution was withdrawn using a injector for GPC testing. The monomer conversion was measured using the gravimetric method, as demonstrated in the following equation:

graphic file with name 13065_2025_1597_Article_Equ1.gif 1

Here, m0 represents the weight of the slide alone, m1 is the combined weight of the slide and the wet sample, and m2 is the combined weight of the slide and the dry PMMA. The sample collected from the reaction system is placed on a glass slide and subsequently dried in a vacuum oven to determine the mass of the PMMA.

1H NMR was used to analyze the structure of the prepared PMMA. Unlike monomer conversion determination and GPC testing, PMMA requires purification and separation prior to 1H NMR and MALDI-TOF analysis. This step is crucial because lower molecular weight PMMA allows for clearer resolution of the polymer’s end-group structure. Additionally, MALDI-TOF can accurately measure molecular weights up to 10,000 Da. However, its accuracy diminishes when the molecular weight exceeds this value. To obtain a pure PMMA, the low molecular weight PMMA was first dissolved in THF, and then n-hexane was used as a precipitant to separate the product. The PMMA product was subsequently dried in a vacuum oven before conducting the 1H NMR analysis, using acetone-d6 (C3D6O) as a solvent to dissolve the PMMA. Subsequently, the dissolved PMMA was analyzed using a Bruker Avance III 300 MHz spectrometer (Bruker). To determine the precise structure and molecular weight of low molecular weight PMMA, MALDI-TOF was employed in positive ion mode. The accuracy of the MALDI-TOF mass spectrometer was confirmed using peptide standards, with 2,5-dihydroxybenzoic acid (DHB) serving as the sample matrix for characterization.

The nano-Cu collected after the polymerization was examined using XPS under a working voltage of 12 kV and a pressure of 10− 7 Pa. To capture the radical intermediates formed during polymerization, a PBN solution was injected into the reaction system. Subsequently, 60 µL of the mixture was encapsulated in a capillary tube for EPR measurement.

Bulk polymerization of MMA initiated by nano-Cu/organic halides

Typically, a Schlenk reaction tube containing a stirrer and nano-Cu was degassed through eight cycles of vacuum evacuation and nitrogen filling. Under a nitrogen atmosphere, the degassed MMA and a certain amount of initiator were then introduced into the reaction tube. The reaction tube was then placed in an oil bath at an appropriate temperature for polymerization. To explore the kinetics of the polymerization, samples were periodically extracted from the reaction tube to evaluate monomer conversion. To obtain high molecular weight PMMA, polymerization was stopped after a predetermined polymerization time. The resulting PMMA was collected for molecular weight analysis (using GPC) and monomer conversion determination.

Synthesis of low molecular weight PMMA for end group analysis

A Schlenk reaction tube equipped with a stirrer and nano-Cu (40 mg, 0.625 mmol) was degassed through eight cycles of vacuum extraction and nitrogen filling. MMA and BPA ([MMA]: [BPA] = 8.25: 1, VMMA = 1 mL) were then injected into the tube. The reaction tube was transferred to an oil bath (80 °C) for a 3 h polymerization. The resulting samples were diluted with THF and then precipitated with n-hexane. The mixture was centrifuged at 3000 rpm to remove impurities and obtain the PMMA precipitate. Sometimes, to obtain low molecular weight PMMA, fractional precipitation was performed to separate the sample. The PMMA precipitate was dried under a vacuum at 100 °C for 8 h to eliminate monomers and solvents. Ultimately, the dried low molecular weight PMMA was ready for subsequent 1H NMR analysis and MALDI-TOF detection.

Results and discussion

Bulk polymerization of MMA with organic halides/nano-Cu as initiator

To obtain a preferable initiating system, organic halides, including EBiB, EBP, C4H9Br, and BPA, were used to initiate the polymerization (Table 1). The result revealed that EBP was unable to initiate the polymerization at 80 °C (Table 1, Entries 1–3). The polymerizations also failed when using C4H9Br as an initiator (Table 1, Entries 4–6). Successful polymerization was achieved using EBiB as the initiator under a MMA/EBiB/nano-Cu molar ratio of 6.9 × 102: 1: 4.8 at 80 °C (Table 1, Entry 7). However, the monomer conversion was only 5.5%, and Mn (4.33 × 104 Da), which was not satisfactory. The monomer conversion can be improved by increasing the EBiB concentration (Table 1, Entries 7–9). However, it barely reached 19.4%, falling short of a satisfactory value. Compared to EBiB, the polymerization of MMA was successfully achieved by using BPA as an initiator under the MMA/BPA/nano-Cu molar ratio of 8.2 × 102: 1: 5.7 at 80 °C (Table 1, Entry 10). Surprisingly, the monomer conversion improved significantly, reaching 78.1%. Furthermore, the Mn of PMMA reached 5.75 × 105 Da. As shown in Table 1, BPA was a satisfactory initiator for the polymerization (Table 1, Entries 10–12). Increasing the concentration of BPA improved the monomer conversion but caused a decrease in molecular weights (Table 1, Entries 11 and 12). The BPA exhibited the highest initiating activity and produced PMMA with the highest Mn (5.75 × 105 Da). Therefore, the MMA/BPA/nano-Cu system was selected for further study to produce ultra-high molecular weight PMMA. While BPA effectively initiates the polymerization of MMA, it is important to note that its initiation efficiency in this system is relatively low. Given that the polymerization proceeds via bulk polymerization, the viscosity rises rapidly. This high viscosity not only exacerbates the cage effect but also hinders the diffusion of active species and monomers, ultimately diminishing the initiation efficiency. This polymerization is a heterogeneous catalytic process. The initiator must be activated on the surface of nano-copper to initiate the reaction. This requirement may result in an excessively high local concentration of active species, which in turn intensifies the cage effect and further reduces the initiation efficiency.

Table 1.

Bulk polymerizations of MMA initiated by organic halides/nano-Cu a

Entry Initiator MMA/organic halide/nano-Cu (Molar ratio) T
(°C)		 t
(h)		 Conv.
(%)		 Mn
(Da)		 Mw
(Da)		 PDI
1 EBP 4.3 × 102: 1: 3.0 80 6.1 / / / /
2 EBP 4.3 × 102: 2: 3.0 80 6.1 / / / /
3 EBP 4.3 × 102: 4: 3.0 80 6.1 / / / /
4 C4H9Br 5.1 × 102: 1: 3.6 80 6.1 / / / /
5 C4H9Br 5.1 × 102: 2: 3.6 80 6.1 / / / /
6 C4H9Br 5.1 × 102: 4: 3.6 80 6.1 / / / /
7 EBiB 6.9 × 102: 1: 4.8 80 6.2 5.5 4.33 × 104 7.65 × 104 1.77
8 EBiB 6.9 × 102: 2: 4.8 80 6.2 12.9 3.60 × 104 5.64 × 104 1.57
9 EBiB 6.9 × 102: 4: 4.8 80 6.0 19.4 6.68 × 103 1.29 × 104 1.93
10 BPA 8.2 × 102: 1: 5.7 80 5.4 78.1 5.75 × 105 1.69 × 106 2.94
11 BPA 8.2 × 102: 2: 5.7 80 5.4 86.5 4.82 × 105 1.24 × 106 2.57
12 BPA 8.2 × 102: 4: 5.7 80 5.4 91.6 2.95 × 105 6.61 × 105 2.24
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a VMMA = 5 mL; [MMA] = 9.3 mol/L; T = temperature; t = time; Conv. = monomer conversion

Preparation of ultra-high molecular weight PMMA with BPA as initiator

To prepare ultra-high molecular weight PMMA, the effects of BPA concentration, nano-Cu concentration, and temperature on polymerization rate and polymer molecular weight were explored (Table 2). It shows that the polymerization of MMA cannot be achieved using BPA or nano-Cu alone (Table 2, Entries 1, 2). However, with the facilitation of nano-Cu, BPA successfully initiates the polymerization of MMA (Table 2, Entries 3–10). The data presented in Table 2, Entries 3–5, indicate that the molecular weight of synthesized PMMA decreases as the BPA concentration increases. At 80 °C, PMMA was obtained with a monomer conversion of 77.9% and Mn of 1.39 × 106 Da, using an MMA/BPA/nano-Cu molar ratio of 1.6 × 103: 1: 11.4 (Table 2, Entry 3). The effect of nano-Cu concentration on monomer conversion and molecular weight was also examined (Table 2, Entries 6–8). Increasing the nano-Cu concentration can improve the monomer conversion. However, this will lead to a decrease in molecular weight (Table 2, Entries 6–8). This implies that reducing the initiator and catalyst concentrations is conducive to the synthesis of high-molecular-weight PMMA. Comparing the results of Entry 3 with Entry 6, it is evident that a catalyst concentration that is too low does not improve the molecular weight of PMMA. The optimal molar ratio appears to be 1.6 × 103: 1: 11.4 for the ingredients with an MMA/BPA/nano-Cu. Additionally, temperature has a significant influence on monomer conversion and molecular weight (Table 2, Entries 3, 9, 10). Increasing the temperature can improve the monomer conversion. Unfortunately, the increase in temperature will cause a significant decrease in molecular weight. Polymerization was conducted at 70 °C with an MMA/BPA/nano-Cu molar ratio of 1.6 × 103: 1: 11.4 for 12 h (Table 2, Entry 9). The results demonstrate that PMMA with a monomer conversion of 73.3%, PDI of 1.81, and Mn of 1.91 × 106 Da was successfully prepared. These results indicate that the combination of BPA and nano-Cu can provide an effective route to synthesizing ultra-high molecular weight PMMA.

Table 2.

Preparation of ultra-high molecular weight PMMA initiated by BPA/nano-Cu a

Entry Initiator MMA/BPA/nano-Cu
(Molar ratio)		 T
(°C)		 t
(h)		 Conv.
(%)		 Mn
(Da)		 Mw
(Da)		 PDI
1 BPA 1.6 × 103: 1: 0 80 12.0 / / / /
2 BPA 1.6 × 103: 0: 11.4 80 12.0 / / / /
3 BPA 1.6 × 103: 1: 11.4 80 12.0 77.9 1.39 × 106 2.90 × 106 2.09
4 BPA 1.6 × 103: 2: 11.4 80 12.0 85.7 5.92 × 105 1.69 × 106 2.85
5 BPA 1.6 × 103: 4: 11.4 80 12.0 91.3 4.92 × 105 1.25 × 106 2.54
6 BPA 1.6 × 103: 1: 5.7 80 12.0 69.6 1.28 × 106 3.21 × 106 2.51
7 BPA 1.6 × 103: 1: 22.8 80 12.0 86.8 1.13 × 106 2.69 × 106 2.38
8 BPA 1.6 × 103: 1: 34.2 80 12.0 91.7 6.39 × 105 2.36 × 106 3.69
9 BPA 1.6 × 103: 1: 11.4 70 12.0 73.3 1.91 × 106 3.46 × 106 1.81
10 BPA 1.6 × 103: 1: 11.4 90 12.0 93.7 8.95 × 105 2.53 × 106 2.83
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a VMMA = 10 mL; [MMA] = 9.3 mol/L; T = temperature; t = time; Conv. = monomer conversion

Kinetics study of the polymerization of MMA initiated by BPA

The kinetics study was conducted using the MMA/BPA/nano-Cu reaction system (Fig. 1). The findings indicated that the reaction rate was influenced by BPA concentration (Fig. 1a), nano-Cu concentration (Fig. 1b), MMA concentration (Fig. 1c), and polymerization temperature (Fig. 1d). The polymerization rates were improved by increasing the BPA concentration (Fig. 1a). Similarly, the polymerization rate was also improved by increasing the nano-Cu concentration, MMA concentration, and temperature (Fig. 1c and d). The initial polymerization rates presented in Fig. 1a and c were used to determine the reaction order of the polymerization. The initial reaction rate constant (k) was calculated based on the conversion-time data in Fig. 1a. Subsequently, the reaction order related to the BPA concentration was established through an ln(k) - ln[BPA] plot. The reaction orders for nano-Cu concentration and MMA concentration can also be determined using the same method. Ultimately, it was found that the reaction orders related to the concentrations of BPA, nano-Cu, and MMA are 0.49, 0.77, and 0.76, respectively. Similar to the process used for calculating the reaction order, the initial reaction rate obtained from Fig. 1d was applied to determine the activation energy. Finally, the activation energy was calculated to be 59.6 kJ/mol, which was significantly lower than that of some emulsifier-free emulsion polymerizations of methyl methacrylate [31].

Fig. 1.

Fig. 1

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Effects of BPA, nano-Cu, MMA concentration and temperatrure on polymerization. Conditions: (a) [MMA]: [nano-Cu] = 75.2: 1, T = 80 °C, VMMA = 5 mL; (b) [MMA]: [BPA] = 2.06 × 102: 1, T = 80 °C, VMMA = 5 mL; (c) [BPA]: [nano-Cu] = 1: 2.74, T = 80 °C, VMMA + Vtoluene = 5 mL; (d) [MMA]: [BPA]: [nano-Cu] = 2.06 × 102: 1: 2.74, VMMA = 5 mL

Regrettably, the deviation from a linear growth relationship between Mn and monomer conversion, coupled with a relatively broad molecular weight distribution, indicates that the polymerization does not possess the characteristics of living polymerization such as anionic polymerization (Fig. 2a). The PMMA samples withdrawn at different monomer conversions were analyzed by GPC (Fig. 2b). The results show that the GPC elution curves did not shift to a higher molecular weight region, implying that the molecular weight did not increase with the increase in monomer conversion. This also suggests that the polymerization lacks controllability. As shown in Table 2, the synthesized high and ultra-high molecular weight PMMA was also analyzed by GPC, with three representative GPC elution curves presented in Fig. 2c.

Fig. 2.

Fig. 2

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(a) Molecular weight evolution with conversion for MMA bulk polymerization with BPA as an initiator in the presence of nano-Cu; (b) GPC elution curves of PMMA obtained at different polymerization times; (c) Representative GPC elution curves of high molecular weight PMMA. Conditions: (a), (b) [MMA]: [BPA]: [nano-Cu] = 3.0 × 102: 1.46: 1, VMMA = 5 mL, [M] = 9.3 mol/L; (c) The polymerization condition was presented in Table 2

Structure analysis of the low molecular weight PMMA

To determine the structure of the synthesized PMMA, a sample with a low Mn of 2862 Da, as determined by GPC was subjected to 1H NMR analysis (Fig. 3). The spectrum in Fig. 3 reveals that peaks c, d, and e represent protons of the methylene (-CH2-), methyl (-CH3), and methoxy (-OCH3) groups in the PMMA repeating units, respectively [32]. Additionally, peaks a and b correspond to the phenyl (-C6H5) and methylene (-CH2-) protons of the BPA initiator residue. The proton peaks of the PMMA macromolecular chain cover the other proton peaks of BPA initiator residue. This implies that the BPA initiator residues were successfully attached to the end of the PMMA macromolecular chain, and BPA did play the role of an initiator. Furthermore, as referenced in the literature [33, 34], the small peak f with δ at 5.54 and 6.14 ppm was from the proton of the vinylidene group of PMMA. This observation suggests that the polymerization may have been terminated by disproportionation and that some synthesized PMMA exhibits a terminal vinylidene structure. The molecular weight of PMMA, calculated from the integral ratio of peaks b to e is 2564 Da, which does not agree well with the molecular weight (2862 Da) measured by GPC. It is unavoidable for the existence of molecular weight error obtained from 1H NMR. Therefore, it is essential to characterize the precise structure of PMMA with accurate instruments.

Fig. 3.

Fig. 3

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1H NMR analysis result of low molecular weight PMMA

The precise structure of the synthesized PMMA was analyzed using MALDI-TOF, as illustrated in Fig. 4. A distinct set of ion peaks with m/z values of 3088.72, 3188.72, 3288.75, 3388.76, and 3488.79 (Fig. 4a) was detected. According to the literature [35], these values correlate with the ion peaks of the Na+ adducts of structure 1 and structure 2 (Fig. 4c). Upon magnifying the spectrum, a small number of additional ion peaks were observed (Fig. 4b). Notably, the observed main ion peaks with m/z values of 2788.66 and 2888.68 also correspond to the Na+ adducts of structure 1 and structure 2. Furthermore, the ion peaks with m/z values of 2804.66 and 2904.67 correspond to the K+ adducts of structures 1 and 2. The ion peaks with m/z values of 2750.65 and 2850.64 correspond to the Na+ adducts of structure 3. This suggests that approximately three different polymer structures have been formed during the polymerization of MMA. Structures 1 and 2 represent the disproportionation termination products of the PMMA macromolecular growth chain. Conversely, structure 3 corresponds to the combination termination product of the PMMA macromolecular chain.

Fig. 4.

Fig. 4

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Full MALDI-TOF result of PMMA (a); Partially zoomed MALDI-TOF result of PMMA (b); Structures of prepared PMMA (c)

Mechanism study of the polymerization of MMA initiated by BPA

The polymerization of MMA can be achieved through a radical mechanism or an anionic polymerization mechanism. As shown in Fig. 2, the polymerization in this work does not possess the characteristics of living polymerization. Since anionic polymerization is a typical living polymerization, the polymerization in the present study should not proceed via an anionic polymerization mechanism. To elucidate the specific mechanism at play, the active species during the polymerization initiated by BPA/nano-Cu were analyzed using EPR, with results shown in Fig. 5. A clear radical signal was observed, which, according to the literature [36], corresponds to a carbon-centered radical signal trapped by PBN. This result indicates that this polymerization occurs via a radical mechanism, and the active species in the polymerization is the carbon-propagating radical of the PMMA chain.

Fig. 5.

Fig. 5

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EPR spectra obtained at reaction time of 1.5 h. Conditions: [MMA]: [BPA]: [nano-Cu] = 2.06 × 102: 1: 2.74, T = 80 °C, VMMA = 5 mL, [MMA] = 9.3 mol/L

In the later stage of the polymerization, a pale green color appeared in the reaction system, indicating that some nano-Cu had changed into monovalent copper ions (CuI). To investigate this phenomenon, the nano-Cu was collected after the polymerization and then characterized using XPS (Fig. 6). Figure 6a reveals that peaks belonging to Cu element and Br element were both detected in the samples. The spectral peaks of Cu 3p and Br 3d were observed in Fig. 6b. Additionally, two prominent peaks were observed in Fig. 6c, where the binding energies of 952.3 eV and 932.5 eV correspond to the peaks of Cu 2p1/2 and Cu 2p3/2, respectively [37]. It is important to note that the difference in binding energy between Cu0 and CuI is minimal. Therefore, the peaks with a binding energy of 952.3 eV and 932.5 eV originate from Cu0 or CuI. To further investigate the presence of copper element as Cu0 or CuI, the peaks of Cu LMM were characterized (Fig. 6d). This reveals the presence of Cu0 and CuI on the surface of nano-Cu. The peak at a binding energy of 571.1 eV is consistent with CuIBr [38], while the peak at 568.0 eV represents Cu0.

Fig. 6.

Fig. 6

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XPS result of nano-Cu with a binding energy from 0 to 1100 eV (a); XPS result of nano-Cu with a binding energy from 63 to 77 eV (b); XPS result of nano-Cu with a binding energy from 926 to 965 eV (c); Auger’s energy spectrum of nano-Cu (d)

Consequently, according to the 1H NMR, EPR, MALDI-TOF, and XPS results, a speculative polymerization mechanism is proposed (Fig. 7). Initiating free radicals can be obtained through a one-electron transfer from metal (such as Raney metals) to the carbon-halogen bond of organic halides [39, 40]. This result indicates that the MMA adsorbed on the surfaces of the nano-Cu could be initiated by the BPA residual carbon radicals, which formed owing to one-electron transfer from nano-Cu to the C-Br bond of BPA. Subsequently, a chain propagating process occurred around the surface of nano-Cu due to the additional reaction between chain propagating radicals and MMA. It appears that the chain termination reaction and chain transfer reaction were somewhat suppressed due to the protection provided by nano-Cu, allowing for the easy synthesis of ultra-high molecular weight PMMA in this polymerization system. The electron transfer process from nano-Cu to BPA was also confirmed by XPS (Fig. 6c and d), resulting in the formation of CuI on the surface of nano-Cu. The polymerization was mainly terminated by disproportionation reaction, generating a vinyl-terminated PMMA (structure 1 in Fig. 4) and an end-saturated PMMA (structure 2 in Fig. 4). The polymerization could also be terminated via a combination reaction, resulting in the formation of structure 3 as shown in Fig. 4.

Fig. 7.

Fig. 7

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The postulated mechanism of the polymerization of MMA initiated by BPA in the presence of nano-Cu

Conclusions

This study explores the polymerization of MMA initiated by combining nano-Cu with organic halides. Ultra-high molecular weight PMMA with an Mn of 1.91 × 106 Da, Mw of 3.46 × 106 Da, and PDI of 1.81 was synthesized by using BPA as an initiator. The orders of polymerization for MMA, BPA, and nano-Cu concentration were determined to be 0.76, 0.49, and 0.77, respectively. Mechanism studies showed that a one-electron transfer from nano-Cu to the C-Br bond of BPA results in the formation of BPA residual carbon radicals to initiate the polymerization of MMA. The disproportionation reaction mainly terminates the polymerization. Additionally, the polymerization can also be terminated by a combination reaction.

Acknowledgements

The authors would like to express their gratitude to EditSprings (https://www.editsprings.cn) for the expert linguistic services provided.

Author contributions

Ming Yuan: Conceptualization, methodology, writing– review and editing, funding acquisition, project administration. Shengrong Guo: Funding acquisition, project administration. Xiaofang Han: Investigation. Zekai Zhang: Writing - original draft. Ruijia Wang: Supervision.

Funding

The work was supported by the Lishui Science and Technology Bureau (No. 2024GYX05 and No. 2023KJTP07).

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Ming Yuan, Email: ym@lsu.edu.cn.

Shengrong Guo, Email: guosr9609@lsu.edu.cn.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

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