# Surfactant-free synthesis of polyethylene nanoparticles: toward more realistic model nanoplastics

Lea Jacquin, Edgar Espinosa, Pierre-Yves Dugas, Muriel Lansalot, Elodie

Bourgeat-Lami, Vincent Monteil, Fabrice Brunel

► To cite this version:

Lea Jacquin, Edgar Espinosa, Pierre-Yves Dugas, Muriel Lansalot, Elodie Bourgeat-Lami, et al. Surfactant-free synthesis of polyethylene nanoparticles: toward more realistic model nanoplastics. Polymer Chemistry, In press, 10.1039/D5PY01074H. hal-05469572

HAL Id: hal-05469572

https://hal.science/hal-05469572v1

Submitted on 21 Jan 2026

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ARTICLE

# Surfactant-free synthesis of polyethylene nanoparticles: toward more realistic model nanoplasics

Lea Jacquinª, Edgar Espinosaª, Pierre-Yves Dugasª, Muriel Lansalotª, Elodie Bourgeat-Lamiª, Vincent Monteilª*, Fabrice Brunelª*

Nanoplastics have recently been identified across diverse environmental compartments and are now considered a new class of emerging pollutants. Among them, polyethylene (PE) is the most predominant polyolefin detected in both micro- and nanoplastic forms. Accurate environmental and toxicological assessment of PE nanoplastics requires representative model particles; however, their development remains challenging due to PE's semi-crystalline nature, which hinders top-down fabrication, and the gaseous state of its monomer, which complicates bottom-up synthesis involving high pressure and temperature. In this study, we report the surfactant-free radical aqueous emulsion polymerization of ethylene as a viable route to synthesize model PE nanoparticles with tunable surface charges (anionic or cationic) and narrow size distributions (50-150 nm, extended up 200 nm via seeded polymerization). To mimic oxidative environmental aging, copolymer nanoparticles incorporating polar monomers (vinyl acetate or carbon monoxide) were also produced. These materials exhibit variable surface charges, particle sizes, and carbonyl indices, offering structurally and chemically relevant models for investigating the environmental behavior and toxicity of PE-based nanoplastics.

## Introduction

Thanks to their many advantages (low cost, lightweight, ease of production), plastics have become ubiquitous in our society. The estimated production of plastics in 2022 was 400 MT, a figure that is expected to keep growing.¹ Primarily due to poor waste management, plastics end up in terrestrial environments, from where they are carried by rivers and streams, eventually reaching the seas and oceans² where they can interact with the environment. Microplastics, defined as plastic fragments smaller than 5 mm, a definition formalized in 2009 by the National Oceanic and Atmospheric Administration (NOAA)³, were first observed in North America as spherules along the New England coast in the 1970s by Carpenter *et al.*⁴. The term ‘microplastic’ was later introduced by Thompson in 2004.⁵ Primary microplastics originate directly from industrial production, such as pellets and microbeads, while secondary microplastics result from the fragmentation of larger plastics such as macro- and mesoplastics. Under the combined action of photo-oxidation,⁶ mechanical⁷ and biological forces,⁸ microplastics degrade into increasingly smaller pieces. Nanoplastics, which are defined as plastic particles smaller than 1 µm with colloidal behaviour,⁹ were first detected in the North Atlantic gyre,¹⁰ highlighting a major issue of plastic contamination in the environment.

Currently, micro- and nanoplastics are found in various environmental compartments such as compost and soil,¹¹⁻¹³ air and atmosphere,¹⁴,¹⁵ freshwater¹⁶ and marine environments,¹⁷ drinking water,¹⁸ and food.¹⁹ There is a growing interest in understanding the impacts of microplastics on flora and fauna. Due to their small size, micro- and nanoplastics are likely to be easily introduced into living organisms through ingestion, inhalation, or skin contact.²⁰ Nanoplastics may exert even more pronounced toxic effects due to their ability to interact with biological membranes,²¹ induce cytotoxicity and genotoxicity,²² and disrupt metabolic processes. They have often been considered merely as an extension of microplastics. However, their properties, which vary with size, set them apart from microplastics in several key aspects, including transport behavior, interactions with light and natural colloids, analytical challenges,²³ bioavailability, potential toxicity,²⁴ and the leaching times of additives.²⁵,²⁶

To gain deeper insights into these impacts and optimize analytical procedures, it is essential to utilize model particles of varying sizes and polymer types.²⁷ Current biological and physicochemical studies on nanoparticles (NPs) and microparticles (MPs) predominantly use commercially available or easily synthesized polystyrene (PS) particles.²⁸,²⁹ However, polyethylene (PE) is the most environmentally relevant polymer, comprising 30 % of European demand in 2019¹ and representing 90 % of plastic waste due to its extensive production and exceptional chemical resistance. The synthesis of model PE nanoparticles is particularly challenging due to the gaseous nature of the monomer, which complicates bottom-up synthesis, and the polymer's semi-crystalline structure and poor solubility in solvents, which make top-down approaches difficult. Top-down methods, like grinding techniques, have been used to reduce particle size to the submicron range (100-200 µm).³⁰,³¹ However, the low glass transition temperature of PE (-100 °C to -70 °C) necessitates cryogenic conditions, making these methods costly and technically demanding. More recently, dissolution of PE in hot toluene followed by emulsification in water has enabled the production of nanoparticles and microparticles with diameters between 500 nm and 5 µm.³²,³³ To enhance particle production, surfactants such as Tween or biosurfactants can

<footnote>ª University Claude Bernard Lyon 1, CP2M, UMR 5128, CNRS, 69616 Villeurbanne,
France</footnote>

<footnote>* E-mail: fabrice.brunel@univ-lyon1.fr; vincent.monteil@univ-lyon1.fr
Supplementary Information available</footnote>
be added. However, commercial polyethylene granules used in top-down approaches often contain unknown additives, posing challenges for toxicity studies. Additionally, particles produced this way typically display broad particle size distribution, with sizes rarely below 500 nm.

To obtain small nanoparticles, a bottom-up approach is necessary starting with the polymerization of gaseous ethylene monomer in water to provide dispersed PE NPs. Radical polymerization of ethylene leads to a highly branched macromolecular architecture, resulting in low-density polyethylene (LDPE), whereas catalytic polymerization yields a more linear polymer chain typical of high-density polyethylene (HDPE). These architectural variations strongly affect the polymer physicochemical properties such as density and degree of crystallinity, and may also impact their environmental fate and behaviour, particularly their buoyancy and photo- oxidative degradation. However, the catalytic polymerization of ethylene in water is strongly restricted by the catalyst system employed, making the synthesis of HDPE nanoparticles particularly challenging.³⁴ The high water sensitivity of commercially used Ziegler-Natta or Phillips catalysts for polyolefin production has motivated the development of new late transition metal-based catalysts. Neutral nickel(II) complexes with bidentate ligands³⁵,³⁶ have been shown to produce stable, low molar mass polyethylene latexes. To obtain higher molar mass PE, Bauers and Mecking employed a miniemulsion process using a nickel-based catalyst in toluene droplets with surfactants such as sodium dodecyl sulfate, Lutensol AT80, and Triton X-100, yielding particles with diameters ranging from 90 to 350 nm.³⁷ The main limitation of these particles for subsequent toxicity studies is the presence of residual metal traces.

Ethylene free radical emulsion polymerization provides an alternative approach, particularly relevant since LDPE is widely employed in plastic packaging and constitutes one of the predominant forms of plastic waste in the environment. Early pioneering studies already demonstrated the feasibility of aqueous ethylene polymerization, but these approaches were generally limited by harsh reaction conditions.³⁸⁻⁴² Based on these foundations, we successfully produced PE nanoparticles from free radical emulsion polymerization under relatively mild ethylene pressure and temperature conditions. The use of either anionic or cationic surfactants allowed the formation of nano-sized particles (<100 nm) with controlled surface charges and relatively high polymer contents (~30 wt%).⁴³⁻⁴⁵ However, the presence of surfactants raises concerns regarding desorption and potential toxicity. The novelty of our work lies in the adaptation and optimization of the synthesis method for environmental applications, enabling the production of more realistic model PE NPs without surfactants. Implementation of a seeding strategy enabled access to a wider range of particle sizes, producing PE particles with diameters above 200 nm, sizes that, to our knowledge, have not previously been achieved with this synthetic route. In addition, to mimic the introduction of polar functionalities generated during environmental photo-oxidation and to enhance the degradability of polyethylene, ethylene was copolymerized with selected polar comonomers, vinyl acetate (VAc) and carbon monoxide (CO). The introduction of these polar monomers allows, on the one hand, control over particle size dispersity (secondary nucleation in the presence of VAc) and, on the other hand, tuning of the final particle morphology (non-spherical and core–shell structures with CO).

# Experimental section

## 1. Materials

Unless otherwise stated, all the reagents were used as received: 2,2'-Azobis[2-methyl-N-(2-hydroxyethyl)propionamide] (VA 086, Fujifilm Wako, 98%), ammonium persulfate (APS, Aldrich, 99%), 2,2'-Azobis[2-methylpropionamidine) dihydrochloride] (AIBA, Aldrich, 97%), sodium hydrogenocarbonate (NaHCO₃, ACS reagent grade, Aldrich, 99%), Disponil® A 3065 (Cognis), ethylene (Air liquide, 99%), carbon monoxide (CO, Air liquide, 99%), and vinyl acetate (VAc, Aldrich, 99%) was purified on an inhibitor remover column (Aldrich, 17 mm × 240 mm, ref 306312). Water was deionized before use (Purelab Classic UV, Elga LabWater).

## 2. Nanoparticle synthesis

*Caution, all polymerizations involve high pressure, explosive and toxic gas.*

### 2.1. Polyethylene nanoparticle synthesis

An aqueous solution of the initiator and pH buffer was first degassed for 30 min under a continuous argon flow. Simultaneously, a 160 mL stainless steel reactor (Parr Instrument Co.) was preheated under an inert argon atmosphere. The degassed solution was then injected into the reactor through a cannula under argon flow, followed by the introduction of ethylene, which marked the beginning of the reaction. Stirring, temperature and ethylene pressure were maintained constant throughout the entire reaction. At the end of the desired reaction time, the reactor was cooled down with water and slowly degassed to atmospheric pressure. The resulting latex was collected and various characterizations were carried out, either directly on the latex or on the dry polymer obtained after water evaporation.

### 2.2. Seeded experiments

For seeding experiments, the initial latex was produced following the procedure described in the previous section. It was then diluted in deionized water to reach the desired initial polymer content and the initiator and pH buffer were added. This dispersion was then degassed for 30 min under a continuous argon flow and injected into the high-pressure reactor through a cannula under argon flow. Ethylene was then introduced in the reactor, which marked the beginning of the reaction. Stirring, temperature and ethylene pressure were maintained constant throughout the reaction. The resulting secondary latex was collected as described above, and could then be diluted again. Initiator and pH buffer were again added, and a tertiary latex could be formed. This process could be repeated several times. All the latexes and PE polymers were characterized as described in section 3.
## 2.3. Synthesis of ethylene-based copolymer nanoparticles

For the copolymerization of ethylene with vinyl acetate (VAc), the procedure was identical to that described in section 2.1, except that the comonomer was added to the aqueous solution containing the initiator and pH buffer after degassing under argon and before introduction into the reactor.

For the copolymerization of ethylene with carbon monoxide, the procedure was identical to that described in section 2.1, except that carbon monoxide was first introduced at the desired pressure after the introduction of the aqueous solution containing the radical initiator and pH buffer. Ethylene was then rapidly added until reaching the desired pressure, marking the start of the reaction.

# 3. Latex and polymer characterizations

## 3.1. Dynamic light scattering

Hydrodynamic particle diameter ($Z_{ave}$) and the polydispersity index (PDI) was measured by dynamic light scattering (DLS) at 20 °C, using a Malvern Zetasizer Nano ZS equipped with a 633 nm laser and a detector set at an angle of 173°. A drop of latex was diluted in deionized water filtered through a 0.45 µm nylon syringe filter, then the dispersion was placed in a polystyrene cuvette.

From the DLS data, the number of particles ($N_p$, in mL⁻¹) was estimated as follows:

$$N_p = \frac{6\tau}{\pi Z_{ave} d 10^{-21}} \quad (\text{mL}^{-1}) \quad (1)$$

with $\tau$, the particle concentration (g mL⁻¹), $Z_{ave}$, the hydrodynamic particle diameter (nm) and d, the polymer density (g cm⁻³). The average density of an LDPE is estimated at 0.86 g cm⁻³.

## 3.2. Zeta potential

Particle stability was determined by measuring the zeta potential using Malvern Zetasizer Nano ZS. The nanoparticles were diluted in deionized water and the pH was kept the same as during the synthesis (pH between 7.8 and 8). The zeta potential is calculated from the Smoluchowski equation as follows:

$$\zeta = \frac{3\eta U}{2\epsilon_0 \epsilon_r} \quad (\text{mV}) \quad (2)$$

with, $\eta$ the solution viscosity (m² s⁻¹), U the electrophoretic mobility (m² V⁻¹ s⁻¹), $\epsilon_0$ the vacuum permittivity (F m⁻¹) and $\epsilon_r$ the relative permittivity of the medium (F m⁻¹). A high absolute zeta potential value (± 30 mV) generally indicates good colloidal stability.

## 3.3. Gravimetric analysis

Gravimetric analysis was used to determine the solid content (SC) of the latex. A known mass of latex was dried for 24 h in an oven at 100 °C to evaporate the water. The solid content is the ratio of dry polymer mass ($m_{dry}$) to latex mass ($m_{lates}$).

$$SC = \frac{m_{dry}}{m_{latex}} \times 100 \quad (\text{wt}\%) \quad (3)$$

The polymer content (PC) was calculated from the solid content subtracting the salt content ($X_{salts}$), which corresponds to the mass of solids introduced initially, including the initiator, the pH buffer and the surfactant, which do not evaporate during the latex drying step.

$$PC = SC - X_{salts} \quad (\text{wt}\%) \quad (4)$$

## 3.4. Transmission electronic microscopy

Transmission electron microscopy (TEM) analyses were carried out at the CTμ (Centre Technologique des Microstructures, platform of the Université Claude Bernard Lyon 1, Villeurbanne, France) on the JEOL 1400Flash at 120 kV. The sample was diluted in deionized water, then 4 µL were deposited on a copper grid covered with a 200-mesh Formvar film. The grid was left in the open air to allow the solvent to evaporate for at least 24 h before microscopic observation.

For cryo-TEM analysis, the diluted latex samples were dropped onto 300 mesh holey carbon films (C-Flat 2/1) and immediately quench-frozen in liquid ethane using a cryo-plunge Vitrobot (ThermoFisher). The grid was then mounted on a precooled Fischione 2550 specimen holder and transferred to a JEOL 1400 Flash microscope and observed in low dose condition with a Gatan Rio16 camera at an accelerating voltage of 120 kV.

From the microscopy images obtained, a particle diameter can be measured using Fiji software, which associates the number of pixels with a metric measurement. Particle diameters can be averaged by number ($D_n$) or weight ($D_w$) and a dispersity of the size distribution ($D_w/D_n$) can be deduced. The closer the dispersity value is to 1, the narrower the particle size distribution.

$$D_n = \frac{\sum_i n_i \times d_i}{\sum_i n_i} \quad (\text{nm}) \quad \text{and} \quad D_w = \frac{\sum_i (n_i \times d_i^3)}{\sum_i (n_i \times d_i^2)} \quad (\text{nm}) \quad (5)$$
### 3.5. Synchrotron-small angle X-ray scattering measurements

The characterization of PE nanoparticles also involved conducting small angle X-ray scattering (SAXS) experiments at SOLEIL synchrotron facility on SWING beamline in BioSAXS or ChemSAXS configuration. A wavelength of $\lambda = 1.03$ Å (12.04 keV) was used for these experiments. Measurements included the scattering intensity of the deionized water or buffer references, which were always measured before the sample. This allowed to subtract the scattering contributions arising from various sources, such as the solvent, the air, the quartz capillary walls (2x10 µm) and instrument windows, that are present while measuring the primary beam without the sample. The water scattering and its known isothermal compressibility were used to calibrate the signal in absolute scale of X-ray scattering (cm⁻¹). Data analysis was conducted using SASfit software. Most of the curves fit very well with a spherical form factor (up to seven oscillations are visible), which is consistent with the very low polydispersity of our particles. The model used is the same as in our previous SAXS analysis of these nanoparticles.[45] We only added a thin interfacial layer, which does not affect the extracted particle radius. The scattered intensity ($I_{\text{LinShell}}$) of a sphere with a core radius R and a shell thickness $\Delta R$ is given by:

$$I_{\text{LinShell}} = [(\eta_c - \eta_{\text{sol}} - b)F_{\text{sph}}(QR, R) - mF_{\text{shlin}}(QR, R) + mF_{\text{shlin}}(Q(R + \Delta R), R + \Delta R) + bF_{\text{sph}}(Q(R + \Delta R), R + \Delta R)]^2 \quad (6)$$

With the form factor of the sphere ($F_{\text{sph}}$) and the shell ($F_{\text{shlin}}$) being defined as:

$$F_{\text{sph}}(A, x) = \frac{4}{3}\pi x^2 3 \frac{\sin(A) - A\cos(A)}{A^3} \quad (6a)$$

$$F_{\text{shlin}}(A, x) = 4\pi x^4 \frac{2\cos(A) + 2A\sin(A) - A^2\cos(A)}{A^4} \quad (6b)$$

$$\Delta\eta(r) = \begin{cases} \eta_c & \text{for } r < R \\ mr + b & \text{for } r \in [R, R + \Delta R] \\ \eta_{\text{sol}} & \text{for } r > R + \Delta R \end{cases} \quad (6c)$$

$$m = \frac{(\eta_{\text{shout}} - \eta_{\text{sh}})}{\Delta R} \quad \text{and} \quad b = -mR + \eta_{\text{shout}}$$

With $\eta_{\text{sh}}$ the scattering length density of the pure shell material:

$$\eta_{\text{sh}} = (1 - x_{\text{sol}})\eta_{\text{sh}} + x_{\text{sol}}\eta_{\text{sol}} - \eta_{\text{sol}}, \quad \text{the scattering length density at R,} \quad (6d)$$

$$\eta_{\text{shout}} = (1 - x_{\text{out,sol}})\eta_{\text{sh}} + x_{\text{out,sol}}\eta_{\text{sol}} - \eta_{\text{sol}}, \quad \text{the scattering length density at R + } \Delta R, \quad (6e)$$

and $\eta_c$ the scattering length density of the core.

With:

* R: core radius

* $\Delta R$: shell thickness

* $\eta_c$: scattering length density (SLD) of the core, $\eta_c = 9.25 \times 10^{10} \text{ cm}^{-2}$ [45]

* $\eta_{\text{sh}}$: scattering length density of the non-swollen shell, $\eta_{\text{sh}} = 7.9 \times 10^{10} \text{ cm}^{-2}$ [45] (calculate using the SASfit SLD calculator, taking into account the presence of sulfate charge from the initiator at the interface)

* $\eta_{\text{sol}}$: scattering length density of the solvent, $\eta_{\text{sol}} = 9.44 \times 10^{10} \text{ cm}^{-2}$ [45]

* $x_{\text{sol}}$: fraction of solvent at the core-shell interface, located at radius R and fixed to 0

* $x_{\text{out,sol}}$: fraction of solvent at the outer shell-solvent interface, located at radius R + $\Delta R$ and fixed at 1

Based on the fit results, the diameter of the nanoparticles ($D_{\text{SAXS}}$) is obtained as follow:

$$D_{\text{SAXS}} = 2 \times (R + \Delta R) \quad (7)$$

and the log-normal standard deviation ($\sigma$) characterizes the width of a log-normal size distribution.

For the PE NPs, we additionally applied the Teubner-Strey model to account for the broad correlation peak observed around 0.05 nm⁻¹. This feature arises from the coexistence of crystalline, semi-crystalline and amorphous domains within the particles, leading to strong electron density fluctuations. The crystalline phase is unlikely to be a unique homogeneous entity; rather, it most likely consists of multiple crystalline domains embedded within the amorphous matrix of PE.

$$I(q) = \frac{8\pi\eta\xi}{a^2 - 2abq^2 + q^4} \quad (8)$$
where: $a^2 = (k^2 + 1 \ \xi^2)^2$ is a positive quantity, and $b = k^2 - 1 \ \xi^2$ can be a positive or negative depending on the relative magnitude of $d = 2\frac{\pi}{k}$ and $\xi$. The length scale $d$ represents a quasi-periodic repeat distance between amorphous and crystalline regions within the polymer, while the correlation length, $\xi$, corresponds to a characteristic length for positional correlation.

For nanoparticles obtained from the copolymerization of ethylene with carbon monoxide, a spherical core/shell form factor was used. This model fits the morphology observed in TEM. The scattered intensity ($I_{shell}$) is given by:

$$I_{shell}(Q, R_1, R_2, \Delta\eta, \mu) = [K(Q, R_1, \Delta\eta) - K(Q, R_2, \Delta\eta(1-\mu))]^2 \quad (9)$$

$$\text{with :} \qquad K(Q, R, \Delta\eta) = \frac{4}{3}\pi R^3 \Delta\eta 3 \frac{\sin QR - QR \cos QR}{QR^3} \qquad (9a)$$

The forward scattering at $Q = 0$ is then given by:

$$I_{shell}(Q, R_1, R_2, \Delta\eta, \mu) = \left( \frac{4}{3} \pi \Delta\eta [R_1^3 - R_2^3(1-\mu)] \right)^2 \qquad (10)$$

The input parameters for the spherical core-shell model are defined as follows:

* $R_1$: shell radius

* $R_2$: core radius

* $\Delta\eta$: SLD contrast between the shell and the matrix

* $\Delta\mu$: relative difference in SLD between the core and the matrix normalized to the shell contrast.

In this case, the very broad size distribution of the crystallites was instead fitted using the Debye-Anderson-Brumberger (DAB) model, which describes scattering from a randomly distributed (i.e. non-particulate) two-phase system. The corresponding form factor is given by:

$$I(q) = I_0 \frac{1}{[1 + (q\xi)^2]^2} \qquad (11)$$

### 3.6. Differential scanning calorimetry

Differential scanning calorimetry (DSC) measurements were carried out on a Mettler Toledo DSC 3+. Approximately 10 mg of dry polymer was introduced into a 40 µL aluminium crucible. The sample was subjected to two successive heating cycles (from -80 °C to 160 °C, 10 °C min⁻¹). The first cycle allows the material's thermal history to be erased, and from the second heating cycle onwards, the melting temperature ($T_m$) and possibly the glass transition temperature ($T_g$) are measured. Between these two cycles, a cooling ramp (from 160 °C to -80 °C, 10 °C min⁻¹) recrystallizes the material, and the crystallization temperature ($T_c$) is measured. Based on this analysis, the crystallinity ratio ($X_c$) is calculated as the ratio between the heat of fusion of the polymer ($\Delta H_{f-measured}$), measured by DSC, and the heat of fusion of 100% crystalline polyethylene ($\Delta H_{f-100\%} = 293 \ J \ g^{-1}$).

$$X_c = \frac{\Delta H_{f-measured}}{\Delta H_{f-100\%}} \times 100 \ (\%) \qquad (12)$$

### 3.7. High temperature size exclusion chromatography

Polymer molar mass was determined by high-temperature size exclusion chromatography (HT-SEC) using a Tosoh HLC-8321 GPC/HT system equipped with three ultra-high-temperature TSKgel columns of mixed pore sizes. The exclusion limit of the columns was 4.0 x 10⁸ g mol⁻¹. Samples were prepared at a concentration of 0.5 mg mL⁻¹ in 1,2,4-trichlorobenzene (TCB) containing 0.2 g L⁻¹ BHT to stabilize the mobile phase. The elution flow rate was set at 1 mL min⁻¹ at 150 °C, and on-line detection was performed using a differential refractive index detector. Calibration was performed using polystyrene standards (200 – 13 000 000 g mol⁻¹) and corrected with Mark-Houwink parameters to obtain the true molar masses distribution. The molar masses in number ($M_n$), in weight ($M_w$) and the dispersity ($D = M_w/M_n$) were obtained.

### 3.8. Fourier-transform infrared (FTIR) spectroscopy

Infrared analysis was performed on a Thermo Scientific Nicolet IS50 in ATR mode. Data were processed using Omnic software, and the carbonyl index (CI) calculated using the following formula:

$$CI = \frac{A_{1800-1650cm^{-1}}}{A_{1500-1400cm^{-1}}} \qquad (13)$$

with A the area under the peak.
### 3.9. Nuclear magnetic resonance (NMR)

The mole fraction of VAc in the copolymer was determined from ¹H NMR spectra of the dry polymer, recorded on a Bruker Avance III spectrometer at 400 MHz. Analyses were carried out in a TCE/C₆D₆ (2:1, v/v) solvent mixture at 90 °C using a 5 mm diameter NMR tube. The hydrogen proton in the alpha position of the ester group with a chemical shift of 4.8 ppm, was used as the reference for integration ($I_{ref}$). The peaks between 0.7 and 2.5 ppm, corresponding to the protons of the aliphatic chain and pending group, were integrated ($I_{0.7-2.5}$) to determine the molar fraction of VAc units in the polymer chains ($n_{vac}$):

$$n_{VAc} = \frac{I_{ref}}{I_{ref} + \frac{I_{0.7-2.5} - I_{ref} \times 5}{4}} \quad (mol\%) \quad (14)$$

The VAc conversion rate ($X_{VAc}$) is calculated using the following equation:

$$X_{VAc} = \frac{m_{polymer} \times x_{VAC}^{m}}{m_{VAC}^{v}} \quad (\%) \quad (15)$$

$$\text{With: } m_{polymer} = PC \times V_{tot} \quad (g) \quad (15a)$$

Mass fraction :

$$x_{VAC}^{m} = \frac{n_{VAc} \times M_{VAC}}{(n_{VAc} \times M_{VAc}) + (n_{ethylene} \times M_{ethylene})} \times 100 \quad (\text{wt}\%) \quad (15b)$$

$$\text{Initial mass of VAc: } m_{VAC}^{i} \quad (g)$$

## Results and discussion

### 1. Aqueous emulsion polymerization of ethylene

Surfactant-free radical emulsion polymerization of ethylene was investigated using anionic or cationic initiators, and a summary of the syntheses is presented in Table 1. Regardless of the initiator used, both the polymer content (from 0.2 % up to 3.7 %) and the average particle diameter (from 50 nm up to 150 nm) increase with ethylene pressure. Cationic particles generally have a smaller diameter and a higher polymer content than anionic particles at a given pressure. In both cases, the particle size distributions remain narrow (PDI < 0.1), highlighting the suitability of these nanoparticles for studying the impact of particle size on transport in the environment, within organisms or across cell membranes. Particle stabilization is ensured exclusively by the charges introduced by the initiator, which induce electrostatic repulsion in the absence of surfactants.

At the lower ethylene pressure ($P_{ethylene}$ = 50 bar), differences are observed between the two types of initiator. The anionic particles show a lower zeta potential value, while the cationic ones exhibit a broader size distribution as evidenced by slightly higher PDI. In both cases, the polymers obtained were of low molar masses ($M_n$ = 2 300 g mol⁻¹ for cationic NPs) and displayed low melting temperatures ($T_m$ < 80 °C) (Table 1). The observation of PE nanoparticles by conventional TEM was impeded by particle melting under the electron beam. To overcome this limitation, cryo-TEM was employed (Supporting information, Figure S2 a), revealing particles with non-spherical, ill-defined morphology. This observation was corroborated by synchrotron-SAXS measurements, which confirmed the poorly defined nature of the particles at $P_{ethylene}$ = 50 bar (Figure 1). The SAXS profiles are markedly attuned compared to other samples, showing only two visible oscillations from the spherical form factor. This led to relatively high standard deviations in the particle size distribution ($\sigma$ = 0.15 and 0.18 for anionic and cationic particles, respectively). At low ethylene pressure, the propagation rate is minimal, most likely limited by the low monomer concentration in the aqueous phase, resulting in the synthesis of low molar mass polymers likely below the critical entanglement molar mass of PE (~1000 g mol⁻¹) even after 4 h of reaction (Table 1). This is particularly evident for the samples synthesized using the APS initiator at pressures below 75 bar. Under these conditions, we also observe a reduction in $T_c$, $T_m$, and $X_c$, indicating that these materials deviate from realistic PE chains and instead approach PE-wax behavior.
<table><caption>Table 1: Effect of ethylene pressure on the synthesis of PE NPs using cationic and anionic initiators in the absence of surfactants</caption><thead><tr><th>P<sub>ethylene</sub><br>(bar)</th><th>PC<sup>[a]</sup><br>(%)</th><th>ζ<sup>[b]</sup><br>(mV)</th><th>Z<sub>ave</sub><sup>[c]</sup><br>(nm)</th><th>PDI<sup>[c]</sup></th><th>N<sub>p</sub><sup>[d]</sup><br>(mL<sup>-1</sup>)</th><th>D<sub>n</sub><sup>[e]</sup><br>(nm)</th><th>D<sub>w</sub>/D<sub>n</sub><sup>[e]</sup></th><th>D<sub>SAXS</sub><sup>[f]</sup><br>(nm)</th><th>σ<sup>[f]</sup></th><th>T<sub>m</sub><sup>[g]</sup><br>(°C)</th><th>T<sub>c</sub><sup>[h]</sup><br>(°C)</th><th>X<sub>c</sub><sup>[i]</sup><br>(%)</th><th>M<sub>n</sub><sup>[j]</sup><br>(g mol<sup>-1</sup>)</th><th>M<sub>w</sub><sup>[j]</sup><br>(g mol<sup>-1</sup>)</th><th>D<sup>[j]</sup></th></tr></thead><tbody><tr><td>50*</td><td>0.2</td><td>-20</td><td>53</td><td>0.03</td><td>2.9x10<sup>13</sup></td><td>37</td><td>1.10</td><td>42</td><td>0.150</td><td>63</td><td>53</td><td>14</td><td>500</td><td>5 200</td><td>10.9</td></tr><tr><td>75*</td><td>0.6</td><td>-44</td><td>62</td><td>0.02</td><td>5.7x10<sup>13</sup></td><td>60</td><td>1.03</td><td>57</td><td>0.068</td><td>80</td><td>72</td><td>23</td><td>800</td><td>8 100</td><td>10.4</td></tr><tr><td>100*</td><td>1.7</td><td>-44</td><td>91</td><td>0.02</td><td>5.0x10<sup>13</sup></td><td>60</td><td>1.01</td><td>58</td><td>0.028</td><td>93</td><td>69</td><td>26</td><td>5 300</td><td>14 000</td><td>2.6</td></tr><tr><td>150*</td><td>2.3</td><td>-55</td><td>105</td><td>0.01</td><td>4.4x10<sup>13</sup></td><td>96</td><td>1.02</td><td>91</td><td>0.035</td><td>98</td><td>82</td><td>27</td><td>8 500</td><td>24 400</td><td>2.8</td></tr><tr><td>200*</td><td>3.7</td><td>-49</td><td>155</td><td>0.13</td><td>2.2x10<sup>13</sup></td><td>122</td><td>1.01</td><td>124</td><td>0.039</td><td>101</td><td>83</td><td>35</td><td>6 200</td><td>38 500</td><td>6.2</td></tr><tr><td>50**</td><td>0.3</td><td>34</td><td>40</td><td>0.12</td><td>1.1x10<sup>14</sup></td><td>28</td><td>1.14</td><td>30</td><td>0.179</td><td>76</td><td>66</td><td>15</td><td>2 300</td><td>4 700</td><td>2</td></tr><tr><td>75**</td><td>0.9</td><td>44</td><td>44</td><td>0.07</td><td>2.5x10<sup>14</sup></td><td>46</td><td>1.06</td><td>44</td><td>0.066</td><td>96</td><td>67</td><td>25</td><td>3 600</td><td>8 800</td><td>2.4</td></tr><tr><td>100**</td><td>1.3</td><td>45</td><td>57</td><td>0.03</td><td>1.5x10<sup>14</sup></td><td>47</td><td>1.08</td><td>54</td><td>0.066</td><td>90</td><td>75</td><td>27</td><td>3 700</td><td>10 000</td><td>2.7</td></tr><tr><td>150**</td><td>3.0</td><td>43</td><td>75</td><td>0.04</td><td>1.6x10<sup>14</sup></td><td>58</td><td>1.04</td><td>66</td><td>0.044</td><td>97</td><td>78</td><td>29</td><td>6 400</td><td>17 000</td><td>2.6</td></tr><tr><td>175**</td><td>3.2</td><td>44</td><td>80</td><td>0.04</td><td>1.7x10<sup>14</sup></td><td>67</td><td>1.01</td><td>68</td><td>0.044</td><td>98</td><td>77</td><td>30</td><td>7 400</td><td>17 900</td><td>2.3</td></tr><tr><td>200**</td><td>5.3</td><td>45</td><td>90</td><td>0.02</td><td>1.7x10<sup>14</sup></td><td>67</td><td>1.04</td><td>75</td><td>0.040</td><td>100</td><td>80</td><td>31</td><td>7 600</td><td>26 900</td><td>3.5</td></tr></tbody></table>

Synthesis conditions: *[APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, 75 °C, 250 RPM, 4 h, pH = 7.5 and **[AIBA] = 1 g L⁻¹, 75 °C, 250 RPM, 4 h, pH = 8. [a] Polymer content, determined by gravimetry using equation (4), [b] Zeta potential of particles in deionized water, determined using equation (2), [c] Hydrodynamic diameter and polydispersity index, determined by DLS, [d] Particle number, determined using equation (1), [e] Number-average diameter and associated dispersity, determined by TEM using equation (5), [f] Average diameter and log-normal standard deviation, determined by SAXS using equation (7), [g] Melting temperature, determined by DSC, [h] Crystallization temperature, determined by DSC, [i] Crystallinity, determined by DSC using equation (12), [j] Molar masses in number, in weight and dispersity, determined by HT-SEC.

With increasing ethylene pressure, its solubility in the aqueous phase rises,⁴⁶ which accelerates polymer chain growth and leads to higher molar masses and increased crystallinity. TEM images of particles synthesized at pressures above 50 bar revealed the formation of larger particles with well-defined spherical morphology (Supporting information, Figure S2 b-e). The mean particle diameters determined from TEM or SAXS were slightly smaller than those obtained by DLS. This discrepancy likely arises because DLS provides intensity-weighted averages, with the scattering intensity scaling with the sixth power of particle diameter, while TEM and SAXS give number-average values. Despite this difference, all three techniques (DLS, TEM and SAXS) consistently indicate very narrow particle size distributions.

<figure><img src="image_2.png" /><figcaption>Figure 1: SAXS curves of a) anionic and b) cationic PE NPs obtained by radical emulsion polymerization at different ethylene pressures. Dashed lines represent raw data, while solid lines correspond to fitted curves (a multiplicative factor was applied to the intensity for clarity). Synthesis conditions: a) [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, 75 °C, 250 RPM, 4 h, pH = 7.5 and b) [AIBA] = 1 g L⁻¹, 75 °C, 250 RPM, 4 h, pH = 8.</figcaption></figure>
It is thus possible to obtain PE NPs with diameters ranging from 50 to 150 nm exhibiting remarkably narrow size distributions and either positive or negative surface charges. However, obtaining smaller particles is crucial for assessing their potential nanotoxicity. Alternatively, and quite effectively, 50 nm particles could also be obtained at 100 bar by shortening the polymerization time (Table S1 and Figure S1, Supporting information). Monodisperse particles with such small diameters are not attainable by top-down methods, which generally produce particles no smaller than approximately 200 nm. To extend the range of accessible sizes above 150 nm via bottom-up methods, seeded radical emulsion polymerizations were carried out (Latex 1-4, Table S3 in Supporting information) using an anionic seed latex (Latex 0) with 2 % initial polymer content. The first seeding increased the hydrodynamic diameter from 153 to 197 nm without affecting the PDI indicating uniform particle growth. Subsequent seedings further increase the hydrodynamic diameter with broader size distributions, indicating secondary nucleation and the presence of several particle populations (Table S3). TEM images (Figure 2a-d,) clearly show the emergence of a second particle population of around 50 nm in diameter as early as the first seeding. This secondary nucleation likely arises because the rate of oligoradical formation exceeds their rate of entry into the NPs, causing many oligoradicals to undergo homogeneous nucleation.

<figure><img src="image_3.png" /><figcaption>Figure 2: TEM images of anionic PE NPs obtained by successive seeding of polyethylene latexes (≥ 150 particles counted for size measurement): a-d) Latex 0-3, and e-h) Latex 0'-3'. Synthesis conditions : Latex 1-3 : [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, P ethylene = 100 bar, 75 °C, 250 RPM, 4 h, pH = 7.8, PC initial = 2 %, Latex 0': [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, P ethylene = 200 bar, 75 °C, 250 RPM, 6 h, pH = 7.8 and Latex 1'-3': [APS] = 0.5 g L⁻¹, [NaHCO₃] = 0.5 g L⁻¹, P ethylene = 100 bar, 75 °C, 250 RPM, 4 h, pH = 7.8, PC initial = 4 %.</figcaption></figure>
Thus, with this method, polydisperse PE nanoparticles can be obtained. To avoid secondary nucleation, the initiator concentration was halved and the initial polymer content increased to 4 % (Latex 1'-3', Table S2 in Supporting information). Under these conditions, particle diameters increased progressively with successive seedings (160 nm after one seeding to 208 nm after three seedings), while the PDI remained low (PDI ≤ 0.03). DLS measurements confirmed an unchanged particle number (Np ≈ 2.1×10¹³ mL⁻¹), indicating that no new particles were formed. TEM (Figure 2e-h) and SAXS (Supporting information, Figure S3) corroborated the presence of a single monodisperse nanoparticle population, which grew steadily up to 200 nm. In this series, the lower radical flux combined with higher initial particle concentration increased the probability of radical entry into existing particles, preventing secondary nucleation. However, particle growth was slower: three seedings were required to reach 200 nm, compared with a single seeding in the previous series. During the seeding polymerization, the crystalline phase initially present (formed upon cooling of the seed latex) did not melt (Tpolymerization < Tmelting), as evidenced by the persistence of a broad SAXS peak at ~ 0.7 nm⁻¹. New crystallites generated during each seeding step gave rise to a second, very broad peak, indicative of more polydisperse crystallite domains.

In conclusion, surfactant-free radical emulsion polymerization of ethylene is an effective bottom-up strategy to produce well-defined PE NPs with controlled size (50–200 nm) and tunable surface charge. Unlike top-down methods, it enables precise control over colloidal properties while avoiding surfactants that could bias toxicological studies. Monodisperse particles offer a platform to investigate size-dependent effects, while polydisperse or charged populations can serve as models for environmental nanoplastics. Although negative surface charges better mimic nanoplastics in the environment, positive surface charges remain highly relevant for studying the effect of surface charge on toxicity, as cationic nanoparticles have been shown to interact more strongly with cellular membranes.⁴⁷,⁴⁸ These model nanoparticles have already been employed in a toxicological study on human intestinal cells, underscoring the critical need for well-defined and tunable model PE NPs.²² However, environmental nanoplastics undergo significant weathering, especially photo-oxidation, which alters their surface chemistry and structural integrity. Incorporating artificial aging steps into synthesis protocols is therefore essential to develop even more representative model particles, a point addressed in the following section.

## 2. Aqueous emulsion copolymerization of ethylene with polar monomers

To better mimic environmental nanoplastics, it is necessary to move beyond purely hydrophobic, crystalline polyethylene. Environmentally relevant nanoplastics should ideally be more polar, reflecting aged polyethylene, and more degradable, to reflect their breakdown into oligomers. This requires lowering crystallinity, which limits photodegradation,⁶ and introducing cleavable chemical groups. Two comonomers are particularly relevant for this purpose: vinyl acetate, which introduces ester groups to lower crystallinity, increase material polarity and promote photodegradation via Norrish reactions, and carbon monoxide, which incorporates in-chain ketone groups resembling oxidation products of aged polyethylene. Together, these two approaches enable the design of surfactant-free copolymer nanoparticles with tailored hydrophobicity, crystallinity, and degradability, providing valuable model systems for studying the environmental fate of nanoplastics.

### 2.1. Ethylene/vinyl acetate copolymerization

Aiming to introduce polarity and enhance degradability of PE NPs, the most straightforward comonomer is VAc. Reactivity ratios are favourable to a statistical copolymerization producing semi-crystalline copolymers for low VAc content.⁴⁹ In addition, VAc units in the polyethylene backbone increases sensitivity to UV light and can act as cleavage points, producing acetic acid and an alkene via the Norrish type II reaction. This degradation pathway not only accelerates polymer breakdown but also facilitates the study of small oligomers generated and released during leaching.⁵⁰

Firstly, aqueous dispersions of poly(ethylene-co-vinyl acetate) (EVA) copolymers were obtained by adding small amounts of VAc in the initial medium with the anionic initiator. VAc is a liquid monomer that exhibits a relatively high solubility in water (26 g L⁻¹ for VAc at 50 °C⁵¹ vs. 4.6 g L⁻¹ for ethylene at 110 bar and 55 °C⁴⁶). As shown in Table 2, increasing the VAc concentration from 0.07 to 1.7 mol L⁻¹ leads to a higher final polymer content (up to 9 %) and larger particle diameters (up to 500 nm). However, the slightly concurrent increase in PDI suggests the occurrence of secondary nucleation. This is likely due to faster polymerization, generating oligoradicals that renucleate instead of entering existing particles. TEM images reveal a noticeable polydispersity in particle size (Figure S4, Supporting information). A closer examination of the size distribution histograms indicates the presence of at least two distinct populations, which is further supported by SAXS measurements (see Figure S5 in Supporting Information). The SAXS profiles display well-defined oscillations, characteristic of monodisperse systems for nVAc = 1 mol%. However, the positions of the scattering minima do not fit a single population when the molar fraction of VAc increases, but rather suggest multiple populations. This phenomenon could arise from differences in monomer composition between the aqueous phase, which is richer in VAc, and the polymer particles, which is ethylene-rich. At the start of the polymerization process, oligomers with a high proportion of VAc units are formed in the aqueous phase due to the higher solubility of VAc in water compared to ethylene. These oligoradicals become progressively more hydrophobic, ultimately precipitating as particles. Once formed, these particles act as a hydrophobic medium, promoting further incorporation of ethylene into the polymer and thus amplifying their hydrophobicity. The new oligoradicals, which are less hydrophobic than the particles, will then precipitate on themselves rather than penetrating existing particles. To reduce the formation of multiple particle populations, a series of experiments was carried out, in which the initial VAc concentration was decreased and the ethylene pressure was varied (Table S6, Supporting information). Decreasing the VAc concentration was found to effectively prevent secondary nucleation, as evidenced by TEM (Figure S6 in Supporting information), which shows a single population of particles, irrespective of the pressure applied. This corroborates the hypothesis of a
compositional drift between the aqueous phase and the particles due to the higher water solubility of vinyl acetate compared with ethylene.

<table><caption>Table 2: Influence of VAc concentration on the surfactant-free radical emulsion copolymerization of ethylene and VAc to form anionic EVA nanoparticles</caption><thead><tr><th>C<sub>VAc</sub><br>(mol L<sup>-1</sup>)</th><th>Z<sub>ave</sub><sup>[a]</sup><br>(nm)</th><th>PDI<sup>[a]</sup></th><th>D<sub>n</sub><sup>[b]</sup></th><th>D<sub>w</sub>/D<sub>n</sub><sup>[b]</sup></th><th>PC<sup>[c]</sup><br>(%)</th><th>ζ<sup>[d]</sup><br>(mV)</th><th>n<sub>VAc</sub><sup>[e]</sup><br>(mol%)</th><th>X<sub>VAc</sub><sup>[f]</sup><br>(%)</th><th>T<sub>m</sub><sup>[g]</sup><br>(°C)</th><th>T<sub>g</sub><sup>[h]</sup><br>(°C)</th><th>X<sub>c</sub><sup>[i]</sup><br>(%)</th><th>M<sub>n</sub><sup>[j]</sup><br>(g mol<sup>-1</sup>)</th><th>M<sub>w</sub><sup>[j]</sup><br>(g mol<sup>-1</sup>)</th><th>Φ<sup>[j]</sup></th><th>Cl<sup>[k]</sup></th></tr></thead><tbody><tr><td>0.07</td><td>113</td><td>0.01</td><td>90</td><td>1.05</td><td>1.4</td><td>-57</td><td>1</td><td>6</td><td>80</td><td>-35</td><td>30</td><td>1 500</td><td>11 200</td><td>7</td><td>0.5</td></tr><tr><td>0.12</td><td>116</td><td>0.05</td><td>92</td><td>1.07</td><td>1.6</td><td>-49</td><td>2</td><td>7</td><td>80</td><td>-32</td><td>29</td><td>1 500</td><td>12 700</td><td>8</td><td>0.8</td></tr><tr><td>0.23</td><td>155</td><td>0.06</td><td>112</td><td>1.19</td><td>2.0</td><td>-48</td><td>6</td><td>16</td><td>80</td><td>-31</td><td>28</td><td>1 600</td><td>14 900</td><td>9</td><td>1.6</td></tr><tr><td>0.47</td><td>163</td><td>0.06</td><td>182</td><td>1.57</td><td>3.2</td><td>-53</td><td>14</td><td>28</td><td>65</td><td>-28</td><td>23</td><td>1 900</td><td>17 900</td><td>10</td><td>3.5</td></tr><tr><td>0.69</td><td>185</td><td>0.05</td><td>230</td><td>1.72</td><td>3.7</td><td>-56</td><td>17</td><td>30</td><td>58</td><td>-31</td><td>18</td><td>2 600</td><td>25 400</td><td>10</td><td>3.5</td></tr><tr><td>0.92</td><td>291</td><td>0.14</td><td>560</td><td>1.10</td><td>6.2</td><td>-60</td><td>22</td><td>38</td><td>40</td><td>-28</td><td>7</td><td>2 200</td><td>34 700</td><td>16</td><td>5.3</td></tr><tr><td>1.16</td><td>490</td><td>0.09</td><td>510</td><td>1.32</td><td>8.9</td><td>-23</td><td>27</td><td>53</td><td>38</td><td>-27</td><td>9</td><td>2 100</td><td>43 800</td><td>21</td><td>6.1</td></tr></tbody></table>

Synthesis conditions: [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, P<sub>ethylene</sub> = 100 bar, 75 °C, 250 RPM, 4 h, pH = 7.8. <sup>[a]</sup> Hydrodynamic particle diameter and polydispersity index, determined by DLS, <sup>[b]</sup> Number-average diameter and associated dispersity, determined by TEM using equation (5), <sup>[c]</sup> Final polymer content, determined by gravimetry using equation (4), <sup>[d]</sup> Zeta potential of particles in deionized water, <sup>[e]</sup> Molar fraction of VAc in polymer, determined by ¹H NMR using (14), <sup>[f]</sup> VAc conversion, determined by ¹H NMR using equation (15), <sup>[g]</sup> Melting temperature, determined by DSC, <sup>[h]</sup> Glass transition temperature, determined by DSC, <sup>[i]</sup> Crystallinity rate of polymer, determined by DSC using equation (12), <sup>[j]</sup> Molar mass in number, in weight and associated dispersity, determined by HT-SEC, <sup>[k]</sup> Carbonyl index, determined by FTIR using equation (13).

As expected, increasing the initial VAc concentration proportionally increases the molar fraction of VAc units incorporated into the polymer chains up to 27 mol%. This allows tuning the amount of polar functions in the chains, which can be used to mimic the ageing state of the polymer by increasing its hydrophilicity and thus accelerating its degradation. Increasing the molar fraction of VAc leads to a simultaneous increase in both particle size (from 113 nm to 490 nm) and carbonyl index (from 0.5 to 6.1). Conversely, increasing the ethylene pressure at a fixed VAc concentration results in larger particle sizes but a lower carbonyl index (Table S6, Supporting information). Therefore, it is possible to produce a series of NPs with different sizes and a constant carbonyl index, or vice versa (Table 2 and Table S6). However, generating small EVA NPs (Z<sub>ave</sub> < 100 nm) with high carbonyl content remains challenging. According to the literature, the carbonyl index of polyethylene degraded under environmental conditions ranges from 0.2 to 2, depending on the exposure conditions,<sup>52–54</sup> making nanoparticles with VAc contents below 14 mol% the most representative. Especially since, below this VAc content, the thermal properties remain comparable to those of LDPE (T<sub>m</sub> = 80 °C and 28 % < X<sub>c</sub> < 30 %, (Table 2), which is the model polymer of interest. Beyond 6 mol% of VAc, both melting temperature (from 80 °C to 38 °C) and crystallinity (from 30 % to 9 %) decrease due to ester groups in the side chains that hinder chain rearrangement into a crystalline structure. When the material becomes poorly crystalline, almost amorphous, a slight increase in T<sub>g</sub> is observed, which is expected because the T<sub>g</sub> of poly(vinyl acetate) (PVAc) is higher than that of PE. Consequently, when the VAc content exceeds 6 mol%, the polymer ceases to behave as PE simply modified with polar groups and instead exhibits characteristics of a structurally distinct copolymer. To further characterize these copolymers, the molar masses of EVA NPs were compared to those of PE under identical synthesis conditions and a comparable size (≈ 100 nm). Weight-average molar masses are similar, but EVA exhibits lower number-average molar masses, likely due to water-initiated VAc-rich chains that later recombine. These lower molar masses resemble those of partially degraded PE nanoplastics, highlighting the relevance of EVA NPs as realistic environmental nanoplastic models.
<figure><img src="image_4.png" /><figcaption>Figure 3: FTIR spectra of EVA copolymers incorporating different molar fractions of VAc, obtained by surfactant-free radical emulsion copolymerization of ethylene and VAc. Synthesis conditions: [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, *P*<sub>ethylene</sub> = 100 bar, 75 °C, 250 RPM, 4 h, pH = 7.8</figcaption></figure>

In summary, copolymerizing VAc with ethylene introduces polar groups that enhance PE degradability while also modifying NPs properties. Increasing the initial VAc concentration enlarges particle size, making NPs smaller than 100 nm difficult to obtain, but enabling a broader and more environmentally relevant size range (100–500 nm) with increased polydispersity. At low VAc content, the copolymers remain semicrystalline, similar to polyethylene, whereas at higher levels they exhibit the distinct behaviour of EVA. This transition occurs at around 14 mol% VAc (CI ≈ 3.5), above the range typically reported for environmentally aged nanoplastics.⁵⁴ Below this threshold, the nanoparticles can be considered realistic PE-based models. However, although VAc increases polarity and degradability, its chemistry does not mimic that of photo-oxidized PE. Therefore, CO was selected as an alternative comonomer to introduce in-chain carbonyl groups that more closely reproduce oxidation products of aged polyethylene.

## 2.2. Ethylene/carbon monoxide copolymerization

To further mimic the chemical structure of aged polyethylene, carbon monoxide (CO) was selected to create in-chain keto groups which are particularly well-suited to promote photo-degradation, as they enable chain scission by Norrish type I and II reactions⁶ and more closely resemble the keto groups formed during UV-induced photodegradation of PE.

The synthesis of PE incorporating a small amount of in-chain keto groups (e.g., 1 %wt.) has been recently reported by Mecking and coworkers.⁵⁵ The polymers were prepared by free radical emulsion copolymerization of ethylene and CO, both with and without surfactant. This approach was evaluated in the present work in a surfactant-free system. At 150 bar of ethylene and 1 bar of CO, very small nanoparticles of poly(ethylene-co-carbon monoxide) (ECO) were obtained exhibiting a diameter of 25 nm and a PDI of 0.22 suggesting a relatively broad particle size distribution (Table 3). However, the polymer content remained very low (< 0.1 %). For comparison, in the case of ethylene polymerization carried out at 150 bar, the resulting particles exhibit an average diameter of 105 nm (PDI = 0.01) and a polymer content of 2.3 %. Increasing the ethylene pressure to 200 bar then 220 bar while maintaining $P_{CO}$ = 1 bar, significantly improved the polymer content (up to 0.7 %, see (Table 3)). Under these conditions, the nanoparticle size increased, reaching a diameter of 69 nm with a PDI of 0.04, indicating a narrower and more uniform size distribution. When the CO pressure was increased to 2 bar while maintaining the ethylene pressure at 200 bar, very small nanoparticles ($Z_{ave}$ = 22 nm, PDI = 0.22) were obtained with again low polymer content (0.3 %). This behaviour can be attributed to the retardation of chain growth, likely caused by the stability of the acyl radical intermediate compared to the alkyl radical. This stabilization effect reduces the chain propagation rate, leading to lower polymer yields and consequently smaller particle sizes. These findings are consistent with observations previously reported by Mecking and coworkers, who highlighted similar influences of CO on polymerization and nanoparticle properties.⁵⁵
<table><caption>Table 3: Influence of CO and ethylene pressures on the synthesis of ECO copolymers obtained by surfactant-free radical emulsion polymerization</caption><thead><tr><th>P<sub>CO</sub><br>(bar)</th><th>P<sub>ETH</sub><br>(bar)</th><th>Z<sub>ave</sub><sup>[a]</sup><br>(nm)</th><th>PDI<sup>[a]</sup></th><th>PC<sup>[b]</sup><br>(%)</th><th>ζ<sup>[c]</sup><br>(mV)</th><th>N<sub>p</sub><sup>[d]</sup><br>(mL<sup>-1</sup>)</th><th>T<sub>m</sub><sup>[e]</sup><br>(°C)</th><th>T<sub>c</sub><sup>[f]</sup><br>(°C)</th><th>X<sub>c</sub><sup>[g]</sup><br>(%)</th><th>M<sub>n</sub><sup>[h]</sup><br>(g mol<sup>-1</sup>)</th><th>M<sub>w</sub><sup>[h]</sup><br>(g mol<sup>-1</sup>)</th><th>Φ<sup>[h]</sup></th><th>Cl<sup>[i]</sup></th></tr></thead><tbody><tr><td>0.5</td><td>200</td><td>74</td><td>0.01</td><td>0.8</td><td>-53</td><td>2.7x10<sup>13</sup></td><td>97</td><td>72</td><td>22</td><td>1 200</td><td>51 400</td><td>41</td><td>0.3</td></tr><tr><td>1</td><td>200</td><td>69</td><td>0.04</td><td>0.7</td><td>-50</td><td>4.7x10<sup>13</sup></td><td>83</td><td>70</td><td>26</td><td>800</td><td>29 500</td><td>37</td><td>0.6</td></tr><tr><td>1.5</td><td>200</td><td>42</td><td>0.01</td><td>0.6</td><td>-56</td><td>1.8x10<sup>14</sup></td><td>69</td><td>53</td><td>9</td><td>500</td><td>4 200</td><td>9</td><td>0.9</td></tr><tr><td>2</td><td>200</td><td>22</td><td>0.22</td><td>0.3</td><td>-56</td><td>6.5x10<sup>14</sup></td><td>52</td><td>25</td><td>5</td><td>300</td><td>2 900</td><td>9</td><td>1.0</td></tr><tr><td>1</td><td>150</td><td>25</td><td>0.22</td><td>0.3</td><td>-56</td><td>3.6x10<sup>14</sup></td><td>58</td><td>24</td><td>3</td><td>300</td><td>1 900</td><td>7</td><td>0.8</td></tr><tr><td>1</td><td>220</td><td>62</td><td>0.06</td><td>0.6</td><td>-45</td><td>5.3x10<sup>13</sup></td><td>85</td><td>64</td><td>16</td><td>700</td><td>21 100</td><td>32</td><td>0.6</td></tr></tbody></table>

Synthesis conditions: [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, 75 °C, 250 RPM, 6 h, pH = 7.8. [a] Hydrodynamic particle diameter and polydispersity index, determined by DLS, [b] Final polymer content, determined by gravimetry using equation (4), [c] Zeta potential of particles in deionized water, [d] Particle number determined using equation (1), [e] Melting temperature, determined by DSC, [f] Crystallization temperature, determined by DSC, [g] Crystallinity rate of polymer, determined by DSC using equation (12), [h] Molar mass in number, in weight and associated dispersity, determined by HT-SEC, [i] Carbonyl index, determined by FTIR using equation (13).

Beyond these kinetic effects, the incorporation of carbon monoxide into the polymer chains also modifies the thermal properties of the material, notably by reducing the degree of crystallinity compared with polyethylene obtained under the same synthesis conditions ($X_{C-ECO}$ = 20 % at most, while $X_{C-PE}$ = 30 %). According to the literature, despite the differences in reactivity between alkyl and acyl radicals, radical copolymerization yields a copolymer with randomly distributed ketone functions in the chains, whereas catalysis yields most often alternating copolymers.⁵⁵ Ketone functions present in the polymer chains prevent crystallite formation, like branching originating from transfer reactions does in homopolyethylene. Indeed, Mecking and coworkers reported that polyketones obtained by aqueous emulsion polymerization were much more branched than those obtained in solution, probably due to the confined reaction environment within the particles, which favors transfer reactions.⁵⁵ In addition, the crystallization temperature of ECO copolymers is lower than the synthesis temperature; thus, the copolymers do not form semi-crystalline particles during emulsion polymerization. Instead, they would be in the form of molten polymer particles which crystallize only when the reaction is stopped.⁵⁵ In our case, the rapid cooling at the end of the reaction likely triggers a faster and less controlled crystallization with small and highly polydispersed crystallites than polyethylene particles. This hypothesis appears to be substantiated by the SAXS data, which demonstrate that while the particles exhibit a crystalline phase in DSC, no peak of sufficient sharpness characteristic of the crystallites is observed (Figure S7, Supporting information). The CO content and the crystallinity also strongly impacts the nanoparticle morphology. When the CO pressure was low ($P_{CO}$ = 0.5 bar), the resulting nanoparticles were spherical. However, as the CO pressure increased, this morphology became less defined, as evidenced by cryo-TEM images showing the emergence of core-shell and irregular structures (Figure 4). Since crystalline domains are denser than the amorphous phase, they appear darker on the TEM images and therefore thought to be on the outside of the particle, forming the shell. This structure may be further stabilized by strong intermolecular interactions between carbonyl groups at the particle surface, which would promote local crystallization.

<figure><img src="image_5.png" /><figcaption>Figure 4: Cryo-TEM images of ECO copolymers obtained by surfactant-free emulsion copolymerization of ethylene and carbon monoxide at different CO pressures. a) $P_{CO}$ = 0.5 bar, b) $P_{CO}$ = 1 bar, c) $P_{CO}$ = 1.5 bar, d) $P_{CO}$ = 2 bar. [APS] = 1 g L⁻¹, [NaHCO₃] = 1 g L⁻¹, $P_{ethylene}$ = 200 bar, 75 °C, 250 RPM, 6 h, pH = 7.8</figcaption></figure>

The FTIR spectra of these copolymers are provided in the supporting information, with an inset zooming on the carbonyl region (Figure S8). An increase in the intensity of the characteristic carbonyl band confirms an increasing incorporation of CO into the copolymer, with a parallel decrease in the intensity of the characteristic band of the -CH₂- groups of the aliphatic chains. Moreover, according to the literature, absorption bands at 1 408 and 1 369 cm⁻¹ have been attributed to methylene structures adjacent to a carbonyl.⁵⁶ Carbonyl indices calculated from the areas under the peaks show an increase in Cl with CO pressure, in the range 0.3
to 1, which is comparable to Cls measured for environmental nanoplastics.⁵⁴ The copolymer molar masses are generally below 1 000 g mol⁻¹ (Table 3). They are therefore mainly composed of short chains, but it remains difficult to assess how realistic such systems are. To our knowledge, there is no detailed characterization of environmental nanoplastics in this size range. However, it is plausible that plastic debris eroded to sizes below 100 nm would consist of heavily photodegraded polymers with numerous chain scissions, and thus relatively low molar mass. These samples could therefore be viewed as analogues of oligomeric degradation products present in the environment.

In this copolymerization, the resulting nanoparticles exhibit smaller dimensions compared to those obtained from the homopolymerization of ethylene, with diameters not exceeding 75 nm. Moreover, the particle size distribution remains narrow, facilitating studies of size-dependent effects. The main difference lies in particle morphology: these nanoparticles are slightly less spherical than PE NPs, making them more representative of environmental nanoplastics, which are typically abraded, degraded, and characterized by highly heterogeneous morphologies. Furthermore, these copolymers have lower crystallinity and molar masses compared to PE, while incorporating a realistic oxidation state through the direct introduction of carbonyl groups along the polymer backbone. Notably, PE biodegradation is expected to be faster when the molar mass is below 500 g mol⁻¹,⁵⁷ but in the environment it may take several decades to reach this stage depending on additive composition.⁵⁷ As a result, these nanoparticles with various oxidation states are well-suited for studying late-stage degradation mechanisms, such as enzymatic or metabolic processes, which may affect nanoplastics after a photo-oxidation phase.

Conclusion

This work paves the way for the production of polyethylene model nanoparticles with controlled and environmentally relevant
properties, including size, surface charge, morphology and oxidation state. PE nanoparticles with diameters ranging from 50 to
250 nm were synthesized without any surfactant, providing well-characterized and reliable models for studying the environmental
impact of nanoplastics. These nanoparticles were prepared with tunable surface charges, allowing stable dispersion in aqueous
media, and all particles exhibited a spherical morphology with a remarkably narrow size distribution. Particle diameter could be
precisely controlled by adjusting ethylene pressure, reaction time or by carrying out seeded polymerization to produce larger
particles, providing well-defined systems for investigating size-dependents effects. Successive nucleation events during seeded
polymerization produced multiple particle populations, resulting in more polydisperse batches that better mimic the wide range
of sizes observed in environmental nanoplastics.

To further enhance the realism of these models, polar ester and ketone functionalities were introduced either in side positions or along the polymer backbone to modulate the oxidation state and polarity of polyethylene. Copolymerization with vinyl acetate enabled the incorporation of photosensitive groups into the polyethylene chains, increasing polarity and potentially accelerating its naturally slow degradation. Similarly, copolymerization with carbon monoxide produced particles with broader size distributions and more heterogeneous morphologies. In both cases, varying monomer concentration (i.e., gas pressures) enabled precise control of particle size and oxidation state, producing variable and environmentally relevant carbonyl indices.
The method developed here complements top-down approaches by producing polymer nanoparticles below 200 nm in size, free from additives or surfactants that could interfere with toxicity studies. It also improves on existing bottom-up strategies by producing more polydisperse batches with less spherical morphologies and incorporating features of aged nanoplastics, yielding more realistic environmental models. These nanoparticles provide well-defined systems that could serve in future studies to explore the environmental fate, degradation pathways and potential impact of aged polyethylene and its oxidative by-products.

Author contributions

Lea Jacquin: Investigation and validation, Conceptualization, Formal analysis, Writing – original draft

Pierre-Yves Dugas: Cryo-TEM images, Writing – reviewing

Edgar Espinosa: Investigation, Writing – reviewing

Muriel Lansalot: Conceptualization, Writing – reviewing, Supervision

Elodie Bourgeat-Lami: Conceptualization, Writing – reviewing, Supervision

Vincent Monteil: Conceptualization, Writing – reviewing, Supervision

Fabrice Brunel: SAXS analysis, Conceptualization, Writing – original draft and reviewing, Supervision

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the
manuscript.

Conflicts of interest

There are no conflicts to declare

Acknowledgements

The authors would like to thank the Agence Nationale de la Recherche for the financial support of this project (ANR-21-CE06-0019 POEM), the Centre Technologique des Microstructures at the Université Lyon 1 for microscopic observations and the NMR Polymer
center of Institut de Chimie de Lyon (FR5223), for assistance and access to the NMR characterization. The authors also thank Mr. Aurelien Thureau from the SWING beamline at Synchrotron SOLEIL and Mr. Guillaume Brotons for all fruitful discussions.

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