One-pot strategy for obtaining magnetic PMMA particles through ATRP using Fe(CO)5 as co-initiator

https://doi.org/10.1016/j.eurpolymj.2021.110446Get rights and content

Highlights

  • ATRP process for methyl methacrylate (MMA) polymerization using Fe(CO)5 as co-initiator;

  • The process displays a slow initiation step and bimolecular termination reactions;

  • Production of magnetic PMMA particles through a facile one-pot approach, using only an ammonia treatment stage;

  • The obtained PMMA particles, characterized by SEM, XPS, TEM, and XRD analyses.

Abstract

The first aim of this study was to develop an ATRP process for methyl methacrylate (MMA) polymerization using Fe(CO)5 as co-initiator. The kinetics analysis of the reaction in solution confirmed the controlled characteristic of the polymerization process. The process displays a slow initiation step and presents bimolecular termination reactions at higher molecular weights, confirmed by NMR analysis. Also, these observations are sustained by GPC, FT-IR, and XPS analyses, explaining the rather unusual dispersity values (1.4) for an ATRP reaction. The use of formic acid, which shifts the equilibrium towards Fe2+ species affording a quasi-controlled controlled polymerization process, is a key element in achieving control over the reaction. The Fe2+/Fe3+ ratio at the end of the reaction, determined by XPS analysis, suggests the possibility to generate magnetic iron oxide particles. Therefore, the synthesis strategy was adapted for use in the suspension polymerization process of MMA which permitted the production of magnetic PMMA particles through a facile one-pot approach, using only an ammonia treatment stage. The obtained PMMA particles, characterized by SEM, XPS, TEM, TGA, and XRD analyses, have porous characteristics and magnetic properties which makes them good candidates as catalyst supports, separation processes, or biomedical applications.

Introduction

“Controlled/living” radical polymerization, more properly termed by IUPAC [1] as reversible-deactivation radical polymerization method, allows the design of polymers with controllable composition and microstructures [2], [3]. A variety of different methods have been developed for this purpose including atom transfer radical polymerization (ATRP) [4], [5], [6], [7], reversible addition-fragmentation chain transfer [8], [9], [10], nitroxide-mediated living free-radical polymerization [11], [12], [13], and iodine transfer polymerization [14], [15]. Amongst these methods, ATRP has attracted a large interest due to its features such as controllable polymer molecular weight, narrow molecular weight distribution, adaptability to most monomers, designable molecular structures (i.e. telechelic, poly-brushes, star-shaped, etc.), and the commercial availability of ATRP initiators [16], [17]. Also, the method is suitable in the presence of many functional groups and the high chain end-functionalized resulting polymers, permit facile post-polymerization modifications reactions. The control over the polymerization process in ATRP is based upon the reversibility of the redox process involving the catalyst and initiator which must sustain the efficient activation and deactivation of polymer chain ends.

The catalyst plays a critical role in metal-mediated ATRP, thus a wide range of systems based on copper [4], [18], [19], [20], iron [4], [21], [22], [23], ruthenium [21], [24], [25], [26], other metals [27], or metal-free organic molecules [28], [29] were developed for use in ATRP. The motivation for transition metal use as catalyst resides in the d-orbitals capacity to interact with different types of ligands based on an electron-exchange process. When considering the metal selection, the most explored options are copper and ruthenium. Nevertheless, iron has also received significant attention due to its low toxicity, biocompatibility as well as environmental, sustainability, and cost-effectiveness aspects [22], [23], [30].

The use of free radical polymerization initiation systems based on metal carbonyl-organic halides has been explored since the 1960s [31], [32], [33]. Thus, the use of such initiation systems has been studied for vinyl monomers of which MMA received significant attention. The kinetic analyses of the polymerization indicated that the rate of polymerization is proportional to the square root of the halide concentration and the monomer concentration [33]. More recently, the use of iron carbonyls in different controlled polymerization reactions was explored with good results for the photomediated processes involving vinylidene fluoride [34] butadiene [35].

The removal of the catalytic system from the polymer is an important disadvantage of ATRP processes. Furthermore, a key step for the successful implementation of ATRP is the choice of solvent and ligand used together with the metal salt to obtain a homogenous system or to achieve a good interaction between the components.

The aim of this study consisted in the development of an ATRP process for methyl methacrylate (MMA) using Fe(CO)5 as a co-initiator. The advantage of using Fe(CO)5 source resides in its solubility in organic media, which facilitates the interaction with the R-Br co-initiator, and its purity that guarantees pure iron or iron oxide particles to obtained by its thermal decomposition. As far as we know this is the first example of a Fe0 carbonyl derivative employed for MMA quasi-controlled radical polymerization reaction. Further, this study presents the polymerization both in solution and in dispersed media, which proves the versatility of the approach. Thus, by employing a suspension polymerization procedure using Fe(CO)5 as part of the initiation system, the obtained poly (methyl methacrylate) (PMMA) particles were converted to magnetic PMMA particles by simple processing using an ammonia solution. The synthesis procedure and the obtained materials could find a wide range of applications since the presence of magnetite inside polymer particles by simple/facile processing can be highly desirable considering the high purity of iron source (Fe(CO)5).

Section snippets

Materials

Methyl methacrylate (MMA) (Sigma-Aldrich) has been purified through vacuum distillation. Methanol (Fluka), ethyl alpha bromoisobutyrate (Sigma-Aldrich), 1,4 dioxane (Sigma-Aldrich), dimethyl sulfoxide (Sigma Aldrich), iron pentacarbonyl -Fe(CO)5 (Sigma-Aldrich), formic acid (Acros), poly(vinyl alcohol) (PVA) Mw 130 000 g/mol (99+% hydrolyzed), dimethyl formamide (Sigma Aldrich), tetrahydrofuran (THF) (Sigma Aldrich), ammonia solution 25% (Fluka) were used as received.

MMA solution polymerization

In a three-necked round

Results and discussion

In Scheme 1 are presented the main steps involved in the synthesis procedure developed with the scope of obtaining a controlled polymerization of MMA using an R-Br/ Fe(CO)5 initiation system. The experimental conditions are based on the following considerations (see Scheme 2):

  • (i)

    Fe(CO)5 can undergo thermal or photo-induced decomposition with the generation of reactive Fe0 and Fe2+ species [37], [38];

  • (ii)

    Formic acid can react with Fe0 facilitating MMA initiation principally by Fe2+;

  • (iii)

    Formic acid can act

Conclusions

In conclusion, this study presented a polymerization procedure for the polymerization of MMA using Fe(CO)5 in solution. The kinetic analysis confirmed that through the combined use of Fe(CO)5 and formic acid, a quasi-controlled polymerization process was achieved, with a slow initiation characteristic and a termination step, predominantly by coupling (as confirmed by NMR analysis). The FT-IR analysis confirmed the presence of carbonyl species at short reaction periods explaining the slow

Data availability statement

All data related to the study is available in the manuscript and in the Supplementary information which includes NMR spectra and a video demonstration of PMMA- Sample B response to an external magnetic field.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank prof. Bogdan Mărculescu for the fruitful and insightful discussions.

This work was financially supported by the project Smart polymers obtained by novel photo-ATRP and 3D printing strategies (SmartPhoto-ATRP), PN-III-P1-1.1-TE-2019-1387 (contract number TE 141/2020), financed by the Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI).

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