By Hyungjun Cho, member-at-large for the GCI
In April of 2019, the International Union of Pure and Applied Chemistry (IUPAC) named reactive extrusion as part of the 10 chemical innovations that could have high impact in society . One of the reactive extrusion technique that is of interest is the use of ball mills instead of solvents to mix reagents. An exciting example of such a reaction is solvent-less polymerization. This method of making polymers in a ball mill has been blossoming in recent years but has not attracted much mainstream attention in the chemical community. This article will present the pivotal publication that re-invigorated interest the field of solvent-less polymerization.
There is the typical list of ingredients required for making polymers: 1) monomer, 2) an initiator and/or catalyst, 3) a constant input of energy, and 4) a solvent. Like in organic chemistry, the solvent is almost always critical to the success of the process. Although some solvent-less polymerization techniques are have known for (such as bulk  and solid-state polymerization ), these methods are not widely applicable and often requires elevated temperatures, which needs a lot of energy. In contrast, ball milling polymerization is a solvent-free technique that is at its early stages of development, but may avoid these issues.
Figure 1. A cartoon diagram of the vibrational ball milling process.
Ball milling is a mechanochemical process, where solid chemical samples are added to a grinding jar with solid metal balls (Figure 1). The grinding jar is shaken or spun which results in collisions between the balls and the chemical material. This generates high instant pressure and heat through impact and friction .
Although ball mills are conventionally used to break down polymers , a handful of reports demonstrated the synthesis of polymers using ball mills [6-10]. In 2014, Ravnsbaek et al. synthesized the polymer poly(p-phenylene vinylene) using a ball mill . This work seems to have reinvigorated the field of ball mill polymerization, and is briefly presented here.
Figure 2. Solvent-less synthesis of poly(2-methoxy- 5-2′-ethylhexyloxy phenylene vinylene) in a ball mill. ACS Macro Lett. 2014, 3 (4), 305–309 . Copyright 2014 American Chemical Society.
Briefly, the solid chemicals bis(chloromethyl)-methoxy-ethylhexyloxy-benzene and potassium tert-butoxide were added to a zirconium oxide grinding jar, along with a zirconium oxide ball (diameter = 10 mm) (Figure 2). The reaction mixture was shaken horizontally at a frequency of 30 Hz at room temperature to synthesize poly(2-methoxy- 5-2′-ethylhexyloxy phenylene vinylene) (MEH-PPV). The authors found that after ca. 10 minutes of shaking, the polymers grew to its maximum average molecular weight (Mn = 35 kDa) (Figure 3). The size dispersity of the polymer was broad (Đ ~ 4) which is typical of stepwise growth polymerizations. The yield was ca. 60% after 10 minutes of shaking and did not improve with longer shaking time. This was especially remarkable because similar polymerization in solution typically requires several hours and elevated temperature [11-13].
Figure 3. The synthesis and degradation of MEH-PPV with respect to ball milling time Reprinted with permission from Ravnsbæk J. B. and Swager, T. M. ACS Macro Lett. 2014, 3 (4), 305–309 . Copyright 2014 American Chemical Society.
In another set of experiments, Ravnsbaek et al. obtained a pre-made sample of MEH-PPV with a larger Mn (Mn =150 kDa) and exposed it to the same grinding conditions used for their synthesis of MEH-PPV. The authors found that the Mn of the larger polymer sample decreased over time to 35 kDa (Figure 3), which suggested that there exists a maximum size of polymer that can be synthesized using a ball mill.
Since this report by Ravnsbaek et al., other research groups have been using ball mills for solvent-less polymerizations. The Borchardt group [14-16], the Kim group [17-18], and the Song group  published their findings within the last 4 years. These reports collectively demonstrate many novel ball mill polymerizations, including co-polymerization (more than one monomer) [15-16,19], metal catalyzed polymerization (palladium-catalyzed polymerization) [15-16], and the synthesis of non-conjugated polymers (i.e. poly(lactic acid) and polyurethanes) [17-19].
In 2019, Christensen et al. published an article in Nature Chemistry showing their work regarding a new type of polymer called the vitrimer , which was synthesized as the product of the reaction of ketones and amines in a ball mill. While the focus of this article was on the chemistry of the polymer, it is noteworthy that the ball mill polymerization has been highlighted in a high impact journal. This will likely encourage more polymer research groups to look into this unconventional technique as it has a lot of room for growth and improvement, and even more potential to reduce the use of solvent in polymer production.
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