When considering a collection of polymer beads, it is important to consider the distribution of particle sizes in the collection. Polymer beads produced by aqueous suspension polymerization often have a distribution of particles sizes that is unimodal. That is, when the occurrence of particles is plotted versus diameter, only a single significant peak is observed. In such cases, the peak is often approximated by a Gaussian distribution, in which the occurrence as a function of diameter (d) is proportional to the function exp(−(d−μ)2/(2σ2)), where μ is the mean and σ is the standard deviation. Such distributions of particle size are also characterized by the uniformity coefficient (“UC,” defined below). Unimodal distributions having UC greater than 1.5 are referred to herein as “typical Gaussian,” while unimodal distributions having UC less than 1.3 are referred to herein as “uniform.”

A drawback associated with a typical Gaussian distribution is that typical Gaussian distributions of polymer particles made in suspension polymerization normally contain undesirable levels of very small and very large particles. Thus the typical Gaussian collection of particles must be sifted to remove both the very large particles and the very small particles from the collection, and the sifting process reduces the yield of polymer particle production. Often, the loss of polymer particles due to sifting of a typical Gaussian collection of particles is 20 to 30% by weight. A drawback associated with a uniform distribution is that, in a collection of spherical particles that are all the same size, the particles do not pack efficiently to fill a volume of space. It is desired to provide a collection of polymer particles, and a method of making those particles, that provide one or more of the following benefits: there is small or no loss of production yield due to sifting, and/or the packing efficiency of the particles is good. It is also desired to provide polymer particles that are useful as ion exchange resins and/or as adsorbent resins.

One method of making particles is described in U.S. Pat. No. 7,727,555, which describes a method of making particles that involves forming a stream of a mixture including a first material and a second material (for example, a polymer and a gelling precursor), exposing the stream to vibration, and treating the stream to form particles including the first material. It is desired to provide a process of making particles that is consistent with aqueous suspension polymerization, and it is desired to provide a collection of particles suitable as ion exchange resin and/or as adsorbent resins.

The following is a statement of the invention.

A first aspect of the present invention is a method of making a collection of polymer beads comprising(a1) forming a first collection of monomer droplets in an aqueous medium in a container, wherein the first collection of monomer droplets has volume-average diameter DAV1 and has uniformity coefficient less than 1.3;(a2) forming a second collection of monomer droplets in the aqueous medium in the container, wherein the second collection of monomer droplets has volume-average diameter DAV2 and has uniformity coefficient less than 1.3, wherein DAV1 and DAV2 differ by 10 μm or more;
wherein either step (a1) and step (a2) are performed simultaneously; or step (a2) is performed after step (a1), while the first collection of monomer droplets remains in the container; or a combination thereof; and(b) after steps (a1) and (a2), polymerizing the monomer droplets by suspension polymerization to form the polymer beads.

A second aspect of the present invention is a collection of polymer beads, wherein the volume distribution expressed as a function of bead diameter comprises(A1) a first peak having a maximum of PM1 at diameter PD1 and having full width at half maximum of 75 μm or less,(A2) a second peak having a maximum of PM2 at diameter PD2 and having full width at half maximum of 75 μm or less; and(B1) a valley having a minimum value of VM1 at a diameter between PD1 and PD2, wherein VM1 is less than 0.25*PM1 and VM1 is also less than 0.25*PM2.
wherein the polymer beads comprise polymerized units of one or more monofunctional vinyl monomer and one or more multifunctional vinyl monomer.

