Recall from Lesson 2 that we learned about two types of polymerizations: step growth and chain growth. In Lesson 3, we explored in depth the monomers that undergo step growth polymerization and Carothers theory for linear step polymerization. In Lesson 4, we now begin to examine the details of chain growth polymerization. More specifically, in this lesson, we are going to focus on free radical polymerization, which is a specific type of chain growth polymerization.
First, we should probably refresh our memory of what a radical actually is! A radical species is characterized by having an unpaired electron. From general chemistry, we recall that it is much more stable to have electrons paired in orbitals rather than unpaired — so as you may expect, radicals tend to be highly reactive and relatively short-lived. Radicals are denoted by a single dot (representing the electron), such as in Figure 4.1.
Introduction to Polymers, Third Edition, CRC Press, 2011.
The radical is shown as the single dot in Figure 4.1. In (i) the geometry and the hybridization of the central carbon is emphasized. The carbon is sp2 hybridized, and the radical exists in the unhybridized p orbital that is perpendicular to the plane containing the substituents. Usually, the orbital is not drawn, as in (ii) or (iii).
Free radical polymerization makes use of this unstable radical by sequentially adding unsaturated monomers to the active center (radical end) of a growing polymer. Let's break that idea down further. As a refresher, recall that unsaturated molecules are characterized by not having the greatest possible number of hydrogens based on the carbon content. Usually, this means that the molecule contains a double or triple bond. Monomers that are most frequently used for free radical polymerization have a double bond, and more specifically, often a vinyl group or acryloyl group (Figure 4.2). Examples of a few common monomers are shown in Figure 4.3.
We already stated that we are going to use the radical to react with unsaturated monomers and this is going to create a polymer — but how? We can represent this reaction, and where the electrons go within the molecules, by using arrow pushing mechanisms which you learned in organic chemistry. In free radical polymerization, monomers are sequentially added together and the reactive radical end (the active center) attacks double bonds of monomers as shown in Figure 4.4.
There is actually a lot going on in Figure 4.4. First of all, you may want to refresh your memory regarding the conventions of arrow pushing mechanisms. The arrows show where the electrons start (at the arrow tail), and where they go (the arrow head); single headed arrows represent the movement of one electron, while double-headed arrows represent movement of an electron pair. Here, we will be using lots of single headed arrows because we are dealing with radicals, which are unpaired electrons. As shown in Figure 4.4, we have broken down the "overall reaction" into two steps for clarity. Let's look in detail at Step 1 in Figure 4.4. Note that the π bond of the double bond is broken (the σ bond still remains) and that the π bond is broken homolytically. By homolytically, we mean that the two electrons in that bond are split evenly between the two carbons in the bond, one electron going to each carbon. Then, one of those new radicals that is generated can react with the radical on the other monomer, forming a new carbon-carbon bond (step 2). In the future, when writing reaction mechanisms, all you would need to show is the "overall reaction" as shown in bottom of Figure 4.4. Notice that after this reaction takes place, our product now has one additional monomer linked to the polymer chain, and our radical is still at the end of the molecule (this is called the active center). Building upon this general mechanism of free radical polymerization, we move on to more in depth understanding of the process. For example, how do we even get these radicals in the first place? How do we get rid of the radicals if we want to stop our reaction to make polymers of a specific length? These are the sorts of questions that we will soon address.