Chapter 1
Mechanisms and Methods of Polymerization

1.1 Introduction

Polymers are chain-like molecules consisting of smaller repeat units. They are synthesized from smaller molecules that are chemically linked together. The smaller molecules are called monomers, and the repeat units on a long polymer chain are called monomer residues. Polymers are divided into natural polymers and synthetic polymers. Natural polymers or biopolymers are made by nature as part of living organisms, including microorganisms, plants, animals, and humans. They include polysaccharides from smaller saccharide or sugar monomers, proteins from smaller amino acid monomers, and polynucleotides from smaller nucleotide monomers. Biopolymers, due to having different types of repeat units in their molecular structure, like 20 different amino acids in proteins, display multiple and specific functions in living organisms. Synthetic polymers are designed and produced by mankind by linking small molecules with at least two reactive functional groups to form long chains. Unlike natural polymers, synthetic polymers are made of one, two, or at most three different types of monomers. Most synthetic polymers are homopolymers, that is, they are made of one monomer, like polyethylene and polymethylmethacrylate. Examples of copolymers include polyethylene-vinyl acetate and polystyrene-methyl methacrylate. An example of a terpolymer is polyacrylonitrile-butadiene-styrene or ABS terpolymer. There are very few, if any, synthetic polymers made up of four different monomers. Another difference between biologic and synthetic polymers is that biopolymer chains are, in general, monodisperse, whereas chains in synthetic polymers are polydisperse. For example, the chains in a sample of albumin protein have identical molecular weights, whereas chains in a polyethylene sample have a distribution of molecular weights with a specified average value and a specified polydispersity. The unique property that distinguishes polymeric materials from other man-made materials like metals and ceramics is the aspect ratio of the molecules making up the material. The aspect ratio of a molecule is defined as the ratio of its contour length to its diameter. The aspect ratio for materials made of small molecules is between 1 and 10. Conversely, the aspect ratio of polymer molecules is generally greater than one thousand. The high aspect ratio imparts unique properties to polymers. For example, unlike metals and ceramics, polymers do not exhibit a boiling point with increasing temperature because the long chains in a polymer degrade prior to gaining sufficient energy to evaporate. Instead, polymers exhibit a glassy temperature below which the chains in the polymer sample are frozen with no translational diffusion, even though the polymer is above its melting temperature. As a result of their high aspect ratio, polymers possess unique physical, mechanical, chemical, and biological properties. The aspect ratio of polymers is characterized by the number of monomeric repeat units or degree of polymerization , chain length , which is or multiplied by the length of a repeat unit, or molecular weight , which is N or DP multiplied by molecular weight of the repeat unit. As chains in a sample have a distribution of or , polymers are characterized by their average number of repeat units , average degree of polymerization , average chain length , or average molecular weight . The of a polymer depends on reaction conditions for polymerization. These include temperature, pressure, pH, ionic strength, concentrations of monomers, solvent, initiator, co-initiator, catalyst, activating agent, crosslinker, chain transfer agent, chain terminating agent, retarder and inhibitor, impurities and oxygen, ratios of monomers and solvents, among others. Reaction conditions not only affect molecular weight and its distribution but also density of short and long branching, chain-end functional groups, unsaturated groups along the chain in multifunctional monomers, intra- and inter-molecular branching, and their distributions. Reaction conditions also affect chain architecture (linear, short-branch, long-branch, star-like, comb-like), distribution of monomers in copolymers (ideal, random, alternate, block), and polymer tacticity. As there is a wide range of reaction conditions affecting properties, there is a need to develop theoretical models and simulation methods to relate molecular weight and its distribution to reaction conditions. As the first step, this chapter covers mechanisms and methods of polymerization, which include addition and step-growth polymerization. Addition polymerization further includes conventional and reversible-deactivation radical polymerization (RDRP), anionic and cationic polymerization, ring-opening, and group transfer polymerization.

1.2 Mechanisms of Polymerization

1.2.1 Addition Polymerization

In the addition polymerization, monomer molecules react with active species formed by initiator dissociation to form propagating chains [1, 2]. Polymerizations in which the active species on propagating chains is a radical are called radical polymerization, whereas those with an anion or cation are called anionic polymerization and cationic polymerization, respectively [3-6].

1.2.1.1 Conventional Radical Polymerization

Monomers with unsaturated vinyl groups like styrene, methyl methacrylate, ethylene, propylene, vinyl alcohol, vinyl chloride, and alkyl vinyl ethers, as well as monomers with unsaturated triple bonds, polymerize by addition polymerization according to the following reaction:

where is the rate constant for the propagation reaction. In the above reaction, the radical attached to the terminal carbon atom of the propagating chain reacts with a vinyl monomer by dissociating the -bond of the vinylic carbon. Following the reaction, the radical is transferred to the terminal carbon atom of the added monomer to increase the number of monomer residues on the propagating chain radical by one residue. The radical on the propagating chain is normally positioned on the carbon atom with a side group, shown by "X" in the above reaction, as the side group increases the stability of the radical by resonance stabilization [7]. Radical polymerization requires an initiator to form the primary reactive species. Common reactions in addition polymerization are chain initiation, chain propagation, and chain termination. In the initiation step, the initiator dissociates by the addition of a reducing or oxidizing agent, or by a change in temperature, pH, or other external factors like ultraviolet radiation to form one or more reactive species [8, 9]. The initiation reaction for a typical temperature-activated initiator like benzoyl peroxide (BPO) is as follows [10]:

In the above reaction, the weak bond of BPO dissociates upon raising the reaction temperature to 80-100 °C, with a rate constant to generate two benzoyloxy radicals. As the benzoyloxy radical is highly unstable, it further disintegrates into a more stable but transient benzyl radical with the liberation of a molecule of carbon dioxide [10]. Some initiators dissociate into two active species while others dissociate into one active species and an inactive fragment [9]. Prior to escaping the dissociation cage, the benzyl radicals may recombine to form an inactive biphenyl compound as follows [11-14]:

In the chain initiation step, the generated radical reacts with a vinyl monomer in the reaction mixture to form a 1-mer propagating radical as follows [11-13]:

where is the rate constant for initiation of a propagating radical. In the propagation step, vinyl monomers react with a 1-mer propagating chain in propagation reaction steps, as shown in Rxn. 1.1, to generate a propagating chain radical with repeat units [15]. As the propagating chain radical is transient, it ultimately abstracts a hydrogen from a chain or combines with another propagating chain to terminate the propagating radical, and forms one or two terminated chains as follows [16, 17]:

where is the rate constant for termination. The termination reaction may take place by termination by disproportionation or termination by combination (shown above), or transfer reactions, which will be discussed in Chapter 2. The defining feature of radical polymerization is that the molecular weight of the synthesized polymer is relatively independent of the extent of polymerization or monomer conversion [18]. Conventional radical polymerization is not a living polymerization unless termination reactions are suppressed, as will be discussed in the reversible deactivation radical polymerization section [19, 20]. Initiation in radical polymerization is generally not instantaneous; that is, initiator dissociation is time-dependent, which leads to time-dependent chain initiation/termination reactions and a relatively broad chain molecular weight distribution.

1.2.1.2 Anionic Polymerization

Vinyl monomers with electrophilic or polar groups that stabilize a propagating anion undergo anionic polymerization [21-23]. Examples include styrene, butadiene, and isoprene with electrophilic groups and acrylonitrile, methyl methacrylate, and cyanoacrylate with polar groups [24]. A commonly used anionic polymerization initiator is butyl lithium [25, 26]. The carbon-lithium bond in...