Chain transfer

In the field of polymer chemistry, scientists study how long chains of molecules form. This process is called polymerization. During this process, a specific event called a chain transfer reaction can occur. In this reaction, the activity of a growing polymer chain is transferred to another molecule. This activity is often held at an active center, which is represented by a symbol in chemical equations. When this transfer happens, the original chain stops growing. A new molecule then takes on that active center to start a new chain. This process is vital because it reduces the average molecular weight of the final polymer.

The mechanism of chain transfer follows a specific sequence of chemical steps. It begins with a growing polymer chain, which we can call P. This chain has an active center that allows it to keep adding molecules. During the reaction, this chain interacts with another molecule, labeled XR. The active center moves from the polymer chain to the substituent, which we call R. As a result, the original polymer chain gains an end group, X, and becomes a finished PX molecule. This leaves the R molecule with the new active center. This new center can then begin growing its own separate polymer chain.

Chemists categorize these reactions based on the molecule that receives the active center. One type is transfer to a chain transfer agent. These agents are sometimes called modifiers or regulators. They contain at least one weak chemical bond, which makes the transfer reaction easier to happen. Common examples of these agents include thiols, such as dodecyl mercaptan (DDM), or halocarbons like carbon tetrachloride.

Another type is transfer to a monomer. This occurs when a growing chain abstracts an atom from an unreacted monomer in the reaction medium. This specific type of transfer is very important in cationic addition polymerization and ring-opening polymerization. It actually determines the theoretical maximum molecular weight a monomer can achieve.

Chain transfer can also involve the solvent or existing polymer chains. In solution polymerization, the solvent may act as a chain transfer agent. If the solvent is not chosen to be inert, the reaction might result in very low molecular weight polymers called oligomers.

A different process occurs during transfer to polymer. This happens when a growing chain reacts with a polymer chain that already exists. This often occurs at the end of a radical polymerization when most of the monomer is gone. When the new radical site forms along the polymer backbone, it creates branched polymers. The specific properties of low-density polyethylene are critically determined by how much this transfer to polymer occurs.

The history of this concept began in 1930. Scientists Hugh Stott Taylor and William H. Jones first proposed the idea of chain transfer. They were studying how to produce polyethylene from ethylene and hydrogen. They used ethyl radicals created from the thermal decomposition of (Et)2Hg and (Et)4Pb. They realized their product mixture could only be explained if the radical character transferred between reactants. In 1937, a scientist named Flory added the concept to the mathematical study of vinyl polymerization. He coined the official term "chain transfer." He used it to explain why polymer chains were usually shorter than mathematical predictions suggested.

Practical applications of this science grew rapidly during World War II. The US Rubber Reserve Company utilized chain transfer for making styrene-butadiene rubber. Their "Mutual" recipe was based on the Buna-S recipe developed by I. G. Farben in the 1930s. The original Buna-S recipe produced a very tough, high molecular weight rubber. This rubber was difficult to process on standard rubber mills without intense heat. Researchers at Standard Oil Development Company and the U.S. Rubber Company found a solution. They discovered that adding a mercaptan modifier produced a more tractable rubber. This modifier also increased the overall rate of polymerization.

Since the 1980s, research has moved into highly controlled versions of these reactions. Scientists now study forms of free radical living polymerizations. These include catalytic chain transfer polymerization and iodine transfer polymerization (ITP). Another major area is Reversible Addition-Fragmentation chain Transfer, or RAFT, polymerization. In these advanced processes, the transfer reaction produces a chain that keeps the same activity as the original agent. This means there is no net loss of chain transfer activity. This allows scientists to build complex materials with great precision through modern chemical systems.