Synthetic Polymers

Homopolymers are polymers made from identical monomer units. They contain only one repeating unit in the polymer chain. A clear example is polypropylene, which is built from propylene monomers.

The name of a homopolymer is usually formed by adding the prefix poly to the monomer name. That is why the polymer from propylene is called polypropylene. When the monomer has multiple parts or substituents, the monomer name may be placed in parentheses after poly. Examples include...

Copolymers are polymers made from more than one kind of monomer. When different monomer species are polymerized together, the chain can contain multiple repeating units. This process is called copolymerization.

Copolymers can be named by the number of monomers involved. A polymer made from two monomers is a bipolymer. Polymers made from three monomers are called terpolymers, and those made from four monomers are quaterpolymers. A common example is poly(styrene-co-acrylonitrile), also called...

Polymer molecular weight is often described with averages because a polymer sample contains chains of many different lengths. Unlike a small molecule, which has one definite molecular weight, each polymer chain can have a unique molecular weight. The result is a sample made up of chains with varying sizes.

The number average molecular weight, or Mn, uses the number fraction of each unique chain. A number fraction is the number of chains with a given molecular weight divided by the total number...

Polymer chain size and dispersity describe how uniform the chains are in a polymer sample. The weight average molecular weight, Mw, is usually higher than or equal to the number average molecular weight, Mn. These two values are equal only when every chain has the same molecular weight.

That exact match is rare in synthetic polymers. It is difficult to control polymerization at the molecular level with complete accuracy. Because of this, the ratio Mw to Mn is used to describe the molecular...

Polymer chains can be classified by their shape and by how their chains connect. Linear polymers have long chain-like structures with little or no branching. Even when a monomer has large substituent groups that look like branches, the polymer is not considered branched if those groups are part of the main chain structure.

Branched polymers contain secondary chains that grow out from the main polymer chain. This branching happens when polymer growth shifts away from the edge of the growing...

Crystallinity in polymer structure depends on how polymer chains are packed. Unlike ionic solids or small covalent molecules, polymers usually do not form fully crystalline solids because their long chains move and diffuse less easily. Instead, many polymers contain microscopic crystalline domains separated by amorphous domains.

Crystalline domains are the ordered regions of a polymer. In these areas, the chains line up closely and stay near each other because of intermolecular forces. In...

Polymer tacticity describes how side groups are arranged along a polymer chain. In polymers made from monosubstituted alkene monomers, polymerization creates chiral centers at every other carbon in the backbone. That stereochemistry can strongly affect the final properties of the polymer.

There are three main configurations: isotactic, syndiotactic, and atactic. In isotactic polymers, the substituents are mostly on the same side of the backbone. In syndiotactic polymers, the substituents...

Radical chain-growth polymerization builds a polymer by repeated addition of monomers to a growing chain. It uses a free-radical intermediate, which is the reactive species that keeps the chain moving forward.

The process starts with a radical initiator. These initiators generate free radicals on their own by homolytic fission, or splitting a bond so each atom keeps one electron. Common initiators include organic peroxides such as dibenzoyl peroxide and azo compounds. A low...

Radical chain-growth polymerization builds polymers through initiation, propagation, and termination. A free radical from the initiator adds to the unsaturated bond in a monomer. The unpaired electron from the radical and one pi electron from the double bond form a sigma bond. The other pi electron then becomes a new free radical on the growing molecule.

The initiation step can be shown with a phenyl free radical reacting with vinyl chloride. After initiation, the new radical adds to another...

Radical polymerization can create branched polymer chains. A clear example is ethylene polymerized by a radical route, which forms a low-density type of polyethylene with a heavily branched skeletal structure. During growth, the radical site can remove a hydrogen atom from the chain and move from the end of the chain, a primary carbon center, to a point within the chain, a secondary carbon center.

When that happens, the part of the chain from the hydrogen-removal site to the end becomes a...

Anionic polymerization is a chain-growth process that uses a carbanion as an intermediate. It begins when a strong nucleophile, such as an organolithium compound or a Grignard reagent, starts the reaction. Butyl lithium is the most commonly used initiator for this process.

The monomers used in anionic polymerization must have a vinyl group attached to one or two electron-withdrawing groups. Acrylonitrile, vinyl chloride, and styrene each have one electron-withdrawing group and can form anionic...

Anionic chain-growth polymerization builds a polymer through initiation, propagation, and termination. It begins when a nucleophilic anion, such as butyl lithium, attacks the pi bond of a vinylic monomer. This creates a carbanion, which is a carbon atom with a negative charge. The electron-withdrawing group on the monomer helps stabilize that carbanion.

The carbanion then drives the propagation step. It acts as a Michael donor and attacks a second vinylic monomer, which serves as the Michael...

Cationic polymerization builds a polymer chain through three stages: initiation, propagation, and termination. It starts when a monomer’s pi bond is protonated by a Lewis acid catalyst formed from boron trifluoride and water. This protonation creates a carbocation, which is stabilized by an electron-donating group.

During propagation, the pi bond of a second monomer acts as a nucleophile, meaning it donates electrons. It attacks the carbocation and forms a dimer, which becomes a new...

Ziegler-Natta polymerization uses a special catalyst to make linear polymers. It is a type of addition polymerization, also called chain-growth polymerization. This method favors straight polymer chains over branched ones.

The catalyst is called a Ziegler-Natta catalyst. It is named after Karl Ziegler and Giulio Natta, who developed it in 1953. The catalyst is an organometallic complex made from titanium tetrachloride and triethyl aluminum. Its active form is an alkyl titanium compound.

With...

Step-growth polymerization is a condensation process that builds long-chain polymers from bi- or multifunctional monomers. In this reaction, the monomers are reactive from the start. Early in the process, most monomers form short oligomers, which are small reactive chains. These oligomers then link together later to make long polymer chains.

Because the chains grow step by step, the reaction must run for a long time to reach high molecular weight. Many natural and synthetic polymers come from...

Step-growth polymerization builds polymers by linking bi- or multifunctional monomers. Bifunctional monomers form linear step-growth polymers. Multifunctional monomers can form non-linear or branched polymers.

As the reaction continues, the molecular weight keeps building up step by step. High molecular weight polymers usually appear at the late stages of the polymerization. By that point, about 99% of the monomers have been consumed.

The extent of the reaction can be found with the Carothers...

Polyesters are widely used step-growth polymers, and PET is one of the most important examples. These materials helped transform the textile industry because polyester blends resist wrinkles. That reduced the need for starching and ironing clothes.

Industrial PET is made from dimethyl terephthalate and ethylene glycol by transesterification at 150 °C. In this process, methanol is the by-product. It vaporizes at the reaction temperature, which helps drive the reaction to completion. PET can...

Olefin metathesis polymerization creates polymers by rearranging double bonds in olefins with a catalyst. The metathesis reaction moves substituents between two alkene molecules, and that same chemistry can be used to build polymer chains. Ruthenium-based Grubbs catalyst is the most common catalyst for this process.

Grubbs catalyst contains a carbon-metal double bond called a carbene. The reaction is reversible and starts when the catalyst reacts with an alkene in a [2+2] cycloaddition. This...

Ring-opening metathesis polymerization, or ROMP, is a polymer method that starts with strained cycloalkenes. A cycloalkene is a ring-shaped alkene, which means it contains a carbon-carbon double bond inside a ring. In ROMP, the ring strain helps push the reaction forward.

The reaction begins when a cycloalkene reacts with Grubbs catalyst. This forms a metallacyclobutane intermediate, which then opens to make a new carbene. That new carbene can react with another cycloalkene molecule, and the...

ADMET polymerization is a type of olefin metathesis used to build linear unsaturated polymers from terminal dienes. In this reaction, molecules such as 1,8-nonadiene react with each other and release ethylene gas.

The process is reversible, so the ethylene must be removed from the reaction mixture for polymerization to continue. ADMET works through a metallacyclobutane intermediate, which forms when one double bond in a terminal diene undergoes [2+2] cycloaddition with a Grubbs catalyst. The...