付加重合

Addition Polymerization

Addition polymerization, also called chain-growth polymerization, is the process by which monomers containing a carbon-carbon double bond (C=C) link together to form long chains without the loss of any atoms. Every atom present in the monomers ends up in the polymer. The result is a polymer whose repeating unit has the same empirical formula as the original monomer.

The General Mechanism

An addition polymerization reaction proceeds through three distinct stages:

1. Initiation — An active species (a free radical, cation, or anion) is generated and reacts with the first monomer, opening its double bond and creating a new active center at the chain end.
2. Propagation — The active chain end attacks another monomer, adding it to the chain. This step repeats thousands of times in rapid succession, each addition regenerating the reactive center.
3. Termination — The growing chain loses its reactivity through combination with another radical, disproportionation, or reaction with an impurity, ending the chain.

Because each monomer adds individually to a growing chain, the polymer reaches high molecular weight early in the reaction. Even at low overall monomer conversion, fully formed long chains coexist with unreacted monomers.

Free Radical Polymerization

Free radical polymerization is the most widely used variant. A radical initiator — typically a peroxide like benzoyl peroxide (BPO) or an azo compound like azobisisobutyronitrile (AIBN) — decomposes when heated or exposed to UV light, producing radicals with unpaired electrons.

The radical attacks the electron-rich double bond of a vinyl monomer, forming a new carbon-carbon bond and transferring the radical to the other carbon. This chain reaction continues at remarkable speed; a single polyethylene chain can grow to 10,000 units in under one second.

Key products of free radical polymerization:

Cationic Polymerization

In cationic polymerization, the active chain end carries a positive charge. Initiation requires a strong Lewis acid catalyst such as AlCl₃ or BF₃, often combined with a proton source (a co-initiator like water or HCl). The resulting carbocation attacks the monomer's double bond, propagating the chain.

Cationic polymerization works best with monomers whose substituents can stabilize a positive charge through electron donation — for example, isobutylene (which polymerizes to butyl rubber) and vinyl ethers. The reaction is sensitive to temperature and must often be carried out at very low temperatures (-80 to -100 degC) to control chain length and prevent side reactions.

Anionic Polymerization

Anionic polymerization uses a nucleophilic initiator — typically an organolithium compound like n-butyllithium (n-BuLi) — to generate a carbanion at the chain end. The carbanion attacks the monomer, and the chain grows as long as monomers are available.

A remarkable feature of anionic polymerization is the absence of inherent termination: the chain ends remain "living" (reactive) indefinitely in the absence of impurities. This living polymerization enables precise control over molecular weight and produces polymers with very narrow molecular weight distributions (polydispersity index close to 1.0). Chemists exploit living anionic polymerization to make block copolymers — polymers with two or more distinct segments in each chain — by sequentially adding different monomers.

Styrene and butadiene are common monomers for anionic polymerization. The technique is central to producing styrene-butadiene block copolymers used in shoe soles, adhesives, and high-impact plastics.

Coordination Polymerization

In the 1950s, Karl Ziegler and Giulio Natta discovered that transition-metal catalysts (now called Ziegler-Natta catalysts) could polymerize ethylene and propylene with extraordinary control over the stereochemistry of the resulting chains. Their catalysts — typically TiCl₃ combined with an aluminum alkyl co-catalyst — produce isotactic polypropylene, in which all methyl groups are on the same side of the chain. This regular arrangement allows the polymer to crystallize, giving it the stiffness and heat resistance needed for automotive parts, food containers, and textiles.

Ziegler and Natta shared the 1963 Nobel Prize in Chemistry for this breakthrough, which transformed polypropylene from a laboratory curiosity into one of the world's most important plastics.

Controlled Radical Polymerization

Traditional free radical polymerization produces polymers with broad molecular weight distributions (PDI of 1.5-2.0) because initiation, propagation, and termination occur simultaneously. In the 1990s, several controlled radical polymerization (CRP) techniques were developed that combine the versatility of radical chemistry with the precision of living polymerization:

• ATRP (Atom Transfer Radical Polymerization) — uses a transition-metal catalyst (typically copper) to establish a rapid equilibrium between dormant and active chain ends, reducing the concentration of active radicals and suppressing termination.
• RAFT (Reversible Addition-Fragmentation Chain Transfer) — employs a chain transfer agent (usually a dithioester or trithiocarbonate) to mediate exchange between growing chains.
• NMP (Nitroxide-Mediated Polymerization) — uses a stable nitroxide radical (like TEMPO) as a reversible capping agent.

These techniques produce polymers with narrow PDIs (1.05-1.3) and well-defined architectures — block copolymers, star polymers, and gradient copolymers — that are essential for drug delivery nanoparticles, advanced coatings, and self-assembling materials.

Industrial Significance

Addition polymerization accounts for the majority of global plastic production. Polyethylene alone — produced through both free radical and coordination processes — represents roughly one-quarter of all plastics made each year. The ability to fine-tune chain length, branching, and stereochemistry through choice of initiator, catalyst, temperature, and pressure gives chemists enormous control over the final material's properties.