Azobisisobutyronitrile AIBN (molecular weight equal to 164) forms nitrogen and two cyanopropyl radicals, has no induced decomposition activity, has low activity and is prone to cage effects.

Azobisisoheptanitrile (ABVN) has higher activity and medium activity.

Inorganic: Persulfate is used in emulsion and aqueous solution polymerization to form a redox system with ferrous ions and can be initiated at room temperature

Organic: Alkyl hydroperoxide, dialkyl peroxide, diacyl peroxide (low activity) Dibenzoyl peroxide BPO molecular weight 242 Features: two-step decomposition to form two free radicals with low activity It is easy to induce decomposition, making the initiator efficiency low, but it is not easy to cause the cage effect.

Low activation energy, fast initiation rate, low initiator efficiency BPO＋NN Dimethylaniline

The first two types can produce free radicals through light decomposition. In addition, disulfides, benzoin, and diphenylethylenedione are also photoinitiators.

Strong selectivity, light controllability, mild reactivity

The time required for the initiator to decompose to half its initial concentration

Initiator efficiency f: the fraction of primary free radicals generated by the decomposition of the initiator that actually participate in the chain initiation reaction It is equal to the decomposed (start-last-decomposition initiator) divided by the total amount of decomposition (start-last), generally 0.5-0.8

The transfer reaction of free radicals to the initiator produces new free radicals, the number remains unchanged, but the initiator is consumed, reducing the efficiency.

Affects reaction rate, time, and molecular weight

Looking at the polymerization method, bulk polymerization (oil phase) suspension, the solution should choose an oil-soluble initiator such as azo peroxide Emulsion aqueous solution in aqueous phase, choose persulfate or redox system

Second, look at the polymerization temperature. If the temperature is high, choose low activity. If the temperature is low, choose high activity. If you want uniform polymerization, you need high or low (medium activity).

Third, look at other factors

First and last bonus: accounting for 98-99% of the set. Determined by the conjugation effect and steric hindrance of side groups The resonance effect of the substituent on the chain free radicals, the resonance is stable, and there is no The steric resistance of the head-to-tail bonding is small, but the head-to-head bonding resistance is large.

Head structure: When monomers with less steric hindrance are polymerized due to resonance effects of substituents, the content of heads increases. As the polymerization temperature increases, the activation energy increases, and the head structure content will increase.

The chain radicals of free radical polymerization are planar sp² hybridization, and there is no orientation factor.

When the monomer adds to the chain radical, it can be added randomly from the top or bottom of the plane. After the chain radical reaction, it changes from sp² hybridization to sp3 hybridization. The substituent has no selectivity for the spatial configuration and is random. Atactic polymers are obtained. Therefore, it is difficult to achieve directional polymerization by free radical polymerization.

The termination reaction in which the single electrons of two chain radicals combine with each other to form a covalent bond

When initiated by an initiator and without chain transfer, both ends of the macromolecule are initiator residues

The stronger the conjugation effect, the more likely it is to occur

A termination reaction in which a free radical in a chain seizes a hydrogen atom or other atom from another free radical

The greater the steric hindrance effect, the more likely it is to occur

Increasing temperature leads to increased disproportionation termination

Monosubstituted vinyl monomers, acrylonitrile, and styrene, are all coupling-terminated A special case is the complete disproportionation termination of vinyl acetate.

11 Disubstituted vinyl, MMA disproportionation terminated

Reduce the heat of polymerization (a methyl substituent reduces the heat of polymerization by 9kJ/mol)

Reduce the heat of polymerization. There are π-π, p-π and σ-π conjugation in the olefinic monomer, which averages the electron cloud and reduces the thermodynamic energy. Conjugation averages the electron cloud, The thermodynamic energy of the monomer decreases.

The heat of polymerization is increased, and the bond energy substituted by strongly electronegative atomic groups is significantly increased.

It reduces the heat of polymerization and hydrogen bonding reduces the internal energy. Acrylic acid has stronger intermolecular interaction than polyacrylic acid, so the heat of polymerization is lower. Methacrylic acid reduces it more due to steric hindrance and hydrogen bonding.

Reduce the heat of polymerization