What Is a Polymer Chain?

The polymer chain structure refers to the chemical structure of the structural unit, the bonding mode, geometric isomerism, stereoisomerism, the conformation of the chain, the branched and crosslinked structure of the chain, the end group structure, molecular weight and molecular weight distribution, and the composition of the copolymer And sequence distribution.

The polymer chain structure refers to the chemical structure of the structural unit, the bonding mode, geometric isomerism, stereoisomerism, the conformation of the chain, the branched and crosslinked structure of the chain, the end group structure, molecular weight and molecular weight distribution, and the composition of the copolymer And sequence distribution, etc. ^[1]

(1) Bonding method: There are many possibilities for bonding between structural units. For example, when olefin monomer (CH ₂ = CHX) is polymerized, there are three possibilities: head-to-tail, head-to-tail and tail-to-tail.

(2) Geometric isomerism: Diene monomers such as butadiene, the polymer chains generated after polymerization can produce cis 1,4-, trans 1, 4-, and 1, 2- geometric bonding differences Constituency.

(3) Stereoisomerism: When there are asymmetric carbon atoms in the structural unit, optical isomerism (such as D configuration and L configuration) will occur. When the two structural units are bonded, there will be the same configuration and different bonding There are two possibilities for configurational bonding. There can be three types of stereoisomers on the polymer chain, namely isotactic, syndiotactic or heteroisotactic with isotactic and syndiotactic mixtures. Study the distribution of various stereoisomers on the polymer chain, that is, the sequence structure of the polymer chain.

(4) Conformation of the chain: The valence bond of the polymer chain (such as carbon-carbon bond) has internal rotational freedom, which will cause the conformation problem of adjacent bonds. Adjacent bonds on carbon-carbon bonds have trans (T), left There are three possible conformations of formula (G) and right formula (G '). The angles between adjacent bonds are 0 °, 120%, and -120 ", respectively. The potential barrier is not high, and the conformation of the chain changes at room temperature, so the conformation of the molecular chain is statistically significant. At low temperatures or in the crystal region, the polymer chain will also have a conformational sequence structure.

(5) Branching and cross-linking: There are often branched chains in free-radical polymerized linear polymer chains. Branched chains can be divided into short and long branches, which have different effects on polymer properties. The parameters characterizing the branched chains are the number of branching points and the length of the branches. For crosslinked polymers, it is necessary to characterize the structure of the crosslinking points and the density and distribution of the crosslinking points throughout the crosslinked polymer chain.

(6) Molecular weight and molecular weight distribution: The molecular weight of a polymer is uneven (from several hundred to several million), and it has molecular polydispersity. Generally, the molecular weight polydispersity is represented by a molecular weight distribution curve. Therefore, the molecular weight of polymers is only statistically significant. The definition of statistical average molecular weight varies with statistical methods.

(7) Sequence distribution of the copolymer: The copolymer is formed by polymerizing two or more monomers. The monomer bonding modes in copolymers containing AB monomers can be divided into four types: random copolymerization, alternating copolymerization, block copolymerization, and graft copolymerization. There is also a sequence distribution problem for the connection of the two units of the random copolymer AB ^[2]

Polymer compounds have almost no volatility and often exist in solid or liquid state at room temperature. Solid polymer can be divided into crystalline and amorphous according to its structure. The former molecular arrangement is regular and orderly; the latter molecular arrangement is irregular. The same polymer compound can have both crystalline and amorphous structures. Most synthetic resins have an amorphous structure ^[3] .