What is a Copolymer?

A polymerization reaction involving two or more monomers together is called copolymerization. The polymer formed contains two or more monomer units. Such polymers are called copolymers and copolymers. For example, styrene-butadiene rubber is a copolymer of butadiene and styrene. According to the arrangement of various monomers in the copolymer molecular chain, it can be divided into random copolymers, alternating copolymers, block copolymers and graft copolymers.

Chinese name: Copolymer

Foreign name: Copolymer, Interpolymer
Alias: Interpolymer

Copolymer introduction

A polymerization reaction involving two or more monomers together is called copolymerization, and the polymer formed contains two or more monomer units, and such polymers are called copolymers. The name of copolymerization is mostly used for chain polymerization, such as free radical copolymerization and ionic copolymerization. In the stepwise polymerization reaction, for example, two kinds of monomers are mostly involved in the condensation polymerization. The polymer formed includes two monomer units, but the term copolymerization cannot be used. ^[1]

Copolymer Copolymer Type

Since the copolymer contains at least two kinds of structural units, it can be divided into four kinds of copolymers according to the arrangement order of the structural units. When the structural units are A and B, the copolymer is named by adding a connection number between the two monomers. Then enclose it in parentheses, preceded by a poly word, such as poly (butadiene-styrene). In international nomenclature, alt, co, b, and g are often added between the two monomer names to represent alternate, random, block, and graft copolymers, such as Poly (Styrene-alt-Maleic Anhydride), which is styrene-maleic anhydride. Alternating copolymer. ^[1]

Alternating copolymers: The two structural units A and B in the copolymer are in alternating phases, and the mole fractions of both in the copolymer are about 50%.

Random copolymers: The two structural units A and B in the copolymer appear randomly. Among them, A and B have a small number of continuous units by themselves, usually several to a dozen. From a statistical point of view, the content of a certain structural unit of the random polymer in the polymer chain is equal to its content in the entire polymer [3].

Block copolymers (block copolymers) are composed of longer segments with only structural unit A and longer segments with only structural unit B, where each segment can reach several hundred to several thousand structural units. With the development of controlled radical polymerization, gradient polymers have emerged. The composition of the structural units of A and B gradually changes with the extension of the main chain. It is not basically the same as random and alternating copolymers, and it does not show a sudden relationship like block copolymers.

Graft copolymers. Graft copolymers are structurally branched polymers. They not only have a main chain, but also have longer branches. The main chain and branch chains are composed of different kinds of structural units. , The main chain is all structural unit A, and the branch chain is all structural unit B. Sometimes, the main and branch chains of the grafted polymer may be copolymers. For example, the random copolymers of which the main chains are A and B, and the alternating copolymers of the branches are A and B. The whole is still a graft copolymer. .

Copolymer Synthesis and Structure

Copolymer graft copolymerization

The most common method of graft copolymerization is to generate free radicals on the main chain of the polymer through a chain transfer reaction, and then initiate the polymerization reaction of another monomer to generate a branched chain composed of another monomer. For diene polymers, such as polybutadiene, styrene-butadiene rubber, natural rubber, etc., there are double bonds on the main chain, and free radicals can be generated at the double bonds or allyl groups to generate branched chains. For example, high-impact polystyrene (HIPS) is produced by an initiator that generates free radicals at the double bond of polybutadiene.

In addition, pre-synthesizing the main chain and the branch chain, and then linking the branch to the main chain through a chemical reaction between the two is also a commonly used synthetic method. For example, polymers with electrophilic side groups such as ester groups and anhydride groups can easily interact Anion at the terminal end of the living polymer is coupled to prepare a graft copolymer of a predetermined structure.

Copolymer block copolymerization

There are two main methods for synthesizing block copolymers: First, the polymerization reaction of one monomer is performed first, and the formed active segment is reacted with another monomer. The difficulty in synthesis mainly lies in how to retain the activity of the active species and control the molecular weight of the obtained blocks. Usually, atom transfer radical polymerization, ring-opening polymerization, and living ion polymerization are used. A common example is the synthesis of styrene-butadiene-styrene resin (SBS), the use of butyl lithium to initiate living anionic polymerization, styrene, butadiene and styrene are added in order, usually forming a benzene with a molecular weight of 1-1.5 Ethylene segments and butadiene segments with a molecular weight of 50,000 to 100,000. ^{[2] A} similar method can also be used to synthesize block copolymers such as polystyrene-b-polymethyl methacrylate (PS-b-PMMA).

The second is to form a block copolymer through the reaction between two active segments (free radical coupling, chemical reactions between end groups). For example, two homopolymers of polystyrene and polyethylene are plasticized to form two segments with free radicals. After the two segments are coupled, a polystyrene-polyethylene block polymer is produced, but mixed with Homopolymer components. Since 2006, researchers have used the chain shuttle process to synthesize block polymers and have successfully synthesized tetrablock polymers. The chain shuttle reaction is very useful in the synthesis of special "stereoblock copolymers" of block copolymers. Stereoblock polymers refer to the chemical composition of the two structural units, but are stereoisomers of each other.

Phase separation of copolymers

The phase separation phenomenon of block polymers occurs at a certain temperature, but the general phase separation system such as water and oil, the size of the oil droplets is on the micrometer scale, and the block structure of the block polymer has chemical bond links to form a phase structure. Only tens to hundreds of nanometers, so it is called microphase separation. The phase transition temperature of block copolymers mainly depends on several parameters, total polymer, topological structure parameters (functionality), the volume fraction occupied by a block mainly affects the entropy in phase behavior, and the The Lori-Huggins interaction coefficient affects enthalpy. It is generally believed that when it is larger than 10.5, microphase separation occurs in the block polymer, and when it is much larger than 10.5, the phase interface becomes more steep.

With different volume fractions occupied by blocks, the morphology of the microstructure of the phase structure formed after the phase separation of the block polymer is also different. There are spherical phases, hexagonal columnar phases, layered phases, and bicontinuous revolutions. After proper processing of the structural micro-regions, a morphological structure similar to a crystal lattice can be formed, which has potential application value in nano-templates, nano-scale separations, catalysts, and semiconductor devices. The solution of the block polymer is similar. Some blocks tend to mix with the solution, and the other block tends to aggregate with this block of other chains to form micelles. At different concentrations, various blocks can be formed. Ordered phase.