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copolymer

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Two or more monomers participate in the polymerization reaction, called copolymerization, the polymer contains two or more monomer units, this kind of polymer is called copolymer, also known as copolymer. For example, styrene butadiene rubber is a copolymer of butadiene and styrene. According to the arrangement of monomers in the molecular chain of copolymers, they can be divided into random copolymers, alternating copolymers, block copolymers and graft copolymers.

introduce

Copolymerization is a polymerization reaction in which two or more monomers take part. The polymer formed contains two or more monomer units. This kind of polymer is called copolymer. Copolymerization is often used in chain polymerization, such as free radical copolymerization and ionic copolymerization. But in the step-by-step polymerization, such as condensation polymerization, there are mostly two kinds of monomers involved in the reaction. The polymer formed contains two kinds of monomer units, but the term copolymerization can not be used.

Copolymer type

Because the copolymer contains at least two kinds of structural units, it can be divided into four kinds of copolymers according to the order of their structural units. When the structural units are a and B, the name of the copolymer is to add a connection number between the two monomers, then enclose it in brackets, and add the word "poly" before it, such as poly (butadiene styrene). In the international nomenclature, alt, Co, B, G and are often added between the two monomer names to represent alternating, random, block and graft copolymers respectively, such as poly (styrene ALT maleic anhydride).

Alternating copolymer

(alternating copolymers): the two structural units a and B in the copolymer are strictly alternating, and their mole fraction in the copolymer is about 50%.

Random copolymer

Random copolymers: two structural units a and B appear randomly in copolymers, in which the number of continuous units of a and B is not much, generally from a few to more than ten. From the statistical point of view, the content of a structural unit of random polymer in one segment of the polymer chain is equal to its content in the whole polymer.

Block copolymer

Block copolymers is composed of long chain segments with only structural unit a and long chain segments with only structural unit B. each chain segment can reach hundreds to thousands of structural units. With the development of controlled radical polymerization, gradient polymers have emerged. The composition of the structural units a and B gradually changes with the extension of the main chain, which is not the same as random and alternating copolymers, nor the mutation relationship as block copolymers.

graft copolymer

Graft copolymers are branched polymers in structure. They have not only main chains, but also long branched chains. The main chain and branched chains are composed of different structural units. The main chain is all structural unit a, and the branched chains are all structural unit B. Sometimes, the main chain and branch chain of the graft polymer may be copolymers. For example, the main chain is a random copolymer of a and B, and the branch chain is an alternating copolymer of a and B. the whole graft polymer is still a graft copolymer.

Synthesis and structure of copolymers

graft copolymerization

The most common method of graft copolymerization is to produce free radicals on the main chain of polymer by chain transfer reaction, and then initiate the polymerization of another monomer to produce a branched chain composed of another monomer. For diene polymers, such as polybutadiene, styrene butadiene rubber, natural rubber, etc., there are double bonds in the main chain, which can produce free radicals at the double bond or allyl group, and then produce branched chains. For example, high impact polystyrene (HIPS) is prepared by the free radical generated by initiator at the double bond of polybutadiene. in addition, pre synthesis of main chain and branched chain, and then connecting the branched chain to the main chain through chemical reaction between them are also common synthetic methods, such as polymers with electrophilic side groups such as ester group and anhydride group, which are easy to couple with anions at the end of active polymer, so as to prepare graft copolymers with predetermined structure.

Block copolymerization

There are two main synthetic methods of block copolymers: one is to polymerize one monomer first, and the active chain formed reacts with the other monomer. The main difficulty in synthesis is how to keep the activity of the active species and control the molecular weight of the obtained block. Atom transfer radical polymerization, ring opening polymerization and living ion polymerization are usually used. A common example is the synthesis of styrene butadiene styrene resin (SBS), which is polymerized by active anion initiated by butyllithium. Styrene, butadiene and styrene are added in turn to form styrene segments with molecular weight of 1-15000 and butadiene segments with molecular weight of 5-100000. Block copolymers such as PS-b-PMMA can also be synthesized by similar methods. The other is to form block copolymers through the reactions between two kinds of active chain segments (free radical coupling, chemical reaction between end groups). For example, polystyrene and polyethylene homopolymers are plasticized to form two kinds of chain segments with free radicals. After the coupling of the two chain segments, polystyrene polyethylene block polymer will be produced, but the components of homopolymer will be mixed. Since 2006, researchers began to use the chain shuttle process to synthesize block polymers, and tetrablock polymers have been successfully synthesized. Chain shuttle reaction is very useful in the synthesis of special "stereoscopic block copolymers" in block copolymers. Stereoblock polymer refers to two kinds of structural units which have the same chemical composition but are stereoisomers of each other.

Phase separation of copolymers

Block polymer will have phase separation phenomenon at a certain temperature, but in general phase separation system, such as water and oil, the size of oil droplets is in micron scale, and there are chemical bonds between the blocks of block polymer, forming a phase structure of only tens to hundreds of nanometers, so it is called microphase separation. The phase transition temperature of block copolymers mainly depends on several parameters, such as total polymer, topological structure parameter (functionality), the volume fraction of a block mainly affects the entropy of phase behavior, while the Flory Huggins interaction coefficient between blocks affects the enthalpy. It is generally believed that microphase separation will occur when it is greater than 10.5, and the phase interface becomes steeper when it is much greater than 10.5. with the different volume fraction of the block, the morphology of the phase structure micro area formed by the phase separation of the block polymer is also different. There are spherical phase, hexagonal columnar phase, layered phase and bicontinuous rotary structure. After proper treatment, these phase structure micro areas can form crystalline lattice like morphology, which can be used in nano template, nano scale separation, catalyst and semiconducting It has potential application value in bulk devices. The solution of block polymer is similar in that some blocks tend to mix with the solution, while the other block tends to aggregate with other blocks to form micelles, and various ordered phases can be formed at different concentrations.

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