What is a polymer compound?


polymer compound

Macromolecular compounds, referred to as macromolecules, also known as macromolecular polymers, generally refer to compounds with a relative molecular mass of several thousand to several million. The relative molecular mass of a molecular compound is the average relative molecular weight. Polymer compounds are made up of thousands of atoms connected to each other by covalent bonds. Although their relative molecular mass is very large, they are all connected by simple structural units and repeats.

The molecules of high molecular weight compounds are much larger than those of low molecular weight organic compounds. Generally, the relative molecular mass of organic compounds does not exceed 1000, while the relative molecular mass of polymer compounds can be as high as 104-106. Due to the large relative molecular mass of high molecular compounds, they are quite different from low molecular weight compounds in physical, chemical and mechanical properties.

Although the relative molecular mass of polymer compounds is large, their composition is not complicated, and their molecules are often formed by repeatedly connecting specific structural units through covalent bonds.

The molecular chain of the same polymer compound contains different number of links, so the polymer compound is essentially a mixture of many compounds with the same link structure but different degrees of polymerization. is the average.

Polymer compounds are almost non-volatile and often exist in solid or liquid state at room temperature. Solid polymers can be divided into crystalline and amorphous states according to their structural forms. The former molecular arrangement is regular and orderly; while the latter molecular arrangement is irregular. The same polymer compound can have both crystalline and amorphous structures. Most synthetic resins have an amorphous structure.

The atoms that make up the polymer chain are combined by covalent bonds, and the polymer chain generally has two different shapes: chain type and body type.

A large number of macromolecular compounds used as materials in the world today are made of low-molecular organic compounds obtained from coal, petroleum, natural gas, etc. as starting materials, and then made by polymerization. These low-molecular compounds are called “monomers”, and the high-molecular compounds produced by their polymerization are also called high polymers. Polymerization is usually divided into two categories: addition polymerization and condensation polymerization, referred to as addition polymerization and polycondensation.

Sort by source
According to the source, macromolecules can be divided into two categories: natural macromolecules and synthetic macromolecules.

Classified by performance
Polymers can be divided into three categories: plastics, rubbers and fibers.
Plastics can be divided into two categories according to their hot melt properties: thermoplastics (such as polyethylene, polyvinyl chloride, etc.) and thermosetting plastics (such as phenolic resins, epoxy resins, unsaturated polyester resins, etc.). The former is a polymer with a linear structure, which can soften and flow when heated, and can be plasticized and molded repeatedly. The defective and waste products can be recycled and processed into products. The latter is a macromolecule with a body-shaped structure, which solidifies once molded, cannot be softened by heating, and cannot be repeatedly processed and molded. Therefore, defective products and waste products have no value for recycling. The common feature of plastics is that they have good mechanical strength (especially polymers with body-shaped structures) and are used as structural materials.
Fibers can be divided into natural fibers and chemical fibers. The latter can be divided into man-made fibers (such as viscose fiber, acetate fiber, etc.) and synthetic fibers (such as nylon, polyester, etc.). Man-made fibers are chemically processed and drawn from natural polymers (such as short cotton linters, bamboo, wood, hair, etc.). Synthetic fibers are synthesized from low-molecular-weight raw materials. The characteristics of the fiber are that it can be drawn and formed, and has good strength and flexibility, and is used as a textile material.
Rubber includes natural rubber and synthetic rubber. Rubber is characterized by good high elastic properties and is used as an elastic material.

Classification by use
It can be divided into general polymer, engineering material polymer, functional polymer, bionic polymer, medical polymer, polymer drug, polymer reagent, polymer catalyst and biopolymer, etc.
The “tetraene” in plastics (polyethylene, polypropylene, polyvinyl chloride and polystyrene), the “tetralen” in fiber (nylon, polyester, acrylic and vinylon), and the “four glue” in rubber (styrene-butadiene) Rubber, cis-butadiene rubber, isoprene rubber and ethylene propylene rubber) are polymer materials with a wide range of uses and are general-purpose polymers.
Engineering plastics refer to polymer materials with special properties (such as high temperature resistance, radiation resistance, etc.). For example, polyoxymethylene, polycarbonate, polysulfone, polyimide, polyarylether, polyarylamide, fluorine-containing polymer, boron-containing polymer, etc. are relatively mature varieties, which have been widely used as engineering materials.
Ion exchange resins, photosensitive polymers, polymer reagents and polymer catalysts are all functional polymers.
Medical polymers and pharmaceutical polymers have special requirements in medicine and sanitation, and can also be regarded as functional polymers.

Classification by main chain structure
It can be divided into four categories: carbon chain polymers, heterochain polymers, elemental organic polymers and inorganic polymers.
The main chain of carbon chain polymers is formed by connecting carbon atoms.
In addition to carbon atoms, the main chain of the heterochain polymer also contains other elements such as oxygen, nitrogen and sulfur, such as polyester, polyamide, cellulose, etc. Easily hydrolyzed.
The main chain of elemental organic polymers is composed of atoms of carbon and other elements other than oxygen, nitrogen, sulfur, etc., such as silicon, aluminum, titanium, boron and other elements, but the side group is an organic group, such as polysiloxane.
Inorganic polymers are the main chain and side chain groups are composed of inorganic elements or groups. Natural inorganic polymers such as mica, crystal, etc., synthetic inorganic polymers such as glass.
The systematic naming of polymer compounds is relatively complicated, and in fact, it is rarely used, and it is customary to use the common names of natural polymers. Synthetic polymers are usually named according to the preparation method and the name of the raw materials. For example, polymers prepared by addition polymerization are often named by adding the word “poly” in front of the name of the raw materials. For example, the polymer of vinyl chloride is called polyvinyl chloride, the polymer of styrene is called polystyrene, etc. For example, the high polymer obtained by polycondensation reaction is mostly named after the simplified raw material name with the word “resin”. For example, phenolic resin, epoxy resin, etc. Addition polymers are often called “resins” before they are made into products. For example, polyvinyl chloride resin, polyethylene resin, etc. In addition, high molecular substances are often given trade names in commerce. For example, polycaprolactam fiber is called nylon-6, polyethylene terephthalate fiber is called polyester, polyacrylonitrile fiber is called acrylic, etc.

Compared with low molecules, macromolecules have the following characteristics:
1. From the perspective of relative molecular mass and composition, the relative molecular mass of macromolecules is very large and has “polydispersity”. Most polymers are polymerized from one or several monomers.
2. From the point of view of molecular structure, there are basically only two molecular structures of polymers, one is linear structure and the other is body structure. The characteristic of linear structure is that the atoms in the molecule are connected to each other by covalent bonds to form a long “chain” (called a molecular chain) in a coiled state. The characteristic of the body structure is that there are many covalent bonds between the molecular chains and the molecular chains are cross-linked to form a three-dimensional network structure. These two different structures have great differences in performance.
3. In terms of performance, polymers are usually in a solid or gel state due to their large relative molecular mass, and have good mechanical strength; and because their molecules are formed by covalent bonds, they have relatively Good insulation and corrosion resistance; due to its long molecular chain, the ratio of molecular length to diameter is greater than one thousand, so it has good plasticity and high elasticity. High elasticity is a unique property of polymers. In addition, solubility, melting, solution behavior and crystallinity are also very different from low molecular weights.

writing in short form
1. Writing method of the structural formula of the addition polymer: When writing the structural formula of the addition polymer, write the chain link in square brackets, write the degree of polymerization in the lower right corner of the square bracket, and use the horizontal line “-” to indicate the end group .
2. Writing method of the simplified structure of the polycondensate: when writing the simplified structure of the polycondensate, write the chain links in square brackets, write the degree of polymerization in the lower right corner of the square brackets, and write the remaining ends of the chain links outside the square brackets. base atom or group of atoms.

The molecular structure of polymers can be divided into two basic types: the first is linear structure, and the polymer compound with this structure is called linear polymer compound. The second is the body type structure, and the polymer compound with this structure is called the body type polymer compound. In addition, some polymers are branched, called branched polymers, which also belong to the category of linear structures. Although some polymers have cross-links between molecular chains, there are few cross-links. This structure is called a network structure, which belongs to the category of body structure.
There are independent macromolecules in the linear structure (including branched) macromolecules, and the macromolecules can be separated from each other in the solvent of this type of polymer or in the heated and melted state. However, there is no independent macromolecules in macromolecular substances with bulk structure (a large number of cross-links between molecular chains), so there is no meaning of relative molecular mass, only the meaning of cross-linking degree. Macromolecules with little cross-linking may also exist in network-structured macromolecules.
Two different structures, exhibiting opposite properties. High polymer with linear structure (including branched structure) has the characteristics of elasticity and plasticity due to the existence of independent molecules, it can be dissolved in solvent, can be melted by heating, and has less hardness and brittleness. Since there are no independent macromolecules, the body-shaped polymer has no elasticity and plasticity, cannot dissolve and melt, but can only swell, with high hardness and brittleness. Therefore, from a structural point of view, rubber can only be a polymer with a linear structure or a network structure with few crosslinks, and fibers can only be a linear polymer, while plastics have both types of polymers.

Synthesis of Macromolecules
There are two most basic reactions in the synthesis of polymer compounds: one is called condensation polymerization (referred to as polycondensation reaction), and the other is called addition polymerization (referred to as polyaddition reaction). The monomer structures, polymerization mechanisms and specific implementation methods of these two types of synthesis reactions are different.

polycondensation reaction
The polycondensation reaction refers to the polymerization reaction in which monomers with two or more functional groups condense with each other and produce small molecular by-products (water, alcohol, ammonia, hydrogen halide, etc.) to generate polymer compounds. like:
In the monomer, terephthalic acid and ethylene glycol each have two functional groups. When a macromolecule is formed, it extends in two directions, and a linear polymer is obtained.
Although phenol and formaldehyde are monofunctional compounds, the initial product of their reaction is multifunctional, and these multifunctional molecules are polycondensed into linear or bulk polymers, namely phenolic resins.

Addition polymerization refers to the reaction of synthesizing high polymers from one or more monomers. During the reaction, no low-molecular substances are generated, and the resulting high polymers have the same chemical composition as the raw materials, and their relative molecular mass. It is an integer multiple of the relative molecular mass of the raw material, and the addition polymerization reaction that only occurs by one monomer is called a homopolymerization reaction. For example, vinyl chloride to synthesize polyvinyl chloride:
The co-polymerization of two or more monomers is called a copolymerization reaction. For example, styrene is copolymerized with methyl methacrylate.
Copolymerization products are called copolymers, and their properties tend to be better than homopolymers. Therefore, product properties can be improved by the copolymerization method.

Addition polymerization has the following two characteristics:
(1) The monomer used in the polyaddition reaction is a compound with an unsaturated bond with a double bond or a triple bond. For example, ethylene, propylene, vinyl chloride, styrene, acrylonitrile, methyl methacrylate, etc. are commonly used important monomers, and the polyaddition reaction occurs on the unsaturated bond.
(2) The polyaddition reaction is accomplished by a series of mutual addition reactions between monomer molecules:
And once the reaction occurs, it will proceed quickly in a chain reaction to obtain a polymer compound (usually called an addition polymer). The relative molecular mass increase was almost independent of time, but the monomer conversion increased with time.
The above two characteristics are the most basic difference between polyaddition and polycondensation.
Addition polymerization can be divided into two categories: radical addition polymerization and ionic addition polymerization.