What is cross-linking reaction?
Cross-linking reaction refers to the reaction of two or more molecules (generally linear molecules) bonding to each other to form a more stable molecule (body molecule) in a network structure. This reaction transforms the linear or lightly branched macromolecules into a three-dimensional network structure to improve the strength, heat resistance, wear resistance, solvent resistance and other properties. It can be used for foamed or non-foamed products. .
Response classification
- Physical cross-linking: It is formed by the combination of physical forces such as hydrogen bonding.
- Chemical cross-linking: formed by covalent bonding.
Reaction type
- In order to improve the polymer performance, artificial crosslinking is performed.
- Aging cross-linking during use.
There are two main types of methods for synthesizing bulk polymers. One is to synthesize from monomers. For example, a chain polymerization reaction is used to synthesize the three-dimensional network skeleton styrene and divinylbenzene copolymer of the ion exchange resin; the second is to synthesize the linear or branched prepolymer first and then carry out the cross-linking reaction. It is chemical cross-linking or physical cross-linking, such as vulcanization of rubber, curing of unsaturated polyester by chain polymerization, reaction of epoxy resin and curing agent, tanning process of leather (interaction of protein and formaldehyde), etc. Chemical cross-linking; the use of light, heat, and radiation can also cause cross-linking of linear polymers. Polyethylene radiation cross-linking is one example, which is physical cross-linking. Linear polymers are moderately cross-linked, and have improved mechanical strength, elasticity, dimensional stability, solvent resistance, or chemical stability. Therefore, cross-linking reactions are often used to modify polymers.
Examples of reactions
The cross-linking reaction is widely used in the synthesis and modification of polymers, such as plastics, resins, and rubbers.
When the plastic monomer is polycondensed, branched chains are generated first, and then cross-linked into a body structure. This type of polycondensation process is called body polycondensation. Bulk polymers that have been crosslinked are insoluble, infusible, dimensionally stable, and cannot be molded. The linear or branched stage can be melted and plasticized. After being heated, the potential functional groups are further cross-linked and solidified. This polymer is called a thermosetting polymer.
Due to the particularity of the cross-linking reaction, thermosetting polymer production is generally carried out in two stages: the first stage is first made into a prepolymer with incomplete polymerization. The prepolymer is generally a linear or branched chain oligomer with a molecular weight of about 500～5000, can be liquid or solid; The second stage is the molding and curing of the prepolymer. The prepolymer still has the flow ability at the beginning under heating and pressurization conditions, can fill the mold cavity, and after the crosslinking reaction, serve into a fixed shape.
In industrial paints and automotive paints made of alkyd resin, epoxy resin, acrylic resin, etc., self-drying paints and paints are generally due to the direct oxidation and cross-linking reaction between the linear molecules in the resin and the oxygen in the air, making the paint The film is dried and fixed. Two-component coatings and paints generally require the addition of curing agents to form thermosetting resins. The curing agent can also be called a cross-linking agent.
Development status
According to genetic theory, if the genetic molecules of an organism remain unchanged, the phenotype of the organism should also be fixed, but the phenotype of various organisms in nature is not completely fixed, but varies with differentiation, development time and age Growth has changed regularly. For example: human hair changes from black to white regularly; various biological tissues and cells always show different phenotypic changes as they age to different degrees of differentiation to aging. Why do biological tissues gradually become tough and old with age? Why does human hair turn white? These irreversible function reduction and phenotypic changes used to be considered only as dynamic regulation of the strength of gene activity without emphasis on intermolecular structure naturally changing views could not explain the problem. According to Russia's A.M. Butrelov's theory of chemical structure, the above phenomenon is undoubtedly the result of the progressive structural changes of biomolecules, indicating that natural substances undergo certain regular natural chemical changes throughout the life process. This kind of thinking is the starting point of a new insight extended to a variety of life phenomena. This new insight is called "the doctrine of natural cross-linking of biomolecules", hereinafter referred to as "cross-linking doctrine".
The so-called cross-linking refers to the chemical bonding and molecular bonding caused by the interaction between the active groups of biological molecules. The author believes that in terms of chemical activity, an organism is a chemical system that is far from achieving chemical balance. Each biomolecule in the system has a large number of active groups, and they must interact with each other to cause a chemical reaction, so that the biomolecules slowly crosslink to trend The stability of chemistry, the degree of cross-linking continues to increase over time, the active groups of biomolecules continue to decrease, and the original molecular structure and intermolecular structure gradually change. The continuous accumulation of this change will undoubtedly make the biological tissue gradually become tough and old, and at the same time it will cause two changes in the expression of biological genes: on the one hand, it may express gene products with different activities or even completely changed effects; another On the one hand, it also affects the recognition and binding of RNA polymerase to reduce its transcriptional activity, showing that the activity of genes is gradually reduced and lost in a regular and orderly manner, thereby causing progressive and regular phenotypic changes in organisms and their cells and tissues. Even aging and death. The author believes that this mechanism is the molecular basis of gene regulation and cell differentiation, and the fundamental cause of cell and biological aging.
According to the above thinking, the basic point of view of cross-linking is:
(1) Various biomolecules are not immutable but naturally cross-linked according to a certain natural pattern over time;
(2) Progressive natural cross-linking makes the biomolecules slowly connect, the intermolecular bond energy continues to increase, gradually polymerizes, and the solubility and surging ability gradually decrease and lose, so that the biological tissues and cells gradually show the old state;
(3) Progressive natural cross-linking leads to the orderly inactivation of genes so that the cells grow and differentiate in a specific pattern and the organism exhibits a dynamic process of programmed and patterned growth, development, senescence, and death.
These points of view are compared with Driesch-Morgan and Caplan-ordahl's gene differentiation theory. It is not simply from full inhibition to separate activation or from full activation to separate inhibition, but it absorbs the essence of multiple theories and is inactivated by molecular crosslinking. The concept of natural movement makes a unified interpretation of various life phenomena.
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