1. Monomer structure of Nonionic polyacrylamide, The monomer structure of acrylamide contains an unsaturated structure between carbon and carbon atoms, and an unsaturated structure between carbon and hetero atoms, Its structural formula is as follows:
There is only one substituent in the monomer molecule of acrylamide--NH2, and the radius of a NH2 substituent is smaller, and the carbon atom, The unsaturated structure between them is asymmetric, so it can be polymerized regardless of the degree of substituents. The combined reaction can happen instantly.
2. Monomer polarity of acrylamide
After introducing -NH2 polar substitution group in the monomer molecule of acrylamide, asymmetric CH2 = CHCONH2 is formed, The dipole moment of the type compound, due to the polarization of the double bond, the polymerization ability of the monomer molecule is significantly improved.
However, in the monomer molecule of acrylamide, the influence of the carbonyl compound >C=0 C=C The difference is that because carbonyl is a polar group itself, if a -NH2 polar substituent is introduced on its carbon atom will reduce the monomer, The polarity of the molecule reduces the polymerization ability.
3. Conjugated part inside acrylamide monomer molecule, The electrons on the double bond in the monomer molecule of acrylamide form a conjugated system with the electrons on the amide group, making the electric charge on the double bond, The density of the child cloud is reduced, so it is easy to open, and the monomer molecules are very lively and not easy to polymerize.
The initiator Cu20 and the ferrous iron salt constitute an oxidation-reduction system, which decomposes to produce primary free radicals. Acrylamide monomers combine to form free radicals of acrylamide monomers. Proceed as follows: In the first step, Cu20 decomposes into primary radicals.
In the second step, the primary radical has high reactivity, and it reacts with the monomer molecule to form a monomer with an electron Free radicals (Cu on the end).
Cu + CH2 -CHCONH, one → +CuCH2- CHCONH2
Because the decomposition of Cu20 is an endothermic reaction, the reaction rate is small, but it is the main step to control the polymerization reaction. Primary dissociation The free radical of the monomer formed by combining Cu and acrylamide monomer is an exothermic reaction, the reaction is very easy to proceed, and the reaction rate is also large.
4. Free radical polymerization of acrylamide
Polyacrylamide is obtained by free radical polymerization of acrylamide, and the structural chain formed by it is:
In the formula, n can range from a few to 40,000. It can use various kinds of radiation, light (visible and ultraviolet), ultrasonic, current or easily decomposed under polymerization conditions, The compound initiates polymerization. The PAM chain structure is a linear macromolecule. In commercial use, it is often called the polymer is named PAM. Compounds commonly used in industry that easily decompose into free radicals are peroxides containing a 0--0- bond and those containing C-N bond Azo compounds, such as K2S2O8, (NH4) 2S2O8, H2O2 and other inorganic peroxides, benzoyl peroxide and other organic peroxides Substances, such as azobisisobutyl cyanide, azobiscyanovalerate and other azo compounds. In peroxide, it is often used to add a small amount of A redox initiation system composed of two-component oxidation-reduction systems such as FeSO4, NaHSO3, and FeCl2. The whole process of free radical polymerization-generally consists of three steps of chain initiation, chain growth and chain termination. In addition, companion There are different degrees of chain transfer reactions. Acrylamide undergoes free radical polymerization under the action of an initiator to obtain a polymer with high molecular weight, The process of acrylamide follows the polymerization reaction mechanism, and each elementary reaction is as follows.
(1) Chain initiation.
The reaction that forms free radicals is called the initiation reaction. Initiator and thermal energy, light energy, radiant energy can make the monomer into single, Body free radicals. When using an initiator, chain initiation involves a two-step reaction.
①Initiator I splits to form-to-primary radical R.:
In the formula, Ks is the rate constant of initiator decomposition, for example:
② Primary radical R. Addition with monomer to generate monomer free radical:
Where K- - the rate constant of initiation. After the formation of monomer free radicals, it continues to polymerize with other monomers and enters the chain growth phase.
(2) Chain growth.
The monomer free radicals generated by chain initiation continuously combine with monomer molecules to generate chain free radicals. This repeated process is called chain, Growth reaction, the reaction process is as follows:
In the formula, Kp-chain growth rate constant. The chain growth response is very rapid, K. The activation energy of chain growth is low within the range of 1 x10*L/ (mol●s), which is 21 ~
23kJ/mol, so the chain growth rate is extremely high. Monomer free radicals can combine thousands or even tens of thousands of monomers in a very short time, Generate free radicals in the polymer chain and then terminate into macromolecules. Therefore, in the reaction mixture, there are almost only two parts, monomer and polymer. Composition. The chain growth reaction is an exothermic reaction, and the heat of polymerization of acrylamide is 82.8 kJ/mol. The structure of the polymer depends on the chain growth reaction. In the growth process, there are two ways to combine chain radicals with monomers:
The results of infrared spectroscopy, nuclear magnetic resonance analysis and chemical determination indicate that the head-to-tail is mainly obtained in free radical polymerization Combined arrangement. There are two reasons for the head-to-tail structure: electronic effect and steric effect:
① When connected head-to-tail, the single electron on the free radical forms a conjugate with the substituent to make the generated free radical more stable; However, when connected head-to-head (tail-to-tail), there is no conjugation effect and the generated free radicals are unstable. Therefore, the resulting polymer The chain is mainly combined head-to-tail.
② The spatial steric hindrance when combining head-to-tail is smaller than that of head-to-head (tail-to-tail), so it is conducive to head-to-tail combination. During free radical polymerization, although the macromolecular chains are dominated by the first-to-tail structure, they cannot be absolutely regular in sequence structure. From From the perspective of the three-dimensional structure, the arrangement of macromolecular chain substituents prepared by free radical polymerization in the spatial structure is random.
(3) Chain termination.
The reaction of chain radicals losing their activity to form stable polymer molecules is chain termination reaction. Chain termination and chain growth are competing Contention reaction. Chain termination is related to the concentration of free radicals in the system. To obtain a sufficiently high molecular weight product, keep the system in Low free radical concentration is very important.
(4) Chain transfer. .
In free radical polymerization, in addition to the three-step elementary reaction of chain initiation, chain growth, and chain termination, chain transfer is often accompanied reaction. The so-called chain transfer reaction, that is, a growing chain of free radicals seizes an atom from other molecules and terminates Stable macromolecule. Other molecules can be monomers, initiators, solvents or special chain transfer agents. After the chain transfer occurs, the original The coming free radicals are terminated, the degree of polymerization is reduced, and a free radical is formed. If the new free radicals are sufficiently active, -The step initiates the transfer of the monomer chain and then continues to grow. The chain transfer constant of acrylamide polymerization in aqueous solution is shown in Table 2-3.