Chain-growth polymerization
Chain polymerization (English IUPAC chain polymerization, in the German language and often not IUPAC - compliant polymerization; see polyreaction ) is a collective name of chemical reactions in which the same or different monomers react via a chain growth polymers. In the growth reaction is a continuous addition of the most unsaturated organic monomers to the growing chain. Neither an elimination of by- products, nor a migration of molecular groups within the reactants takes place. No chain polymerizations are so polycondensation and polyaddition. Chain polymerizations can be subdivided according to their active sites in radical, cationic, anionic and coordinative chain polymerizations. A distinction is made between simple homopolymerization, in which only one type of monomer is converted, and the copolymerization can be caused to react with the at least two different monomers.
Mechanisms
Radical chain polymerization
The free radical polymerization comprises three sub-steps:
1 Start reaction in which the active site is formed.
Second growth reaction in which the macromolecular chain of a chain reaction grows (repeated addition of the monomers ), and
3 termination reaction, at which the growth of the chain by disproportionation combining or terminates irreversibly.
Mechanism
To start chain breaks a radical the multiple bond ( for example, a C = C double bond of a vinyl group ) and creates a growth- capable primary radical:
Now constantly Store in the primary radical in a growth reaction with low activation energy of monomers.
Through the meeting of two radicals, so by combining a chain termination is caused.
Further, a chain terminator be introduced by disproportionation.
To stop the reaction targeted to specific reagents, so-called free radical scavengers may be added. Besides, a certain control of the molar mass will be achieved by the polymerization, adding a chain transfer agent.
Kinetics of free-radical chain polymerization
For each of the partial reactions of the free-radical polymerization equations for the reaction rates can be formulated. The knowledge of this relationship makes it possible to control the average degree of polymerization of the resulting polymer, and to indicate some of the effects occurring during the polymerization.
1 initiation
The speed of the initiation is dependent upon the concentration of the initiator and of the relevant speed coefficient from:
However, the initiator decomposition does not necessarily provide fully effective radicals R *, as some of them do not cause a chain initiation, but otherwise react or recombine. Accordingly, a factor is introduced, which represents the effectiveness of the initiator decomposition for initiation:
Second chain growth
The speed of the chain growth, ie, the monomer consumption is dependent on the monomer concentration, the concentration of the polymer radicals and the corresponding rate constants:
3 recombination
The rate of recombination, ie the consumption of polymer radicals P *, is a quadratic function of the concentration and of course the coefficients:
4 transmission
The speed of transmission, is dependent on the concentration of the carrier, the concentration of polymer radicals, and the coefficient from:
Fifth root - I- Law
From the given rate laws can derive a formula for the rate of growth. This applies only at medium conversions, but are then basically the overall rate of reaction to:
6 Mayo equation
Similarly, one can determine the number-average degree of polymerization. It is derived from the ratio of the growth rate to the speeds of all reactions in which stops the growth:
Mayo inferred from a formula with which one can either calculate the propagation constant for a mixture of monomer and transfer agent, or the average degree of polymerization of the polymer formed when you mix concrete volumes monomer and transfer agent:
Therein is the number-average degree of polymerization in the absence of a transfer agent.
Of polymerization
Course of the reaction of radical polymerization is as follows:
- Sales volume < 0.01 %: Initiation - Non Clinical course in
There are increasingly emerging initiator radicals, primary radicals and a few oligomeric radicals. The overall reaction rate is increasing rapidly, the average degree of polymerization is low.
- Turnover from 0.01 to 5 %: Ideal Stationary reaction course
At such low conversion, the monomer can be regarded as constant, as is the number of available radicals, the overall reaction rate remains constant while the rate of polymerization increases. She released her first macroradicals. The polymerization- is relatively narrow.
- Sales from 5 to 20 %: the end of the stationary reaction process
The monomer concentration increases the consumption sharply, so that the reaction rate decreases. There are now many macroradicals present and termination reactions take place.
- Turnover 20 to 60%: gel or Trommsdorff-Norrish effect
The polymerization is subject to a self-acceleration. There are now facing very large polymer radicals, which are diffusionsgehindert because of their size. As a result, fewer termination reactions take place, while the monomers are still mobile enough to maintain the reaction in the life and the initiator breaks down to form further new radicals. In this phase, the degree of polymerization increases sharply, and the polymerization-degree is very broad.
- Sales 60% max.: Glass effect
The polymer molecules and polymer radicals are now so large and immobile, that the reaction mixture slowly solidifies (hence "glass" ). The other monomers in the reaction mixture is increasingly stationary, therefore, the polymerization proceeds more slowly. A complete conversion is i.d.R. not achieved.
Current Trends
Since a few years it is possible to control the termination reactions of free radical polymerization. Using the techniques of Controlled Free Radical Polymerization ( LFRP ), it is possible to synthesize polymers with narrow molecular weight distribution by adding a ' control reagent '. Depending on the use of the control reagent is far different between the atom transfer radical polymerization ( ATRP), nitroxide mediated polymerization (NMP) and chain transfer methods such as reversible addition - fragmentation chain transfer process ( RAFT). The common principle of these methods is, at any time to have only a very small number of reactive chain ends, whereby the radical recombination is severely hampered as a termination step.
Cationic chain
As a starter here, for example, functions as an acid borofluoric which protonated monomer unit at the double bond in the starting reaction.
In the growth reaction, the resulting reaction from the starting cation is added to another monomer, in turn, produces a cation.
A termination reaction such as the free-radical polymerization, does not occur. However, can be transferred to a new monomer unit by an elimination reaction, a proton.
Thus, the growth of the chain is interrupted and a proton is a further start reaction.
Anionic polymerization
Here, the growing chain is an anion, such a reaction is initiated by organolithium compounds or Grignard compounds. Frequently, may not transfer or termination reaction can be formulated, one then speaks of a living polymerization. This can be obtained ( polydispersity close to unity ) often polymers having a very uniform chain length. Anionic polymerization reactions are generally sensitive to slightest traces of water, and therefore require very thorough absolutierte starting materials.
Coordinative chain polymerization
The catalysts are transition metal compounds whose structure can be characterized in such a way that a central atom ( the metal ion ) is surrounded by ligands such that one monomer and the polymer chain attach it ( coordinate ) can. The principle is based on activation of the monomers due to the interaction of the monomer with the metal. The double bond in the monomer is thereby weakened and initiates the addition of a second monomer. For stabilization of the resulting complex compound to the monomer pushes into the already existing polymer chain and another monomer is attached, etc. The polymerization reaction is initiated. The process is also referred to as insertion polymerization.
The advantage of the coordination is that depending on the choice of catalyst and monomers, the tacticity of the resulting polymer can be controlled, which can have significant impact on the polymer property.
There are various types of Koordinationspolymerisationen with different mechanisms. The most important is named after their discoverers Ziegler -Natta polymerization, which allows to implement at low temperatures and low pressures, for example, ethene to linear high density polyethylene ( HDPE ). Of great importance also is the polymerization using metallocene catalysts.
More specific types of coordination as the ring-opening metathesis polymerization ( ring -opening metathesis polymerization, ROMP) find their application in the preparation of special polymers that are produced by means of ring-opening cyclic monomers, and combination with transition metal catalysts.
Copolymerization
Unlike simple homopolymerization, in which only a single monomer species is used, different monomers can be simultaneously used in the copolymerization, or are reacted successively. The product of copolymerization is a copolymer, the different monomer has in a molecule. Depending on the distribution of the different types of monomers in the polymer, a distinction between statistical, alternating, block and graft copolymers.
Twin Polymerization
The twin polymerization chemists have developed the TU Chemnitz 2007. A specially designed compound ( Zwillingsmonomer ), which consists of two covalently linked components AC, reacts in a process coupled to two different end products, for example, a polymer with the block A and a polymer with the block C. In the simultaneous twin polymerization are reacted together, wherein two polymers of the blocks A and B or of only the component C results in two different starting compounds ( AC and BC ). However, this method is still under development and may be for the manufacture of nanostructured polymeric hybrid materials is important.
Technical Procedures
- Bulk (also called mass or bulk polymerization ): monomer reacts with the initiator in pure form without solvent
- Solution: monomer and polymer dissolved in solvent
- Precipitation polymerization: monomer dissolved in solvent, polymer precipitates
- Emulsion polymerization: monomer solved by an emulsifier in water, polymer precipitates
- Suspension polymerization: monomer stabilizers and suspended by stirring in water ( small drops), polymer precipitates
- Gas phase: gaseous monomer keeps polymer granules in a fluidized bed
- Polymerization of monomolecular layers by the Langmuir -Blodgett