Don't worry that we've gone back to a simpler diagram. The sigma bond between the carbon atoms isn't affected by any of this. The other electron returns to the right hand carbon. You can show this using "curly arrow" notation if you want to:. If you aren't sure about about curly arrow notation you can follow this link. You would get more energy out when the new bond is made than was used to break the old one.
The more energy that is given out, the more stable the system becomes. What we've now got is a bigger free radical - lengthened by CH 2 CH 2.
That can react with another ethene molecule in the same way:. So now the radical is even bigger. That can react with another ethene - and so on and so on. The polymer chain gets longer and longer.
The chain does not, however, grow indefinitely. Sooner or later two free radicals will collide together. That immediately stops the growth of two chains and produces one of the final molecules in the poly ethene. It is important to realise that the poly ethene is going to be a mixture of molecules of different sizes, made in this sort of random way. Because chain termination is a random process, poly ethene will be made up of chains of different lengths.
A large number of important and useful polymeric materials are not formed by chain-growth processes involving reactive species such as radicals, but proceed instead by conventional functional group transformations of polyfunctional reactants.
These polymerizations often but not always occur with loss of a small byproduct, such as water, and generally but not always combine two different components in an alternating structure.
The polyester Dacron and the polyamide Nylon 66, shown here, are two examples of synthetic condensation polymers, also known as step-growth polymers.
Although polymers of this kind might be considered to be alternating copolymers, the repeating monomeric unit is usually defined as a combined moiety. Formulas for these will be displayed below by clicking on the diagram. Condensation polymers form more slowly than addition polymers, often requiring heat, and they are generally lower in molecular weight.
The terminal functional groups on a chain remain active, so that groups of shorter chains combine into longer chains in the late stages of polymerization. The presence of polar functional groups on the chains often enhances chain-chain attractions, particularly if these involve hydrogen bonding, and thereby crystallinity and tensile strength.
The following examples of condensation polymers are illustrative. Note that for commercial synthesis the carboxylic acid components may actually be employed in the form of derivatives such as simple esters. Also, the polymerization reactions for Nylon 6 and Spandex do not proceed by elimination of water or other small molecules.
A typical of particle consist of 1—10, macromolecules, where macromolecule contains about — monomer units. Recent Research in Polymerization. Emulsion polymerization is a unique process involves emulsification of hydrophobic monomers by oil-in water emulsifier, then reaction initiation with either a water soluble initiator e.
These emulsion polymers find a wide range of applications such as synthetic rubbers, thermoplastics, coatings, adhesives, binders, rheological modifiers, plastic pigments [ 1 ]. Emulsion polymerization is a rather complex process because nucleation, growth and stabilization of polymer particles are controlled by the free radical polymerization mechanisms in combination with various colloidal phenomena [ 1 ].
Aside from other polymerization techniques, emulsion polymerization affords increasing molecular weight of the formed latexes through decreasing polymerization rate by either decreasing initiator concentration or lowering reaction temperature [ 5 , 6 ].
Systems of emulsion polymerization involve 1 conventional emulsion polymerization, in which a hydrophobic monomer emulsified in water and polymerization initiated with a water-soluble initiator [ 5 ]. Miniemulsion, microemulsion and conventional emulsion polymerizations show quite different particle nucleation and growth mechanisms and kinetics [ 1 ]. Many articles discuss different types of emulsion polymerization found in literature [ 1 , 11 , 12 , 13 , 14 , 15 , 16 ].
The main components of emulsion polymerization media involve monomer s , dispersing medium, emulsifier, and water-soluble initiator [ 5 , 17 , 18 , 19 ]. The dispersion medium is water in which hydrophobic monomers is emulsified by surface-active agents surfactant.
When surfactant concentration exceeds critical micelle concentration CMC it aggregate in the form of spherical micelles, so surface tension at the surface decrease, as a result hydrophobic monomers enter in to the vicinity of micelle and reaction continue until all monomer droplets are exhausted and micelle containing monomers increase in size.
Typical micelles have dimensions of 2—10 nm, with each micelle containing 50— surfactant molecules [ 5 ]. Water-soluble initiators enter into the micelle where free radical propagation start.
In general, monomer droplets are not effective in competing with micelles in capturing free radicals generated in the aqueous phase due to their relatively small surface area [ 1 ], so the micelle act as a meeting site of water-soluble initiators and hydrophobic vinyl monomers.
As polymerization continue inside micelle, the micelle grow by monomer addition from monomer droplets outside and latex are formed. Schematic representation of emulsion polymerization shown in Figure 1. Emulsion polymerization carried out through three main intervals as shown in Figure 2.
Schematic representation of emulsion polymerization. Emulsion polymerization intervals. There is a separate monomer phase in intervals I. The particle number increases with time in interval I and particle nucleation occurs in interval I. At the end of this stage most of surfactants are exhausted i. About one of every — micelles can be successfully converted into latex particles [ 1 ]. Particle nucleation process is greatly affected by surfactant concentration, which in turn affect particle size and particle size distribution of latex [ 1 ].
The lower the surfactant concentration, the lower the nucleation period the narrow the particle size distribution. At interval II Particle growth stage , the polymerization continue and polymer particles increase in size until monomer droplets exhausted. Monomer droplets act as reservoirs to supply the growing particles with monomer and surfactant species. At interval III, the polymer size increase as latex particles become monomer-starved and the concentration of monomer in the reaction loci continues to decrease toward the end of polymerization [ 1 ].
Initiator act to generate free radicals by thermal decomposition, or redox reactions. These initiators initiates emulsion polymerization without the need of stabilizers. Act to decrease interfacial tension between monomer and aqueous phase, stabilize the latex and generate micelles in which monomers emulsified and nucleation reaction proceed. Surfactants increase particle number and decrease particle size, these surfactants may be 1 Anionic surfactants such as fatty acid soaps sodium or potassium stearate, laurate, palmitate , sulfates, and sulfonates sodium lauryl sulfate and sodium dodecylbenzene sulfonate ; 2 Nonionic surfactants such as poly ethylene oxide , poly vinyl alcohol and hydroxyethyl cellulose; 3 Cationic surfactants such as dodecylammonium chloride and cetyltrimethylammonium bromide [ 5 , 21 ].
For ionic surfactants, micelles formed only at temperatures above the Krafft point. For a nonionic surfactant, micelles formed only at temperatures below the cloud point. Emulsion polymerization carried out below the cloud temperature and above the Krafft temperature [ 5 ]. Polymerizable surfactants surfactants with active double bond such as sodium dodecyl allyl sulfosuccinate [ 13 , 22 , 23 , 24 ] also used to produce latexes with chemically bound surface-active groups [5, 25—30, 31].
Polymerized surfactants surfactants with active double bond consist of amphipathic structure comprising hydrophobic tail and hydrophilic head group [ 32 ], in addition to polymerized vinyl groups [ 33 ] in their molecular structure, which acquire them unique physicochemical properties other than traditional surfactants moieties [ 34 ] such as;.
They have surface activity like ordinary surfactants and polymerized vinyl group like vinyl monomers, so they have the ability to undergo polymerization reactions.
Small unsaturated ethene monomers join up by the opening of the double bond allowing them to join up to form a long carbon chain. Polymers made in this way are called addition polymers. The monomers used to make other addition polymers are drawn in a similar shape to ethene, for example, propene. Although this is the usual way to draw the structural formula for propene, for the purposes of showing how the molecule acts as a monomer and can form a polymer it should be drawn in a different way.
Examples of other monomers are given below. Given the structure of the monomer the structural formula of the addition polymer can be drawn. Three propene monomers.
The molecules link together by partially breaking their double bonds.
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