Polymers Lec 2

Polymers

Polymers 5/5/2010Department of Polymer & Process Engineering, UET, Lahore1Polymerization In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains.

5/5/2010Department of Polymer & Process Engineering, UET, Lahore2In more straightforward polymerization, alkenes, which are relatively stable due to bonding between carbon atoms form polymers through relatively simple radical reactions.In contrast, more complex reactions such as those that involve substitution at the carbonyl group require more complex synthesis.

5/5/2010Department of Polymer & Process Engineering, UET, Lahore3Polymer SynthesisThere are two major classes of polymer formation mechanisms

Addition Polymerization Step-Growth ( Condensation) Polymerization5/5/2010Department of Polymer & Process Engineering, UET, Lahore4Addition Polymerization The polymer grows by sequential addition of monomers to a reactive siteChain growth is linearMaximum molecular weight is obtained early in the reaction

5/5/2010Department of Polymer & Process Engineering, UET, Lahore5General Mechanism

5/5/2010Department of Polymer & Process Engineering, UET, Lahore6Step-Growth PolymerizationMonomers react together to make small oligomers. Small oligomers make bigger ones, and big oligomers react to give polymers.Chain growth is exponentialMaximum molecular weight is obtained late in the reaction

5/5/2010Department of Polymer & Process Engineering, UET, Lahore7General Mechanism

5/5/2010Department of Polymer & Process Engineering, UET, Lahore8

5/5/2010Department of Polymer & Process Engineering, UET, Lahore9Addition Polymerization It isn't difficult to form addition polymers from monomers containing C=C double bonds; many of these compounds polymerize spontaneously unless polymerization is actively inhibited.There are 3 mechanisms through which the addition polymerization can proceedFree-Radical Polymerization ReactionsAnionic PolymerizationCationic Polymerization5/5/2010Department of Polymer & Process Engineering, UET, Lahore10Free-Radical PolymerizationThe simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of afree radicalto the monomer. The termfree radicalis used to describe a family of very reactive, short-lived components of a reaction that contain one or more unpaired electrons.5/5/2010Department of Polymer & Process Engineering, UET, Lahore11The presence of a free radical, addition polymers form by a chain-reaction mechanism.There are 3 main steps involvedChain-initiationChain-propagationChain- termination5/5/2010Department of Polymer & Process Engineering, UET, Lahore12Chain-initiationA source of free radicals is needed to initiate the chain reaction. These free radicals are usually produced by decomposing a peroxide such as dibutyl peroxide or benzoyl peroxide. In the presence of either heat or light, these peroxides decompose to form a pair of free radicals that contain an unpaired electron.5/5/2010Department of Polymer & Process Engineering, UET, Lahore13


Dibutyl peroxideBenzoyl peroxideChain PropagationThe free radical produced in the chain-initiation step adds to an alkene to form a new free radical.

The product of this reaction can then add additional monomers in a chain reaction.


Chain TerminationWhenever pairs of radicals combine to form a covalent bond, the chain reactions carried by these radicals are terminated.


Chain Termination mechanisms

17Anionic PolymerizationAddition polymers can also be made by chain reactions that proceed through intermediates that carry either a negative or positive charge.When the chain reaction is initiated and carried by negatively charged intermediates, the reaction is known asanionic polymerization.5/5/2010Department of Polymer & Process Engineering, UET, Lahore18MechanismThe reaction is initiated by a Grignard reagent or alkyllithium reagent, which can be thought of a source of a negatively charged CH3-or CH3CH2-ion.


The CH3-or CH3CH2-ion from one of these metal alkyls can attack an alkene to form a carbon-carbon bond.


Propagation The product of this chain-initiation reaction is a new carbanion that can attack another alkene in a chain-propagation step.


Termination The chain reaction is terminated when the carbanion reacts with traces of water in the solvent in which the reaction is run.


Cationic PolymerizationThe intermediate that carries the chain reaction during polymerization can also be a positive ion, or cation. In this case, thecationic polymerizationreaction is initiated by adding a strong acid to an alkene to form a carbocation.


Propagation The ion produced in this reaction adds monomers to produce a growing polymer chain.


Termination The chain reaction is terminated when the carbonium ion reacts with water that contaminates the solvent in which the polymerization is run.


Step-Growth Polymerization

26Because high polymer does not form until the end of the reaction, high molecular weight polymer is not obtained unless high conversion of monomer is achieved.

Xn = Degree of polymerizationp = mole fraction monomer conversion

27Formation of Nylon

28Polymerization MethodsA number of different techniques are applied to carry on the polymerization process, important of them are:Bulk polymerizationSuspension polymerizationSolution polymerizationEmulsion polymerization

5/5/2010Department of Polymer & Process Engineering, UET, Lahore29Bulk polymerizationNeat monomer (and initiator)Simplest formulation and equipmentMost difficult in control, when polymerization is very exothermicIf polymer is insoluble in monomer polymer precipitateCommon problems: Heat transfer Increase in viscosityCommercial uses:1. Casting formulations2. Low MW polymers for adhesives, plasticizers, and lubricant additives5/5/2010Department of Polymer & Process Engineering, UET, Lahore30Suspension polymerizationDisperse monomer droplets in a non compatible liquid (e.g. H2O)Polymerize the monomer by an initiatorStabilize the dispersion with a stabilizer (e.g. poly(vinyl alcohol) or methyl cellulose)Heat transfer is efficient and reaction is easily controlledSimilar to bulk polymerization in kinetics and mechanism (micro-bulk polymerization)Not applicable for tacky polymers (e.g. elastomers) due to the tendency of agglomerationCommercial uses:For making granular polymers, e.g. PS, PVC, PMMA

5/5/2010Department of Polymer & Process Engineering, UET, Lahore31Solution polymerizationUse monomer solutionHeat transfer is very efficientMW may be severely limited by chain transfer reaction (probably caused by the solvent molecule or its impurities).Solvent residues difficult to remove completelyEnvironmental concerns organic solvent wasteProduct sometimes directly usableCommercial uses:Acrylic coating, fiber spinning, film casting

5/5/2010Department of Polymer & Process Engineering, UET, Lahore32Emulsion polymerizationWidely used for large-scale preparationFor water-based (latex) paints or adhesivesEmulsion oil-in-water. Inverse emulsion water-in-oil (less stable)Extremely high MW are obtainable, but often too high to be usefulChain transfer reagents are often added to control the MWAlso suitable for tacky polymers (small particles are relative stable and can resist agglomeration)

5/5/2010Department of Polymer & Process Engineering, UET, Lahore33Mechanism Emulsification:Add emulsifying agent (e.g., soap or detergent) in an aqueous solution to form micellesMonomers enter and swell the micellesInitiation:Radicals (redox type) are generated in the aqueous phase and diffuse into micelles

5/5/2010Department of Polymer & Process Engineering, UET, Lahore34Propagation:Polymerization propagated within micellesMore monomers enter micelles to support the polymerizationTermination:Termination by radical combination when a new radical enters the micelle.

5/5/2010Department of Polymer & Process Engineering, UET, Lahore35Sheet1pXn010.11.11111111110.21.250.31.42857142860.41.66666666670.520.62.50.73.33333333330.850.9100.92513.33333333330.95200.975400.991000.9991000

step-growth11.11111111111.251.42857142861.666666666722.53.333333333351013.333333333320401001000

XnMole Fraction Conversion (p)Degree of Polymerization

Sheet2

Sheet3

Polymers Lec 2

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