Question

Describe the step-growth and chain-growth polymerization by using the following terms: monomer, reaction, rate, molecular weight.

Answer 1

Step-growth polymerization is a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters, polyamides, polyurethanes, etc. the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight.

Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers, usually containing heteroatoms such as nitrogen or oxygen. step-growth polymers are classified as condensation polymers, but not all step-growth polymers (like polyurethanes from isocyanate and alcohol bifunctional monomers) release condensation.

The important step-growth polymerizations are condensation and addition polymerization the condensation reaction, two monomers combine with the loss of a small molecule, an alcohol, a water molecule or an acid as an addition reaction involves only the rearrangement of the electrons of a double bond to form a single bond with another molecule.

Chain-growth polymerization or chain polymerization is a polymerization technique where unsaturated monomer molecules add the active site of a growing polymer chain one at a time. Growth of the polymer occurs only at one (or possibly more) ends. Addition of each monomer unit regenerates the active site

Polyethylene, polypropylene, and polyvinyl chloride (PVC) are common types of plastics made by chain-growth polymerization. They are the primary component of four of the plastics specifically labeled with recycling codes and are used extensively in packaging.

Step-growth polymerisations, the polymer molecular weight increases very slowly with percentage of conversion. Because all monomers are capable of reacting at any given time. High molecular weight polymers are produced only at very high conversion.

Chain-reaction polymerisations, high molecular weight species are formed instantly after the beginning of the reaction, i.e. at low monomer conversion rates.

The kinetics and rates of step-growth polymerization can be using a polyesterification mechanism. The esterification is an acid-catalyzed process and protonation of the acid is followed by interaction with the alcohol to produce an ester and water.. The water is efficiently removed. the functional group reactivities are independent of chain length.

Chain –growth polymerization of rate law result in high molecular weight polymer being formed at low conversion. The final weight is determined by the rate of propagation compared to the rate of individual chain termination, which means both chain transfer and chain termination steps.

chain initiation, an initiator which starts the chemical process. Typical initiators include any organic compound with a labile group: e.g. azo (-N=N-), disulfide (-S-S- or peroxide (-O-O-). .

chain propagation

chain transfer, terminates the chain, but the active site is transferred to a new chain.

chain termination it occurs either by combination or disproportionation