lecture 03: Polymerization process or synthesis by Dr. Salma Amir

1. Lecture No. 03Course title:Inorganic PolymersTopic: Polymerization processes Course instructor: Dr. Salma Amir GFCW Peshawar

2. Polymerization • Polymerization, any process in which relatively small molecules, called monomers, combine chemically to produce a very large chainlike or network molecule, called a polymer. • Themonomer molecules may be all alike, or they may represent two, three, or more different compounds. 

3. Synthesis/polymerization processes A wide variety of synthetic methods have been used for synthesizing inorganic polymers. Syntheses can be classified according to • Step-growthpolymerization • Chain-growth polymerization • Ring-opening polymerization

4. Step-Growth polymerization • A sizable number of linear inorganic polymers (including most that have metal ions as part of the polymer backbone) are synthesized by step-growth syntheses. • These polymerizations are primarily condensation reactions, although step-growth addition polymerizations are also known. • The terms step growth polymerization, step-growth condensation, and step-growth addition are redundant because polymerization, condensation, and addition all imply growth; therefore, these terms can be shortened to step polymerization, step condensation and step addition.

5. Step condensation • In a step condensation polymerization, water or another small molecule is displaced from the growing polymer chain in each polymerization step. • Several methods are possible for removing the water that is formed in step condensation polymerizations-such as the esterification nHOOC–M–COOH + nHO–R–OH H[–OOC–M–COO–R–]nOH + (2n -1)H2O where M is a metal ion and its coordinated ligands (including 2 with carboxylic acid functionality) and HO–R–OH is a diol or a second metallic species with dihydroxy functionality.

6. An example of a step condensation reaction where the small molecule is HCl is shown nHO–M–OH + nCl–R–Cl H[–O–M–O–R–]nCl + (2n -1)HCl • where M is a metal ion and its coordinated ligands including diol or dihydroxo functionality and Cl–R–Cl is an activated dichloro species such as ClC6H4SO2C6H4Cl or ClC(O)(C6H4)C(O)Cl. Alternatively, R could be another coordination or organometallic species including two chloro ligands or ligands with activated chloro groups.

7. A large number of step condensation reactions have been used to synthesize organic polymers. Most, if not all, of these reactions have been adapted to inorganic and organometallic polymers. • Step condensation polymerizations of the type require very careful reactant ratio control as well as a high extent of reaction to obtain polymers rather than just short oligomers. nAMA + nBRB A(MR)nB + (2n -1)AB where AB is the condensation byproduct, M is with functional group A, and R is the second component with the complementary functional group B and may be either an organic or an inorganic species. • The average degree of polymerization (DP) is almost twice as large as the average number of repeating units (n), because every MR bond that is formed is concurrent with an increase in the size of the growing polymer unit.

8. The simplest scheme of this polymerization involves the reaction of a difunctional monomer AB, which contains both functional groups A and B in the molecule. For example, A can be an amine and B a carboxylic acid group. Another scheme involves the reaction between two difunctional monomers of the type AA and BB. In any case, each polymer linkage will have involved the reaction of the functional groups A and B coming from two molecules (monomers or chains).

10. Chain Polymerization • Chain polymerizations are often called addition polymerizations. The chain is extended through either a radical or an ionic initiator. The monomers that produce chain polymers must have unsaturated functionality to form the bonds that make up the backbones of the polymer chains. The chain polymerization process is normally considered to consist of four reaction types: • chain initiation, • chain propagation, • chain transfer, and • chain termination.

11. 1. Initiation • Initiation is the mechanism which starts the polymerization process.  • Monomers are quite easily polymerized by a variety of activating methods. for example, simply by heating, and ultraviolet light can have exactly the same effect. Usually, an activating agent is used. This is an unstable chemical which produces active species that attack the monomer. A good example is benzoyl peroxide which splits up when heated:

12. The radical initiator that is formed , combines with the monomer (M) to form a chain initiator as shown (C6H5)C(O)O* + M (C6H5)C(O)O—M* A cationic initiator can be a simple protonic acid (HX), such as sulfuric, perchloric, or hydrochloric, where the proton reacts with the monomer (M) HX H+ + X- H+ + M HM+ An anionic initiator can be as simple as n-butyllithium. The butyl anion then reacts with the monomer (M) n-C4H9Li n-C4H9- + Li+ n-C4H9-+ M n-C4H9M-

13. 2. Propagation • These chain initiation steps are followed by hundreds or thousands of chain propagation steps per initiator until the supply of monomer is exhausted or the initiator is terminated or transferred from the chain. These reactions can be simplified as shown I + M IM IM + M IM2 IM2+ M IM3 …………… IMn-1 + M IMn • where I= initiator (regardless of whether the initiator is a radical, an anion, or a cation) and M=monomer.

14. 3. Chain transfer and termination • The n-mer (polymer with n repeating units) eventually either transfers its active initiator group to another monomer or, in the case of radical initiators, it can react with another radical chain to either combine or simply be deactivated IMn + M Mn + IM ……………chain transfer IMn + IMmMn+m ……………….coupled termination IMn + IMmMn+ Mm………deactivated termination Thousands of metal-containing polymers have been synthesized by chain polymerizations. Virtually all of these polymers are anchored metal polymers that have vinyl precursors.

15. Ring opening polymerization • Ring-opening polymerization is a broad aspect in polymer chemistry that involves chain growth polymerization, whereby the tail end of a polymer acts as a reactive site where other cyclic monomers can bind by opening their ring system for effective elongation of the polymer. The tail end of the polymer where reactions take place can be cationic, anionic, or a radical. The cyclic monomer structures that can be used for ring-opening polymerization include alkanes, alkenes, and compounds containing heteroatoms from different groups, for example, the sulfur family (polysulfur, sulfides, etc.), the nitrogen family (amines, amides, imides, etc.), the oxygen family (acetals, ethers, esters, etc.), and the silicon family (silathers, siloxanes, silanes, and carbosilanes).