The word ‘polymer’ comes from the Greek words poly (meaning ‘many’) and meros (meaning ‘parts’).

Example: POLYBUTADIENE = (BUTADIENE+ BUTADIENE+......)n Where n = 4,000

Polymers are very large molecules made when hundreds of monomers join together to form long chains.

Polymers are complex and giant molecules usually with carbons building the backbone, different from low molecular weight compounds.

The small individual repeating units/moleules are known as monomers(means single part).

Imagine that a monomer can be represented by the letter A. Then a polymer made of that monomer would have the structure:

-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A This kind of polymer is known as HOMOPOLYMER.

In another kind of polymer, two different monomers might be involved.

If the letters A and B represent those monomers, then the polymer could be represented as:

-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A- B-A-B-A A polymer with two different monomers is known as a

COPOLYMER / HOMOPOLYMER.

Linear High Density Polyethylene (HDPE), PVC, Nylon, CottonBranched Low Density - Polyethylene (LDPE)

Cross-l inked Rubber

Network Kevlar, Epoxy

Should be inert and compatible with the environment.

Should be non-toxic. Should be easily administered. Should be easy and inexpensive to

fabricate. Should have good mechanical strength.

The process by which the monomer molecules are linked to form a big polymer molecule is called ‘polymerisation’.

Polymerization is a process of bonding monomer, or “single units” together through a variety of reaction mechanisms to form longer chains named Polymer

As important as polymers are, they exist with monomers, which are small, single molecules such as hydrocarbons and amino acids.

Addition Polymerization=

When monomers just add on to form the polymer, the process is called ‘addition polymerisation’. The polymer is the only product

e.g. Ethylene monomers add on to form polyethylene. (5 Ethylene monomers)

Polyethylene formation

Condensation polymerisation= The molecules do not just add on but also undergo some reaction in

forming the polymer, the process is called ‘condensation polymerisation’.

Here the two molecules condense to form a polymer.The condensation takes place between two reactive functional groups, like the carboxyl group(of an acid) and the hydroxyl group(of an alcohol). While forming the polymer water molecules also get eliminated.

In A. P. mol. weight of polymer is roughly equal to that of all monomers, while in C. P. the mol. weight of polymer is lesser by the weight of simple molecules eliminated during the condensation process. E.g. Condensation polymerization diacid diamine.

Polymers which are isolated from natural materials, are called as ‘natural polymers’.

E.g. : Cotton, silk, wool, rubber.

natural rubber Polymers synthesized from low molecular weight compounds, are

called as, ‘synthetic polymers’. E.g. polyethylene, nylon, terylene.

Polyethylene

A Polymer whose backbone chain is essentially made of carbon atoms is termed an ‘Organic polymer’.Examples- cellulose, proteins, polyethylene, nylons.

A Polymer which does not have carbon atom in their chain is termed as ‘Inorganic polymer’ .

Examples- Glass and silicone rubber

Some polymer are soften on heating and can be converted into any shape that they can retain on cooling.

Such polymer that soften on heating and stiffen on cooling are termed as

`thermoplastic ’ polymers. Ex. Polyethylene, PVC, nylon, sealing wax.

Polymer that become an infusible and insoluble mass on heating are called ‘thermosetting’ polymers. Plastics made of these polymers cannot be stretched, are rigid and have a high melting point.

Polymer is shaped into hard and tough utility articles by application of heat and pressure, is known as ‘plastics’.

E.g. polysterene, PVC, polymethyl methacrylate.

When plastics are vulcanised into rubbery products exhibiting good strength and elongation, polymers are known as ‘elastomers’.

E.g. silicone rubber, natural rubber, synthetic rubber, etc.

Long filament like material whose length is atleast 100 times it’s diameter, polymers are said to be ‘f ibres’.

E.g. Nylon, terylene.

Polymers used as adhesives, potting compounds, sealants, etc., in a liquid form are described as ‘l iquid resins’.

E.g. Epoxy adhesives and polysulphides sealants.

Structure   Chemical Name   Trade Name or CommonName  

      poly(tetrafluoroethylene)   Teflon  

 

polypropylene Herculon  

 

polyisobutylene butyl rubber

polyethylene

There are three significant reactions that take place in addition polymerization:

INITIATOR: A relatively unstable molecule that decomposes into a free

radical. Used to "initiate" a polymer growth reaction. (A molecule with an unpaired electron, making it highly reactive).

The stability of a radical refers to the molecule's tendency to react with other compounds. An unstable radical will readily combine with many different molecules. However a stable radical will not easily interact with other chemical substances.

The first step in chain polymerization- Initiation involves the formation of a free radical. Addition can occur at either end of the monomer. This process is illustrated in the following animation in which a chlorine atom possessing an unpaired electron (often indicated as cl-) initiates the reaction.

.

Propagation is the middle step in chain polymerization where successive monomers are attached to the growing chain. In the propagation stage, the process of electron transfer and consequent motion of the active center down the chain proceeds.

In following reaction(chain), refers to a chain of connected monomers, and X refers to a substituent group (a molecular fragment) specific to the monomer. For example, if X were a methyl group, the monomer would be propylene and the polymer, polypropylene.

The entire propagation reaction usually takes place within a fraction of a second.

Termination of reaction is nothing but stop the further propagation of chain.

In theory, the propagation reaction could continue until the supply of monomers is exhausted. Most often the growth of a polymer chain is halted by the termination reaction. Termination typically occurs in two ways:

Combination occurs when the polymer's growth is stopped by free electrons from two growing chains that join and form a single chain. The following diagram depicts combination, with the symbol (R) representing the rest of the chain.

Combination Disproportionation

Dispropor tionation halts the propagation reaction when a free radical strips a hydrogen atom from an active chain. A carbon-carbon double bond takes the place of the missing hydrogen.

- Disproportionation can also occur when the radical reacts with an impurity. This is why it is so important that polymerization be carried out under very clean conditions.

There exists a type of addition polymerization that does not undergo a termination reaction. This so-called "living polymerization" continues until the monomer supply has been exhausted. When this happens, the free radicals become less active due to interactions with solvent molecules. If more monomers are added to the solution, the polymerization will resume.

Uniform molecular weights (low polydispersity) are characteristic of living polymerization. Because the supply of monomers is controlled, the chain length can be manipulated to serve the needs of a specific application. This assumes that the initiator is 100% efficient.

Mainly used for drug delivery. As a coating material

examples: Hydroxyl propyl methyl cellulose(HPMC), Methyl cellulose,

Propylene glycol. As a binders in tabletting granulation

examples: Acacia, Gelatin, Sodium alginate. As a disintegrants

examples: starch, HPMC As a thickening agent in suspension and ophthalmic preparations

Example: methyl cellulose. To form bases in ointments. In hard and soft capsule gelatin is used. Gelatin also used as suppository base, as an emulsifying agent and

suspending agent.

Thermal analysis of the polymers is the important phenomenon to study the stability and degradation of polymers.

Method :- a) TGA b) DSC

c) Thermo mechanical analysis

Definit ion :Biodegradable polymers are defined as polymers

comprised of monomers linked to one another through functional groups and have unstable links in the backbone.

They slowly disappear from the site of administration in response to a chemical reaction such as hydrolysis.

Material progressively releasing dissolved or dispersed drug, with ability of functioning for a temporary period and subsequently degrade in the biological fluids under a controlled mechanism, in to product easily eliminated in body metabolism pathway.

Biodegradable polymers can be classified in two:1. Natural biodegradable polymer examples: a) Collagen

b) Albumin c) Casein d) gelatin

e) xanthum gum f) gaur gum g) chitosan h) chtin

2. Synthetic biodegradable polymer examples: Polyanhydrides, Poly(ß-Hydroxybutyric Acids) etc.

Synthetic biodegradable polymer are preferred more than the natural biodegradable polymer because they are free of immunogenicity & their physicochemical properties are more predictable &reproducible

Localized delivery of drug

Sustained delivery of drug

Stabilization of drug

Decrease in dosing frequency

Reduce side effects

Improved patient compliance

Controllable degradation rate

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