DEFINITION

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.

INTRODUCTION

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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.

CONT…..

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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.

Molecular Structure of Polymer

Linear

• High Density Polyethylene (HDPE), PVC, Nylon, Cotton

Branched

• Low Density

- Polyethylene (LDPE)

Cross-linked

• Rubber

Network

• Kevlar, Epoxy

CHARACTERISTICS OF IDEAL POLYMER

• 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.

POLYMERISATION

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

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

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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 polym erisation’. The polymer is the only product

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

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Polyethylene formation

It Is useful to dIstInguIsh four

polymerIzatIon procedures fIttIng thIs general descrIptIon.

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Radical Polymerization The initiator is a radical, and the propagating site of reactivity (*) is a carbon radical.

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Cationic Polymerization The initiator is an acid, and the propagating site of reactivity (*) is a carbocation.

Anionic Polymerization The initiator is a nucleophile, and the propagating site of reactivity (*) is a carbanion.

Coordination Catalytic Polymerization The initiator is a transition metal complex, and the propagating site of reactivity (*) is a terminal catalytic complex.

CONT….

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Condensation polymerisation=

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

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

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Here the two molecules condense to form a polymer.The

condensation takes place between two reactivefunctional groups, like the carboxyl group(of an acid) and the hydroxyl group(of an alcohol). While forming the polymer water molecules also get

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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.

1. Natural and Synthetic Polymers

materials,

^ Polymers which are isolated from natural 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

NATURAL RUBBER-

Hevea brasiilensis

2. Organic and Inorganic

Polymers

• A Polymer whose backbone chain is essentially made of carbon atoms is termed an ‘Organic polymer’.

Examples- cellulose, proteins, polyethylene, nylons.

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

Examples- Glass and silicone rubber

3. Thermoplastic and Thermosetting Polymer

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

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• Such polymer that soften on heating and stiffen on cooling are termed as `thermoplastic’ polymers.

Ex. Polyethylene, PVC, nylon, sealing wax.

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• 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.

4. Plastics, Elastomers, Fibres & Liquid resins

^ 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.

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^ 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.

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

E.g. Nylon, terylene.

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

E.g. Epoxy adhesives and polysulphides sealants.

Common Addition Polymers

STEPS FOR SYNTHESIS OF POLYMERS

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

1. INITIATION

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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).

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The stability of a radical refers to the molecule's

tendency to react with other compounds. An unstable

radical will

different

molecules.

readily combine However a stable

with radical

many will

not easily

interact with other chemical substances.

CONT….

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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.

2. PROPAGATION

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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.

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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.

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

3. TERMINATION

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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

CONT….

Disproportionation 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.

LIVING POLYMERISATION

• 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

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become less active due to interactions with solvent molecules. If more monomers are added to the solution, the polymerization will resume.

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Uniform molecular weights (low polydispersity) are characteristic of living polymerization. Because the

supply of monomers is controlled, the chain length can

the needs of a specific that the initiator is 100%

be manipulated to serve application. This assumes efficient.

MOLECULAR WEIGHT DETERMINATION

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There are two ways to calculate the average molecular weight:

1. Number Average Molecular Weight

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• Weight Average Molecular Weight

CONT…

1. Number Average Molecular Weight

• Molecular weight is determined by calculating the total molecular weight of monomer and total number of monomer.

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Mi- total molecular weight of monomer.

Ni- number of monomer molecules.

Mn- number average molecular weight.

_{Mn = ∑}NiMi

_∑Ni

CONT…

2. Weight Average Molecular Weight

_{M w = ∑} NiMi.Mi

_∑ Ni Mi

• Mw- weight average molecular weight.
• Mi- total molecular weight of monomer.
• Ni- number of monomer molecules.

APPLICATIONS

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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

CHARACTERIZATION

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

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Method :-

1. TGA
2. DSC
3. Thermo mechanical analysis

Thermo-gravimetric Analysis (TGA)

• This method provides indication for thermal stability and upper limit of thermal degradation where loss of sample begins.

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• This method only measures loss of volatile content from the polymer.

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• This method has limitation that it can not detect temperature at chain cleavage of chain takes place.

Differential Scanning Calorimetry (DSC)

Parameters measured-

1. Glass transition temperature (Tg)
2. Crystalline melting point
3. Heat of fusion
4. Heat of crystallization

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It requires placing of Reference and test sample for the continuous monitoring in the heating chamber.

Thermo Mechanical Analysis (TMA)

• This method is used for determination of deformation of polymer sample as a function of temperature placed on platform in contact with probe.
• It measures transition from glassy to a rubbery polymer and gives idea about softening temperature.

BIODEGRADABLE POLYMERS

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Definition :

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.

Classification

• Biodegradable polymers can be classified in two:
• Natural biodegradable polymer examples:
1. Collagen
2. Albumin
3. Casein
4. gelatin
5. xanthum gum
6. gaur gum
7. chitosan
8. chtin

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2. Synthetic biodegradable polymer

examples: Polyanhydrides, Poly(ß-Hydroxybutyric Acids) etc.

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

ADVANTAGES

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Localized delivery of drug

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Sustained delivery of drug

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Stabilization of drug

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Decrease in dosing frequency

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Reduce side effects

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Improved patient compliance

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Controllable degradation rate

BIBLIOGRAPHY

• file:///D:/polymerization/polymers%20with

%20biodegradable.htm

• file:///D:/polymerization/Polymerization.htm
• file:///D:/polymerization/synthesis%20of

%20polymerization.htm

• file:///D:/polymerization/types.html