Polymers

Polymers

Basic Concepts

• Polymer: Poly (many) - Macromolecules.
• Individual molecules of very high molecular weight e.g., Hemoglobin, Chlorophyll etc.
• Monomer: The molecule that forms the basic unit for Polymer. e.g., Propene, Styrene, Vinyl chloride etc.

Definition

• Polymers are natural, semisynthetic and synthetic compounds consisting of a number of molecules (monomers). eg, Polypropene, Polystyrene, PVC, Nylon-6 etc.
• The number of monomers which are joined together in a polymer constitutes the degree of polymerization.

Classification of Polymers

• Based upon source: Natural, Semisynthetic, Synthetic
• Based upon structure: Linear, Branched, Crosslinked or three-dimensional
• Based upon Molecular Force: Elastomers, Fibre, Thermoplastic, Thermosetting
• Based upon Polymerization: Addition polymers, Condensation polymers

Classification Based Upon Source

1. Natural Polymers

• Polymers which are found in nature.
• eg. Starch, cellulose and natural rubber, silk, Proteins, Nucleic acid.

2. Semisynthetic Polymers

• Obtained from naturally occurring polymers by chemical treatment.
• Most of the semisynthetic polymers are prepared from cellulose.
• Examples: cellulose acetate, cellulose nitrate, cellulose xanthate and rayon.

3. Synthetic Polymers

• Man-made polymers are known as synthetic polymers.
• eg. PVC, polyethylene, polystyrene, nylon-6, nylon-6,6; nylon-6,10; terylene, synthetic rubbers etc.

Classification Based Upon Structure

1. Linear Polymers

• Monomers are joined together to form long straight chains.
• The various linear polymeric chains are stacked over one another to give a well-packed structure, close packed in nature, having high densities, high melting point and high tensile (pulling) strength.
• All fibers are linear polymers. eg. cellulose, silk, nylon, terylene etc.

2. Branched Chain Polymers

• Polymers in which the monomeric units constitute a branched chain.
• Branched chain polymers have lower melting point, low densities and tensile strength as compared to linear polymers.
• Examples are amylopectin, glycogen, low density polyethylene and all vulcanized rubbers.

3. Crosslinked or Three-Dimensional Network Polymers

• When linear polymeric chains are joined together to form a three-dimensional network structure.
• These polymers are hard, rigid and brittle.
• Crosslinked polymers are always condensation polymers.
• Resins are crosslinked polymers.
• Monomers having three functional groups always gives cross linked polymer. Examples Urea formaldehyde resin, phenol-formaldehyde resin.

Classification Based Upon Molecular Force

1. Elastomers

• Polymers in which the intermolecular forces of attraction between the polymer chains are the weakest (weak van der Waals forces of attraction).
• These polymers consist of randomly coiled molecular chains of irregular shape having a few crosslinks.
• Examples are natural rubber, Buna-S, Buna-N etc.

2. Fibres

• Those polymers in which the intermolecular forces of attraction are the strongest are called fibres.
• These polymers are held together by H-bonding or dipole-dipole interaction.
• Fibres have high tensile strength, least elasticity having high melting point and low solubility.

3. Thermoplastics

• In thermoplastics intermolecular forces of attraction are in between those of elastomers and fibres.
• Thermoplastics become soft and viscous on heating and rigid on cooling.
• Examples are polythene, nylon-6, nylon-6,6 etc.

4. Thermosetting Polymers

• These polymers have low molecular masses and are semi-fluid substances.
• These polymers are hard and infusible.
• Examples are melamine-formaldehyde, bakelite (phenol-formaldehyde) etc.

Classification Based Upon Polymerisation

1. Addition Polymerization

• Addition polymers are those in which addition reaction takes place.
• If monomer is ethylene, then addition polymer may be either linear polymer of branched chain polymer. Examples are polystyrene, polytetrafluoroethylene, polyacrylonitrile etc.
• If monomer is 1,3-butadiene or substituted-1,3-butadiene $(CH<em>2=C(G)-CH=CH</em>2)$, then polymer is always branched chain polymer.
• Addition polymerization takes place in three steps: Initiation, chain propagation and chain termination.
• Addition polymers are also known as chain-growth polymers.

Types of Addition Polymerization

(a) Free Radical Polymerisation

• Free radical polymerisation takes place in the presence of radical initiators such as dioxygen, benzoyl peroxide, acetyl peroxide etc.
• For example, polymerization of ethene is carried out at high temperatures (350-570K) and at high pressure (1000-2000 atm) in the presence of dioxygen or a small amount of benzoyl peroxide as radical initiator.

• Chain initiating steps:
  • $Benzoyl_peroxide \rightarrow 2C<em>6H</em>5COO^. \rightarrow 2C<em>6H</em>5^. + 2CO_2$
  • $C<em>6H</em>5^. + CH<em>2=CH</em>2 \rightarrow C<em>6H</em>5-CH<em>2-CH</em>2^.$
• Chain propagating steps:
  • $C<em>6H</em>5-CH<em>2-CH</em>2^. + nCH<em>2=CH</em>2 \rightarrow C<em>6H</em>5-(CH<em>2-CH</em>2)<em>n-CH</em>2-CH_2^.$
• Chain terminating steps:
  • Step 1: By combination of free radicals
    • $2C<em>6H</em>5-(CH<em>2-CH</em>2)<em>n-CH</em>2-CH<em>2^. \rightarrow C</em>6H<em>5-(CH</em>2-CH<em>2)</em>n-CH<em>2CH</em>2CH<em>2CH</em>2-(CH<em>2-CH</em>2)<em>n-C</em>6H_5$
  • Step 2: By disproportionation of free radicals
    • $2C<em>6H</em>5-(CH<em>2-CH</em>2)<em>n-CH</em>2-CH<em>2^. \rightarrow C</em>6H<em>5-(CH</em>2-CH<em>2)</em>n-CH=CH<em>2 + C</em>6H<em>5-(CH</em>2-CH<em>2)</em>n-CH<em>2-CH</em>3$

(b) Cationic Polymerisation

• Cationic polymerisation takes place in the presence of strong protonic acids such as $H<em>2SO</em>4, AlCl<em>3, BF</em>3$ etc.

(c) Anionic Polymerization

• In anionic polymerisation, it takes place in the presence of strong bases such as $KNH<em>2, Na</em>2OH, KOH$, some organometallic compounds etc.

2. Condensation Polymerization

• They are formed due to condensation reactions.
• Condensation polymerization is also known as step-growth polymerization.
• For condensation polymerization, monomers may should have at least two functional groups (functional groups may be same or different).
• During condensation, elimination of small molecules like ammonia, alcohol, water, HCl takes place.
• Monomers having only two functional group always give linear polymer.
• Monomer having three functional groups always gives cross linked polymer. Examples Urea formaldehyde resin, phenol-formaldehyde resin.

Natural Rubber

• The main source of natural rubber is the brasiliensis tree.
• Natural rubber is obtained from latex.
• Latex is coagulated with acetic acid and formic acid and the coagulated mass is then squeezed.
• Natural rubber is sticky, gummy & soft and insoluble in water, alkalies & dilute acids.
• Natural rubber is soluble in non-polar solvents.
• It has low elasticity and low tensile strength.
• Example: Polyisoprene.
• Natural rubber is a polymer of 2-methyl-1,3-butadiene (isoprene). On average, molecules of rubber contain 5000 isoprene units held together by head to tail. All the double bonds in rubber are cis, hence natural rubber is cis-polyisoprene.
• $nCH<em>2=C(CH</em>3)-CH=CH<em>2 \rightarrow [-CH</em>2-C(CH<em>3)=CH-CH</em>2-]_n$

Structure of Natural Rubber (cis-polyisoprene)

Synthetic Rubber or Polymerisation of Dienes

• Polymers of 1,3-butadienes are called synthetic rubbers because they have some properties of natural rubber.
• Synthetic rubbers are waterproof and have great elasticity.

1. Homopolymers

• Monomer of this class is 2-substituted $(CH<em>2=C(G)-CH=CH</em>2)$, where $G = H, Cl or CH_3$.
• Ziegler-Natta catalyst is used for polymerisation, which gives stereo-regular polymers.
• Neoprene (Polychloroprene): the first synthetic rubber manufactured on a large scale. It is the monomer of chloroprene (2-chlorobutadiene).

2. Copolymers

• Copolymers are derived from two or more types of monomer units.
• Examples of copolymers are: Buna-S, SBR (Styrene-Butadiene rubber); Buna-N, NBR (Nitrile-Butadiene rubber); Butyl rubber; ABS (Acrylonitrile, Butadiene, Styrene).

(a) Buna-S, SBR (Styrene-Butadiene Rubber)

• Buna-S rubber is a copolymer of three moles of butadiene and one mole of styrene.
• Buna-S is generally compounded with carbon black and vulcanized with sulfur.
• It is extremely resistant to tear & wear and therefore used in the manufacture of tires and other mechanical rubber goods.
• It is obtained as a result of free radical copolymerisation of its monomers.

(b) Buna-N

• Buna-N is obtained by copolymerisation of butadiene and acrylonitrile (General purpose Rubber acrylonitrile or GRA)
• Buna-N is rigid and resistant to the action of organic solvents, lubricating oil and petrol.
• It is also used for making fuel tanks.

Vulcanisation of Rubber

• Natural rubber is soft & sticky and becomes even more so at high temperatures and brittle at low temperatures.
• Therefore, rubber is generally used in the temperature range 283-335 K, where its elasticity is maintained.
• It has a large water absorption capacity, low tensile strength and low resistance to abrasion.
• It is also not-resistant to the action of organic solvents and is also easily attacked by oxidizing agents.
• These properties can be markedly improved by a process called vulcanization.
• It consists of heating raw rubber with sulfur at 373-415 K.
• Since this process is slow, additives like zinc oxide are used to accelerate the rate of vulcanization.
• The vulcanized rubber thus obtained has excellent elasticity, low water absorption tendency, and is resistant to the action of organic solvents and oxidizing agents.
• During vulcanization, sulfur bridges or cross-links between polymer chains are introduced either at reactive allylic positions or at the sites of the double bonds.

Nylon

• Nylons are prepared by the condensationpolymerisation of dibasic acids with diamines.
• Nylon contain amide linkages having a protein-like structure.

(1) Nylon-6,6 (Nylon-six, six)

• It is obtained by condensationpolymerisation of a diamine with six carbon atoms (hexamethylenediamine) and a dibasic acid having 6 carbon atoms (adipic acid).
• $nHOOC(CH<em>2)</em>4COOH + nH<em>2N(CH</em>2)<em>6NH</em>2 \rightarrow [CO(CH<em>2)</em>4CONH(CH<em>2)</em>6NH]<em>n + (2n-1)H</em>2O$

(2) Nylon-610 (Nylon-six, ten)

• It is obtained by condensation polymerization of six carbon atoms (hexamethylenediamine) and a dibasic acid with 10 carbon atoms (sebacic acid).
• These fibres are light, very strong, flexible, and elastic having retain creases and drip-dry properties.
• These polyemrs are inert towards biological and chemical agents.
• These polymers can be blended with wool to make carpets, garments, tire cords, ropes, etc.

(3) Nylon-6 (Perlon L)

• A polyamide closely related to nylon is known as perlon L (Germany) or Nylon-6 (USA).
• During prolonged heating of caprolactum at 260-270°C, it is formed by self-condensation of a large number of molecules of amino caproic acid.
• Since caprolactum is more easily available, it is used for polymerization, which is carried out in the presence of $H_2O$, that first hydrolyzes the lactam to amino acid.
• Subsequently, the amino acid can react with the lactam and the process goes on and onto formed by polyamide polymer.
• Caprolactam is obtained by Backman rearrangement of cyclohexanone oxime.

Polytetrafluoroethene (PTFE) or Teflon

• Teflon is manufactured by heating tetrafluoroethene in presence of peroxides or ammonium persulphate catalyst at high pressures.
• $nFC=CF<em>2 \rightarrow [-F</em>2C-CF<em>2-]</em>n$

Polyacrylonitrile (PAN) or Orlon

• During Polymerisation of acrylonitrile in the presence of a peroxide catalyst gives polyacrylonitrile.
• $nCH<em>2=CH-CN \rightarrow [-CH</em>2-CH(CN)-]_n$
• PAN is used as a substitute for wool in the manufacture of orlon and acrilan fibres which are used for making clothes, carpets and blankets.

Melamine-Formaldehyde Resin

• This resin is formed by condensation polymerization of melamine and formaldehyde.
• It is a quite hard polymer and is used widely for making plastic crockery under the name melamine.
• The articles made from this polymer do not break even when dropped from considerable height.

Bakelite

• Phenol-formaldehyde resins are obtained by the reaction of phenol and formaldehyde in the presence of either an acid or a basic catalyst.
• It starts with the initial formation of ortho and para hydroxymethyl phenol derivatives, which further react with phenol to form compounds where rings are joined to each other with -$CH_2$ groups.
• This reaction involves the formation of methylene bridges in ortho, para or both ortho and para positions.
• Linear or cross-linked materials are obtained depending on the condition of the reaction.

Polyesters

• Dacron is a common polyester, prepared using ethylene glycol and terephthalic acid. The reaction is carried out at 140°C to 180°C in the presence of zinc acetate and $Sb<em>2O</em>3$ as catalyst.
• The dacron is crease-resistant and has low moisture absorption. It has high tensile strength.
• It is mainly used in making wash and wear garments, in blending with wood to provide better crease and wrinkle resistance.

Molecular Mass of Polymer

• Normally, a polymer contains chains of varying lengths and therefore, its molecular mass is always expressed as an average.
• In contrast, natural polymers such as protein contains chains of identical length and hence, have a definite molecular mass.

Number average molecular mass $(M_n)$

• $M<em>n = \frac{\sum N</em>iM<em>i}{\sum N</em>i}$
• Here $N<em>i$ is the number of molecules of molecular mass $M</em>i$

Weight average molecular mass $(M_w)$

• $M<em>w = \frac{\sum N</em>iM<em>i^2}{\sum N</em>iM_i}$
• Definition: The ratio of the weight and number average molecular masses $(M<em>w/M</em>n)$ is called Poly Dispersity Index (PDI).
• The number average molecular mass and mass average molecular mass of a polymer are 30,000 and 40,000 respectively. the PDI of the polymer is:$PDI = \frac{40000}{30000} = 1.33$
• Methods such as light scattering and ultracentrifuge depend on the mass of the individual molecules and yield weight as average molecular masses.
• $M_n$ is determined by osmotic pressure measurement (colligative properties).
• Some natural polymers, which are generally monodispersed, the PDI is unity (i.e., $M<em>w=M</em>n$).
• In synthetic polymers, which are always polydisperse, PDI>1 because $M<em>w$ is always higher than $M</em>n$.

Biodegradable Polymers

• Many polymers are quite resistant to the environmental degradation processes and are thus responsible for the accumulation of polymeric solid waste materials (environmental pollution).
• For general awareness and concern, the problems created by the polymeric solid wastes, certain new biodegradable synthetic polymers have been designed and developed.
• These polymers contain functional groups similar to the functional groups present in biopolymers.
• By far the most important class of biodegradable polymers are aliphatic polyesters and polyamides.

(I) Nylon-2-Nylon-6

• It is an alternating polyamide of glycine (containing two carbon atoms) and z-aminocaproic acid or 6-aminohexanoic acid (containing six carbon atoms).
• $nH<em>2N-CH</em>2-COOH + nH<em>2N-(CH</em>2)<em>5-COOH \rightarrow [-NH-CH</em>2-CO-NH-(CH<em>2)</em>5-CO-]<em>n + (2n-1)H</em>2O$

(ii) PHBV

• It is a thermoplastic co-polymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid in which the two monomer units are connected by ester linkages.
• $nHO-CH<em>2=CH-COOH + nHO-CH-CH</em>2-COOH \rightarrow [-O-CH-CH-C-O-CH-CH<em>2-C-]</em>n + (2n-1)H_2O$

Chapter Summary

• Polymers have high molecular mass and formed by union of monomers.
• Chain growth polymers or addition polymers are formed by successive addition of monomers without loss of simple molecules like $H<em>2O, NH</em>3$ etc.
• Step-growth or condensation polymers: these are formed through series of independent steps. Each step involves condensation between two monomers leading to the formation of polymers. More than one monomer unit is involved.
• Homopolymers: Polymers made of the same monomer.
• Copolymers: Polymers made of different types of the same monomers.
• Elastomers: The strands of polymer are held together by weak intermolecular forces (van der walls). e.g. - Vulcanized rubber.
• Fibers are held together by hydrogen bonds. e.g. - nylon, polyester, polyamide.
• Thermoplastics can be easily molded on heating. They don't have cross-links, eg-polyethene.
• Thermosetting polymers have cross links, cannot be remolded on heating. e.g. - Bakelite.
• Plastisizer are high boiling esters which are added to plastics to make it soft and rubber like.

Polymer Tables

Some Common Addition Polymers/ Chain Growth Polymer (Homopolymers)

Some Condensation POLYMERS (Step-Growth Polymers)