Polymer chemistry

Classification of polymer1- Based on monomer source Addition and Condensation Polymers The addition polymer is often named according to the monomer that was used to form it Example : pol

Polymer Chemistry

Part 1

Polymer Characteristics and Classifications

Polymer

Poly mer mer

many repeat unit (building blocks)

C C C C C C

HHHHH

H

HHHHH

H

HHHHHH

Poly(vinyl chloride) (PVC)

HH

HH

CH3

CH3H

repeat unit

repeat unit

repeat unit

Carbon chain backbone

composed of repeating units

connected by covalent bonds

polymer The reactant for the

polymerization reaction

Characterizing a Polymer

 Classification

Skeletal Structure

Skeletal Structure

Skeletal Structure

other chains

Classification of polymer

1- Based on monomer source

Addition and Condensation Polymers

The addition polymer is often named according to the monomer that was used to form it

Example : poly( vinyl chloride ) PVC is made from vinyl chloride

ex , poly (vinyl acetateex , poly (vinyl acetate ) -CH2-CH(OCOCH3)-

The most common method for condensation polymers since the polymer contains different functional groups than the monomer

Classification by Origin

 Synthetic organic polymers

 Biopolymers (proteins, polypeptides, polynucleotide,

polysaccharides, natural rubber)

 Semi-synthetic polymers (chemically modified synthetic polymers)

 Inorganic polymers (siloxanes, silanes, phosphazenes)

Classification by Monomer Composition

Consists of two or more constitutional repeating units (A.B )

 Statistical copolymer (Random)

ABAABABBBAABAABB

two or more different repeating unit

are distributed randomly

 Alternating copolymer

ABABABABABABABAB

are made of alternating sequences

of the different monomers

AAAAAAAAABBBBBBBBB

long sequences of a monomer are

followed by long sequences of another

Consist of a chain made from one type of

monomers with branches of another

type

(d)

Classification by Chain structure (molecular architecture)

 Linear chains :a polymer consisting of a single continuous chain of

repeat units

Branched chains :a polymer that includes side chains of repeat units connecting onto the main chain of repeat units

Hyper branched polymer consist of a constitutional repeating unit

including a branching groups

Cross linked polymer :a polymer that includes interconnections between chains

Net work polymer :a cross linked polymer that includes numerous

interconnections between chains

Direction of increasing strength

Classification by Chain Configuration

Configuration : Is defined by polymerization method A change in configuration require the rupture of covalent bonds

Classification by Thermal Behavior

Thermoplastics - materials become fluid and processible upon heating, allowing them to be transformed into desired shapes that are stabilized by cooling

Thermosets - initial mixture of reactive, low molar mass compounds reacts upon heating in the mold to form an insoluble, infusible network

which can be melted when heat is applied

processing techniques such as

injection molding or extrusion

water piping, rope, fishing line, car parts

cellulose (proteins), polylactic acid

Codes for Plastics

 1 – PETE – soft drink bottles

polymers that can be stretched

easily (3-10x original size)

dimensions when applied stress is released

crosslinking

bands, bouncing balls

motion is greatly restricted by a high degree of crosslinking

once formed

epoxy

applications, super glue, counter top laminates, epoxy resins, tires

Polymer Chemistry

Part 2

Polymer Synthesis

The reaction mixture consists

of high polymer and

unreacted monomers with

very few actively growing

Step Growth Step Growth

Reaction can occur independently between any pair of molecular species

The reaction mixture consists of oligomers of many sizes in a statically calculable

distribution Monomer disappear early in favor

of low oligomer

Chain growth

High polymer appears

immediately , average

molecular weight does

not change much as

reaction proceeds

Increased reaction time

increases overall

product yield , but does

not affect polymer

Long reaction time are essential to produce polymer with height average molecular weight

Polymerization mechanisms

Polymers are large molecules made up of repeating units called Monomers

The synthetic process is Polymerization.

Note – define repeating unit in terms of monomer structure

However, for synthetic polymers it is more accurate to state average degree of polymerization ( ¯ ) DP

Two main classifications of Polymerization

Addition reaction or Chain growth

Molecular weight increases by successively adding monomers to a reactive polymer chain end resulting in high molecular weights at low conversions.

STEP reaction or growth

Polymers are formed by linking monomer molecules to form dimers, trimers and higher species in a step-wise fashion The most abundant species react, and thus high molecular weight formed only beyond 99% conversion.

Polymerization Conversion (p)

P

M0 = initial number of monomer molecules

Mt = Number of monomer molecules at time t

Ionic Chain (addition)-Growth Polymerization

The choice of ionic procedure depends greatly on the electronic

nature of the monomers to be polymerized

2 R CN

N

Vinyl monomers with electron-donating groups

Vinyl monomers with electron-withdrawing groups

Monomers and reagents should be scrupulously purified; water and oxygen should be removed.

Polymerizations carried out at very low temperatures

Anionic Polymerization

Cationic Polymerization

Anionic Polymerizations

Initiators include alkyl lithiums and sodium amide

Cationic Polymerization the formed carbocation must be quite stable

Stable tertiary carbocation

stable oxonium ion

BF 3 /H 2 O

n

E.g proton initiates polymerization of isobutane (2-methylpropene)

Adhesive, sealant, insulating oil, lubricating oil

CN CN

CN

O H H

CN CN

OMe OMe OMe n

+

base

n

+ H 3 O +

Reactions of water with reactive carbanions and carbocations

Note – viable substrates for anionic polymerizations do not have α-protons

Chain Reaction: Free Radical Polymerization

Conventional Radical Polymerization

Advantages –

1/wide range of vinyl monomers polymerizable

2/can be carried out in bulk, water, organic solvents and other solvents

3/no rigorous purification or drying of reagents required

Conditions: Usually heat required for initiation

Initiator decomposition time should be considered

- Amount of initiator, reaction temperature and initiator half-life (slow decomposition)

Initiation Rate = Termination Rate - “steady state” kinetics apply

Overall,

[radical concentration] = low

Since termination (disproportionation and coupling mechanism) is random, a broad MWD results This polymer is dead (cannot initiate new monomer additions).

Examples of Polymers Prepared by Radical Polymerization

CH 2 CH

O OMe C

CH 2 CH O

C O Me

n n

n

n

Ziegler-Natta Chain (Addition) Polymerization

C C H

Milder conditions than radical polymerization

HDPE (high density poly(ethylene) is 3-10 times stronger than LDPE

Less cross-linking, as terminal DBs less reactive than substituted DBs of radical polymerization

Cl 3 Ti

H H

Ti Cl Cl

Cl H

+

Termination reaction

Few monomers polymerized by Z/N

Ziegler-Natta Addition Polymerization

Step-growth Polymerization

Step-polymers are made by allowing difunctional monomers with

complementary functional groups to react with one another

Condensation between two molecules

O H

This is an example of a poly(ester)

The reaction is a transesterification Recyclable plastic

bottles and textile fabrics

Using a condensation reaction

Step-growth Polymerization

Self-Condensation or Ring-Opening Polymerization

First patented by Dupont

Nylon 6 is made by heating caprolactam to about 250 ºC with about 5-10% water

These are poly(amides) – bristles of toothbrishes, stockings, rope, tires, carpet fibre

First patented by BASF

260-280 °C

250 psi

- H2O

MW = 10,000, m.pt 250 °C, fibres stretched (to increase strength) to 4 times their length

High temp to drive off water

Also opened by

cations & anions

Molten nylon spun into fibres

Step-addition – “no by-products”

Lower Temp than condensation reactions

Impurity found in diazomethane

the same molecule having two different functional groups.

Ring opening: caprolactum to nylon-6