Polymers

Question 1

What are:

• Hydrocarbons, their formula
• Unsaturated and saturated
• Polymer
• Macromolecule
• Oligomer

Answer 1

• Compunds made of H and C with covalent bonds
  CnH2n+2, simple ones belong to the paraffin family
• Un: molecules with double and triple covalent bonds, Sat: all bonds are single
• A polymer is a substance composed of molecules which have long sequences of one or more species of atoms linked to each other by primary and usually covalent bonds
• Polymer and macromolecule are used interchangeably, strictly, polymers are composed of macromolecules
• Oligomer: low molecular weight polymer (dimers, trimers, tetramers)

Question 2

How are macromolecules formed?

Answer 2

By linking together monomer molecules through polymerisation

Question 3

Examples of:

• Natural polymers
• Tough and hard (thermoset) polymers
• Thermoplastic polymers

Answer 3

• Natural: Starch, cellulose, protein
• Thermoset: PVC, protein, polystyrene
• Thermoplastic: Polyester, silicone

Question 4

Polyethylene (PE)
Polytetrafluoroethylene (PTFE)
Poly(vinyl chloride) (PVC)

Question 5

What are:

• Homopolymer
• Copolymer
• Functionality

Answer 5

When all the repeating units along a chain are of the same type, polymer is a homopolymer
When chain is composed of two or more different repeat units, polymer is a copolymer
Functionality is the no of bonds a given monomer can form

Question 6

How are diff conformations achieved>

Answer 6

Single chain bonds are capable of rotating and bending in three dimensions, leading to different conformation

Question 7

What is:

• Stereoisomerism
• Isotactic configuration
• Syndiotactic configuration
• Atatic configuration

Answer 7

• Sterisomerism: denotes the situation in which atoms are linked together in the same order but differ in their spatial arrangement
• Isotactic: R groups are situated on the same side of the chain
• Syndiotactic: R groups alternate sides of the chain
• Atactic: Random positioning of the R groups

Question 8

Cis and trans Isomers

Answer 8

Cis: R groups on same side of the double bond
Trans: R groups on opposite sides of the double bond

Question 9

Molecular structures of polymers with increasing strength

Answer 9

• Linear polymers: Long, flexible chains, with only 2 ends. Some van der Waals or hydrogen bonding between chains. Eg PVC, polystyrene, nylon, High density PE
• Branched polymers: Side-branch chains are connected to the main ones, they may occur as a result of from side reactions that occur during the synthesis. Chain packing efficiency and polymer density are reduced. E.g Low density PE
• Cross linked polymers: Cross linkage happens either during synthesis or in a separate process, typically involving addition of impurities which bond covalently (vulcanisation in rubber)
• Network polymers: 3D networks made from trifunctional mers. These are thermosetting materials which are very hard. E.g epoxies, phenolformaldehyde

Question 10

What is vulcanisation?

Answer 10

Crosslinks can be created with additional compounds.

Question 11

Types of copolymers

Answer 11

• Random
• Alternating
• Block
• Graft polymers

Question 12

Molecular forces in polymers, which important when crystalline? which important when in rubbery amorphous state?

Answer 12

• Intramolecular: generally strong covalent bonds, forces between atoms in one chain
• Intermolecular: van der Waals (PE)
  Hydrogen Bridges (PS) (stronger)
  Can become very strong as M increases
  Forces between two chains
• Entanglements (physical)
  In the crystalline state, the van der Waals bonds r v important, in rubbery amorphous state, entanglements r v important

Question 13

Thermosets and Thermoplastics

Answer 13

Thermoplastics: can be processed by melting, most are linear.
Thermosets: cannot be melted or dissolved to be processed: chemical decomposition occurs before softening. Most crosslinked and network polymers. Cannot change mechanical properties

Question 14

What are:

• Number-average molar mass
• Weighted-average molar mass
• Molar mass dispersity

Answer 14

• Number average: = sum of mole fraction x molar mass
• Weighted-average molar mass: = sum of weight fraction x molar mass
• Molar mass dispersity = DM = Mw/Mn

Question 15

How do polymers form crystal structures? Amorphous regions? % crystalinity

Answer 15

Polymers can form crystal strcutures due to packing of molecular chains to produce an ordered atomic array. Some parts of the structure align during cooling to form crystalline regions. Around crystals get amorphous regions
% crystalinity = ρc(ρs-ρa)/ρs(ρc-ρa) x 100
densities of completely crystalline polymer, sample, amorphous polymer

Question 16

What does % crystallinity depend on?

What does high crystallinity say about strength?

Answer 16

-Rate of cooling: faster cooling, less crystallinity
-Type of polymer: simple structures, more, copolymer, less
-Linear: linear polymers more easily form crystals
Higher % = higher strength

Question 17

What types of polymers do condensation and addition polymers yield?

Answer 17

• Condensation yields polymers with repeat units have different atoms than present in the monomers due to elimination of small molecules
• Addition yields polymers with repeat units having identical molecular formulae to those of monomers

Question 18

Step-growth and chain-growth polymerisation

Answer 18

• Step: polymer chains grow step-wise by reactions that can occur between any two molecular species
• Chain: polymer chains grow only by reaction of monomer with a reactive-end group on the growing chain

Question 19

Step polymerisation description

Answer 19

Links in step polymerisation contain a heteroatom (not C or H). Difficult to obtain high MW, normally 15-30 kg/mol compared to 50-10,000 kg/mol with chain-growth
Involve successive reaction between pairs of mutually-reactive functional groups

Question 20

Step polymerisations, polycondensations

Answer 20

Step polymerisations involving the elimination of small molecules are called polycondensations. eg production of polyesters

Question 21

How are polyesters produced?

Answer 21

Polycondensations e.g carboxylic group + alcohol or carboxylic acid halide + alcohol
First one is slow and requires high temp and acid catalyst, second one happens at room temp
These are RA2 + RB2 step polymerisations
or can be prepared from single monomers, ARB step polymerisation, monomer contains alcohol and carboxylic group, easier to obtain higher degree of polymerisation, since no imbalances can occur

Question 22

How are polyamides produced?

Answer 22

Polycondensation using carboxylic group + amine group

Question 23

How are polyethers produced?

Answer 23

Polycondensation from diols, but this is difficult to produce
Polycondensation from dihalides and dialkoxides

Question 24

What are aromatic polymers, advantages, how produced?

Answer 24

Aromatic polymers: They contain hydrocarbons with sigma bonds and delocalized pi electrons between C atoms forming a circle
Improved mechanical properties and resistance to heat or radiation. High performance polymers. Typically obtained by polycondensation, look at examples

Question 25

Why do polysiloxanes have higher thermal stability?

Answer 25

They have an inorganic backbone

Question 26

How are polysiloxanes prepared?

Answer 26

Polycondensation through hydrolysis of highly reactive dichloroalkylsilanes (see notes)

Question 27

What is a polyaddition and what is it used for?

Answer 27

Step polymerisation without elimination of small molecules Used for polyurthanes and epoxy resins See notes

Question 28

Free radical polymerisation

Answer 28

Chain polymerisation We need an initiator which forms free radicals which allows the chain to form/grow or a catlyst to start the chain polymerisation The most widely used polymerisation type for unsaturated monomers (having C=C bonds) Polymer grows by sequential additions of unsaturated monomer to an active centre (a terminal free-radical reactive site)

Question 29

``` Case study Polypropylene Issues Routes Priorities Processing steps Catalyst used ```

Answer 29

Issues: Complicated due to stereospecificity of the repeating unit affecting final properties Free radical polymerisation results in low MW and atatic product. Ziegler-Natta catalyst used instead. Atactic product and limited commercial interest, low melting point and poor solvent resistance. kept below 10 wt%. No comercial interest in sydiotactic. Isotactic there in commercial interest. Two continuous routes: - Slurry process eg. Mitsui petrochemical montecatini process - Gas phase processes eg, BASF novolen gas phase process Very exothermic so reactor temp control is a priority Processing steps: - catalyst prep - polymerisation - recovery and recycling of unreacted propylene and comonomers - Recovery and recycling of diluent - Destruction and separation of the catalyst from the final polymer Removal of atactic and low MW fractions Finishing operations: product drying, additives blending, extrusion, pelletising Catalyst: TiCl4

Question 30

Free radical polymerisation steps

Answer 30

- Initiation - Propagation - Termination

Question 31

Bulk Polymerisation

Answer 31

It uses no solvets, quite simple. Only monomer and initiator High concentration of monomer so high rates and degrees of polymeristion. Obtained polymer is pure Large castings may be prepared directly (PMMA transpaDroprent sheets) Difficult to reproduce because of increase of viscosity and poor heat dissipation (polymerisation is exothermic). Problems of autoacceleration Broad molecular weight distribution

Question 32

Solution polymerisation

Answer 32

Solvent selected to dissolve initiator, monomer and polymer water for polyacrylic polymers or organic solvents used Solvent used to tackle bulk polymerisation problems (viscosity, heat transfer, autoacceleration) However, it decreases monomer conc, so lower rates and degree of polymerisation Isolation of the polymer requires evaporation or precipitation Limited for industrial scale

Question 33

Suspension polymerisation

Answer 33

It is a bulk polymerisation where the reaction mixture is suspended as tiny droplets in an inert medium Initiator, monomer and polymer must be insoluble in the inert medium (typically water) Limited by the inert medium boiling point Droplets are maintained by vigorous agitation, dispersion stabilisers Low viscosity and high thermal conductivity provided by the water and high surface area of droplets

Question 34

Emulsion polymerisation

Answer 34

Very used in industry Same principles as in suspension but in this case, the initiator is only soluble in the aqueous medium Results in different kinetics and colloidally-stable particles in water called latex Particles are three order of magnitude small than those in suspension Anionic surfactants are commonly used as dispersion stabilisers: - they are amphiphilic molecules: hydrophobic chain, hydrophilic head - low solubility in water -> they form micelles above the critical micelle conc