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Polymers Flashcards

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1
Q

What are:

  • Hydrocarbons, their formula
  • Unsaturated and saturated
  • Polymer
  • Macromolecule
  • Oligomer
A
  • 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)
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2
Q

How are macromolecules formed?

A

By linking together monomer molecules through polymerisation

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3
Q

Examples of:

  • Natural polymers
  • Tough and hard (thermoset) polymers
  • Thermoplastic polymers
A
  • Natural: Starch, cellulose, protein
  • Thermoset: PVC, protein, polystyrene
  • Thermoplastic: Polyester, silicone
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4
Q

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

A
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5
Q

What are:

  • Homopolymer
  • Copolymer
  • Functionality
A

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

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6
Q

How are diff conformations achieved>

A

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

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7
Q

What is:

  • Stereoisomerism
  • Isotactic configuration
  • Syndiotactic configuration
  • Atatic configuration
A
  • 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
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8
Q

Cis and trans Isomers

A

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

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9
Q

Molecular structures of polymers with increasing strength

A
  • 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
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10
Q

What is vulcanisation?

A

Crosslinks can be created with additional compounds.

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11
Q

Types of copolymers

A
  • Random
  • Alternating
  • Block
  • Graft polymers
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12
Q

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

A
  • 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
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13
Q

Thermosets and Thermoplastics

A

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

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14
Q

What are:

  • Number-average molar mass
  • Weighted-average molar mass
  • Molar mass dispersity
A
  • 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
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15
Q

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

A

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

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16
Q

What does % crystallinity depend on?

What does high crystallinity say about strength?

A

-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

17
Q

What types of polymers do condensation and addition polymers yield?

A
  • 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
18
Q

Step-growth and chain-growth polymerisation

A
  • 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
19
Q

Step polymerisation description

A

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

20
Q

Step polymerisations, polycondensations

A

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

21
Q

How are polyesters produced?

A

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

22
Q

How are polyamides produced?

A

Polycondensation using carboxylic group + amine group

23
Q

How are polyethers produced?

A

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

24
Q

What are aromatic polymers, advantages, how produced?

A

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

25
Q

Why do polysiloxanes have higher thermal stability?

A

They have an inorganic backbone

26
Q

How are polysiloxanes prepared?

A

Polycondensation through hydrolysis of highly reactive dichloroalkylsilanes (see notes)

27
Q

What is a polyaddition and what is it used for?

A

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

28
Q

Free radical polymerisation

A

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)

29
Q 
Case study Polypropylene
Issues
Routes
Priorities
Processing steps
Catalyst used
A

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

30
Q

Free radical polymerisation steps

A
  • Initiation
  • Propagation
  • Termination
31
Q

Bulk Polymerisation

A

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

32
Q

Solution polymerisation

A

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

33
Q

Suspension polymerisation

A

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

34
Q

Emulsion polymerisation

A

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

ENG CHEM (2 decks)

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