Lecture 12 Flashcards
What are polyolefins?
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene
What are the general properties of PE and PP?
They are both semi-crystalline with typical properties as shown in the digital notes.
Important things to mention are:
- The Tg of both PE and PP are very small, therefore they would have some chain flexibility at room temp (for example plastic)
-Tm of both PE and PP are not that high, so they are limited to applications with low temperatures
- At high molecular weight PE shows a rubbery plateau, But pp doesn’t
What are the different types of PE polymers and how are they formed?
Types of PE polymers are:
i) formed by coordination polymerization:
- High-density PE (HDPE) - Hardly has any branches, which allows for a larger crystallization making it have a higher density
-Linear Low-Density PE (LLDPE) - It has short chain branching, making it difficult to crystallize which leads to it having a lower density
ii) Free radical polymerization
Low-density PE - It has long and short branching making it much harder to crystallize leading to lower densities.
Note that the densities of LDPE are comparable to the densities of LLDPE.
What are the two requirements needed for polymerization of polyolefins?
- Symmetry:
For crystallite formation, close-packing in 3D is required. Any Irregularities (such as BRANCHING) which disturb linearity/symmetry will lower the packing efficiency thereby reducing the crystallinity (this is shown in digital notes)!
To show how important symmetry is, I put an image to see the top view of PE (it is really symmetrical)
- Tacticity: Shown in the coming slides.
What is tacticity?
Tacticity refers to the arrangement of side groups (or substituents) along the polymer chain in relation to the main polymer backbone.
These said side groups will affect the flexibility of the chain, the larger the are the more rigid the chain becomes, but the more difficult it is to pack!
There are three common arrangements:
- Isotactic: Same-side arrangements, can crystallize. Usually forms helical structures with the side groups sticking out, forming very nice crystals as shown in the notes
- Syntactic: Alternating side group arrangements, can crystallize
- Atactic: Random side group arrangements, can’t crystallize –> amorphous.
What effect does branching have on Tm?
Generally branching of side groups, will lead to stiffer bonds that will then increase the Tm. The bulkier the side group the larger the Tm
Important note
In the exam, we will be asked to compare between polymers. Which has a larger melting point and which is more likely to crystallize.
What is the molar mass effect on the melting point?
Chains end in polymers are able to move freely. This mobility causes them to be in the amorphous region. The more chain ends present, the more amorphous character and therefore the lower the melting temperature is.
Therefore when the molar mass of the polymers is increased, the number of chain ends decreases. This of course leads to an increase in the melting temperature.
This is depicted within the notes
How are HDPE and LLDPE made? (No mechanism yet)
HDPE - Coordination polymerization of ethylene
LLDPE - Coordination copolymerization
of ethylene with 1-butene, 1-hexene, 4-methyl-1-pentene or 1-octene
How can we find the density of PE?
First, it is important to note that any semi-crystalline material is made up of both a crystalline region and an amorphous region. These regions have different densities (with crystalline ofc having larger densities). So from this, we can derive an equation that gives the value of the overall density:
ρ = φc⋅ρc + (1−φc)⋅ρa, where φc is the volume fraction of crystalline materials.
What is the relationship between density, crystallinity and mol% comonomer?
As the percentage of comonomer increases, the stiffer the chains get, the lower the crystallinity will be and the lower the density will be.
This is depicted in digital notes
What is the relation between Young’s modulus and crystallinity?
as the young modulus of the polymer increases the crystallinity increases exponentially
This is depicted in notes under flashcard 11
Imp note
Okay, I am getting a bit confused so let’s make things clear:
As chain stiffness increases the polymers will be easier to back together increasing the crystallinity. BUT if comonomers are involved, even though they increase the stiffness, they will introduce steric hindrance that can completely disrupt the packing of the polymers, making it not crystallize
In LLDPE and LDPE, where does the branching originate from?
In LLDPE Branches originate from the comonomer in coordination polymerization.
In LDPE Branches originate from inter- and intramolecular chain transfer in free-radical polymerization.
What are the different processing techniques that apply to LDPE, LLPDE, and HDPE?
Look at the digital notes
Why is having control over the crystalline region important?
Because the number of crystalline regions determines the rubbery plateau, which is essential in many process techniques
How do we make syndiotactic and isotactic polymers?
Syndiotactic is made via attack of the double bond from the monomer from under causing an alternating side groups
Isotactic is made via attack of the double bond from the monomer from above causing a same-side side group.
Atactic is formed when the attack has no stereoselectivity for the attack
What does tacticity determine/influence?
i) tacticity determines the crystallinity - of course, as mentioned before that depends mainly on the fact that activity determines the structure of the polymer (atactic, syntactic, isotactic)
ii) tacticity determines the solubility - Again the more something is nicely packed, the harder it would be for the solvent molecule to disrupt its structure causing solubility to decrease.
iii) Tacticity determines the glass-transition temperature (Tg) - the more crystalline, the less space there is in the amorphous regions causing them to become less mobile, therefore the Tg increases
What is the main analysis method of tacticity for molecules?
NMR:
i) Analysis via Dyads
ii) Analysis via Triads
What are dyads?
A diad refers to the stereochemical relationship between two neighbouring monomer units in the polymer chain. There are two types of diads:
- meso (m) diad: This occurs when two adjacent monomer units have the same stereochemistry — that is, their side groups are on the same side of the polymer backbone.
- racemic (r) diad: This occurs when two adjacent monomer units have opposite stereochemistry — that is, their side groups are on opposite sides of the polymer backbone.
This is shown in the notes.
How is a quantitative analysis of tacticity determined by Dyads?
¹³C-NMR (Carbon-13 NMR) is often used to analyze the diads because the chemical shifts of the carbon atoms will be different depending on whether the adjacent monomers form an m diad or an r diad.¹H-NMR (Proton NMR) can also detect diads by analyzing the chemical shifts of the hydrogen atoms in the polymer backbone or side groups.
m diads will produce distinct NMR peaks corresponding to the regular, same-side arrangement of the methyl groups.
r diads will produce different peaks because of the alternating stereochemistry of the adjacent methyl groups.
By integrating the peak areas in the NMR spectrum corresponding to m diads and r diads, you can quantify the proportion of each type of diad in the polymer, which gives insight into the polymer’s tacticity:
Isotactic polymers: Will predominantly show m diads in the NMR spectrum, since all the side groups are on the same side.
Syndiotactic polymers: Will predominantly show r diads, since the side groups alternate regularly.
Atactic polymers: Will show a mixture of m diads and r diads, reflecting the random placement of side groups.
What are triads?
triads refer to groups of three consecutive monomer units in a polymer chain, and they are used to describe the stereoregularity (tacticity) of a polymer. There are three types of triads:
mm (meso-meso): Indicates two adjacent units with the same stereochemistry (both are on the same side, corresponding to isotactic configuration).
rr (racemic-racemic): Indicates two adjacent units with opposite stereochemistry (alternating pattern, corresponding to syndiotactic configurations).
(random orientation, corresponding to atactic configuration).
mr (meso-racemic): Indicates a mixed stereochemistry(random orientation, corresponding to heterotactic configuration).
This shown in the notes under flashcard 20
How is a quantitative analysis of tacticity determined by Triads?
Very similar to dyads:
¹³C NMR is commonly used because it can detect the carbon atoms in the polymer backbone and side groups. ¹H NMR can also be used, especially to monitor the protons of the methyl or methylene groups along the polymer chain.
By integrating the areas under the peaks corresponding to different triads (mm, mr, and rr), the relative amounts of isotactic, syndiotactic, and atactic sequences in the polymer can be quantified
What is the effect of tacticity on Tm?
The higher the stereoregularity (i.e the higher the isotacticity which in this case corresponds to the higher m ratio) the higher the Tm.
This is depicted within the notes.
The reason of course is due to better crystallization and more close packing
