Polymers (Topics 1-3) Flashcards
1
Q
Dp is the
A
Degree of polymerisation
2
Q
p (greek ro) is
A
conversion of a polymer synthesis
3
Q
p (greek ro) is expressed as
A
either a percentage, or a number between 0 and 1 with 1 being full monomer consumption
4
Q
Dm (funky line through the bottom of D) is called the
A
polydispersity index
5
Q
Dm (funky line through the bottom of D) tells us
A
how broad the distribution is
6
Q
Mn is the
A
number average of the polymer weights (true average calculated by the total dived by how many)
7
Q
Mw is the
A
Weight average
8
Q
What will a polymer look like on a graph of Mw vs number of molecules
A
a bell curve to represent that some chains are long and some are short but the majority are in the middle.
9
Q
Where would you find Mn on a graph of Mw vs number of molecules ?
A
at the highest point of the bell curve. Then read down to the x axis to get the weight of that partiucular length.
10
Q
Dm =
A
Mw/Mn
11
Q
Mw is always —– than Mn
A
bigger / larger
12
Q
true or false:
it is possible to have two polymers with different Dm but the same Mn?
A
true
13
Q
true or false:
it is possible to have two polymers with different Mn but the same Dm?
A
True
14
Q
Number averaged molecular mass (Mn) =
A
(sumNi)
15
Q
Weight averages molecular mass (Mw) =
A
(sumNiMi)
16
Q
Dm is always —— than 1
A
bigger/ larger
due to Mw being larger than Mn
Dm= Mw/Mn
17
Q
Polymers in solution:
In solution polymers coil or kink, why?
A
its beneficial for energy
18
Q
Polymers in solution:
We can measure the size of a polymer by its ….
A
hydrodynamic volume (Dh)
19
Q
Polymers in solution:
In a bad solvent, what does the polymer look like?
A
In a bad solvent the polymer would look like a really small tight contracted ball of string. This is due to bad solvent/polymer interactions so it contracts and coils away to limit the amount of these interactions.
20
Q
Polymers in solution:
In a good solvent, what does the polymer look like?
A
In a good solvent the polymer is extended and takes a much loser ball of string type form. This is because the solvent/polymer interactions are good so the polymer doesnt have to coil away to avoid these interactions, it is a bit more open to allow for these interactions to occur throughout the chain.
21
Q
Polymers in solution:
what is the usual trend between Mw and Average diameter, what effect does this have on transparency of solutions?
A
usually a larger Mw means a larger diameter in the same solvent. The larger Mw polymers will be cloudy in solution as they scatter the light more as they are larger.
22
Q
Polymers in solution:
GPC/SEC:
What do GPC and SEC stand for?
A
Gel permeation chromatography
size exclusion chromatography
23
Q
Polymers in solution:
GPC/SEC:
What do we use GPC and SEC for?
A
To calculate Mw and dispersity of a polymer.
From this we can calculate Mn
24
Q
Polymers in solution:
GPC/SEC:
Basic Principle of GPC/SEC
A
A polymer solution is washed down a chromatographic column with porous particles.
25
Polymers in solution:
Polymer size in solution is dependent on what two factors ??
solvent and
temperature
26
Polymers in solution:
GPC/SEC:
How does GPC/SEC work?
A polymer solution is washed down a chromatographic column with porous particles.
The porous particles have a rough surface with lots of little gaps for the polymers to permeate into. Depending on the size of the polymer, some will fully permeate, others will partially permeate and some will be excluded and continue past the porous particles.
the small polymers will get embedded into the porous particles and fully permeate whilst the larger ones will be too big and move past.
27
Polymers in solution:
GPC/SEC:
What order do polymers of different size and the solvent come out of GPC/SEC ?
Largest polymers come out first as they pass the porous particles as they are too large to be embedded.
Then the smaller ones will come after as they have been slowed by being embedded into the porous particles.
The solvent will come out last as it is super small and is very permeable into the pores.
28
Polymers in solution:
GPC/SEC:
We need to use a calibration curve to obtain a value of Mw from our sample, how would we interpret this curve to do this?
We would get a calibration curve of retention time (x) logM (y). We would measure the retention time of a certain polymer given by GPC/SEC and read off the calibration line to get logM which can be used to get Mw.
29
Polymers in solution:
GPC/SEC:
True or false, when doing a calibration, it is important to match the calibration polymer to the one being measured.
why?
true because different polymers will interact differently in the same solution and have different sizes (different Rms) which will effect their retention times.
30
Polymers in the solid state:
Polymers are rarely fully crystalline as they often have a mix of crystalline and ------- regions
amorphous
31
Polymers in the solid state:
What does Tg indicate?
Glass transition temperature
32
Polymers in the solid state:
What does Tm indicate ?
Crystalline Melting (transition) temperature
33
Polymers in the solid state:
What is Tg?
Temperature at which a material undergoes a transition from a glass state to a rubbery state or vice versa. Allows molecular chains to slide past eachother when a force is applied.
34
Polymers in the solid state:
What is Tm?
The temperature at which the material melts . in polymers this will be from a rubbery state to melting.
35
Polymers in the solid state:
As temperature increases how does the rigidity of a polymer material change?
low temps - glassy until tg.
after tg - rubbery until tm
after tm- melted
36
Polymers in the solid state:
Useful rubbers will have a Tg of what?
more than 0
37
Polymers in the solid state:
What is the definition of a rubber/ elastomer?
a material that can undergo much more elastic deformation under stress than most materials can and still return to its previous size without permanent deformation.
38
Polymers in the solid state:
Useful plastics will have a Tg of what?
they are ofeten molded into shape above this temperature and cooled below it to harden.
above 60 degrees celcius
39
Polymers in the solid state:
Crystalline polymers:
A polymer can crystallise when ...
total E of the molecules has dropped to a point when translational and rotational energies are zero.
Symmetry requirements need to be met also.
40
Polymers in the solid state:
Crystalline polymers:
True or false ?
A completely crystalline polymer will only have a Tm and not a Tg?
True
41
Polymers in the solid state:
Crystalline polymers:
True or false ?
A fully amorphous polymer will only have a Tg and not a Tm?
True
42
Polymers in the solid state:
At Tg, what properties about the polymer change ? (2)
specific heat capacity and specific volume changes.
43
Polymers in the solid state:
What would we see on a graph of temperature vs specific volume chnage of a polymer to represent Tg?
there would be a point on the line in which a change in gradient will occur.
44
Polymers in the solid state:
At what point do we consider the polymer frozen?
at Tg
45
Polymers in the solid state:
How does the concept of free volume explain the glass transition temperature?
free volume is the space in a solid or liquid that is not occupied by polymer molecules.
As temperature decreases, free volume decreases until there is not enough free volume to allow for molecular rotation or translation.
It is at this te,perature where no movement is possible that we say the polymer is frozen.
This is Tg as it has become hard and glassy.
46
Polymers in the solid state:
More glassy polymers have ---- free volume.
less /low/ small amounts of
47
Polymers in the solid state:
More rubbery polymers have ---- free volume.
more/ high/ larger amounts of
48
Polymers in the solid state:
Factors that effect Tg?
(8)
Sterics
Tacticity
Mw
Polarity
Symmetry
Branching
Crosslinking
Stereochemistry of polymer
49
Polymers in the solid state:
How do sterics effect Tg?
Increased steric bulk means it is harder for polymer chains to move past eachother which means they have a higher Tg as it needs much more energy to move past eachother.
50
Polymers in the solid state:
How does Mw effect Tg?
Assumption that there is more free volume at an end of a polymer than in the middle parts. Shorter polymers have more ends so more free volume.
More free volume lower Tg.
51
Polymers in the solid state:
How does tacticity effect Tg?
52
Polymers in the solid state:
How does polarity effect Tg?
Increase in polarity increases Tg.
Due to an increased amount of polar bonding. Bonds are stronger , less free volume, Tg increased
(i havent checked this logic but it does make sense)
53
Polymers in the solid state:
How does symmetry effect Tg?
(not to be confused with symmetry eg a1g t2g etc but rather symmetry across a polymer)
increase in symmetry reduces Tg.
harder to pack therefore larger free volume
reduces Tg
eg
h h h cl
-c c - - c c -
h cl h cl
Tg = 87C Tg= -17C
54
Polymers in the solid state:
How does branching effect Tg?
Low levels of branching increases free volume as it leads to more end groups. therefore reduces Tg
High levels of branching restricts chain mobility and has the same effect as steric bulking so raises tg.
55
Polymers in the solid state:
How does crosslinking effect Tg?
Crosslinking reduces volume of the polymer overall and therefore reduces the amount of free volume.
less free volume higher Tg.
56
Polymers in the solid state:
More free volume, ------ Tg
lower
57
Polymers in the solid state:
How does the sterochemistry of the polymer as a whole effect Tg?
Growing polymer chain can attack from one of two sides so we end up with three posibilities of polymer:
Isotactic, syndiotactic and atactic.
isotactic polymers in which all same groups are on the same side means that layers are easier to move past one another
therefore a lower Tg than syndiotactic and atactic polymers.
58
Tg of mixtures formula:
Tgmix(AB)= ( wa/Tga + wb/Tgb ) ^-1
59
Tg of mixtures formula: Temperature is measured in .....
Kelvin
60
Tg of mixtures formula: Wa+Wb =?
1
Wa and Wb are weight fractions
61
Polymers in the solid state:
What would we see on a graph of temperature vs specific heat capacity of a polymer to represent Tm?
a slope as such :
With Tm in the middle of the slope
-----------
/
/ ---------tm here
/
-------/
62
Polymers in the solid state:
At Tm the liquid and solid phases are in -----?
Equilibrium
63
Polymers in the solid state:
Equation for Tm relating dH and dS:
Tm= (h2-h1)/(s2-s1) = dH/dS
Comes from G1=G2 as liquid and solid phases are in equilibrium at Tm.
64
Polymers in the solid state:
What happens with entropy at Tm?
increase in disorder as the crystal melts.
This is controlled by the number of conformations of the chains upon melting.
65
Polymers in the solid state:
What happens with enthalpy at Tm?
There is a big difference in enthalpy between crystal and liquid which is the heat (enthalpy) of melting.
this is due to alot of energy being required to disturb the crystal lattice.
66
Polymers in the solid state:
Factors that effect Tm?
(8)
Symmetry
Intermolecular interactions
Flexibility
Tacticity
Branching
Mw
Polarity
Size of side group
67
Polymers in the solid state:
How does symmetry effect Tm?
Increase symmetry reduces conformation freedom and increases Tm.
^Symmetry ^Tm
68
Polymers in the solid state:
How do Intermollecular Interactions effect Tm?
Increased intercations increase Tm
^Interactions ^Tm
69
Polymers in the solid state:
How does flexibility effect Tm?
Rigidity increases Tm
Rigidity^Tm^
a polymer would be more rigid with an aromatic group in the central chain rather than a few ch2s for example.
eg c-c-aromatic-c-c
much stronger than
c-c-c-c-c-c-
aromatic group is stuck in place whereas c chain can move around a bit with bond rotation.
70
Polymers in the solid state:
How does Tacticity effect Tm?
71
Polymers in the solid state:
How does branching effect Tm?
A small amount of branching lowers Tm
72
Polymers in the solid state:
How does MW effect Tm?
Larger Mw lower Tm, less bonds to overcome.
73
Polymers in the solid state:
How does polarity effect Tm?
Increase in polarity increases Tm because bonds are stronger and take more energy to overcome
74
Polymers in the solid state:
How does size of side group effect Tm?
Larger side group higher Tm
75
Polymers in the solid state:
Which is larger Tm or Tg?
Tm>Tg
76
Polymers in the solid state:
We can use Tg/Tm ratios to correlate the symmetry of a polymer. eg
tg/tm = ?? for symmetrical polymers
0.5
77
Polymers in the solid state:
We can use Tg/Tm ratios to correlate the symmetry of a polymer. eg
tg/tm = ?? for unsymmetrical polymers
0.75
78
Polymers in the solid state:
Tg is often ----------- times the size of Tm
0.5-0.8
79
Polymers in the solid state:
To make a polymer more rigid we could....
Add an aromatic group to the main central chain.
Add an amine group to allow for polar interactions with other polymer chains to increase tg and reduce free volume.
Do anything else that will increase Tg
80
Polymers in the solid state:
Does a more rigid material have a higher Tg or a lower Tg?
Higher Tg
Higher Tg means less free volume
less free volume more rigid
81
Polymers in the solid state:
To make a polymer more flexible we could....
Take away an aromatic group
Take away any amines or polar groups to reduce polar interactions
Do anything else that will reduce Tg
82
Polymers in the solid state:
How do we measure where Tg and Tm lie for a polymer?
Differential Scanning Calorimetry (DSC).
Measures the change in heat capacity of a sample as a function of temperature.
Obtain a temp vs power graph:
First increase in power -Tg.
Top of tallest peak =Tm.
83
Step growth polymerisation:
Two types when we assume an equal amount of A and B.
TYPE 1:
A----BA----B
TYPE 2:
A----AB----B
84
Step growth polymerisation:
How do the chains grow?
From both ends one step at a time
85
Step growth polymerisation:
What is carothers equation and what does it help us calculate?
DP= 1/(1-p)
DP= Degree of polymerisation
p= greek ro = conversion 0
86
Step growth polymerisation:
How does DP vary with increasing p?
Dp increases with increasing p (greek ro) due to carothers equation.
87
Step growth polymerisation:
What can we define to relate Dm and p?
Dm= p+1
88
Polymers in the solid state:
TRUE OR FALSE:
Polymeric material always posseses a Tg and a Tm?
False, not all have a glass transition temperature
89
Polymers in the solid state:
TRUE OR FALSE:
"Melted polymeric materials are always above their Tg"
TRUE
90
Polymers in the solid state:
TRUE OR FALSE:
The glass transition (Tg) is an exothermic transition
False. its endothermic
91
Polymers in the solid state:
TRUE OR FALSE:
The melting transition (Tm) is an exothermic transition
False, its endothermic as it takes in the energy that is needed to break the bonds.
92
Step growth polymerisation:
Which of the following can be made using step growth polymerisation?
1) polyamides
2) polyurethanes
3) polyesters
ALL OF THEM!
93
Polymers in the solid state:
A rubber band gets how when stretched because of ....
Crystallisation
94
Step growth polymerisation:
What is the rate of change in the molecular weight of a step growth polymer at lower conversion and higher conversion respectively?
Decreases then increases
Use carothers eqn
95
Step growth polymerisation:
Acid catalysed condensation:
What assumptions do we make when considering the kinetics of this reaction?
that the reaction is irreversible
that each step uses one OH and one CO2Me group
that all steps have the same Kp.
Ignore:
diffusion
viscocity
entanglement
96
Step growth polymerisation:
Acid catalysed condensation:
What do we ignore when considering kinetics of the reaction?
diffusion
viscocity
entanglement
97
Step growth polymerisation:
Acid catalysed condensation:
What is the rate equation for this type of condensation?
Rate = K [reactants]
98
Step growth polymerisation:
Acid catalysed condensation:
What is the catalyst in this reaction?
[H+]
99
Step growth polymerisation:
Acid catalysed condensation:
what can we say about the concentrations of A and B throughout the reaction?
They are equal,
start from a 1:1 ratio and they get used up at the same rate so have equal concentrations at any given point
100
Step growth polymerisation:
Acid catalysed condensation:
What Graph can we draw to obtain the rate constant for this type of reaction?
What will it look like?
A graph of Dp vs time
time on x axis
Dp on y axis
slope = kp'[CO2Me]i
i=initial conc
Dp=Kp'[CO2Me]t+1
y = m x+c
should be linear
101
Step growth polymerisation:
Acid catalysed condensation:
What experiment can we do (measurements can we take) to obtain the rate constant for this reaction?
Do the reaction and measure Dp at different time points to find kp' using the slope of the graph and the initial concentration of a starting material.
102
Step growth polymerisation: Uncatalysed condensation:
What assumptions do we make when considering the kinetics of this reaction?
The reaction is irreversible
each step uses 1 OH and 2 CO2H groups
1:1 ratio of reactants
C02H acts as the catalyst
all steps have the same Kp
103
Step growth polymerisation: Uncatalysed condensation:
What conditions does that reaction take place in?
The reaction is heated
104
Step growth polymerisation: Uncatalysed condensation:
What acts as the catalyst in this reaction?
CO2H
105
Step growth polymerisation:
Acid catalysed condensation:
What experiment can we do (measurements can we take) to obtain the rate constant for this reaction?
Do the reaction and measure Dp at different time points to find kp' using the slope of the graph and the initial concentration of a starting material.
106
Step growth polymerisation:
Acid catalysed condensation:
What Graph can we draw to obtain the rate constant for this type of reaction?
What will it look like?
From the experiment we get Dp at different timepoints but we need Dp^2 on the y axis of our graph vs time.
Dp^2=2Kp'[CO2H]^2t+1
y = m x+c
should be linear
107
Step growth polymerisation:
How can we calculate the concentration of a reactant at a given time ?
[reactant]t = [reactant]i x (1-p)
p = greek ro
108
Stoichiometric balance:
When [A]=[B]
how does the chain look if we have A and B as monomers ?
-ABABABABABAB-
109
Stoichiometric imbalance:
[A]≠[B]
AB + A'
How does the chain look in this situation?
-ABABABA'-
A' means you cant continue the chain at that end. The end is capped and it no longer propagates.
110
Stoichiometric imbalance:
[A]≠[B]
AA + xsBB
How would the chain look in this situation?
B-(BAAB)-n B
End groups of the repeating units are B and the terminus of the polymer itself are also B.
111
Stoichiometric imbalance:
[A]≠[B]
AA + BB
What would the chain look like in this situation?
-AABBAABB-
One end is A and one end is B.
112
Stoichiometric imbalance:
[A]≠[B]
Total number of chains (N) =
N= 1/2 (Na+Nb)
where Na and Nb are number of unreacted a and b groups respectively.
1/2 because the chain has 2 ends.
113
Stoichiometric imbalance:
[A]≠[B]
How do we work out how many groups are unreacted at a given time?
Na= (1-p)Nai
Nb = (1-p)Nbi
Where Na and Nb are number of unreacted a and b groups respectively.
Where Nai and Nbi are the number of a and b groups at t=o respectively
114
Stoichiometric imbalance:
[A]≠[B]
What is r?
How is it calculated?
the ratio of two functionalities.
r=Nai/Nbi
115
Stoichiometric imbalance:
[A]≠[B]
What happens to Dp when [A]≠[B]?
What equation do we use to calculate this?
Extended carothers equation:
Dp= (r+1) / (r+1-2pr)
where p = greek ro
116
Multifunctional Monomers:
What is the functionality of a monomer?
The number of functional groups that participate in the polymerisation.
eg
A-B + A ==== ABA
f=1 for this as the A can only attach to the B so the B is the only functional group
A-A + B-B ==== AABBAABB
f=2 for this as both A and B groups can participate in polymerisation
117
Multifunctional Monomers:
How is a star polymer created? And what is the f value for the reactant?
A 3 pronged reactant like this has a f=3 when its with A-B
a a
\ /
l + A-B
a
The B ends attach onto the A ends of the triangular thing and continue in an BABABA form.
118
Multifunctional Monomers:
What trend does the value of Dm follow on increasing functional units?
Dm decreases on increasing functional units.
eg
AB f=1 Dm=2
AA+BB f=2 Dm=1.5
AAA+AB f=3 Dm=1.33
119
Multifunctional Monomers:
what equation can we use to calculate a value for Dm using a value of functional units on a monomer?
Dm= Mw/Mn = 1 + 1/f
120
Multifunctional Monomers:
How does a hyperbranched system form and what does it look like?
b b
\ /
l
a
(AB2 monomer)
These monomers join onto themselves to create a hyperbranched system. As join to Bs and vice versa.
(ABn)
b b b b
\ / \ /
l l
a a
b b
\ /
l
a
b
/ \
b a
121
Multifunctional Monomers:
How does a crosslinked loop system form and what does it look like?
AnBm
b b
\ / + A-A
l
b
(B3 monomer) + (A2 monomer)
Loop / crosslinked system created
b baab b
\ / \ /
l l
b b
a a
a a
b b
\ /
l
b
122
Crosslinked material:
What is a crosslinked material / polymer?
A polymer/ material which has many paths to get from one side to another.
123
Crosslinked material:
How is a crosslinked material different to a hyperbranched material?
a hyperbranched material only has 1 path from one end to the other but a crosslinked material has many paths across it.
124
Crosslinked material:
What are the advantages of crosslinked materials?
Improves physical properties
Heat resistant
Chemically inert
Higher Mw
125
Crosslinked material:
What is the biggest problem with crosslinked materials?
They are hard to process
126
Crosslinked material:
At what point does total crosslinking occur?
Gel point (Pg).
This is when the Mw tends to infinity. and p tends to 1.
127
Crosslinked material:
What is the gel point (Pg)?
The point at which total crosslinking occurs.
the point at which you get a lump of useless polymer.
128
Crosslinked material:
Mathematically what do p (greek ro) and Mw have to tend to for the gel point to occur?
p tends to 1
Mw tends to infinty
129
Crosslinked material:
What is the gel point equation to find the ratio of two functionalities (r)?
r= (sumNaFa) / (sumNbFb)
where Na and Nb are equivalents of A and B respectively, Fa and Fb are the number of functional groups of a and b respectively.
r is the ratio of the two functionalities.
130
Crosslinked material:
What is the gel point equation to find favg?
favg= (sumNaFa+sumNbFb ) / (sum Na + sum Nb
131
Crosslinked material:
What does favg stand for?
Average number of FGs (functional groups) per monomer unit
132
Crossslinked material:
What is the Carothers gel point equation?
Pg= 2/favg
133
Crossslinked material:
What is the Stockmayer gel point equation?
Pg= [r(fwa-1)(fwb-1)]^-1/2
fwa = weight fraction of a
134
Crossslinked material:
In the Stockmayer gel point equation, how do you calculate fwa and fwb (weight fractions of a and b)?
fwa= (sumNaFa^2)/(sumNaFa)
135
Crossslinked material:
In the Stockmayer gel point equation, what magnitude of answer will you get and what will this represent?
we should get a number between 0 and 1 which represents the conversion% at which we expect the system to have gelled.
eg 0.88 = 88% conversion is the conversion at which we expect the system to have gelled.
136
What is the degree of polymerization (DP) of a polymerization using a 2:1 equivalence of ethylene glycol to terephthalic acid at 75% conversion?
2:1 ratio therefore concs of A and B are not equal
therefore we use the extended carothers eqn
Dp= (r+1)/(r+1-2pr)
r=Na/Nb but r must be less than 1 therefore r=1/2 in this case
2
137
Step Growth polymerisation:
How is the conversion, r, of a step growth polymerisation related to the average functionality of the monomers in the system?
It is proportional to 1/favg^2
It is proportional to favg^2
It is not related
It is proportional to favg
it is proportional to 1/favg
it is proportional to 1/favg
p=2(No-N)/NoFavg
is the same as
p=2(No-N)/No x 1/Favg
138
Step growth polymerisation:
Why is the assumption that the propagation rate is the same at all points during a step growth polymerisation a poor assumption?
The chemistries are different
It is not a poor assumption
Propagation depends on time
Propagation depends on availability of the polymer end group
Propagation depends on reactivity of the polymer end group
Propagation depends on availability of the polymer end group
The availability of the polymer end group will change during the polymerisation.
139
Step growth polymerisation:
In a step growth polymerisation the first 95% conversion takes the same time as the last 2-3%?
TRUE OR FALSE
TRUE
Step growth polymerisations are characterised by very fast initial growth, with the last additions taking much longer.
140
Crosslinked material:
Carother’s and Stockmayer’s gelation theory assumes that there are no intramolecular reactions during gelation?
TRUE OR FALSE
TRUE
141
Crosslinked material:
The addition of monomers with more functional units leads to gel formation at lower conversion?
TRUE OR FALSE
TRUE
Addition of functional units causes more branching and therefore an earlier onset of gelation.
142
Step growth polymerisation:
In a step growth polymerisation if one monomer is in excess, it is this monomer which determines the DP of a step growth polymerisation?
TRUE OR FALSE
FALSE
The monomer that is not in excess determines the DP because it is the only one that is completely consumed.
143
Crosslinked material:
TRUE OR FALSE
The act of crosslinking increases the solubility of a polymeric material?
FALSE
144
Step growth polymerisation:
How does the reactivity of an acid depend on the DP of the polymer it is attached to in a step growth polymerisation?
It decreases with decreasing DP
It is independent of DP
It increases with increasing DP
It increases with decreasing DP
Cannot be determined
It is independent of DP
The reactivity is assumed to be independent of DP. However, the availability of the end group might change - as polymers get longer the accessibility of the end group might reduce.
145
Crosslinked material:
More branching means an earlier or later onset of gelation?
Earlier
146
Multifunctional monomers:
Why does addition of a multifunctional monomer lead to a narrowing of the dispersity of the resultant polymer in a step growth polymerization?
It leads to fewer chains so less coupling.
