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Polymer Polymer chemistrychemistry



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Chapter 2

Step-Growth Polymerization



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Part 1 An Introduction to Polycondensation



One way is from monomer

Suitable Monomer & Suitable method (polymerization)

Another way is from a given polymer

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2.1.1How to Prepare Polymer ? 2.1.1How to Prepare Polymer ?

2.1 Introduction



① No low molecular weight material is produced. The composition of the newly produced material is the same as the monomer.

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② There are only reactions between increasing chains and the monomers



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By the reaction mechanism

A. Step-Growth Polymerization

B. Chain Polymerization

Classification 2Classification 2



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• Great Industrial Value

Examples

Polyester Linear saturated polyester: plasticizer, 可塑剂

Linear unsaturated polyester: glass fiber laminate, casting resin, solventless lacquer

Network polyester: surface coating

2.1.3 Why do we study the Step-Growth Polymerization ? 2.1.3 Why do we study the Step-Growth Polymerization ?



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Polycondensation reactions are taken as examples to illustrate the Step-Growth Polymerization.



B. Polycondensation

Polycondensation is the abbreviation of

condensation polymerization.

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Monomers with functional group

Polymer

many times of repeated condensation



B. Functionality ( f ) ：

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the number of functional groups in the

molecule which take part in the reaction.

(monomer,oligomer).



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general reaction ：

n aRa + n bR’b a[R － R’]nb + (2n － 1)ab

a ， b － functional group ； R’ ， R － construction unit ； ab － micromolecule

These reactions involve two different functional groups.

One type of the functional group in each monomer.

Forming linear polycondensation polymer admission.edhole.com


1) monomer’s f ≥2 ；2) Changeable factors : functional groups ( － OH, － COOH, － COOR, － Cl,

－ NH2 ，…… ), f (linear or cross linking polycindensation) R, and R’ Huge sorts of polycondensation polymers

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3) The polycondensation polymers are usually the heterochain polymers with N,O,S,P in the backbone and the functional groups in the end. eg. － O － , － CONH － , － COO － etc

Summary Summary



Industrially, polycondensation can be divided into many types according to the group in the polymer chain.

polyester, polyamide, polyether reaction

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4) The compositions and structures of the polymers are different from those of the monomer, because of the byproducts in the reaction.

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5) The conversion of monomer does not increase with

the reaction time in the polycondensation reactions.



Essentially, the key of polycondensation is the reactions between the function groups. Only with the efficient reaction, the macromolecules can be prepared.

Practically, polycondensation should be described by the extent of reaction.

(Notes: conversion for the chain polymerization. Chapter 3)

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(2) mixed polycondensation, f = 2,2

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Two kinds of bifunctional monomers.

Only one type of functional group in each monomer.

Example: diamine / diacid, dihydric alcohol / diacid

n H2N(CH2)6NH2 + HOOC(CH2)4COOH →

[NH(CH2)6NHOC(CH2)4CO]n + (2n － 1)H2O

2.2.4 Classifications of Polycondensation 2.2.4 Classifications of Polycondensation



① linear polycondensation Bifunctional monomer The chain increases to two directions along the ends of the

chain.

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2. By Structure of polymer2. By Structure of polymer2.2.4 Classifications of Polycondensation 2.2.4 Classifications of Polycondensation



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♦ Systems of f = 2 and f = 2, 2 are linear polycondensation.

♦ The key of linear polycondensation is to control the molecular weight of the products.




② cross linking polycondensation At least one monomer has more than two functional

groups (f = 2, 3 or 2,4 ， 3,3 …)

The molecule increases towards more than two directions. The crosslinking polymer forms.

Example: glycerol / phthalic anhydride → alkyd resin,

phenol / formaldehyde → phenolic-formaldehyde resin.

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The viscosity will be suddenly increased as reaction goes to the certain degree, forming the gel. --------gelation

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♦ The key of cross-linking polycondensation is to forecast and crontrol the gel point

This critical point is called the gel point.




balanced polycondensation

unbalanced polycondensation

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the rate of reverse reaction is not equal to zero

K ＜ 103 .

the rate of reverse reaction is little or equal to zero.

K ＞ 103 .

3. By Thermodynamics2.2.4 Classifications of Polycondensation 2.2.4 Classifications of Polycondensation



The increasement of macromolecule chain is step by step.

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2.3 Characteristic of linear polycondensation

Characteristic I.



Any molecule with different functional group can react to each other.

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There are no particular active centers in the reaction.

The molecular weight of the polymer gra-dually increases with the decrease of the number of the groups.




nX

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The monomers dispear at the early stage of reaction far

before forming any polymer with sufficiently high

molecular weight for practical utility.

High conversion of monomer is reached at early stage of

reaction followed by the reaction between oligomers.

As the time increases, increases instead of the

conversion.

Contrarily, the conversion increases with time in the

chain polymerization.




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HOROH HOOCR`COOH

HOROCOR`COOROH

HOROCOR`COOH

HOROH HOOCR`COOH+

+ H2O

HOOCR`COOROCOR`COOH

trimertetramer

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dimer

trimer



Firstly, the diol and the diacid monomer reacts to form dimer. 二醇

.

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aAa + bBb a [ A B ] b + ab

Then the dimer reacts with itself to form tetramer or with unreacted monomer to yield trimer.

a[A B]b + aAa a[A B A]a + ab

a[A B]b + bBb b[B A B]b + ab

a[A B]b + + aba[A B]b a[A B A B]b

aAa: dihydric alcohol(diol); bBb: diacidadmission.edhole.com


The tetramer and trimer continues to react with themselves, with each other, and with monomer and dimer.

The polymerization proceeds in the stepwise manner, resulting the continuously increases of the molecular weight of the polymer.

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All polycondensations are characterized by the stepwise.

The mechanism of polycondensation is rather different to that of chain polymerization which will be discussed in Chapter 3.

The reactivity of a functional group is independent of the size of the molecule.

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the degree of polymerization

P, the fraction of the functional groups that have

reacted

00

0 1N

N

N

NN−=

−

� where,

� N0 ： the total of the certain groups at the initial stage

� N ： the quantity of unreacted groups at time of tadmission.edhole.com

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� ： the number everage of construction units in

� each macromolecule

nX

leculesof macromonumberthe

sction unitof construthe total X n

=

＝

PP ＝＝ 0. 90. 9 ，， = 10= 10

= 100 ~ 200= 100 ~ 200 ，， P P 0. 99 ~ 0. 9950. 99 ~ 0. 995

P−1

1nX

nX

11P －=

00

0

N

N1

N

NNP －

－==

nXnX


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Linear polycondensation is

reversible equilibrium.

The equilibrium constants (K) of different

linear polycondensation is different.

Characteristic II.



① K ＝ 4 ～ 10, e.g kinds of polyester ， the existance of micromolecule will greatly affect the degree of polymerization.

② K ＝ 300 ～ 400, e.g kinds of polyamide ， the existance of micromolecule will affect the degree of polymerization to some extent.

③ K≥103 ， e.g phenol ~ methanal the reaction is irreversible ，

It is clear that the synthesize art depends on the equilibrium constant, K, will affect.

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In the closed system, the high molecular weight polymer is hardly obtained, due to the existence of byproducts and residual micromolecule.

Industrially, the micromolecules can be removed by reducing pressure method in order to change the equilibrium for preparing high molecular weight polymer.

e.g. The vacuum degree of the system, i.e., the quantity of residual micromolecules, control the molecular weight of terylene. 涤纶

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P, the fraction of the functional groups that have reacted

00

0 1N

N

N

NN−=

−where,

N0 ： the total of the certain groups at the initial stage

N ： the quantity of unreacted groups at time of t

2.5 The degree of polymerization2.5 The degree of polymerization

： the number everage of construction units in

each macromolecule

nX



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Example 1 ： polyester reaction nHO － R － COOH [ORCO]→ n + (n － 1)H2O

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t=0 ， the total of the initial groups ：－ COOH ：N0

t=t ， the quantity of unreacted groups ：－ COOH ：N

P － OH ＝ P － COOH ＝＝ P (2 － 2) 00

0 1N

N

N

NN−=

−

＝ (2 － 3) N

N0



=


from (2 － 3) ：＝ (2 － 4)

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nX

1

0N

N

P ＝ 1 －nX

1

＝ (2 － 5) P−1

1nX

substitutes (2 － 2) for (2 － 4) ：

＝ (2 － 3) leculesof macromonumberthe


= N

N0

P － OH ＝ P － COOH ＝＝ P (2 － 2) 00

0 1N

N

N

NN−=

−


Example 2 ： HOROH + HOOCR’COOH

Case : the same mole ratio

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t=0 － OH: N0 ，－ COOH ： N0,

the total of construction units ： N0

t=t － OH: N ，－ COOH ： Nthe quantity of macromolecules ： N

P － OH ＝ P － COOH ＝＝ P 00

0 1N

N

N

NN−=

−

nX N

N0

P−1

1 ＝＝


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In the polycondensation reaction ， increase of the degree of reaction depends on

1. prolonging the reaction time

2. increasing the reaction temperature

3. removing the micromolecule intensively

4. using high active monomer


The equilibrium polycondensation reactions consist of a series of equilibrium reactions.

As the reactivity of functional groups are assumed to be equal to each other, all reactions can be expressed by the same K ：

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～－ COOH ＋～－ OH ～－ OCO －～＋ H2O

K ＝＝ 1

1

−k

k]][[

]][[ 2

OHCOOH

OHOCO

−－－－

2.5.2 The equilibrium constant (K) and the degree of polymerization2.5.2 The equilibrium constant (K) and the degree of polymerization


～－ COOH ＋～－ OH ～－ OCO －～＋ H2O

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t ＝ 0 C0 C0

t ＝ t C0(1-P) C0(1-P) C0P C0P

the closed system

)1()1( 00

00

PCPC

PCPC

−•−•

2

2

)1( P

P

− K = ＝

＝＝

K P

P

−1 nX P−1

1


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P ＝ (2 － 6)1+K

K

＝ + 1 (2 － 7)K

Thus,

has relations not only with P ， but also with K.nX

nX


To polyester ： K ＝ 4 ， P(equilibrium) ＝ 2/3 ，＝ 3

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nX

To polyamide: K ＝ 400 ， P(equilibrium) ＝ 0.95 ，＝ 21

nX

K ＝ 104 ，＝ 100

nX

Thus ：1. In the closed systems especially that with small K,

the high molecular weight polymer is hard to be prepared.

2. Try to remove the micromolecules is key for incre-asing the molecular weight of the polymer.


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