Ionic, Ring-Opening, & Controlled Polymerization. Chapter 4. 4.3 Anionic Polymerization. 4.3.1 Initiator and Monomer 4.3.2 Mechanism 4.3.3 Living Polymer 4.3.4 Kinetics 4.3.5 Comparison between Radical & Ionic Polymerization. Overview.
4.3.1 Initiator and Monomer
4.3.3 Living Polymer
4.3.5 Comparison between Radical & Ionic
liquid ammonia with potassium amide as a
initiator, which bring out a mechanism of the
Anionic polymerizable initiators are listed in the following table.
Type of Initiator Molecular Formula or Examples
alkali metals suspension system Na suspends in THF or fluid chlorin
organic radical anion naphthalene natriuminitiator
alkyl or aryl lithium reagent n-C4H9Li
Grignard eagent RMgX( R－alkyl or aryl group)
alkyl aluminium AlR3
In these initiators:
The occurrence of bottle green solution indicates the produce of radical anionic initiator.
Water or wet air must be strictly removed， or the activity of initiator will be destroyed.
So these anionic polymerization initiators are used
in the form of a solution, in which inert organic solvents are chosen.
In many anionic polymerizations, the length of the chain depends on the quantity of the initiator. In principle, every initiator molecular produce a polymer chain. So the more the quantity of the initiator adds,the lower the MW of produced polymer is. The Polymer of narrow MWD could be prepared by this kind of polymerization.
The Anionic polymerization still belongs to the chain polymerization. It consists of 3 elementary reactions：
The initiating progress is that the initiator is added to the monomer double bond to produce an anionic monomer active center:
In the above equation, M represent metal，and Y group is an
Electron attracting group, which is apt to be attacked by the car-
boanion of the initiator.
an Initiator molecule is often in the state of association in the solution. For example:
(LiC4H9)4 4 LiC4H9Li+/C4H9-
high reaction activity with the monomer double
bond carbon atom, of which the electron
cloud density is brought low by the influence
the active center at the chain end.
The chain propagation is that the monomer continuously inserts the ion pairs and adds to the anion at the chain. This progress carries along until the monomer is used up or the chain termination occurs.
1.Reaction active centers are of different types,
like intimate ion pairs， loose ion pairs， free
ion, so that the corresponding propagation
reaction activities are greatly different.
2. In the solvents of different polarities， the
relative contentof each type of the active center
differs， which affects the total propagation reaction rate.
In the solvent of strong polarity or solvating power， the content of the incompact or free ion and the propagation reaction rate is high.
3.The volume and negativity of the gegenion
also affect chain propagation.
The less the volume of the gegenion is, the stronger its electrophilicity is.
In the solvent of low polarity there are more compact ion pairs.
In the solvent of high polarity, for the higher solvated power, more incompact ion pairs gain.
The effect of the gegenion is very complicated。
The chain transfer occurs less in anionic polymerization, especially under low temperature.
ation in anionic polymerization
Some kinds of end groups gradually loses activity in isomerization, for example, the Isomerizatin of the styrene anion in THF solvent.
The polymerization of butadiene initiated by metal Na, in which buna rubber gains, is one of the living polymerizations. Naphthalene natrium initiator transfer electrons to the monomer to produce double anions. The polymerization propagates in both sides of molecule as the following equation:
(1)The initiation is very fast, in which all the initiators
immediately join in the initiating progress and
are transferred to active centers.
The feature of the living polymerization is that the MWD of product is quite narrow.
The reasons are:
(2)All chains propagates at the same rate until monomers are used up. Every active chain has the same chance to share all of the monomers. The MW of the product is nearly similar.
(4)Depolymerization can be overlooked.
Anionic initiator nearly quantitatively and immediately dissociate into positive and negative ions which have initiation activity. All of the carboanions are added to monomers at once, and at the same time the propagation starts:
According to the solvating power of the solvent and the character of the gegenion, the active center could be a free ion or an ion pair. Ion pairs differ in compact degree of dissociation. Mostly several kinds of active centers exist proportionally.
Compactly only the primary form is considered, for example, the free ion, so the propagation can be written as:
The total polymerization rate equation is the propagation reaction rate equation（4－18）
The total concentration of all chain active centers
The concentration of the added
Integrating this equation, the change of monomer
concentration with varied time results.
It`s a first order reaction.
In the living polymerization,for there is no chain termination, the kinetic chain propagation will not stop until monomers are used up.
The average kinetic chain length is defined as:
When t goes infinite, that is to say the monomers are used up, the longest kinetic chain length is:
Average DP is the number of monomers consumed by each polymer molecule. If every active center forms one polymer molecule, then:
Taking naphthalene natrium or metal Na as the initiator, the polymerization is called double anion propagation polymerization, in which one polymer molecule has two active centers and consume two initiator molecule. So,
The MW of this kind of polymer obeys Poisson distribution. Mol fraction of its x-polymer, nx, is:
can be determined, or the rate constant kp can be
directly determined by the monomer transfer ratio.
In principle during the polymerization progress the equilibrium between ion pairs and free ions always exists:
Diluted by a solvent, that is to say, the concentration of [R-G]0 reduces，the dissocia-tion equilibrium moves rightward. Extrapolating the experimental value of kp to the direction of solution infinite dilution, the chain propagation rate constant of the free ion, k－, gains. It proved experimentally that the k－value of the free ion is much larger than that of the ion pair.
Polarity and solvating of the solvent affect the dissociation degree of the ion pair. So the comprehensive value of the apparent propagation rate constant kp must be affected, too.
Table 4－6 shows the effect of the solvent on kp of styrene anionic polymerization.
Solvent dielectric constant /D Kp/ (L/mol.s)
benzene 2.2 2
1,4-dioxy hexatomic ring 2.2 5
THF 7.6 550
1,2-dimethoxy ethane 5.5 3800
※Determined in styrene polymerization initiated
by naphthalene natrium under 25℃.
also affect the dissociation degree of the ion
pair, so as to affect value of the propagation
rate constant of the ion pair.
On the whole，polymerization feature is mainly determined by chain propagation, of which the activation energy is very low, and varies little with the change of the temperature.
But dissociation equilibrium of the ion pair, the solvating power, and even the side reaction are affected, where a current rule is hard to be summed up.
Without chain transfer and termination, polymer MW is not easily affected by the temperature.
Styrene polymerization in benzene solvent initiated by butyl lithium is several orders of magnitude lower than that by naphthalene natrium.
Dissociation number of butyl lithium could be 6, and that of secondary and tertiary butyl lithium are 4.
Elevating polymerization temperature can also weaken association.
1.Monomer and Initiator
push-electron group, the activity of M depends
on the push-election ability of substitute.
proton acids or Lewis acids (coinitiators)
the substances which supply protons or carbo-
Requirement:of which the ability to supply protons or carbocations is strong and the gegenion nucleophilicity is weak.
Initiation: Ei = 8.4 --21 KJ/mol
Propagation: more complicated than the
a) Diversiform spike;
b) Carbocation being apt to isomerize
Transfer: mainly to monomer, solvent and gegenion
Cationic polymerization is apt to transfer,and its CM is larger than that of the radical polymerization
Termination: single-group termination, mainly to
the gegenion reaction
Summary: fast initiation, rapid propagation, greatly easy transfer and comparatively hard termination
The temperature elevated , polymerization reduces.
ER＝Ep - Et<0:
E ＝Ep - Et or E ＝Ep – EtrM<0:
The temperature elevated , polymerization reduces.
d. The effect of the solvent and gegenion
When Solvent polarity increases, the incompact ion pairs multiply, and the polymerization rate enhances
The gegenion with nucleophilicity, polymerization
rate reduces; the volume of gegenion enhances,
the incompact ion pairs multiply and the poly-
merization rate increases.
1. Monomer and initiator
Monomer: With electron attracting groups, the
activity of M depends on the
electron attracting ability of the
Initiator: an alkal reagent
Transfer and termination: no transfer,
no termination-------living polymerization
Initiation: The initiators instantaneously and
quantitatively produce spikes.
Single anion spikes and double anion spikes
Propagation: the form of spikes is diverse
The property of solvent
The solvalrility increases with increase of the solvent between polarity, increasing the distance between ion pairs, the amount of loose ion pairs, the concentration of the free ion and hence the rate of polymerization.
（1）the type of the initiator
（2）the monomer structure
（3）the effect of the solvent
（4）the polymerization temperature
（5）the polymerization mechanism
（1）the type of the initiator
（1）the type of the initiator
（2）The Monomer Structure
The ionic polymerization have high selectivity of the monomer.
（2）The Monomer Structure
（3）the Effect of the Solvent
In the radical polymerization a solvent only takes part in the chain transfer and can affect the initiator decomposition rate.
（3）The Effect of the Solvent
TheIonic polymerization can use a nonpolar hydrocarbon solvent , and other solvents can only be used selectively.
The rules cannot be used inversely, otherwise the chain transfer or termination will occur.
（4）The Polymerization Temperature
（5）The Polymerization Mechanism
nonpolar alkene monomer
compound and chromic oxide catalyst
(1) Isomers of Polymer
What is isomerism?
The atoms or atomic groups in a polymer molecule have different interconnection orders, which bring out isomerism or structural isomerism.
Fox example：Polymerizing same or different monomers can produce polymers of same chemistry constitute and different structures.
Solid isomers of polymer indicate the kind of polymers , which have the same chemistry constitute and connection structure , and differs just in conformation , that is to say , the steric arrangements of them are different.
Solid isomerism can be classified into two categories：
R (Right) form & S (Left) form
Z（cis conformation）& E（trans conformation）
How to distinguishconfigurationandconformation
① Optical Isomers
All monomers with the chiral center in the framework atoms can form polymers of different steric structures.There are chiefly three possibilities when monomer inserts polymer chain.They are:
② Geometry Isomers
When substitute on the double bond or ring have different steric arrangements , several geometry isomers form.
Polydiolefine in the form of 1,4 addition, have
cis 1，4 addition and trans 1，4 addition,
see Chart 4-5
Cis form 1,4-polymer
rity greatly affects the material property.
regular polymers of different conformations
should be produced.
For example , isotactic and atactic PP greatly differs in properties.
The structure units of natural polymers , cellulose and starch , are both glucose unit , of which the steric conformation differs , so properties are greatly divergent.
The differences in the structure cause that:
Polymerization of the alkene monomer initiated by a Ziegler一Natta catalyst can produce an unbranched , stereoregulated polymer . PP gained in this way is linear and has a higher density than that produced in the radical polymerization .
PP synthesized by Ziegler-Natta polymerization is isotactic and of excellent material property . Many nonploar alkene monomers are able to process the coordination complex polymerization .
A Real catalystis not a simple coordination of two compositions. It gets high catalysisactivity through an aging progress , in which there is complicated alkylating reduction reaction .
The most important feature is that the transitional metal is reduced from the high oxidation state to the low oxidation state and gets unfilled coordination value state . It`s the real activation center .
Initiating ability is expressed by catalyz-
ing activity index:
For example , amine , ether and some compounds with phosphor , expressed by B: , have the ability to coordinate kinds of aluminous compounds, though the degrees of the coordination ability are different and the stability follows the subjacent order :
B:→AlCl3 > B:→AlRCl2 > B:→AlR2Cl >B:→AlR3
Alkyl aluminium compound processes alkylating reduction reaction during the forming of the catalyst :
Complex Competition Reaction
With low quantity of the third composition , enough cocatalyst can be reproduced . Usually Al:Ti:B approximately equals 2:1:0.5 in industry .
Duing the producing of Ziegler-Natta heterophase catalyst , only catalyst on the surface can form polymerization catalysis active centers , and the interior of granule doesn`t effect and waste the catalyst .
Highly Dispersion of Catalyst On Carrier Surface
Using Superfine Granule Carrier
Superfine Granule Carrier ----For example , granules of Mg(OH)Cl , MgC12 and Mg(OH)2 make the newly produced catalyst highly disperse on the surface of the carrier .
The active surface increases from 1～5 m2/g to 75～200 m2/g , producing so-called High-Performance Catalyst.
With as high stereo tacticity as more than 95％ , the stability and life will increase .
Alkyl metal compound Transitional metal compound Quantity of isotactic structure
AlEt3 TiCl4 30~60
AlEt3 TiBr4 42
AlEt3 TiI4 46
AlEt3 VCl4 48
AlEt3 ZrCl4 52
AlEt3 MoCl4 50
AlEt3 TiCl3 (α,γ,δ) 80～92
BeEt2 TiCl3 (α,γ,δ) 94
AlEt2I TiCl3 (α) 98
1. Single TiCl3 have no initiating activity;
2. The more the aluminium alkyl associating with Ti in the system is , the high the initiating activity is.
There are two typical coordination polymerization mechanisms , which are accepted commonly.
Alkene monomer coordinates at a voidance of single metal.
Transitional metal coordination octahedron
Forming four membered ring transition state with the
organic group on the ortho position
Chart 4-7 Conventional Diagram of Single Metal Coordination Complex Polymerization Mechanism of Alkene
So if the voidance jump finishes before the new monomer coordinatingly takes the available space , all of the insertingly propagating monomers will coordinate at the same position , and the propagating chain structure unit will have the same geometry conformation.
The Alkyl group locates on the bridge bond between the two metals.
Electron deficient four membered ring
Coordination transition state
The monomer insert titanium-carbon weak bond and alkyl R transposes from Ti metal to the other carbon atoms of alkene.Continuing:
Carbon atom of alkene and double metal then forms new bridge bond.
Six membered ring
The above two mechanisms only explain the basic feature of the coordination polymerization and are commonly accepted , but both have bugs.
Ziegler-Natta coordination polymerization still belongs to chain the polymerization , consisting of every elementary reaction of the chain polymerization.
① Transfer to alkyl metals of family Ⅰ～Ⅲ
③ Transfer to H2 , the MW regulator
Because the metal atom is apt to form a metal hydrogen bond , all the chain transfer, except hydrogen is addition reaction, summed up to be caused by the transfer of unstable β hydrogen . A Metal ion is apt to produce a metal hydrogen bond . All chain transfer except hydrogen addition reaction could be summed up to be composed of the transfer of unstable β hydrogen anion .
Spontaneous chain transfer to β-hydrogen anion
gent will increase activation centers and the
polymerization rate . There is a optimum ratio
between the chief catalyst and the cocatalyst ,
which usually is about 1:2 (Ti:Al) .
For example: High cis-1,4 addition polymer or high trans-1,4 addition polymer , and 1,2 or 3,4 addtion polymers ,.etc.
(1) The Mechanism of the Coordination Polymer-
ization of the π －allyl Complex Compouund
On Ziegler-Natta catalyst , the diene monomer can coordinate to form π——allyl complex compound and bring out the polymerization .
This kind of the activation center of cataly-sis polymerization can be produced by the direct reaction of the allyl compound and the transi-tional metal compound .
transitional metal halide ,like nickel , chro-mium, cobalt, iron, titanium, vanadium, moly-bdenum, tungsten and niobium, can form poly-merization catalysts of high activity.
(π-C4H7)4Ti 或 (π-C4H7)4Zr
Л－allyl catalyst complex compound have its own coordination structure . When the monomer inserts among the metal carbon bonds and take 1,4 addition , as the following reaction equation shows, cis-1,4-polybutadiene is produced .
If the position that the monomer inserts is not on the M-C (1) bond but on the M-C (3) bond , 1,2 or 3,4 addition occurs shown by the following reaction equation.
A Catalyst of orientation, in which chromic oxideis most commonly used, is the metal oxide formed on the surface of inert granula of silica gel or aluminum oxide.
The catalysis function of this kind of catalysts is caused by the ability of the chromium ion to change its oxidation state.
Some monomers, like alkenes and substances with rings, can undertake ionic chain polymerization with a ionic initiator.
（1）The active center of cationic polymerization is a
（2）The active center of anionic polymerization is a
（3）The active center of coordination poly-
merization is coordinated ion with a
metallic carbon bond.
Ring-Opening polymerization is defined as the process in which the ring of monomer opens , and a linear polymer forms. The general formula is:
Hetero atom or functional group in ring monomer
Like O,N,S, -CH=CH- , -CONH-, -COO-,.etc.
With groups like ether,ester, amide, imine and double bond etc.
Controlled Polymerization is the new direction of the late 20 years.
Isobutylene (1) 1,3-butadiene (2) vinyl ether (3)
styrene(4)1-methyl styrene(5)methyl aldehyde(6)
strong proton acid
The typical substances are listed in table 4-1
1．Strong proton acid H2SO4,HClO4,H3PO4,
2．Lewis Acid BF3 , BF3O(C2H5)2,
AlCl3 , SnCl4
almost take place under low temperature，and the
solvent must be inert and very dry.
Cationic polymerization belonging in chain polymerization，contains:
Firstly，the initiator reacts to produce a proton or carbocation;
Then the carbocation adds to the monomer to produce a monomer carbocation.
Loose ion pair
Intimate ion pair
1. Combination with gegenion，for example:
2. The add-terminating-agent termination is to terminate cationic polymerization by adding protonizing agent like water, alcohol and amine etc. The excess amount of the terminator is stabilized by solvating around the proton so that can`t initiate the polymerization to terminate the reaction any more.
The feature of the cationic polymerization is fast initiation, rapid propagation, very extremely easy transfer and comparatively hard to ter-mination, in which a terminator is often indispensable.
(1) The rate equation
Conveniently，the following general equations can be used to show elementary reactions of the cationic polymerization，where:
(2) The degree of polymerization
The termination is a first order reaction. Its rate is directly proportional to the active chain concentration in propagation
According to the following equation:
The polymer producing reaction contains termination and transfer, So that:
Substituting it into (4－10), the expression of DP
Its reciprocal form is more commonly applied,
and ktrM / kp is substituted by the chain transfer
The above expression only deals with the chain transfer to the monomer.Involving transfer to the solvent,the expression has one more term:
The effect of the temperature on the polymerizing rate depends on the relation between kp/kt or kp‘/kt’ and the temperature. The symbols without primes represents ion pairs, and the other represents free ion. So the arrhenius form of the rate constant ratio of one kind of those two symbols is:
kp=(1-α)k(±) +αk(+) (4－17)
α is the degree of dissociation from ion pairs to free ions
The propagating rate constant of free ion, k(+), is 1~6 order of magnitudes larger than that of ion pairs, k(±).
Table 4-2 The Effect of Solvent on the Cationic Polymerization of Styrene（HClO4 as a initiator）
Solvent Dielectric ConstantKp/25℃,l/mol.s
Thus it can be seen that the kp of the cationic poly-merization is not higher than that of radical polymer-ization. However because the kt of the cationic poly-merization is low, the kp/kt1/2 of the cationic polymer-ization is much higher than that of the radical polymer-ization，and the concentration of spike is high, the rate of the cationic polymerization is much higher than that of the radical polymerization.