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Chapter 8.
Stereochemistry of
Polymerization
※ Constitutional (Structural) isomerism
i) From different monomers
Nylon 6
Nylon 6,6
ii) From a single monomer
• head-to-head & head-to-tail
• 1,3-dienes : 1,2- and 1,4- polymers
• monomers with alkene & carbonyl groups (5-7a)
• linear or cyclized polymers (6.6b)
From radicals
From ions
8-1 Types of Stereoisomerism in Polymers
→ arises from different spatial arrangement (configuration)
↔ conformational isomer : interconvertible
8-1a Monosubstituted Ethylenes
8-1a-1 Site of Steric Isomerism
H2C CH
R
⇒ Possible chirality arises from the differences of the length
Chiral property
• Optical rotation, different properties in chiral environment
• Same physical property
→ The carbons at the end of the polymer chain can have optical activity
: Population is very negligible
→ Other carbons : not much optical activity
∴ The sum of optical activity is below the limits of detection
⇒ rather achirotopic (achiral)
8-1a-2 Tacticity
Atactic : R groups on successive stereocenters randomly distributed
Isotactic : the stereocenters has the same configuration
Tactic
Syndiotactic : the configuration of the stereo
Stereospecific polymerization : polymerization that yields stereoregular polymers
※ Stereoregular : isotactic or syndiotactic
8-1b Disubstituted Ethylenes
8-1b-1 1,1-disubstituted ethylenes
R
H2C C
R'
R & R’ are the same : no stereo isomerism
R & R’ are different : atactic, isotactic, syndiotactic
8-1b-2 1,2-disubstituted ethylenes
-Diisotactic : each of the two stereocenters is isotactic
- Disyndiotactic : each of the two stereocenters is syndiotactic
- Erythro : R and R’ is anti, H and H is anti
- Threo : R and H, : R’ and H is anti, then R and R’ is gauche
⇒ four possible isomeric structures; threodiisotactic, erithreodiisotactic,
Threodisyndiotactic, erithreodisyndiotactic
Except the
End group
they are identical,
8-1c Carbonyl and Ring-opening polymerizations
• Polyacetaldehyde : Carbonyl
O
CH3CH
H
C O
CH3 n
Iso & Syndio
8-1c Carbonyl and Ring-opening polymerizations
• Poly(propylene oxide) or Poly(oxy[1-methylethylene])
O
OCHCH2
CH3
Iso or Syndio
8-1d 1,3-Butadiene and 2-substituted 1,3 butadienes
8-1d-1 1,2 and 3,4- polymerization
butadiene
1,2
3,4
Same Product
⇒ iso, syndio, atactic
⇒ three different structures are possible
Isoprene or chloroprene
4
1,2 poly
⇒ iso, syndio, atactic
3
2
1
3,4 poly
⇒ iso, syndio, atactic
⇒ six different structures are possible
8-1d-2 1,4-polymerization
isoprene
∴ transtactic or cistactic
Nomenclature
• trans-1,4-polyisoprene :
trivial name (IUPAC recommended)
• poly(E-1-methylbut-1-ene-2,4-diyl):
(IUPAC name)
8-1e 4-substituted and 1,4-disubstituted 1,3-butadienes
4
3
2
1
1
3,4
also possible
1,2 poly
2
3
4
1,2 & 3,4 are the same
R
CH2 CH
CH
n
CH CHR
CH
n
CH
CH
CH R
CH
CH
2
Similar to the polymerization
CHR'
Similar
to
the
polymerization
Of propylene
of 1,2-disubstituted ethylene
n
* 1,4 polymerization of 4-substitutes
cis
chiral
Still not optically active
possibilities
① Trans
Iso
② Cis
Iso
③ Trans
Syndio
④ Cis
Syndio
① Trans
atactic
① Cis
atactic
8-1e-2 1,4-polymerization of 1,4-disubstituted 1,3-butadienes
possibilities
R&H
anti
Cis & Trans
R & R’ anti
Threo & Erythro
disyndio & diiso
See Fig. 8.2 + cis & trans
See p 631 XIV : erythrodiisotactic
R&R’ anti, R&R iso, R’&R’ iso
Transerythrodiisotatic 1,4-poly(methyl sorbate)
Diisotactic poly[erythro-3-(methoxycarbonyl)-4-E-methylbut-1-ene-1,4-diyl]
8-1f Other polymer
Poly(cyclopetane-1,2-diyl) (IUPAC) or polycyclopetene
Erythro: cis of chain bonds entering and leaving each ring
Threo: trans of chain bonds entering and leaving each ring
cis
trans
8-2 Properties of stereoregular polymers
8-2a-1 Isotactic, Syndiotactic, and Atactic P.P
• Atactic : Amorphous (noncrystalline), soft (tacky) material
With no physical strength
Application : asphalt blends, formulations sealant & adhesives
• Isotactic & Syndiotactic
: highly crystalline ( ∵ ordered structure )
high physical strength
increased solvent & chemical resistance
Application : plastic & fiber
• Isotactic PP
Tm ~176˚C
→ Easily synthesized from stereospecific polymerization
• Syndiotactic PP
Tm ~156˚C
→ not easily synthesized !
20˚C lower than that of isotactic, more soluble in HC solvent
8-2a-2 cis- and trans- 1,4-poly-1,3-dienes
see P633 Table 8-1
Tm, Tg of trans is higher than those of cis
∵ The trans isomer crystallize to a greater extent
as a result of higher molecular symmetry.
Natural rubber : 1,4-polyisoprene
Mostly cis
8-2a-3 Cellulose and Amylose
β-D-glucose
α-D-glucose
H OH
H OH
H
H
O
O
HO
HO
OH
HO
H
H
H
HO
H
OH
H
OH
H
• Cellulose : β-D-glucose units with trans linkage
• Amylose : α-D-glucose units with cis linkage
OH
8-2a-3 Cellulose and Amylose
• Cellulose : fully extended chain hydrogen bonded into sheet
∴ crystalline, physical strength, decreased solubility, stable to hydrolysis
• Amylose : random-coil
8-2b Analysis of Stereoregularity
→ IR & NMR
How :the difference of sequence distribution
① Dyad tacticity
R
R
CH
CH2
CH
R
CH2
CH
CH2
CH
R
meso ≡ m
racemic ≡ r
CH2
8-2b Analysis of Stereoregularity
② Triad tacticity
mm
rr
mr
Example)
6 meso dyads , 2 racemic dyads
∴
∴
High resolution NMR
Tetrads, Pentads are also possible
• Tetrads
• Pentads
8-3 Forces of stereoregulation of alkene polymerizations
depends,
• rates of addition (Temp)
• monomer
• initiator
• active end
⇒ case by case
8-3a Radical polymerization
⇒ freely propagating species : chain end can freely rotate
∴ don’t have specific configuration
⇒ configuration is determined after the monomer is added.
Kr & Km exist (racemic or meso)
ΔΔH+ ∼
= -4 ~ -8 kJ/mol
+
ΔΔS + ∼
= 0 ~ -4 J/mol∙K
+
∴ Syndiotactic is favored over isotactic because of the enthalpy difference.
Ex)
bulkier
+
ΔΔH+
+
ΔΔS +
MMA
-4.5 kJ/mol
-4.2 J/mol∙K
VC
-1.3 kJ/mol
-2.5 J/mol∙K
* Syndiotactic is favored because of
steric and/or electrostatic repulsions between the substituent
∙
T ↑ , Kr/Km → 1
Ex) PVC ,
Since radical polymerization is performed above 60˚C,
→ atactic with syndio rich.
8-3b Ionic and Coordination Polymerization
8-3b-1 Effect of Coordination
• In good solvent : free ion or solvated ion pair
→ Similar to radical polymerization
Syndio favored !
• In poor solvent (coordination is possible)
→ Stereospecific polymerization can occurs
Usually isotactic , sometimes syndiotactic
including Z-N Catalyst ?
To form isotactic polymer,
Key point : coordinates in one direction
8-4 Traditional Ziegler-Natta polymerization of nonpolar alkene monomers
8-4a History
The significance of Ziegler-Natta initiators
→ can polymerize α-olefin
Cannot be polymerized
by radical & ionic mechanism
⇒ TiCl3 with Al(C2H5)2Cl or TiCl4 with Al(C2H5)3 in HC solvent with olefin
⇒ then polymers are obtained in situ as a precipitate
⇒ Many trial and errors
⇒ -TiCl3 (brown color) was found to be effective for the polymerization
(prepared by mixing TiCl4 with H2, aluminium, or alkylaluminiums
Still isotacticity 20 – 40 % with Al(C2H5)2Cl
-, -, -crystals of TiCl3 were found to increase the isoctacticity
Finally used initiator preparation method
① Mixing (by ball milling)of magnesium chloride (or alkoxide) as a solid support
and TiCl4
② Addition of Al(C2H5)3 with an organic Lewis base
(external base; water, ester, phenol)
⇒ high activity (50- 200 kg of polymer per gram of initiator system
ex) initiator 2-4% of Ti, so 1500-6000 kg polymer per gram of initiator
So very small amount of initiator in PE or PP
+ 90~98% isotactic
8-4b Chemical nature of propagating species
⇒ Radical, ionic, or coordination is possible according to the conditions.
ex) ① The use of alkyl lithium as group I-III metal component (early days)
Not AlR3
⇒ anionic polymerization ex) MMA
② Monomers with e--rich double bond cationic by TiCl4 ex) vinyl ether)
③ Radical can be generated
Ionic character
Possibly anioic
Poly rate:
Ethylene>propene>1-butene
The reverse of cation stability
④ Stereospecific polymerization → generally coordination mech.
Evidences for the anionic character
• Polymerization rate: Ethylene>propene>1-butene
• Carbanion attacks at the  carbon to form less substituted (more
stable carbanion)
• Size -substituent ↑, reactivity ↓
• Termination with CH3O3H radioactive, while that with 14CH3OH is
not radioactive
8-4c Primary vs Secondary Insertion
① Primary Insertion (1,2 addition)
less substituted - G
Polymerization of olefin : isotactic formation
olefin
R → e- donating effect
C
C
-δ
G
C
+δ
R
C
-δ
G
+δ
R
+δ
-δ
Less steric hinderence
* Less substituted carbanion is more stable
H
C
R
C
C
C
-δ
-δ
R
⇒ reasonable
G
8-4c Primary vs Secondary Insertion
② Secondary Insertion (2,1-addition)
→ substituted chain end is attached to G
Attack to less substituted
part like radcial
-δ
+δ
When R is very bulky, then secondary insertion is possible
-δ
+δ
C
G
R
due to the size
If R is very bulky, then secondary insertion is possible, like radical polymerization.
So syndioselective polymerization proceeds through secondary insertion
8-4d Propagation at Carbon-Transition Metal Bond
Ziegler-Natta catalyst
Transition metal (Ti, Zr, V ···) + Group I-III metals (Li, Al ···)
which is G
Conclusion
⇒ Transition metal
• No polymerization of olefins with Group I-III metals
• With transition metal alone, olefins can be polymerized
→ less stereospecific
⇒ Transition metal + Group I-III metal
• increased reactivity !
• stereospecific reaction !
8-4e Mechanism of isoselective propagation
Active sites are formed from tatanium chloride and alkyaluminiums
TiCl4 with AlR3
or
TiCl3 with AlR2Cl
① Surface structure of TiCl3 crystal
I.
Four Cl’s are shared with another Ti nearby.
II. R : on the surface of TiCl3
could be polymer chain
III. Empty site ⇒ monomer can approach
Mixing with MgCl2
Mg-Cl···Ti
linkage is helpful.
Mg and Ti are interchangeable in
The metal lattice
The actual possible structure
② Other possible structure
Since Group I-III metals are helpful
Possibility
R
Al
R
Cl
Cl
Ti
Cl
Cl
R
Same as monometallic
R & Cl is shared with Al.
Propagating site is shared !
Polymerization mechanism
for isoselective polymerization
The whole polymer chain
does not migrate, only
the active end of the
Polymer chain migrates.
Ti is always bonded less
substituted carbon
The migratory insertion
only the atoms of the
first few repeat units
move.
migration
different
position
CH3
isotactic
∵ migration regenerate
the original conformation.
⇒ same position addition
Ti
CH2
C
CH3
syndiotactic (∵ steric effect)
alternating position addition
If propagation continued with the vacant site contineously, the syndioselective
propagation can occur. Then the Position changes!
8-4f Syndioselective propagation
Syndiotactic rich polymerization is mostly
possible from soluble initiator, vanadium
comppounds, still the selectivity is not very high
ex)rr=0.9
8-4g Direction of Double-bond opening for 1,2 disubstituted monomers
Isotactic polymerization
⇒ monomer approaches the propagating center with the same face, then rotate to
form threodiisotactic, trans monomer produces diisotatic
incoming monomer is
aligned so as to minimize
steric repulsions between
the R group of the chain
end and the R groups on
the monomer (R groups a
re opposed to each other)
Syn addition: It is possible the transition metal directs the stereochemistry.
8-4a-6 Mechanism of isotactic control
If they coordinate
in the same manner
⇒ isotactic
If not
⇒ atactic
In alternative way
→ active site changes
⇒ syndiotactic
Ti
•
always same position , same direction
⇒ isotactic polymer
R
8-4h Effect of component of Ziegler-Natta initiator
8-4h-1 Transition-metal
TiCl3 → isotactic
VCl3 → syndiotactic
Order of stereospecificity
α, γ, δ – TiCl3 > TiCl2 > TiCl4 ~
= β-TiCl3
layered structure
Composed of bundle of
linear TiCl3 chains
β-TiCl3 : on the surface
Half of Ti ⇒ one vacant site
Half of Ti ⇒ two vacant site
∴ Two sites ⇒ stereospecific polymer is not possible
Actual situation
Use TiCl4 rather than TiCl3
∵ TiCl4 : liquid → handling is easy
→ by adding AlR3 or AlR2Cl, TiCl4 is reduced to TiCl3
Actual initiator
The extent of isotacticity for propene polymerization
or
α-TiCl3 > CrCl3 > VCl3 > FeCl3
TiCl4 ~
= VCl4 ~
= ZrCl4
⇒ reasons are not very clear but there is a trend.
stereoselectivity and activity are dependent on the crystal structure,
steric size, ligands, and the electronegativity of the transition metal.
•
Ti3+ (trivalant oxidation state): more stereospecific and active
•
Ti4+ (tetravalant oxidation state): less stereospecific and active
not much dependent on the ligand
Decreasing the electronegativity of the transition metal (more easily polarized
metal growth bond), increasing the stereoselectivity
Increasing the size of the ligands decreases stereoselctivity probably
because of a decreased coordinating ability.
8-4h-2 Group I-III Metal component
•
not an absolute necessity
•
but need to obtain high stereospecificity and high activity
* Group I : Li , Na, K
→ not very attractive ∵ low solubility in hydrocarbon solvent
Lithium → soluble but aggregation
* Group II : Be , Mg , Zn , Cd
Be : not used ( ∵ toxic )
Zn, Mg : widely used
* Group III
Al : most widely used ( ∵ active )
Ga : active but expensive
8-4h-3 Third component : Lewis base
→ oxygen, water, inorganic halide (KCl, NaF), organic halide, phenols, ethers,
amines, phosphines, aromatics ∙∙∙
⇒ case by case ( ∵
)
Internal base & External base
The preparation of initiators
① Internal base is ball milled with support material and the transition-metal.
(TiCl4 + MgCl2)
② Then group I-III compound + external base are added
Ester : internal base , also external base
Phenyl triethoxysilane : external base
⇒ change the stereospecificity
8-4i Kinetics
→ very complicated (∵ heterogeneous system !)
ball milling
after sometime
large crystal aggregates
Polymer chain cleaves the aggregates.
⇒ Ball milling can reduce the induction period.
Different active sites are existing
TiCl4+AlR3
MgCl2 milling
TiCl2 + MgCl2
milling
TiCl3
Low activity
TiCl3
High activity
8-4i-2 Termination
→ probably no termination
* Active site have a character of living polymer, not the propagating chain
① Spontaneous intramolecular β-hydride transfer
still active
Chain transfer to monomer
active
② Chain transfer to the group I-III metal alkyl
③ Chain transfer to H2 or other compound
④ Many other complicated unknown reactions
8-4i-3 Rate expression
Rp=kp[C*][M]
Conc. Of active site
→ If homogeneous, then true
*
Langmuir-Hinshelwood model: Reaction occurs only after monomer is
absorbed from the solution onto the transition metal active sites.
•
Group I-III metal compounds are also soluble in the solution & competes
with monomers for the same sites, they are normally excess.
Therefore the polymerization of monomer competes with the adsorption of I-III
Metals.
[A] and [M] are the group I-III metals and monomers and KA anf KM are
equilibrium constant for the adsorption
Then, Rp=kp[C*][M] becomes
Xn → same as radical polymerization
and
8-4i-4 Values of kinetics parameters
[C*] : conc. of active site
⇒ determined by quenching with CH3OH,
∵ Ti
14CO, 14CO
2
∙∙
coordinated with O the unpaired e-’s then reaction stops
⇒ measures the consumed MeOH, CO2, CO
Problem : obtained [C*] could be higher than the real value
∵ The products from chain transfer to the group I-III metal alkyl
Ti
+ AlR3
Ti
R + Al
both react with O
∙∙
[C*] is hundredthes of a percent (0.01%) to tenths of a percent (0.1%) of the
metal concentration
0.65 M propene in heptane, 0.0001M Ti (TiCl4/Al Et3 on MgCl2)
Internal base : ethyl benzoate
a : atactic
External base : methyl p-toluate
-specific
i : isotactic -aspecific
With both bases, specificity increases !
* Summary of Ziegler-Natta initiator
Possible monomers
① Ethylene & α-olefins
propylene, 1-butene, 4-methyl-1-pentene
vinyl cyclohexane, styrene
② 1,1-disubstituted ethylene
isobutylene, α-methyl styrene by non-coordination cationic mechanism
③ 1,2- disubstituted
No polymerization using Ziegler-Natta initiators because of steric hinderence
except)
I.
1,2-disubstituted ethylene → isomerization → polymerization
II. cycloalkene
(Prior to the polymerization)
④ Polar monomers
vinyl acetate, acrylate, vinyl chloride
→ by non-coordinated radical or cationic mechanism
How about stereospecific polymerization ? ⇒ Not easy
∙∙
∵ coordination of N an oxygen on the transition metal !
8-5 Metallocene Polymerization of nonpolar alkene monomers
Metallocene : LL’MtX2
where LL’ are η5-cyclopentadienyl (Cp) or alkyl-substituted η5-cyclopentadienyls
1-indenyl (Ind)
4,5,6,7-tetrahydro-1-indenyl (H4Ind)
Mt : group 4 transition metal
ex) zirconium, titanium, nafnium
group 3 transition metal
ex) scandium, yttrium, lanthanum
X : Cl or CH3
9-fluorenyl (Flu)
Advantage of metallocene over Ziegler-Natta system
I.
soluble (homogeneous)
→ 100-fold more active (every site is active)
II. stereoselectivity : by changing ligands & reaction conditions
III. Narrower distributions of molecular weight
∵ same coordination environment of metals : single site initiation
→ Ziegler-Natta : multisite initiation
MAO (methylaluminoxane), [Al(CH3)O,]n,
Lewis acid for the initiation like AlR3 in Z-N system
AlR3 does not work in mtallocene system
Polymer site
Monomer site
Two functions
alkylation,
abstraction of second chloride
8-5a Metallocene symmetry
MAO + Cp2TiCl2 ⇒ non stereoselective
* unbridged metallocene : non stereoselective
⇒ free rotation of η5-cyclopentadienyl ligands to make achiral
environment at the active sites
Stereoselective polymerization is possible from chiral & stereorigid metallocene
ex) η5-cyclopentadienyl ligands are linked by a bridging goups such as CH2,
CH2CH2, (CH3)2C, Si(CH3)2
⇒ called ansa metallocene
See p 667 Table 8-5
next page
CEC
CSC
Catalyst site control
Chem. Rev., 100 (2000) 1223-1252.
Chem. Rev., 100 (2000) 1223-1252.
Bent metallocene
η5-cyclopentadienyl ligands are not parallel
: bite angle 60-75o
: 115-125o
: less than 90o
* Preparation of ansa (bridged) metallocene initiator
E: ex) Si(CH3)2
8-5b C2v-symmetric metallocenes
Ex) (CH3)2SiFlu2ZrCl2
→ unsubstituted biscyclopentadienyl
→ two coordination (active) sites are both achiral & homotopic
⇒ atactic polymer !
8-5c C2-symmetric metallocenes
Ex) rac-(dimethylsilyl)bis(1-indenyl)zirconium dichloride, rac-(CH3)2SiInd2ZrCl2
enantiomers and mesocompound are synthesized, then seperation
(R, R)
(S, S)
or
(R, S)
Meso compound Cs-symmetry
can be separated → different stereoselectivity
8-5c-1 Effect of initiator structure
Angle + steric + bridge
* Bite angle
M
small
M
large
if bite angle is too large or stereorigidity is too low → stereoselectivity ↓
∵ polymer chain & monomer loosely held in plane
if bite angle too small or stereorigidity is too high → stereoselectivity ↓
∵ Polymer chain and monomers have difficulty to coordinate
⇒ general rule but not always
1. Zr, Hf, Ti : metals
Zr : most commonly used
Hf : less active but higher molecular weight polymer produce
Ti : less active & less stereoselective
2. Cp or Ind ligands: Substituents in the 3- and 4-positions have the greatest
effect for increasing isoselectivity, activity, and polymer molecular weight.
3. bridge
Isoselectivity and molecular weight ↑ in the order of
CH2 < CH2CH2 < (CH3)C < Si(CH3)2
4. hetero atoms into ligands such as alkoxy or amine
→ not promising
8-5c-2 Effect of reaction variables
-
Temp ↑ , reaction rate ↑ , stereoselectivity ↓
High temp makes the initiator more flexible.
So the effect for the very stereorigid initiators is less!
ex) propene with rac-C2H4(Ind)2ZrCl2/MAO
20˚C , Mv : 56,000 , regio irregular 0.4%
70˚C , Mv : 19,600 , regio irregular 0.7%
* If the initiator is more stereorigid, the effect of temperature is less.
- Monomer conc. ↓ , stereoselectivity ↓
- Initiator (metallocene) & MAO ↑ , stereoselectivity ↓
same thing
Low monomer concentration lower the propagation rate, then epimerization might
occurs (scrambling the chirality of the last monomer unit inserted into the
propagating chain end)
8-5d Cs-metallocene
Two types Cs Table 8-5 XXX and XXXI
Plane of symmetry: horizontal plane XXX, vertical plane XXXI
XXX: atactic, XXXI: syndiotactic
XXX type Cs -symmetric metallocene is called meso Cs
XXXI type Cs -symmetric metallocene is called Cs
⇒ highly stereoselective , more than C2
8-5e C1-metallocene
No element of symmetry Cs becomes C1 when on of the ligand is unsymetric
Two coordinate sites are both chiral (chirotopic)
⇒ selectivity & molecular weight varies with the structures ! (as usual)
8-5f Oscillating metallocene
During the life time r and m conformers can be made
Ex block type polymer can be prepared
8-5g Coinitiator
8-5g-1 MAO
The most commonly used Lewis acid coinitiator (activator)
→ obtained by hydrolysis of TMA(trimethylaluminium),
→ n = 5 ~ 20 , mixtures of linear, cyclic , and 3D structures,
3D spherical cage like structure is desirable
MAO contains TMA in two form; free TMA or associated TMA
Free TMA decreases the initiator activity, removed by vacuum
associated TMA can help the polymerization
Large excess MAO is used (100-1000 to one metallocene)
MAO is added first into the polymerization system to destroy deleterious
impurities (which can lower the reactivity) before the addition of
metallocene
MAO does not have long term stability (low solubility in HC solvent)
If the bottle is opened or exposed to air, the precipiation observed.
the precipiate does not affect the reactivity.
MMAO has improved stability
prepared by hydrolysis of TMA and triisobutylaluminium
Possible structures for the initiation
Monometallic species
Or bimetallic species
→ structure is not clear !
→ more than one type structures might exists due two more than one
type of MAO structures
New type of coinitiators are desirable
1. To avoid the cost of large excess of MAO
2. Simpler system not various possible structures
Boron type?
8-5g-2 Boron containing coinitiator
⇒ only one equivalent is needed to activate the initiator,
Sometimes even smaller amount can be used
ex) 2:1, initiator: conitiator system (possibly bimetallic system)
Higher activity ascribed to the large and weakly coordiating anoins compared with
MAO system
Anions must be sufficiently week in coordiation to the cation not to compete with
monomer for the coordination
* Order of coordination ability
[CH3MAO]- > [(CH3)B(C6F5)3]- > [B(C6F5)][B(C6H5)]- cannot be used because strong coordiation with cations
Normally activity ↑ streoselectivit↓ but not always
8-5h Kinetics
8-5h-1 Rate of polymerization
Zr + MAO
I*
+M
M* + M
K1
K2
kp
I*
M*
M*
If K2 is irreversible and fast, then [M*] = [I*]
Rp = K1kp[Zr][MAO][M]
If K2 is reversible and fast, then [M*]  [I*]
Rp = K1K2kp[Zr][MAO][M]2
K1
Zr + MAO
I*
+M
K2
I*
M*
Using an isolated metallocenium borate (I*), there is no equilibium
corresponding to K1 (K1 is irreversible)
If K2 is irreversible and fast, then [M*] = [I*]
Rp = kp[I*][M]
If K2 is reversible and fast, then [M*]  [I*]
Rp = K2kp[I*][M]2
8-5h-2 Degree of Polymerization
Same as Z-N system.
Similar chain transfer and termination
Spontaneous intramolecular β-hydride transfer
Vinyilidene
Chain transfer to monomer
Chain transfer to the group I-III metal alkyl
others
Vinyil
Double-bond end group composition varies
Ex) 1-hexane by rac-Me2(H2Ind2) ZrCl2 /MAO
Temp ↑ MW↓ transfer activation E is larger than that of polymerization
At 0˚C, vinylene double bond constitute 85% of all double bond,
less spontaneous termimation
At 80˚C, vinylidene double bond constitute 75% of all double bond
Monomer ↑
vinylidene content ↓ less spontaneous termination
Metallocene ↑
vinylidene content ↑
8-5h-3 Supported Metallocene
Metallocene system follows the Z-N system called drop-in technology
Mix MAO with silica first then add metallocene
Supported metallocenes generally have lower activity than do homogeneous metall
ocenes because of steric hindrance by the support. At the same time, less MAO is
required because deactivation processes are also hindered.
Polymer molecular weight and stereoselectivity are either unaffected or increased,
depending on the specific reaction system.
8-6 Cycloalkenes
8-6a 1,2-disubstituted alkenen, Cycloalkenes
Normally 1,2-disubstituted alkenens are not polymerized except cycloalkene
Or isomerization is involved in the polymerization of 1,2-disubstituted alkenens
Ex) 2-butene yields poly(1-butene) from Z-N system
Cycloalkene can be polymerized due to the relief of ring strain
Two possible polymerization for cyclobutene
Four possible different isomers
See p632
cis or trans
Ring-opening olefin metathesis
•
Vanadium (V) based catalyst ⇒ polymerization through double bond
•
Tungsten (W), Ti, Ru ⇒ Ring-opening olefin metathesis
8-6b styrene
Slighlty polar compared with olefins.
Isoselective polymerization is possible from Z-N systems
Synditactic polymerization is possible from Metallocene system
Partial syndiotactic polymerization form anionic and cationic initiators
8-7 copolymerization
See p 685 Table 8-7
Reactivity: ethene > propene > 1-butene > 1-hexane
Reactivity when using the same initiator system
8-10 polymerization of dienes
8-10a radical polymerization
General trend: 1,4-poly. > 1,2-poly. & 3,4-poly.
Why
1,4-polymerization
① Steric effect
trans-1,4-addition > cis-1,4-addition
~CH2-CH=CH-CH2·
↕
·
~CH2-CH-CH=CH2
1,4-addition
1,2-addition
② Stability of the final product
• Temp ↑ cis↑ (1,4-addition)
• 1,2-addition or 3,4-addition ⇒ do not change much
8-6 Anionic and Anionic coordination Polymerization
No clear explanation!
In polar solvent (THF)
→ polymerization via free ion or solvent separated ion pair favors
1,2-addition for butadiene
Anionic center at carbon 2 is not extensively delocalized because the
double bond is not strong ewg.
3,4-addition for isoprene (due to steric effect, less hindered C is attacked)
In nonpolar solvent (n-alkane)
Strong coordination power favors 1,4-addition
This effect is most pronounced in Li and also unstable cis polyperization is
favored although trans product is more stable.
1. slow polymerization (enough time to have isomerization)
2. When Li is counter ion, mostly cis polymer because cis
monomer is more reactive
In polar solvent this carbon attacks
unassocoated more free ion like
More reactive C- can react
More stable anion, so 1,4 in nonpolar
solvent, assocoated more contact ion pair like
The predominance of cis 1,4-polymerization over trans 1,4-polymerization has been
ascribed to the greater reactivity of cis-LI,
Cis 1,4 is predominant in non solvent system with low concentration of initiator
Becaues isomerization of cis-LI to the more stable trans-LI increases faster than
propagation of cis-LI with an increase in initiator concentration since propagation
proceeds only through unassociated species while isomerization proceeds through
both associated and unassociated species.
In polar solvent this carbon attacks
Unassocoated more free ion like
More stable anion, so 1,4 in nonpolar
Solvent, assocoated more contact ion pair like
Effect of initiator
8-6 Cationic Polymerization
Not practical
Because low MW polymers with cyclized structures are obtained
8-12 Stereospecific polymerization of Polar vinyl monomers
General rule
◎ Soluble (uncoordinated) system
→ propagating species are free
syndiotacticity is increasingly favored w/ decreasing rxn temp.
⇒ radical & ionic poly. in highly solvating media
◎ poorly solvating media (coordinated)
→ isospecific polymerization can occur.
8-12a MMA
In polar solvent (THF, pyridine)
generally syndiotactic polymers
∵ generally free ion or solvent separated ion pair
* the counter ion is removed from the vicinity at the propagating
center and does not exert a stereoregulating influence on the
entry of next monomeric unit
* Syndiotacticity
Li > Na > K
∵ Li+ (smaller ion) most highly solvated !
* Syndiotacticity ↑ Temp. ↓
In nonpolar solvent
=> generally isotactic polymer
isotacticity ↑
Li > Na > K
↓
greater coordinating power
isotacticity ↓ Temp. ↑
Effect of metallocene initiators
8-12b Vinyl ether, styrene
Isotactic polymers were obtained from cationic and Z-N systems
Syndiotactic polymers were obtained from Monomers having bulky groups
6-membered ring could be formed
Aldehydes
Isoselective for anioic polymerization using zinc and aluminium alkyls,
Grignard reagent, and lithium alkoxide
Caionic system ( Ex) BF3etherate) less isoselective max. 70%