Handbook of Polymer Synthesis Second Edition Episode 5 potx

5
Polymeric Dienes
Walter Kaminsky and B. Hinrichs
University of Hamburg, Hamburg, Germany
I. INTRODUCTION
Homopolymers of conjugated dienes such as 1,3-butadiene, isoprene, chloroprene, and
other alkylsubstituted 1,3-butadienes, as well as copolymerzs with styrene and
acrylonitrile, are of great economical importance [1–3]. The conjugated dienes can
polymerize via 1,4 or 1,2 linkage of monomeric units. In addition to this, 3,4 linkage
occurs with butadienes bearing substituents in the 2-position. In the case of 1,4 linkage the
polymer chain can exist as cis or trans type:
ð1À3Þ
1,2 Linkage yields a tertiary carbon atom, thereby making it possible to form
isotactic, syndiotactic, and atactic polybutadiene (3), in analogy to polypropene. The rare
3,4 linkage also gives isotactic, syndiotactic, or atactic configuration. This applies only to
high stereoselectivities. Further isomeric structures are formed when next to head-to-tail
linkages; head-to-head and tail-to-tail linkages also occur. The polymerization of dienes
can be initiated ionically by coordination catalysts or by radicals [4–10].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
II. POLYBUTADIENE
Polybutadiene belongs to the most important rubbers for technical purposes. In 1999 more
that 2 million tons were produced worldwide, that is about 20% of all synthetic rubbers
[11,12]. The cis type made by 1,4-addition is economically the most important
polybutadiene [13,14]. Trans- as well as isotactic, syndiotactic, or atactic 1,2-polybutadiene
can also be synthesized in good purity with suitable catalysts. For anionic polymerization
with butyllithium or the coordinative process with Ziegler catalysts, 1,3-butadiene must be
carefully purified from reactive contaminants such as acetylene, aldehydes, or hydrogen
sulfide.
A. Anionic Polymerization
Metal alkyls, preferably of alkali metals, are used as initiators. The polarization of the
catalyst exerts a strong influence on the stereospecifit y (Table 1) [15,16]. Lithium alkyls

give a polymer with the great est trans-1,4-portion. The stereospecifity is also influenced
by catalyst concentrations, temperatures, and associative behavior [17–34]. In more
concentrated solutions, alkyllithium, especially butyllithium, which is the preferred
initiator, forms hexameric associates that are dissociated in several steps to finally give
monomers [35–53]. Only mon omeric butyllithium is suited for the insertion. Isobutyl-
lithium shows an association grade of 4 in cyclohexane [36]. Branched alkyl groups gave
higher activities than those with n-alkyl groups. As postulated by the kinetic model for
very weak initiator concentration, the reaction order is 1 and less than 1 for higher
concentrations [54–62]. This results in a series of reactions:
Dissociation: ðCH
3
ÀðCH
2
Þ
3
ÀLiÞ
6
! 6CH
3
ÀðCH
2
Þ
3
À Li ð4Þ
Start: CH
3
ÀðCH
2
Þ
3

ÀLi þ CH
2
¼CHÀCH¼CH
2
! CH
3
ÀðCH
2
Þ
3
ÀCH
2
ÀCH¼CHÀCH
2
ÀLi
ð5Þ
Propagation: CH
3
ÀðCH
2
Þ
4
ÀCH¼CHÀCH
2
ÀLi þ CH
2
¼CHÀCH¼CH
2
! CH
3

ÀðCH
2
Þ
3
ÀðCH
2
ÀCH¼CHÀCH
2
Þ
2
ÀLi
ð6Þ
Table 1 Microstructure of poly(1,3-butadiene) in relation to the initiator.
Microstructure (%)
Initiator Solvent cis trans 1,2
C
2
H
5
Li Hexane 43 50 7
C
2
H
5
Li THF 0 6 91
C
4
H
9
Li Hexane 35 55 10

C
10
H
8
Li THF 0 3.6 96.4
C
10
H
8
Na THF 0 9.2 90.8
C
10
H
8
K THF 0 17.5 82.5
C
10
H
8
Rb THF 0 24.7 75.3
C
10
H
8
Cs THF 0 25.5 74.5
Source: Refs. [15] and [17].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
With hydrocarbons as solvents, the rate of the starting reaction is up to a factor of
100 smaller than that of the propagation step. This difference is caused by the absence
of a double bond in conjugation to lithium in butyllithium. In contrast to this, the use of

ether accelerates the starting reaction such that propagation becomes the rate-determining
step [63–67].
In the absence of chain transfer reagents, the molecular weight increases steadily
with increasing conversion of monomer. In this way one gets living polymers with very
narrow molecular weight distribution when the starting reaction is fast or lithium octenyl
is used as a starter (Poisson distribution). The average degree of polymerization is equal
to the ratio of converted moles of monomer (st arting concentration [M ]
0
) over the number
of moles of initiator [I ] reacted:
P
n
¼
½M 
0
½I 
0
À½I 
>
½M 
0
½I 
0
ð7Þ
At the end of the polymerizat ion when no more unreacted initiat or is present ([I ] ¼ 0), the
number average of the molecular weight can be calculated as follows:
M
n
¼
½M 

0
½I 
0
 54 ð8Þ
Equation (9) is valid as long as there is still some monomer in the reaction mixture:
½M À½M 
0
¼ð½I 
0
Þ 1 À
kw
kg

þ
kw
kg
½I 
0
ln
½I 
½I 
0
ð9Þ
To improve the processibility of linear polybutadiene with its narrow molecular
weight distribution, one can continuously add initiator in the course of the polymerization,
vary the reaction temperature, or force long-chain branching by addition of divinyl
compounds [68–74]. Addition of small amounts of ethers or tertiary amines alters the vinyl
content from some 12% to more than 70% (Table 2). Bis(2-methoxy) ethyl ether and
1,2-bis(dimethylamino)ethane as well as crow n ethers [75,76] are particularly effective.
The microstructures of the products are determinated by IR [77–87], NMR [88–99], x-ray

diffraction, and other methods [100,101].
The anionic poymerization of 1,3-butadiene is normally carried out in solvents
[102–109]. Aliphatic, cycloaliphatic, aromat ic hydrocarbons, or ethers as solvents could be
used. Working in ethers requires low temperatures because of the high reactivity and low
stability of the lithium alkyl in this solvent. Using n-hexane as solvent, a butadiene
concentration of 25 wt% and a polymerization temperature of 100 to 200

C is preferred.
Low-molecular-weight polybutadiene oils result when the polymerization is
catalyzed by a mixed system of butyllithium, 1,2-bis(dimethylamino)ethane, and potas-
sium t-butanolate [110–112]. With 1,4-dilithium-1,1-4,4-tetraphenylbutane it is possible
to get bifunctional living polymers (seeding technique) [113–118].
B. Coordination Catalysts
A large number of complex metal catalysts have been employed in the polymerization
of conjugated dienes [119–139]. Table 3 shows a selection of catalyst systems that have
Copyright 2005 by Marcel Dekker. All Rights Reserved.
been used for the polymerization of butadiene. Some systems yield polymers with a high
percentage of cis-1,4 linkage, while others favor the formulation of trans-1,4 or trans-1,2
linkages. As in the case of Ziegler–Natta catalysis of propene, the active centers are
transition metal-carbon bonds. They normally form a 
3
-alloyl bond [140]:
ð10Þ
Table 2 Influence of polar compounds on the microstructure (1,2 content)
a
.
1,2 Structure (wt%) for polymerization temperature
Polar compound Molar ratio 30

C50


C70

C
(H
3
C–O–CH
2
–CH
2
)
2
O 0.10:1 51 24 14
0.45:1 77 56 28
0.80:1 77 64 40
(H
3
C)
2
N–(CH
2
)
2
–N(CH
3
)
2
0.06:1 26 14 13
0.60:1 57 47 31
1.14:1 76 61 46

Source: Ref. [73].
a
Catalyst: C
4
H
9
Li.
Table 3 Catalysts for the polymerization of 1,3-butadiene.
Microstructure (%)
Catalyst cis trans 1,2 Refs.
TiCl
4
/R
3
Al 65 35 [123]
TiJ
4
(R
3
Al 95 2 3 [124]
Co(O-CO-R)
2
/(H
5
C
2
)
2
Al-Cl/H
2

O 96 [125]
Ni(O-CO-R)
2
/F
3
B-O(C
2
H
5
)
2
/R
3
Al 97 [126]
Ce(O-CO-R)
3
/(H
5
C
2
)
3
Al
2
CL
3
/R
3
AL 97 [127]
U(OCH

3
)
4
/AlBr
3
/R
3
AL 98.5 1 0.5 [128]
U(O-CO-C
7
H
15
)
4
/AlBr
3
/R
3
Al 98.2 1.1 0.7 [129]
Nd(O-CO-R)/R
n
AlCl
3-n
/R
3
Al 98 1.5 0.5 [130,132]
VCl
3
(VOCl
3

)/R
3
Al 99 1 [133,134]
Cr(C
5
H
7
O
2
)
3
/R
3
Al 8 2 90 [135]
Rh(C
5
H
7
O
2
)
3
a
/R
3
Al 98 [136]
Cr(allyl)
3
10 90 [120]
Nb(allyl)

3
1 2 97 [121]
Cr(allyl)
2
Cl 90 5 5 [144]
a
2,4-Pentandionato.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
The propagation reaction proceeds via insertion into these carbon–transition metal bonds
after the diene has been coordinated as a p-complex:
ð11; 12Þ
In the transition state a short-lived s-allyl bond is formed, which in the case of cis
migration, restores an alkyl-transition metal bond [141–143].
Various mechanisms for the control of the cis linkage in the propagation step are
discussed [144,145]. Allyl compounds can occur in syn or anti form [Structures (13–16)],
from which double bonds with trans or cis configuration are formed [146,147], respec-
tively. Solvents or cocatalysts as ligands are of great importance for the equilibration.
ð13À16Þ
C. cis-1,4-Polybutadi ene
cis-1,4-Polybutadiene is preferrentially produced with mixed catalysts. Systems on the
basis of titanium (IV) iodine/trialkylaluminum are employed [148–150]. For better dosing
a mixture of TiCl
4
/I
2
/R
3
Al, TiCl
4
/R

2
All, or Ti(OR)I
3
/TiCl
4
/(C
2
H
5
)
3
Al in which all
Copyright 2005 by Marcel Dekker. All Rights Reserved.
compounds are soluble in hy drocarbons, is used. It is essential for a high cis content of
the products that the catalyst contains iodine. Those of TiCl
4
and R
3
Al only lead
predominantly to the formation of trans-1,4-polybutadiene. Aromatic hydrocarbons
(benzene, toluene) are used as solvents. The polymerization is a first-order reaction with
respect to the 1,3-butadiene concentration [150,151]. As TiCl
4
gives living polymers, the
molecular weight increases almost linearly with the conversion of monomer [152]. At
higher degrees of conversion, the molecular weight can be controlled by varying the
catalyst concentration or composition. The molecular weight distribution M
w
/M
n

ranges
from 2 to 4 with a cis content between 90 and 94%. Regulation of molecular weights can
be achieved by the addition of 1,5-cyclo-octadiene [153].
Supported Ziegler catalysts are also used [154–156]. High cis contents up to 98%
can be obtained with cobalt salts [cobalt octanoate, cobalt naphthenate, tris(2,4-penta-
dionato) cobalt] in combination with alumoxanes which are synthesized in situ by hydro-
lysis of chlorodiethylaluminum or ethylaluminum sesquichloride. Only 0.005 to 0.02 mmol
of cobalt salt is needed for the polymerization of 1 mol of 1,3-butadiene [157–159]. At 5

C
the molecular weight varies from 350 000 to 750 000 depending on the alkylaluminum
chloride, while at 75

C the variation is between 20 000 and 200,000. The polymerization
rates are fast over a considerable range of chloride content. The cis-1,4-structure increases
with chlorid e content. The molecular weight increases with the chloride level [160].
Nickel compounds can also be employed as catalysts [161–170]. A three-component
system consisting of nickel naphthenate, triethyl-aluminum, and boron trifluoride
diethyletherate is used technically. The activities are similar to those of cobalt systems.
The molar Al/B ratio is on the order of 0.7 to 1.4. Polymerization temperatures range from
À5to40

C. On a laboratory scale the synthesis of cis-1,4-polybutadiene with
allylchloronickel giving 89% cis, 7.7% trans, and 3.4% 1,2-structures is particularly
simple [8]. In nickel compounds with Lewis acids as cocatalysts, complexes with 2,6,10-
dodecatriene ligands are more active than those with 1,5-cyclooctadiene (Table 4) [171].
The influence of the ligand on cis or trans insertion is particularly obvious for 
3
-allyl
nickel systems.

ð17À20Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Alkanolates or carboxylates of lanthanides and actinides, especially uranium, are
particularly well suited for the production of cis-1,4-polybutadiene [172–187]. Of the
lanthanides, compounds of cerium, praseodymium, and neodymium are combined with
trialkylaluminum and a halogen containing Lewis acid [188,189]. The polymerization can
also be carried out in aliphatic solvents at 20–90

C [190].
The microstructures are influenced primarily by the nature of the alkylaluminum
compound. With triethylaluminum the portion of trans-1,4 double bonds reaches a
relatively high level of 10%, while tris(2-methylpropyl)aluminum and bis(2-methylpropyl)
aluminum hydride yield cis-1,4 contents as high as 99% [190]. Similarly, high cis-1,4
portions are obtained in the polymerization of 1,3-butadiene with 
3
-allyluranium
complexes. The osmometric measured mole mass ranges from 50 to 150 000, the molecular
mass distribution between 3 and 7. The extremely high temperature-induced crystallization
rate of uranium polybutadiene in comparison with titanium or cobalt polybutadiene
corresponds to a greater tendency tow ard expansion-induced crystallization. A technical
application, however, is in conflict with the costly removal of weakly radioactive catalyst
residues from the products [132].
1. Metallocene-catalysts
Different methyl substituted cyclopentadienyl titanium compounds can be employed as
catalysts (Table 5) [191].
At a polymerization temperature of 30

C the chlorinated and the fluorinated
complexes show nearly the same activity. Only the highly substituted fluorinated
compounds (tetra- and pentamethylcyclopentadienyl titanium trichloride Me

4
CpTiF
3
,
Me
5
CpTiF
3
) are significantly more active than the corresponding chlorinated ones.
At higher polymerization temperatures a corresponding behavior can be observed,
however with increasing polymerization temperature also the activity of the complexes
increase. The activities of the 1,3-dimethylcyclopentadienyl titanium trihalides are the
highest and reach about 700 kg Br/mol Ti
*
h. It makes no difference if one of the fluorides is
substituted by another ligand like perfluoroacetic or perfluorobenzoic acid (Me
5
CpTi-
F
2
(OCOCF
3
), Me
5
CpTiF
2
(OCOC
6
F
5

)).
The activity reaches a maximu m value for all catalysts after a short induction period
of 5 to 10 min. After this, the activity decreases to a value being constant for a longer
period of time of up to about 1 h.
The substitution pattern influences the induction period. The most active com-
pounds show the shortest induction period, whereas the less active ones need a clearly
longer period.
Table 4 Polymerization of 1,3-butadiene.
a
Cocatalyst
Molar ratio,
HX/Ni
Reaction
time
(h)
Yield
(%)
cis-1,4
(%)
trans-1,4
(%)
1,2
(%)
HCl 1 3 13 84 13 3
HBr 1 3 4 72 25 3
HJ 1 6 30 0 100 0
Source: Ref. [161].
a
3,4 mol butadiene, 0.014 mol of 2,6,10-dodecatrienylchloronickel at 55


C in heptane.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
The activity increases linear with increasing butadiene concentrations in the starting
phase of the polymerization. The kinetic order of the butadiene concentration is 1. At
constant Al:Ti ratio the polymerization rate is given by
r
p
¼ k
p
Á c
cat
Á c
b
ð21Þ
where c
b
is the concentration of butadiene. The activity increases with an increasing Al:Ti
ratio, reaches a maximum at an Al:Ti ratio of about 700 and decreases slowly with
increasing Al:Ti ratios.
High molecular weights are obtained for the polybutadienes produced with these
catalysts.
The di- and trimethylcyclopentadienyl titanium trichlorides give the highest molecu-
lar weights while the fluorinated compounds have significantly lower molecular wei ghts,
even if their activity is higher, as shown for the Me
4
CpTiF
3
and Me
5
CpTiF

3
complexes
(Table 6).
The glass transition temperatures range of À90.1 and À96.9

C. The polybutadienes
produced with the most active catalysts have the highest content of cis-1,4 units and the
lowest glass transition temperature.
For all catalysts, the cis-1,4 structure units of the polybutadiene range between
a content of 74 and 85.8%, the trans -1,4 between 0.5 and 4.2%, and the 1,2-units between
13.7 and 22.6% (Table 7). The most active systems generate the polymer with the highest
content of cis-1,4 and the lowest content of trans-1,4 and 1,2-units. The fluorinated
compounds show a similar behavior. A mechanism for the formation of these micro-
structures is published by Porri [192].
There is no dependence of the microstructure on the polymerization time
(between 10 and 120 min the cis content is 81.8 Æ 0.3% for MeCpCl
3
) and on the Al:Ti
ratio (between Al:Ti ¼ 500 and Al:Ti ¼ 10 000 the cis content is about 80.7 Æ 1.2 for
MeCpTiF
3
).
D. trans-1,4-Polybu tadiene
Butadiene can be polymerized with Ti/Al catalyst systems. A sharp change in structure
of polybutadiene can be seen by varying the mole ratio of TiCl
4
to R
3
Al. At Ti/Al ratios of
Table 5 Activities of titanium complexes for the polymerization of 1,3-butadiene in 100 ml

toluene, 10 g 1,3-butadiene, 0.29 g MAO, [Ti] ¼ 5 Â 10
À5
mol/l, Al/Ti ¼ 1000, T ¼ 30

C, poly-
merization time ¼ 20 min.
Catalyst Activity
a
Catalyst Activity
a
CpTiCl
3
260 CpTiF
3
260
MeCpTiCl
3
300 MeCpTiF
3
310
Me
2
CpTiCl
3
750 Me
2
CpTiF
3
605
Me

3
CpTiCl
3
340 Me
3
CpTiF
3
350
Me
4
CpTiCl
3
165 Me
4
CpTiF
3
350
Me
5
CpTiCl
3
60 Me
5
CpTiF
3
350
IndTiCl
3
310 Cp*TiF
2

(OCOCF
3
) 330
PhCpTiCl
3
325 Cp*TiF
2
(OCOC
6
F
5
) 340
a
Activity: kg BR/ mol Ti*h.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
0.5 to 1–5, the cis content of the 1,4-polybutadiene increases to about 70% at a ratio of 1,
and then falls off so that trans-1,4-polybutadiene is obtained at Ti/Al ratios of 1.5 to 3.
Under these conditions it is a good catalyst for preparing trans-1,4-polybutadiene.
Also heterogeneous catalysts consisting of TiCl
4
immobilized on MgCl
2
have been
reported [193].
Other catalysts contain the transition metals vanadium, chromium, cobalt, and
nickel as their main components [194–202]. The polymerization activity is usually far lower
than in the synthesis of cis polymers (see Table 2). Addition of a donor such as
tetrahydrofuran, which directs the bonds into a trans-position to the catalyst of titanium
tetraiodide and triethylaluminum, results in the formation of a polybutadiene with 80%
trans-1,4-double bonds [197].

Table 6 Molecular weights of the polybutadienes produced with fluorinated and chlorinated
catalysts.
Catalyst
X ¼ Cl
Molar mass M

[g/mol  10
6
]
X ¼ F
Molar mass M

[g/mol  10
6
]
CpTiX
3
1.2 0.97
MeCpTiX
3
1.6 1.22
Me
2
CpTiX
3
3.1 1.28
Me
3
CpTiX
3

3.6 1.25
Me
4
CpTiX
3
3.3 1.5
Me
5
CpTiX
3
2.6 1.4
IndTiCl
3
1.25 –
PhCpTiCl
3
0.86 –
CyCpTiCl
3
1–
(Me
3
Si,MeCp)TiCl
3
1.5 –
Table 7 Microstructure and glass transition temperatures of polybutadienes produced with
chlorinated and fluorinated catalyst precursors.
Catalyst cis-1,4 [%] trans-1,4 [%] 1,2 [%] T
g
[


C]
CpTiCl
3
81.7 1.1 17.2 À 95.1
MeCpTiCl
3
81.9 1.1 17 À 95.3
Me
2
CpTiCl
3
85.8 0.5 13.7 À 96.9
Me
3
CpTiCl
3
83.8 1.1 15.2 À 95.6
Me
4
CpTiCl
3
80 1.7 18.3 À 91.5
Me
5
CpTiCl
3
74.8 2.6 22.6 À 91
IndTiCl
3

74.3 4.2 21.5 À 90.1
PhCpTiCl
3
80.9 2.1 16.9 À 95.8
CyCpTiCl
3
82.6 0.8 16.7 À 95
CpTiF
3
81.8 1.4 16.8 À 95
MeCpTiF
3
81.9 1.2 16.9 À 92.7
Me
2
CpTiF
3
82 2 16 À 95
Me
3
CpTiF
3
84 1.1 14.9 À 94.1
Me
4
CpTiF
3
80.4 1.9 17.7 À 89.9
Me
5

CpTiF
3
74.6 2.8 22.5 À 87.9
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Another possibility is anionic polymerization with alkyllithium in combination with
barium compounds such as barium 2,4-pentanedionate [192–194, 203–205]. Also,
cobalt(II) chloride in combination with diethylaluminum chloride and triethylamine is
used, yielding a polymer with 91% trans-1,4 and 9% 1,2 structures.
E. 1,2-Polybutadiene
The synthesis of crystalline, syndiotactic 1,2-polybutadiene is also successful with
compounds of titanium, cobalt, vanadium, and chromium [194,206–210]. Alcoholates
[e.g., cobalt(II) 2-ethylhexanoate or titanium(III) butanolate] with triethylamine as
cocatalyst, are especially well suited for this purpose. They are capable of producing
polymers with up to 98% 1,2 structure. Amorphous 1,2-polybutadiene is produced with
molybdenum(V) chloride and diethylmethoxyaluminum [211]. Addition of esters of
carboxylic acids raises the vinyl content of the products [212]. The influence of the
coordination at the center atom is remarkable. Trisallylchromium polymerizes 1,3-
butadiene to 1,2-polybutadiene, while bisallylchro mchloride gives 1,4-polybutadiene.
ð22À23Þ
1. Polymerization Processes
Polybutadiene can be produced in nonaqueous media or by a radical mechanism in an
aqueous emulsion. The field of homopolymerizations is dominated by the processes in
nonaqueous media, as described. Emulsion polymerization is characterized by good
dissipation of the reaction heat. The monomer concentration is on the order of 50 wt %.
The reaction is initiated by free radicals, which are preferably formed from organic
hydroperoxides such as p-menthane hydroperoxide [213,214]. Sodium formaldehyde
sulfoxylate and iron(II) complexes are employed as reducing agents. At reaction
temperatures below 5

C the polymerization is discontinued at a degree of conversion

between 50 and 60%, to avoid cross-linking. The product features low stereospecifity
(14% cis-1,4, 69% trans-1,4, and 17% 1,2 struc tures). At higher temperatures degradation
of the polybutadiene lowers the molecular weight [215,216].
III. POLYISOPRENE
The homopolymerization of isoprene
ð24Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
can take place with a cis-1,4, trans-1,4, 1,2, or 3,4 connection.
In addition, the 3,4- and 1,2-polyisoprenes can both exist in three forms: isotactic,
syndiotactic, and atactic. Thus there are eight possible structures if we disregard head-to-
head possibilities. The part of the structure elements in the polymer depends on the
catalysts. In general, the polymerization activity is lower compared to polybutadiene.
Of the various structures of polyisoprene, only cis - and trans-1,4-polyisoprene and atactic
3,4-polyisoprene are important (Table 8) [217–219].
A. cis-1,4-Polyisoprene
Natural rubber (hevea) is 98% cis-1,4-polyisoprene with 2% 3,4-structure. It can be
synthesized by anionic polymerization with alkyllithium compounds or with Ziegler–
Natta catalysts [220–225]. The polymerization is carried out in solvents. Impurities such as
acetylenes, carbonyl compounds, hydrogen sulfide, and water have to be removed
[217,226–228].
1. Anionic Polymerization
cis-Polyisoprene can be obtained with butyllithium under certain condition. The cis
content depends on the initiator and monomer concentrations as well as on the
temperature [23,49]. In aliphatic solvents up to 97% 1,4-cis polymer could be obtained
(Table 9). The strong influence of the initiator concentration is explained by a two step
mechanism [229].
Table 8 Homopolymerization of isoprene; microstructure of polyisoprenes.
Catalyst cis-1,4 (%) trans-1,4 (%) 1,2 (%) 3,4 (%)
LiC
4

H
9
in heptane 93 0 0 7
LiC
4
H
9
in THF 0 30 16 54
(i-Bu)
3
Al/TiCl
4
97 0 0 3
Nd(2-ethylhexanoate)/Et
2
AlCl/THF 96.9 0 0 3.1
Et
3
Al/VCl
3
09802
Na dispersion 29 29 0 42
K
2
S
2
O
8
22 65 6 7
Source: Refs: [217] and [218].

Copyright 2005 by Marcel Dekker. All Rights Reserved.
ð29À33Þ
First, dissociation of the lithium alkyl association [Structure (29)] takes place,
followed by activation by complexing of the monomer lithium alkyl with the cis-isoprene
[Structure (31)]. For insertion in a second step, a dimer alkyllithium is necessary [Structures
(32) and (33)].
The living polymerization shows no breaking-off or transfer reactions and therefore
gives polymers with a narrow molecular weight distribution [230] . The molecular weight
Table 9 Dependence of polyisoprene microstructure on butyllithium concentration.
Microstructure (%)
Butyllithium (mmmol/l) cis-1,4 trans-1,4 3,4
61.2 74 18 8
178175
0.1 84 11 5
0.008 97 0 3
Source: Ref. [23].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
can be calculated as follows:
M
cal
¼
½isoprene
½RLi
 68 ð34Þ
The polymer is highly linear without branching. For the synthesis of polyisoprenes
with an extremely narrow molecular weight distribution (M
w
/M
n
¼ 1.05) a vacuum or

seeding technique could be used [231, 232]. In the second case the polymerization is started
with separately prepared polyisoprene of low molec ular weight. Polar solvents such as
ethers and amines have an influence on the microstructure [233–236]. The initiation step
increases in relation to the propagation step [237].
The anionic polymerization leads to polymers with an active lithium end group.
This can be used for further reactions. By treatment with chlorsilanes such as 1,2-
bis(dichloromethylsilyl)ethane, a four-star polymer results; with 1,2-bis(trichlorosilyl)
ethane, a six-star polymer. Aromatic divinyl compounds used for the same purpose have
been described [238–240].
2. Coordinative Catalysts
Titanium tetrachloride in combination with aluminum trialkyl (ratio 1:1) gives optimum
activity in isoprene polymerization. The Ziegler system TiX
4
/R
3
Al (X ¼ halides) yields
either cis-1,4, trans -1,4 or 3,4-polyisoprene, while the unmodified lithium systems produce
predominantly cis-1,4-polyisoprene (Table 10). Using TiCl
4
and R
3
Al cis-1,4-polyisoprene
is obtained at Ti/Al ratios of 0.5 to 1.5 [160]. At lower Ti/Al ratios, oligomers are formed.
At ratios of 1.3 to 1.6, mixed cis/trans polymers are obtained; at 1.6 to 2, trans-1,4-
polyisoprenes. Ratios above 2 give resinous materials that are cyclized trans-polymers. The
other titanium halides were found to be equivalent to TiCl
4
in these reactions. Catalyst
efficiency is increased by complexing the R
3

Al with ethers and tertiary amines.
It is important to mix the catalyst components and alter the heterogeneous system
before adding the monomer [241, 242]. Titanium (II) seems to be inactive. Therefore the
catalyst could be stabilized by addinng electron donors such as ethers and esters [243–246].
Instead of alkylaluminum, alane etherates such as HAlCl
2
Á O(C
2
H
5
) are used [247–251].
The best results in obtaining high yields of cis-1,4-polyisoprene are given by rare
earth catalysts [252–257]. Similar to the polymerization of butadiene, three component
catalysts (transition metal compound, Lewis acids, and alkyl aluminum) are used. It is
necessary to have an excess of 4 to 10 times of the aluminum co mponent. Most attractive
Table 10 Ziegler catalysts for isoprene polymerization: influence of the Ti/Al ratio on the
microstructure.
Microstructure (%)
Catalyst Molar ratio Ti/Al 1,2 3,4 cis-1,4 trans-1,4
TiCl
4
/R
3
Al 0.5–1.5 <13 97 <1
TiCl
4
/R
3
Al 1.3–1.6 <1 3 56 41
TiCl

4
/R
3
Al 1.6–2.0 <13 8 97
TiCl
4
/R
2
AlCl/R
3
N 0.1–1.5 <13 97 <1
Ti(OR)
4
/RAlCl
2
1 <199 <1 <1
Source: Ref. [160].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
is neodymium salt. With increasing temperature and the Al/Nd ratio, the molecular weight
of the polymer decreases. The cis-1,4 content is higher than 95% and the 3,4 part is less
than 5% (see Table 8).
B. trans-1,4-Polyisoprene
The natural products gutta-percha and balata consist of trans-1,4-polyisoprene. With the
aid of vanadium trichloride and triethylalumium, trans-1,4-polyisoprene can be produced
with 98% trans-1,4 enchainments [133,258]. The optimal Al/V ratio is the range of 5 to 7.
The activity can be increased by the addition of small amounts of ether, heterogenerization
on supports (kaolin, TiO
2
), or blending with titanium(III) chloride or titanium alcoholates
[259–261]. Further catalysts featuring lower activity, however, are allylnickel iodide,

trisallylchormium on silica, or complexes of neodymium [262–265].
Li½NdðC
3
H
5
Þ
4
Á1,4-dioxane ð35Þ
ðH
5
C
6
ÀCH
2
Þ
3
Nd ð36Þ
Pure trans-1,4-polyisoprenes as well as trans-1,4-polybutadienes can be synthesized
by polyme rization in inclusion compounds [266–269]. As typical hosts for this dienes, the
inclusion compounds or clathrates of urea, thiourea, or perhydrotriphenylene [PHTP;
Eq. (36)] are used [270,271]. The host forms the frame of the crystal and the guest is placed
in the cavities existing in the lattice. Polymerization is generally star ted by subjecting the
inclusion compound to irradiation with a-, g-, or x-rays and proceeds by a radical
mechanism [272,273]. Also, free radical initiators such as di-tert-butylperoxide could be
used [274]. Inclusion in urea yields crystalline trans-1,4 polymers, whereas trans-1,4-
polyisoprene obtained in PH TP is amorphous. There is no trace of 1,4-cis units or of 1,2,
3,4, and cyclic units. The reason for the amorphous product is the presence of a substantial
number of head-to-head and tail-to-tail junctions in addition to head-to-jail junctions
[275, 276].
C. 1,2- and 3,4-Polyisoprene

Pure 1,2-polyisoprene cannot be synthesized. With alkyllithium in polar solvents (THF) or
upon addition of bases such as 1,2-bis(dimethylamino)ethane (TMEDA), a large amount
of 3,4 structures is formed, which can exceed 70% [33,277–279]. Polymers with a
dominating portion of 1,2 structures have not been described so far. Catalysts on the basis
of tris(2,4-pentanedionato)chromium and triethylaluminum are used but feature low
activities [280].
Addition of TMEDA to butyllithium results in the formation of polyisoprenes with
cis-1,4 and 3,4 structures (see Table 8). These equibinary polyisoprenes can also be
synthesized with cobalt halogenides in connection with phenylmagnesium bromide. The
addition of alcohols (e.g ., CoI
2
/H
5
C
6
MgBr/octanole at a ratio of 1:2.2:2) leads to an
increase in activity [281–283]. Polyisoprene with over 99% 3,4 structures can be generated
with (C
4
H
9
O)
4
Ti/AlR
3
catalysts. The product is isotactic.
D. Metallocene-catalysts
Also, half sandwich titanium compounds can polymerize isoprene (Table 11).
Copyright 2005 by Marcel Dekker. All Rights Reserved.
As of steric effects the unsubstituted cy clopentadienyl compound is more active than

the substituted ones. The fluorinated compounds are much more active (up to a factor of
30) than the chlorinated ones. The glass transition temperature of the polyisoprenes is
about À52

C.
IV. CHLOROPRENE
The polymerization of 2-chloro-1,3-butadiene(chloroprene), which is made from acetylene
or 1,3-butadiene [284–287] is strongly exothermic (75 kJ/mol). It can be initiated radically,
anionically, cationically, or with Ziegler catalysts [288] . Only the free-radi cal process,
which is usually run as an emulsion polymerization, is of technical importance [289–294].
Compared with polybutadiene and polyisoprene, polychloroprene features improved gaso-
line and aging resistance, low-temperature flexibility, and is less combustable [295–297].
The properties of polychloroprene are influenced by polymerization conditions as
well as by the nature of the additives. In the radical polymerization the monomer is built
into the polymer in trans-1,4, cis-1,4, 1,2, and 3,4 structures [Structures (37)–(40)] [298].
In addition to head-to-tail-enchainments, also head-to-head and tail-to-tail enchainments
occur, with a probability of 10 to 15% (Table 12) [297, 299]. Polymers with a high trans
(>98%) or cis content (>95%) are both possible [300, 301]. The glass transition
temperature for the trans-polychloroprene is À 45

C; that for the cis polymer is À 20

Cat
a degree of crystallinity of about 12%.
Table 11 Homopolymerization of isoprene. Polymerization conditions: 50 ml toluene, 50 ml
isoprene. [Ti] ¼ 5 Â 10
À5
mol/l, Al/Ti ¼ 200, Tp ¼ 30

C, t

p
¼ 5–24 h.
Catalyst Activity [g IR/mol Ti-h] T
g
[

C]
CpTiCl
3
‘28 À52
Me
5
CpTiCl
3
8 n.b.
CpTiF
3
840 À50.3
MeCpTiF
3
250 n.b.
Me
5
CpTiF
3
29 n.b.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Products with a low degree of crystallinity that can be decreased by comonomers
such as 2,3-dichloro-1,3-butadiene are suitable for applications as rubbers, whereas more
crystalline polymers which are produced at lower polymerization temperatures are used in

applications as components of adhesives. Due to the increased reactivity of the chlorine
atoms in 1,2 structures, they tend to trigger aging reactions [302].
Chloroprene is mainly produced by emulsion polymerization [303–308].
A. Sulfur Modified Chloroprene
Chloroprene is very reactive and can be polymerized with elemental sulfur. The resulting
block copolymer consists of chloroprene and sulfur segments of various lengths. The
sulfur can either be dissolved in the liqui d monomer or added as a dispersion. The sulfur
bridges are easily cleaved by iodoform or other additives, thus permitting a variation of
molecular weights over a wide range [297].
V. SUBSTITUTED POLYBUTADIENES
Next to isoprene, pentadienes and 2,3-dimenthyl-1,3-butadiene are produced as alkylbu-
tadienes on a large scale. Poly-2,3-dimethyl-1,3-butadiene was one of the first synthetical
rubbers [309, 310]. Terminally substituted 1,3-butadienes give 1,4 monomeric units each of
which contains one or two asymmetric carbon atoms [R ¼ H or alklyl group; R
0
¼ alkyl
group; (41) and (42)]. Therefore, monomers of this type can lead to different stereoregular
1,4-polymers: cis-1,4 iso- or syndiotactic, trans-1,4 iso- or syndiotactic [311,312].
A. Poly(2,3-dimethyl-1,3-butadiene)
2,3-Dimethyl-1,3-buta diene is produced by dehydration of pinacol, which in turn is made
by reductive coupling of acetone, followed by purification via sulfur dioxide adducts [313].
It can be polymerized radically (emulsion polymerization), anionically, cationically, or
by coordinative catalysts [314–319]. Due to the sterical hindrance of two methyl groups,
Table 12 Microstructure of polychloroprene as related to polymerization temperature.
trans-1,4
Temp.(

C) Head/tail (%)
Head/head,
tail/tail (%)

cis-1,4
(%) 1,2 (%) 3,4 (%)
12 83.0 11.5 3.8 1.0 0.8
30 81.5 12.0 4.5 1.2 1.0
42 80.5 12.0 5.2 1.2 1.1
57 80.5 11.0 5.8 1.4 1.3
70 75.0 13.5 8.4 1.5 1.4
Source: Refs. [297] and [299].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
1,2 ench ainment is hindered in comparison to 1,3-butadiene [320] [cis-1,4-poly(2,3-
dimethyl-1,3-butadie ne)].
ð43Þ
By analogy with the polymerization of isoprene with Ziegler catalysts, the micro-
structure of the polymer is determined by the aluminum/titanium ratio [321–323]. At Al/Ti
ratios smaller than 1 the portion of trans-1,4 structures goes up to 75%, while the
formation of a cis-1,4 polymer requires a Al/Ti ratio of at least 1. In either case some 10%
of 1,2 structures are formed in the reaction [324]. The polymerization is carried out in
benzene, toluene, or hexane as solvents. The trans-1,4 polymer has a higher melting point
of 260

C, compared to 190

C for the cis-1,4 polymer. Since this is connected with a high
degree of crystallinity, these polymers do not possess any rubber elasticity.
Catalysts on the basis of complexes of cobalt or iron salts [e.g., cobalt(II) chloride/
pyridine, cobalt(II) acetate/AlR
2
Cl] yield mixed structures with more than 20% 1,2
double bonds and rubber elastic-like polymers [325,326]. Rare earth catalysts have
also been described [327]. A crystalline cis-1,4 polymer with a melting point of 198


C
and a molecular weight of 100 000 is obtained with aluminum alkyls/neodymium com-
pounds at a molar ratio of 31:1. The yield is in the range of 30%. Cobalt(II) acetate
in combination with diethylaluminum chloride or rhodium salts also yields a cis-1,4
polymer [328,329].
Large amounts of trans-1,4-poly(2,3-dimethyl-1,3-buta diene) can be prepared by
inclusion polymerization [330–332]. Urea or thiourea are used as templates. Trans-1,4
polymer (99

C) is also obtained with p-allylnickel chlorid es in combination with
tetrachlor-1,4-benzoquinone. Anionic polymerization by butyllithium allows good control
of the products microstructure over a wide range [97].
B. Poly(alkyl-1,3-butadienes)
Polymers of some of the higher 2-alkyl-1,3-butadienes give vulcanizates with tensile
strength and elasticity comparable to that of natural rubber. Poly(2-ethylbutadiene)
and poly(2-phenylbutadiene) are most important. 2-Ethyl-1,3-butadiene can be polyme r-
ized in the same way as isoprene [333,334]. The polymer has a glass transition temperature
of À76

C [335]. A polymer rich in trans-1,4 structures is obtained by catalysis with
vanadium(III) chloride/triisobuthylaluminum. In contrast to trans-1,4-poly isoprene, the
product can be used as rubber, due to its reduced tendency to crystallization [336, 337].
Additional alkyl-substituted polybutadienes are listed in Table 13. Parallel to an
increase in the alkyl sub stituents’ volume and electron donor properties, there is a decrease
in selectivity and activity for cis-1,4 insertions, although the vulcanizing properties of the
products are improved [338–345].
Cationic polymerization of substituted alkyl-1,3-butadienes is accompanied by a
considerable loss of double bonds (up to 80%) due to the formation of cyclic products [346].
Tin(IV) chloride in trichloracetic acid, tungsten(VI) chloride, and boron trifluoride

etherate have be en tested as cationic catalysts [347, 348]. In addition to polymerization,
isomerizations are observed with these catalysts.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Table 13 Polymerization of 2-alkyl-1,3-butadienes with various: influence on the microstructure.
Polymer microstructure (%)
Monomer Catalyst
Temp.
(

C)
Time
(h)
Yield (%)
(loss of double
bond) cis-1,4 trans-1,4 3,4 1,2
2,2-Dimethyl-1,3-butadiene H
9
C
4
Li 0 24 98 52 0 48
2-Ethyl-1,3-butadiene AlR
3
/TiCl
4
/BF
3
10 5 100 98 0 2 0
H
9
C

4
Li 40 18 97 78 14 8 0
AlR
3
/VOCl
3
25 18 16 39 53 8 0
Allyl-Ni-J 25 40 47(3) 0 85 14 1
2-Propyl-1,3-butadiene AlR
3
/TiCl
4
/BF
3
10 18 49 95 0 5 0
H
9
C
4
Li 40 18 91 91 4 5 0
2-Isopropyl-1,3-butadiene AlR
3
/TiCl
4
/BF
3
10 18 50 91 9 0 0
Allyl-Ni-J 25 40 29 0 76 22 2
2-Butyl-1,3-butadiene AlR
3

/TiCl
4
/BF
3
10 18 27 60 33 7 0
H
9
C
4
Li 40 18 88 62 35 3 0
1,3-Pentadiene WCl
6
0 1 (54) 30 0 16
5-Methyl-1,3,hexadiene SnCl
4
/CCl
3
COOH 0 1 (30) 26 44 0
5,5-Dimethyl-1,3-hexadiene SnCl
4
/CCl
3
COOH 0 1 (27) 21 52 0
Source: Ref. [342].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
C. Phenyl-1,3-butadienes
Poly(2-phenylbutadienes) with a high cis content are also produced with the triisobuthyl-
aluminum/titanium tetrachloride catalysts [349]. Phenyl-1,3-butadienes can also be
considered as vinyl-substituted styrenes, which explains the effects on activities and
microstructures. Poly(2-phenyl butadienes) occur in trans-1,4, cis-1,4, 3,4, and 1,2

structures. Maximum conversions are achieved with a molar Al/Ti ratio of 1, which leads
to the formation of 73% cis-1,4 and 27% 1,2 structures. At higher Al/Ti ratios the cis-1,4
content goes up to 96%. The molecular weights are low, ranging from 2000 to 18,000.
In contrast to this, the polymerization of 1-phenyl-1,3-butadiene was found to
produce polymers with high contents of 3,4- but no 1,2-structures.
Generally, the molecular weights are low, with a ceiling of 10 000. The more crowded
the positions of phenyl residue and methyl group, the higher is the 3,4 content. At the same
time, there is an increasing tendency towards partial cyclization of the polymers via 3,4
structures (Table 14).
D. Polybutadienes with Heteroatoms
Next to chloroprene, numerous other polybutadienes with different substitution patterns
and substituents have been synthesized, although they are of no commercial importance
(Table 15) [350–358].
VI. POLY(1,3-PENTADIENE)S
A. Poly-1,3-Pentadiene
1,3-Pentadiene is the most studied terminally substi tuted butadiene. It exists in two
geometric isomers, which have different conformers:
The polymerization leads to four ditactic isomers [359,360]: isotactic trans-1,4 (46),
isotactic-cis-1,4 (47), syndiotactic trans-1,4 (48) and syndiotactic cis-1,4 (49). In addition,
there are also the cis- and trans-1,2 and 3,4 polymers, which can have an iso- or
syndiotactic structure [(50) and (51)].
Table 14 Microstructure of poly(phenyl-1,3-butadienes).
a
Microstructure (%)
Polymer 1,4 3,4 Cyclic
Poly(4-phenyl-1,3-pentadiene) 7 80 13
Poly(2-methyl-1-phenyl-1,3-butadiene) 29 62 9
Poly(3-methyl-1-phenyl-1,3-butadiene) 82 16 2
Poly(1-phenyl-1,3-pentadiene) 100 0 0
Poly(2-phenyl-1,3-pentadiene) 100 0 0

Poly(3-phenyl-1,3-pentadiene) 100 0 0
Source: Ref. [347].
a
Catalysts ZnCl
4
and CCl
3
COOH in dichloromethane at À 78

C.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Trans-1,4 isotactic, cis-1,4 isotactic and cis-1,4 syndiotactic polypentadienes have
been prepared. The cis-1,4-polypentadienes are of technical interest [361,362].
Table 15 Polymerization of different monomers leading to substituted polybutadienes.
Polymer Monomer (structure)
Poly-(1-chlor-1,3-butadiene)
Poly(3-chlor-2-methyl-1,3-butadiene)
Poly(1,2-dichlor-1,3-butadiene)
Poly(2-chlor-1-phenyl-1,3-butadiene)
Poly-(2,3-dichlor-1,3-butadiene)
(Poly(2-fluor-1,3-butadiene)
Poly(1-cyan-1,3-butadiene)
Poly(2-cyan-1,3-butadiene)
Poly(1-methoxy-1,3-butadiene)
Poly(2-methoxy-1,3-butadiene)
Poly(1-dimethoxyphosphoryl-2-methyl-1,3-butadiene)
Poly[2,3-bis(diethylphosphano)-1,3-butadiene]
Copyright 2005 by Marcel Dekker. All Rights Reserved.
B. 1,4-Poly(1,3-Pentadiene)
The polymerization of 1,3-pentadiene with cobalt acetylacetonate and chloralumoxane

(52) or diethylaluminumchloride
ð52Þ
leads to syndiotactic cis-1,4-poly(1,3-pentadiene) (Table 16) [312]. The catalyst is only able
to polymerize the trans-isomer of 1,3-pentadiene (61). The addition of thiophene or
pyridine decreases the amorphic part, which contains a high number of 1,2 structures. In
the reaction between AlEt
2
Cl and Co(acac)
2
, all the acetylacetonato groups are displaced
from the cobalt atom with the formation of a Co(I) species. The polymerization-active
cobalt system (and the nickel system as well) is a cationic system in benzene [363–367].
In systems of this type the mode of presentation of the monomers is probably determ ined
by the steric interaction between the butenyl group and the incoming monomer and forms
by minimizing the steric interaction the syndiotactic polymer [368].
Neodymium catalysts show a different behavior. In this catalytic complex the
neodymium is probably in the trivalent state and at least one Nd–Cl bond is present [369].
The AlEt
3
-Ti(OR)
4
system is also a catalyst in which some alkoxy groups remain bonded
to the titanium. Both catalysts give cis-1,4 isotactic polypentadiene. The anionic ligands
bonded to the neodymium or the titanium atom of the catalytic species force the new
monomer to react to the isotactic structure.
With these catalysts a mixture of cis-andtrans-1,3-pentadienes in a wide range could
be polymerized. But the polymer obtained from the trans isomer is more clean and
crystalline. In the titanium catalyst the Al/Ti ratio plays an important role for the
molecular weight. With an increasing Al/Ti ratio, the molecular weight of the polymer
decreases [370,371]. An optimal values is Al/Ti ¼ 7.

With the optically active aluminum triethyltitanium tetramenthoxide system, an
optically active cis-1,4 isotactic polypentadiene was obtained, a fact that can be accounted
for by assuming that at least one menthoxy group is bonded to the titanium atom of the
catalytic complex [370]. The melting point of cis-1,4-poly(1,3-pentadiene) is in the range
of 40 to 53

C depending on the cristalline part, and the molecular weight in the range of
20 000 to 400 000.
Isotactic trans-1,4-poly(1,3-pentadiene) shows a melting point of 95

C [373]. It can
be synthesized with the heterogen eous catalyst triethylaluminum/vanadium trichloride
[311]. The trans-1,4 units reach nearly 100%. Small amounts (10 to 30%) of an amorphic
polymer can be extracted with ether. AlR
3
/a-TiCl
3
could also be used. This system gives
polymers consisting of trans-1,4-units; hence the butenyl group has a syn configuration
[368]. A syn-butenyl group can derive eithe r from coordination of the monomer with only
one double bond or from coordination with the two double bonds in the cisoid
conformation, forming an anti-butenyl group, followed by an anti–syn isomerization.
The heterogeneous systems AlEt
3
/a-TiCl
3
and AlEt
3
/VCl
3

are capable of polymerizing
both the trans- and the cis-isomers of 1,3-pentadiene. For the cis-isomer the cisoid
conformation is unfavored for steric reasons. It is therefore likely that for these catalysts
the coordination of the monomer occurs with only one double bond for both isomers of
pentadiene. In the presence of CrO
3
only the trans-isomer of pentadiene is polymerized to
a crystalline polymer with 80% trans-1,4 and 20% 1,2 structures.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Table 16 Polymerization of 1,3-pentadiene with various catalysts in benzene.
Microstructure (%)
Catalyst
Al/Metal
(mol/mol) Time (h) Yield (%) cis-1,4(1,2) trans-1,2 3,4
Bis(pentandithionato)-cobalt/Al(C
2
H
5
)
2
Cl 600 20 78 45 55 0
300 15 50 65 35 0
Al(C
2
H
5
)
2
Cl
2

(heptane) 600 20 57 (5) 95 0
Al
2
O(C
2
H
5
)
2
Cl
2
600 20 85 82 18 0
Al
2
O(C
2
H
5
)
2
Cl
2
þ thiophene 600 20 82 93 7 0
Ti(OC
4
H
9
)
4
/Al(C

2
H
5
)
3
3 26 2 61 29 10
726 31 68 20
Source: Ref. [312].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Cationic polymerization provides, independent of the isomer of the 1,3-pentadiene,
high trans-1,4 and trans-1,2 microstructures (Table 17). Studies on the insertion
mechanism and the various side reactions have been carried out [373,374].
In principle, 1,4-disubstituted butadienes can give different types of 1,4-stereoregular
polymers: erythro (or threo) trans-1,4 iso- or syndiotactic.
With the mode of presentation indicated, the new monomer gives, after insertion, a
butenyl group superimposable on the preceding one; hence a diisotactic polymer will be
formed.
C. Poly(methyl-1,3-pentadiene)
2-Methyl-1,3-pentadiene and 4-methyl-1,3-pentadiene are easily polymerized via a
cationic route [376,377]. Even weak acids at low temperatures give high-molecular-
weight polymers that are not cyclized and contain a large trans-1,4 portion. However,
4-methyl-1,3-pentadiene can also give mostly 1,2 structures at low yields. The same
compounds that are used for the polymerization of 1,3-pentadiene are employed as acids
(Table 18).
In comparison, the cationic polymerization of 3-methyl-1,3-pent adiene yields up
to 70% cyclized poly(3 -methyl-1,3-pentdiene) with 1,2- and 1,4-microstructure of the
remaining double-bond portion [378].
Also, Ziegler–Natta catalysts can be used with reduced activities. Most of the
polymerizates feature low degrees of crystallinity. Trans-2-methyl-1,3-pentadiene
polymerizes with the homogeneous Ti(OR)

4
/VCl
3
/AlR
3
catalyst to give amorphous
trans-1,4-poly(2-methyl-1,3-pentadiene), whereas the heterog eneous system consisting
of TiCl
4
/AlR
3
produces a partly crystalline cis-1,4-polymer. In this process the
4-methyl-1,3-pentadiene is converted almost exclusively to isotactic 1,2-sequences. The
trans-3-methyl-1,3-pentadiene polymerizes with the titanium catalyst to cis-1,4-poly
(3-methyl-1,3-pentadiene) with high molecular weights and melting points between 79
and 94

C. In the anionic polymerization of 2- and 4-methyl-1,3-pentadiene with
butyllithium only the trans-isomers give polymers with 60% cis-1,4 and 40% trans-1,4
structures [376]. Optically active poly(2-methyl-1,3-pentadiene) with up to 100% trans-
1,4-double bonds is obtained by inclusion polymerization in desoxy- and apocholic
acid [273].
Using the metallocene-catalysts CpTiCl
3
/MAO it is possible to polymerize 4-methyl-
1,3-pentadiene to a mainly syndiotactic polymer [379].
VII. MISCELLANEOUS DIENES
A. Poly(2,4-Hexadiene)
Hexadiene occurs also in severa l isomers, of which the trans–trans isomer is most reactive
with Ziegler catalysts. Results of polymerization reactions with a number of different

catalysts are compiled in Tabl e 19 [380]. Although Ziegler catalysts are normally not
capable of polymerizing olefins with internal double bonds, this is successful in the case
of 2,4-hexadiene, leading to crystalline polymers with high molecular weights [381].
Also, cationic polymerization gives high-molecular-weight polymers [382]. Anionic
polymerization yields only oligomers [383,384]. Exclusively trans-1,4-threo diisotactic
microstructure is found in crystalline poly(2,4-hexadiene) [385,386]. The melting point is
near 87

C.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Table 17 Polymerization of 1,3-pentadiene with cationic catalysts in benzene at 20

C.
Microstructure (%)
Catalyst mmol Monomer isomer H
2
O/catalyst (mol/mol) Gel (%) Double bond (%) 1,4 1,2 3,4
SnCl
4
0.38 trans 1.00 12 79 47.1 18.6 0
SnCl
4
0.38 cis 1.00 15 85 52.5 19.5 0
TiCl
4
0.53 trans 0.65 0 89 46.5 21.0 0
TiCl
4
0.53 cis 0.70 16 78.5 23.8 21.2 0
AlCl

3
16 trans 0.02 0 76 40 16 0
AlEtCl
2
0.79 trans 0.01 0 91.6 60.4 25.6 0
AlEtCl
2
0.79 trans 0.10 15 60.5 41.5 12.5 3
Source: Ref. [375].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
B. Polyterpenes
Myrcenes and ocimenes are isoprenoids that are occur in plants. They can be considered as
multiply substituted 1,3-butadienes [structures (53)–(57)].
With Ziegler catalysts such as titanium(IV) chloride/triisobutylaluminum they can be
polymerized to rubbery products with high molecular weights [387,388]. Also, Lewis acids
Table 18 Polymerization of methyl-1,3-pentadiene with various catalysts.
Microstructure (%)
Monomer Solvent Catalyst (mmol) cis-1,4 trans-1,4 1,2
2-Methyl- Benzene AlEt
2
Cl/H
2
O 0.12 90 >5
Heptane TiCl
4
0.43 <590<5
Heptane VOCl
3
/AlEt
3

0.11 90 10
Benzene TiCl
4
/Al(i-Bu)
3
0.5 90 10
Benzene TiCl
4
/AlEt
3
0.5 50 45 <5
Heptane H
9
C
4
Li 3.4 52 48
4-Methyl- Heptane AlEt
2
Cl/H
2
O 0.1 93 <7
Heptane TiCl
3
/AlEt
3
3.2 Crystalline, mp ¼ 166

C
Benzene H
9

C
4
Li 1.7 83 17
Source: Refs. [7,376,377].
Copyright 2005 by Marcel Dekker. All Rights Reserved.