transition metal polymers, preparation and properties

1. Lecture No. 07
Course title:
Inorganic polymers
Topic: Transition metal polymers
Course instructor: Dr. Salma Amir
GFCW Peshawar

2. Transition metal polymers
 Transition metal-containing polymers are macromolecules that contain
transition metal complex moieties at the side and/or main chains.
 Most of synthetic polymers consist of main group elements including C, H,
N, O, F, Si, P, S, and Cl. In accordance with the progress of transition metal
catalyst chemistry, various polymers containing transition metals have
been synthesized, and they are finding applications as polymer catalysts,
redox-active materials, electron-conductive materials, photo and
electroluminescent materials, biomimetic materials, and so on.

3. Classification
 Polymers Containing Transition Metals at the Side Chains
 Polymers Containing Transition Metals at the Main Chains

4. Synthesis
 RADICAL POLYMERIZATION: Poly(vinylferrocene) is presumably the first synthetic
polymer containing transition metals synthesized by the radical polymerization of
vinylferrocene
 The analogous metallocene polymers containing titanium, chromium,
molybdenum, manganese, tungsten, rhodium, iridium, are also synthesized.

5. Mechanism
 For free radical polymerization reactions, the initiating radicals must be generated
from azo-initiators because peroxides cause oxidation of the metal.
 In polymerizations of the type shown in reaction, the side group ferrocene units are
the source of both the thermal stability of the product polymers and complications
inherent in the free radical polymerization process. For example, electron donation
from the iron atoms to a growing radical chain end can convert an active radical to
an anion, which terminates the polymerization.
 The Fe+ center then rearranges to form a paramagnetic, ionically bound Fe(III)
species. Ultimately this leads to extensive chain-transfer, limitation of the chain
length, and formation of branched structures.

6. Vinyl ferrocene and other species with electron-rich rings undergo
cationic addition polymerizations quite effectively. With boron trifluoride
etherate as a catalyst, divinyl ferrocene polymerizes to a product with an
average molecular mass of 35,000 . On the other hand, replacement of the
–CH=CH2 vinyl group of vinylferrocene with a –C(CF3)=CH2 group
provides a very sluggish polymerization center.

8. Condensation polymerization
 Condensation polymerization of functional ferrocenes generally yields
medium- or low molecular-weight polymers with broad molecular-weight
distributions.
The molecular weights were in the range of 2,500 to 6,000, which
corresponds to only 13 to 30 repeating units per chain.

10.  Terpyridine is a typical tridentate ligand that coordinates most transition metals.
The resulting complexes exhibit typical optical and electric properties including
charge transfer from metal to ligand, redox, and luminescent properties. Various
polymers containing terpyridine–transition metal complexes are synthesized
Structure of polymer containing terpyridine–transition metal complex

11.  Porphyrin and phthalocyanine coordinate transition metals at their four pyrrole
nitrogen atoms to form stable metal complexes. Various π-conjugated polymers
based on porphyrin/phthalocyanine–transition metal complexes are synthesized by
the Sonogashira–Hagihara coupling polymerization
Structure of polymer containing porphyrin-coordinated transition metal

12. Ring opening reactions
 Polymers containing metal in the main chains are commonly synthesized
by step-growth polymerization such as Heck coupling, Suzuki–Miyaura
coupling, and Sonogashira–Hagihara coupling polymerizations.
 The anionic ring-opening polymerization of silicon-bridged
ferrocenophanes is also possible for synthesizing polymers containing
metals in the main chains.
 Differently from coupling polymerizations, ring-opening polymerization
enables control of molecular weights.

13.  polymerization of strained organometallic arenes such as silicon-bridged
ferrocenophanes via ROP has provided a versatile synthetic route to
polymetallocenes with high molecular weights (i.e. Mn > 100,000) such as
polyferrocenylsilanes. Polymerization via ROP has also been applied to a variety of
similar strained monomers with other single-atom linkers (where A= Sn, S, etc.),
two-atom linkers (where A = CeC, CeP, etc.), transition metals (where M = Ti, V, Cr,
Ru, Co, etc.), and various cyclic para-hydrocarbon rings such as cycloheptatrienyl
ligands and arenes

14. Ring opening reaction
The cyclopentadienyl ligands present in these remarkable molecules are
appreciably tilted with respect to one another by about 16–21°. As these
ligands adopt a preferred parallel arrangement in ferrocene, the tilting is
indicative of the presence of significant ring strain, which we have estimated
to be about 60–80 kJ mol−1. We found that these species polymerized when
heated in the melt in sealed evacuated tubes at 120–150 °C to afford high
molecular weight (Mn > 105) polyferrocenylsilanes (PFSs)

15.  We also showed that analogous germanium‐ and phosphorus‐bridged
ferrocenophanes (E = GeR2 or PR) also polymerize
thermally. Copolymerization of silicon‐bridged ferrocenophanes with other
monomers has also been achieved, and we have also recently expanded
this ROP methodology to a range of analogous strained monomers that
contain other single‐atom bridges ( E = SnR2, S, etc.), two‐atom bridges (E
= CC, CP, CS, etc.), and transition metals (e.g., Ru and Cr) and/or different
π‐hydrocarbon rings (arenes)

16.  The thermal ROP of ferrocenophanes is a chain‐growth process; high
molecular weight polymer is formed even at low monomer conversions. In
the case of silicon‐bridged [1]ferrocenophanes, cleavage of the
cyclopentadienyl (Cp)Si bond during ROP has been shown to occur.
However, the detailed mechanism of the reaction is not yet clear. It is
possible that trace quantities of nucleophilic impurities initiate the
polymerization, and recent studies of tin‐bridged ferrocenophanes (E =
SnR2) that undergo ROP at room temperature suggest that this may
indeed be the case.

17. Synthesis of polyferrocenylsilane (PFS) by anionic ring-opening
polymerization of silicon-bridged ferrocenophane

18. Properties of transition metal polymers
 Magnetism and sensors Transition metal polymers exhibit many kinds
of magnetism. Antiferromagnetism, ferrimagnetism,and ferromagnetism ar
e cooperative phenomena of the magnetic spins within a solid arising
from coupling between the spins of the paramagnetic centers. In order to
allow efficient magnetic, metal ions should be bridged by small ligands
allowing for short metal-metal contacts (such as oxo, cyano, and azido
bridges).

19.  Photosensitive materials: Polymers containing metal–metal (M–M) bonds
represent a very unique subclass of metal-containing polymers. The
metal–metal bonds behave as chromophores and thus produce
characteristic absorption bands. Due to the ease of breaking M–M bonds
using visible light, these photolytically degradable polymers can be used
as photosensitive materials.
 Lithographic materials: Additionally, some polymers containing M-M
bonds may have potential use as conductive or lithographic materials
 Thermal stability: Thermal analysis of majority of polymers revealed that
the polymer backbone was thermally stable.

20.  Sensor capability
These polymers can also show color changes upon the change
of solvent molecules incorporated into the structure. An example of this
would be the two Co coordination polymers that contains water ligands that
coordinate to the cobalt atoms. This originally orange solution turns either
purple or green with the replacement of water with tetrahydrofuran, and blue
upon the addition of diethyl ether. The polymer can thus act as a solvent
sensor that physically changes color in the presence of certain solvents. The
color changes are attributed to the incoming solvent displacing the water
ligands on the cobalt atoms, resulting in a change of their geometry from
octahedral to tetrahedral

21.  Conductors: Transition metal polymers can have short inorganic and
conjugated organic bridges in their structures, which provide pathways
for electrical conduction. example of such coordination polymers
are conductive metal organic frameworks.
 The conductivity is due to the interaction between the metal d-
the pi* level of the bridging ligand. In some cases coordination polymers
can have semi-conductor behavior. Three-dimensional structures
consisting of sheets of silver-containing polymers demonstrate semi-
conductivity when the metal centers are aligned, and conduction
decreases as the silver atoms go from parallel to perpendicular.