lecture 07: Transition metal polymers by Dr. Salma Amir

1. Lecture No. 07Course title:Inorganic polymersTopic: 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, ionicallybound 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 trifluorideetherate as a catalyst, divinylferrocene polymerizes to a product with an average molecular mass of 35,000 . On the other hand, replacement of the –CH=CH2vinyl 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 ferrocenophanesvia 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 ferrocenophaneswith 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 = CC, CP, CS, etc.), and transition metals (e.g., Ru and Cr) and/or different π‐hydrocarbon rings (arenes)

16. The thermal ROP of ferrocenophanesis 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 ofmagnetism. Antiferromagnetism, ferrimagnetism,andferromagnetism are 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-orbital and 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.