Synthesis of Chain-end Functionalized Polyolefins and Fluoropolymers and Applications in Nanocomposites
Open Access
- Author:
- Wang, Zhiming
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 14, 2005
- Committee Members:
- Tze Chiang Chung, Committee Chair/Co-Chair
Evangelos Manias, Committee Chair/Co-Chair
Ian Roland Harrison, Committee Member
Ayusman Sen, Committee Member - Keywords:
- Polyolefins
fluoropolymer
nanocomposite
metallocene catalyst - Abstract:
- In this thesis, we have demonstrated a very useful and simple method (one-pot polymerization process) for synthesis of chain end functionalized polypropylene. The chemistry involves a chain transfer reaction to a styrenic derivative (St-f), with or without hydrogen during propylene polymerization, using an Exxon-Hoechst C2-symmetric catalyst (rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO complex) or a Mitsubishi C1-symmetric catalyst (Me2Si(2-Me-Benz[e]Ind(2-Me-4-Ph-4HAzu)HfCl2 with MAO or trialkylaluminum-treated clay). In the presence of the Exxon-Hoechst catalyst, the propylene propagating chain-end engages in a facile consecutive chain transfer reaction, reacting with St-f and then with hydrogen, with high catalytic activity under the proper reaction conditions. The polymer molecular weight is proportional to the molar ratio of [propylene]/[St-f]. A silane protecting group in St-NSi2 or St-OSi unit can be hydrolyzed in an acidic solution during the sample work-up step to obtain desirable i-PP polymers, such as i-PP with a terminal NH2 or OH group, in one pot. Despite the low concentration, the terminal functional group is very reactive and can serve as an active site for many applications. One example was shown in a chain extension reaction (coupling reaction) with polycaprolactone (PCL) in solution to form PP-b-PCL diblock copolymers that are very effective compatibilizers in PP/PCL polymer blends. Unexpectedly, a Mitsubishi C1-symmetric catalyst exhibits significant polymerization activity even in the absence of hydrogen, indicating that the trialkylaluminum may participate in chain transfer to p-MS (p-methylstyrene) terminated propagating chains. In the case of polymerization using MAO as a cocatalyst at 55oC, the addition of hydrogen increases the activity and regulates the polymer molecular weight. The chain-end structure is solely terminal p-MS. When TEA (triethylaluminium) -treated clay is adopted as an activator and carrier at the optimal polymerization temperature of 75oC, the high concentration of hydrogen suppresses catalytic activity. The chain ends consist of predominately terminal p-MS and a small amount of unsaturated end groups. A higher p-MS concentration or introduction of hydrogen eliminates the undesirable unsaturated chain ends. Furthermore, we also study a new chemical route to prepared side chain functionalized polyolefin, especially the desirable MA (maleic anhydride) -modified PE and PP polymers with well-controlled molecular structures. The chemistry involves a post-polymerization process using borane/O2 stable radical initiators to create polymeric radicals that are simultaneously stabilized by in situ formed *O-BR2 stable radicals. The dormant polymeric radicals do not undergo undesirable side reactions (crosslinking and degradation, etc.), but can react with maleic anhydride. Some MAH-modified PP polymers with high molecular weight and controlled MAH content have been obtained. They have been proven to be the effective compatibilizers to improve the interfacial adhesion in the PP/Nylon 11 blends. In a different approach, when dealing with fluoropolymers, a modified iodine transfer polymerization (ITP) method was also developed, based on the combination of a specific radical initiator (2,2'-azobisisobutyronitrile, AIBN) and a reversible addition-fragmentation chain transfer (RAFT) process involving two iodo-compounds, i.e., a,ù-diiodoperfluoroalkane (I-Rf-I) and mono-iodoperfluoroalkane (Rf-I). We take advantage of the inactive radicals, created by the decomposition of AIBN, which readily react with the iodo-compounds (chain transfer agents). Pure telechelic fluoropolymers with almost all the polymer chains containing two terminal iodo groups have been synthesized. Using diiodoperfluoroalkane as the chain transfer agent is more effective than mono-iodoperfluoroalkane. In turn, the reactive terminal CF2I groups can undergo facile ethylenation to convert to relatively stable CH2I group or readily transform to imidazolium ions that are very effective in forming fluoropolymer/clay nanocomposite. One major application of functional polyolefin and fluoropolymers is the preparation of exfoliated polymer/clay nanocomposites. The process involves melt or solution blending using functional polymer as a surfactant, namely, chain-end functionalized polypropylene containing a terminal hydrophilic functional group (NH3+) and a high molecular weight hydrophobic polymer chain. The chain-end functionalized polypropylene exhibits very high surface activities and results in an exfoliated clay interlayer structure, even with pristine clay minerals without any organic treatment. Furthermore, this exfoliated clay structure maintains its disordered state even after further mixing with neat (unfunctionalized) polypropylene that is compatible with the backbone of the chain end functionalized polypropylene. Mechanical property evaluation shows the addition of PP-t-NH3+ in the system remarkably enhanced flexural modulus. These experimental results demonstrate the advantage of chain-end functionalized PP in the formation of an exfoliated clay layer structure and lead to the proposition that the terminal hydrophilic NH3+ functional groups anchors the PP chains on the inorganic surfaces via ion exchange, and that the hydrophobic high molecular weight and semicrystalline PP chains are repelled from the inorganic surfaces and exfoliate the clay platelets. This chain-end functional polymer technology was extended to fluoropolymers, and we have prepared a new family of fluoropolymer/clay nanocomposites that exhibits an exfoliated and uniformly dispersed clay structure in a polymer matrix. The process involves a specific interfacial reagent-chain end functionalized fluoropolymer containing an unperturbed hydrophobic and oleophobic fluoropolymer chain and a terminal functional group, such as Si(OEt)3, OH, imidazolium, or sulfonium ions. The terminal functional group can anchor fluoropolymer chain to the clay surfaces between interlayers, either by a chemical bond (such as a Si-O-Si bond), strong interaction (such as hydrogen bonding and ion-ion interaction) or ion-exchange with cation (Li+, Na+, etc) located on the surfaces between the clay interlayers. On the other hand, the rest unperturbed high molecular weight hydrophobic and oleophobic fluoropolymer chain, disliking the hydrophilic clay surfaces, exfoliates the clay layer structure and maintains this disordered clay structure even after further mixing with a neat (unfunctionalized) polymer that is compatible with the backbone of the chain end functionalized fluoropolymer.