An efficient and facile method of grafting Allyl groups to chemically resistant polyketone membranes
Graphical abstract
Grafting pending allyl groups onto polyketone polymer chain.
Introduction
Polyketone, which is one of the high performance thermoplastic polymers, is a type of copolymers and terpolymers made of alternating copolymerization of carbon monoxide and ethylene or a mixture with propylene. It is an environmentally friendly polymer making use of carbon monoxide as raw material which is actually a serious air pollutant and toxic gas [[1], [2], [3], [4]]. Due to polar carbonyl group in the main chain, it has a semi-crystalline, occasionally polymorphous, and compact crystal structure, which improves the physico-chemical properties through intramolecular and intermolecular interactions [5,6]. Hence, it demonstrates a useful variety of strong chemical resistance, impact resistance, abrasion resistance and gas barrier properties [7,8].
Initially, the US and Japan companies challenged to commercialize copolymer of polyketone. In 2013, Hyosung Corporation (Korea) became the first in the world to succeed in developing an economically efficient way to deliver copolymer of regularly alternating carbon monoxide and ethylene, and named their product POKETONE. This product is highly promising for thermoplastics applications such as automobile and electronic parts due to its excellent impact resistance, chemical resistance and flame retardancy compared to the conventional engineering plastic nylon. According to Hyosung Corporation's report, POKETONE has a chemical resistance superior to that of nylon by more than 30%, and has higher resistance to chemicals among many other plastic materials. It also has excellent impact resistance compared to PA (polyamide) and PBT, and has excellent impact strength of over 230% compared to Nylon. POKETONE has 14 times better abrasion resistance than polyoxymethylene (POM), and has the advantage of reducing friction noise. Therefore, polyketone can be used for interior and exterior materials in automobile, electric and electronics fields due to aforementioned advantageous characteristics [9].
Researchers have focused on polyketone copolymers composed of other type of monomers [10,11] so far until a convenient method has just recently been invented for synthesis of this type of polyketone. To our knowledge, there is scarce information about chemical functionalization of carbon monoxide and ethylene copolymer since it is rather a new kind. Previously in our group, we used γ-irradiation grafting technique to incorporate functional groups to polyketone chain of POKETONE [12,13]. γ-irradiation can cleave C-H bonds of alpha carbons homolytically, which can perform a radicalic attack towards double bonds of grafting monomers. Vinyl compounds bearing sulfonyl or amine groups were incorporated to polyketone backbone by our group. Only such an energetic method enabled modifying aliphatic part of the chain. Some researchers used Paal-Knorr reaction to modify carbonyl group of polyketone into pyrrols, then they attached some functional groups to the extension of the pyrrole ring for endowing further functionality [6,[14], [15], [16], [17], [18], [19]]. This method apparently changes the intrinsic chemical and physical properties of polyketone since most carbonyl groups are chemically modified, thus it can't be even called a polyketone anymore. It has been revealed to us that there was not satisfactory amount of work in the literature which deals with chemical modification of polyketone (here we refer as PK anymore) without major changes in main chain atoms. If the chemical modification of PK and monomer grafting can be succeeded in a controlled manner, incorporation of charged functional groups to PK chain will be feasible for manufacturing ion exchange membranes which preserve highly advantageous mechanical properties of polyketone backbone adequately. Such engineered membranes derived from PK may be promising for the design of tenacious membranes for long-term use in water treatment [[20], [21], [22], [23]], energy applications, e.g. redox flow batteries [[24], [25], [26], [27], [28]], and material separations [[29], [30], [31]] etc.
Here we introduce a new method which exploits alpha carbon chemistry of carbonyl compounds. A strong base such as potassium tert-butoxide (KOtBu) can easily remove relatively acidic hydrogen of alpha carbon [[32], [33], [34]]. Among negatively charged resonance forms of enolate, carbanion form can bind electropositive centers with strong C-C bonds. We used allyl bromide to attach allyl groups to the alpha carbons of polyketone (Fig. 1) (here we refer this product as PK-allyl anymore). To the best of our knowledge, this is the first time polyketone is modified chemically without changing carbonyl backbone except slightly. Since membrane form of polyketone was used as the starting agent, there will be no need to cast the membrane of the product again. Further functionalizations became possible through double bonds of allyl groups. For example, radicalic polymerization through allyl groups or additions to the allyl double bonds can be used to add extra functionality to the membranes, such as quaternary amines and sulfonic acids which can endow ion-exchanging ability to the membranes. FTIR and NMR measurements confirmed the presence of vinyl groups in the product's structure.
Section snippets
Materials
POKETONE was purchased from Hyosung Corp. (Korea). It is the brand name of a polyketone product, which has a molar weight of ∼200,000 g/mol. Resorcinol (98.0%) used to make the polyketone membrane was purchased from Samchun. Potassium tert-butoxide (98.0%), the strong base used in polyketone synthesis, was purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO, 99.8%) and allyl bromide (99.0%) were purchased from Samchun. All chemicals were used without further purification.
Synthesis of polyketone membrane
Polyketone,
Synthesis and general properties
With the attachment of allyl groups, weight of the membranes increased apparently between varying ratios depending on time and other conditions (between 10% and 50%). We carried out all analyses on the samples which showed weight gain around 28%. This means at least ∼39% of ketone units were allylated (Degree of allylation≈39%). It should be kept in mind that some polyketone may have somewhat dissolved into DMSO or decomposed during enolization.
A fast color change was observed when KOtBu
Conclusions
Functional modification and monomer grafting of polyketone polymer was successfully carried out by simple chemical methods despite the fact that polyketone is among the most chemically resistant thermoplastic polymers. We succeeded to attach unsaturated substituents to the polyketone backbone with allylation process on polyketone enolates. We obtained the PK-allyl product physically in membrane form, since PK reacts in solid state and as a membrane with all solutions of chemicals used in our
Conflicts of interest
None.
Acknowledgements
This work was supported by the Commercializations Promotion Agency for R&D Outcomes Grant funded by the Korean Government (MSIP)” (2017, 2017K000215, Joint Research Corporations Support Program). This work was also supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2015H1C1A1035652).
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