Second year lab reports – feedback sheet

Name: Adam Wdowiak Mark awarded: 158

Good points

Generally well written in good English and easy to follow. Nice to see that a good range of references were used.

Points for improvement

See comments throughout text. Generally try to keep you experimental more concise.

Mark SchemeThe following mark scheme should be used to judge the overall quality of a report.

Level 2 Written Laboratory Reports 200

You would be happy to submit this report to a journal for publication with your name on it. No improvement is needed.

200

An exceptional report. Only minor improvements can be made to this report. The references are thorough and presented in the correct format. All tables and figures are correctly included and formatted in the report. The student has spent considerable time in the library researching the topic fully. The results are presented in a logical and thorough manner. Excellent standard of English with no errors.

180

An excellent report. The references are thorough and presented in the correct format. All tables and figures are correctly included and formatted in the report. The student has spent considerable time in the library researching the topic, presenting the majority of the points of the experiment and also related information expanding on the basic premise of the experiment. The results are presented in a logical and thorough manner. Excellent standard of English.

160

A very good report. The references are thorough and presented in the correct format. All tables and figures are correctly included and formatted in the report. The student has spent some time in the library researching the topic and has covered the majority of the points of the experiment. The results are presented in a logical and thorough manner. Good standard of English, but with a few typographical errors.

140

The student has carried out some research into background material, and has covered the basic points of the experiment. All of the data has been analysed with few errors. Figures and tables are present and are correctly formatted, and correctly referred to in the text. Standard of English is reasonable with some errors.

120

The student has carried out some research into background material, and has covered the basic points of the experiment. Most of the data has been analysed, but with errors. Figures and tables are present and are mostly correctly formatted; however, they have not been referred to correctly in the text. Standard of English is reasonable, but with errors.

100

The student has written up the material in the DLM, and has included some extra information. Key data has not been analysed. Figures and tables, while present, are incorrectly formatted and not referred to correctly in the text. Poor standard of English.

80

The student has simply written up the material presented to them in the DLM. No new information has been supplied. If outside references have been supplied, they are simply the course textbooks or Wikipedia. Poor standard of English.

60

One or two pages, but little or no effort put in. Poor standard of English40

A very brief report, one page or less. Very poor standard of English. 20

The student did not submit a report. 0

Investigation of some silicone polymersAdam Wdowiak

AbstractTwo siloxane polymers were selected to be synthesised and have their properties investigated. Poly(dimethylsiloxane) has been synthesised in 86.6% yield by hydrolysis of dichlorodimethylsilane and condensation, then heated with boron to obtain a polymer known commercially as “Silly Putty”. Octaphenylcyclotetrasiloxane has been synthesised in 43.7% yield by hydrolysis of dichlorodiphenylsilane and condensation in alkaline conditions.

IntroductionThe term “silicones” was first used in 1901 by Kipping to describe a class of siloxane polymers with the general formula:

where R is any organic group1. Whereas the simplest R group is hydrogen, the most commonly used silicone in industry is poly(dimethylsiloxane), also known by the names PDMS and silicone oil, with methyl as the R group. The resulting polymer is rich in properties, such as high resistivity, flexibility, thermal stability and biocompatibility. It is also worth noting that, when compared to hydrocarbon chains, the viscosity of siloxane chains varies significantly less. This allows for a very extensive variety of uses of PDMS and its derivatives in industry, healthcare and engineering. Examples of use include sheet insulation, electrical insulation, hydraulic fluid and prosthetics.1, 2, 7 In 2010, the total silicone demand has beenwas estimated at ca. $12 billion.9 PDMS is easily synthesised by hydrolysis of dichlorodimethylsilane 1. The result is formation of chains where the chain length varies between 20 and 50. Some cyclic compounds are also produced, which can be separated and recycled. To enable industrial use of PDMS, the chain length needs to be increased further; this is done by further polymerisation with an acidic catalyst, extending the typical chain length to ca. 2000-4000.1, 2

The driving force for the reaction is the strength of Si-O bonds. The bond is stabilised by hyperconjugation, which occurs between the filled non-bonding oxygen orbital and a σ* silicon-carbon orbital, as shown in figures Figures 1 and 2. The resulting polymer is very stable thermodynamically, and further polymerisation is favoured.3

Figure 1: Diagrammatic explanation of the orbital overlap

Figure 2: HOMO-6 orbital of dimethylsilanediol calculated by Hückel method, isocontour 0.010 a.u.

One of the ways to change the properties of the polymer is to introduce other elements into the polymer chain, such as boron. This allows a degree of cross-linking to be achieved and extends the range of uses, making borated silicone oil the simplest example of a “viscoelastic material”. This class of materials refers to compounds which are elastic if investigated over a small period of time, yet viscous if observed over a longer period of time.6 The borated version of PDMS is widely known as “Silly Putty”, most widely known as a toy but since has found physical and medical implementations, mainly in astronautical engineering and dentistry.5, 6 Another way of fine tuning the properties of the polymer is by changing the organic R groups, most commonly alkyl, allyl and aryl. This allows for a large variety of polymers with different properties to be synthesised and used. Changing the R groups affects the ability for cross-linking and sliding of the chains, which in turn affects density. The electronic structure is also affected, as the n-σ* overlap efficiency will depend on the R group, and stability might be improved by further conjugation. 1, 2, 3

ExperimentalSynthesis of PDMS and Silly Putty

20 ml of dichloromethane was placed in a 100 ml conical flask, to which 20 ml of 1 was syringed. The resulting pink solution was poured into a dropping funnel. 150 ml of deionised water and 60 ml of dichloromethane was added to a 500 ml three-necked round-bottom flask, earlier equipped with a stirrer bar and placed in an ice-water bath at 0°C. A vertical condenser was attached to the middle neck of the flask, while the condenser was attached to one of the side necks. A glass stopper was used to cover the third inlet. The dropping funnel outlet was open to release the solution into the round bottom flask at about 5 drops per second for 9 minutes. The contents were stirred at 750 rpm throughout the addition and for additional 15 minutes after the whole content of the dropping funnel was transferred. The contents were transferred into a separating funnel and the organic layer collected into a 500 ml conical flask. The top layer was collected into a 500 ml beaker and later disposed. 50 ml of deionised water was added to the conical flask followed by 50 ml of sodium bicarbonate. The pH of the aqueous layer was tested using universal indicator paper to ensure that all hydrochloric acid was neutralised. The contents of the conical flask were transferred into an empty separating funnel and the organic layer was collected into an empty 500 ml

conical flask, while the top layer was collected into a beaker and later disposed. 25 ml of deionised water was added to the conical flask. The mixture was dried with magnesium sulfate (ca. 50 spatulae), filtered and transferred into a clean, pre-weighed 500 ml round bottom flask. The product was placed in a rotary evaporator at 100 mbar for ca. 10 minutes to obtain a viscous yellow liquid – PDMS (10.6g, 86.6%), of which a sample was taken and tested with a few drops of concentrated sulfuric acid to obtain an orange-pink gel. In a separate conical flask, 0.44 g (6.32 mmol, ca. 4% by weight of product) of boric oxide was weighed out. The product was added to the flask and gently swirled. Resulting suspension was poured into an aluminium cup and heated at 200°C for 91 minutes to obtain Silly Putty.Synthesis of octaphenylcyclotetrasiloxane 5

5 ml of toluene, 10 ml of 2-methylbutan-2-ol and 35 ml of deionised water was poured into clean and dry 500 ml three-necked round-bottom flask earlier placed in a water bath at 21°C and equipped with a stirrer bar. 8 ml of diphenyldichlorosilane 3 was syringed into a clean and dry dropping funnel which was then attached to the side neck of the flask. A vertical condenser was attached to the middle neck and the third neck was sealed with a glass stoppered. The solution was stirred at 500 rpm and the contents of the dropping funnel added dropwise at a rate of ca. 5 drops per second over 5 minutes. After stirring the mixture for another 5 minutes, the resulting white precipitate was vacuum filtered, washed dropwise with the following solvents: 10 ml of deionised water, 10 ml of methanol and 10 ml of diethyl ether, all of which were cooled down to 0°C prior to washing, and air dried for ca. 10 minutes to obtain diphenylsilanediol 4 (0.80 g, 9.7%), vmax/cm-1 3180br (OH), 1590w (CH), 1119m (SiO), 741m (SiAr) (lit.10 2940br (OH), 1599m (CH), 1129s and 1117s (SiO), 738s (SiAr)).Product was then also collected from other chemists carrying out the reaction at the same time to make up a 2 g sample of 4 which was weighed into a two-necked round-bottom flask and dissolved in 20 ml of ethanol and 2 ml of deionised water. With a vertical condenser attached to the middle neck and the side neck stoppered, the solution was stirred at 500 rpm and refluxed at 200°C for 15 minutes when it was observed to boil. At that point, 0.5 ml of 6 M sodium hydroxide was added and vigorous fizzing observed. The reaction mixture was refluxed for 16 minutes and the resulting white precipitate vacuum filtered, washed with ca. 10 ml of ethanol at 0°C and air-dried for 16 minutes to obtain the product 5. (0.80 g, 43.7%), vmax/cm-1 3069w (CH), 1590w and 1487w (conj. CC), 1117s and 1089s (SiO), 740m (SiAr) (lit.4 3070w (CH), 1591w, 1487w and 1429m (conj. CC), 1119s and 1103s (SiO), 741s, 717s and 698s (SiAr)).

Results and discussionPDMS and Silly Putty

The first part of the experiment focused on the preparation of PDMS and introducing boron to make the bouncing putty. The reaction was carried out slowly and at a low temperature as the hydrolysis is very exothermic. This was a safety measure to prevent an explosion from occurring in the lab. The ice bath, as well as attaching a vertical condenser, ensured that as much of 2 as possible has been polymerised. The mechanism is shown in scheme Scheme 1.

Scheme 1: Hydrolysis of 1 and condensation of 2As the hydrolysis generated hydrochloric acid, this had to be neutralised with sodium bicarbonate prior to rotary evaporation. Throughout the synthesis the product remained dissolved in dichloromethane which allowed transfers between the various vessels to be very effective, with minor losses at each stage, and resulted in a relatively high yield. To further test the product, concentrated sulfuric acid was used, which aids further polymerisation and increases the chain length of the polymer (Sscheme 2). Upon its addition to a small sample of the polymer, the yellow oil turned into a pink gel. The phase transition suggests that further polymerisation occurred.

Scheme 2: acid catalysed further polymerisation of PDMSThe oil was then heated with boric oxide, introducing boron into the polymer chains. The electron deficiency of boron allows for cross-linking between the chains (fFigure 35), which is where the properties of the putty arise from.

Figure 3: dative cross-linking in borated PDMSThe final product was translucent, had a slight orange tint, was easy to mould when wet and was bouncy. The properties observed match those reported in literature, suggesting that the synthesis was successful.

Octaphenylcyclotetrasiloxane

The aim of the second part of the experiment was to synthesise a siloxane ring. Similarly to the first part, 3 was first hydrolysed. Resulting compound 4 was then polymerised using a basic catalyst. This promoted the formation of an 8-membered ring rather than a long polymer chain (scheme 3).

Scheme 3: polymerisation of 4 and ring closure

Though the hydrolysis of 3 was very successful, the issues arose with collecting and purifying the product. The white precipitate formed in the reaction was very difficult to transfer from the flask, and as a result most of the product remained in the flask and was not collected. Moreover, despite the solvents used for washing were cooled down to 0°C, much of the product dissolved in them as the product was washed. This resulted in a very poor yield of the hydrolysed product. The small, broad OH peak in the IR spectrum suggests that 5 was not completely dry. The IR spectrum matches reasonably well with that found in literature, suggesting the hydrolysis was successful.

Due to such low yield, 5 had to be collected from other chemists working in the lab in order to carry out the next step of the synthesis. This was more efficient and the yield was larger. The IR spectrum also showed absence of OH stretch, meaning that no 4 was present in the final product, meaning that the polymerisation was successful. The IR spectrum matches very closely with the spectrum found in literature, suggesting that the right product was synthesised.

ConclusionSiloxane polymers play a very important role in industry. The vast range of properties unique to this class of compounds make them ideal for a variety of uses, including medicine and engineering. The simplicity of appropriately adjusting the properties by affecting the cross-linking ability and electronic structure through introducing other elements and changing the R groups makes it easy to synthesise a range of polymers for a range of applications. The methods of synthesis require the use of simple chemistry (alkylation, hydrolysis and condensation), are very effective and reliable, with a potential to produce very high yields. Care must be taken during transfers between reaction vessels in order to avoid spillages and large residues in glassware, as large amounts of product can be wasted this way. The synthesis routes followed in this experiment are very similar to industrial routes, confirming that the industrial methods are efficient and effective.

References1. H. W. Post, Silicones and other Organic Silicon Compounds, Reinhold Publishing, New York,

1949, 102-122.Secondary source with information about synthesis of silicones and their use in industry. Slightly dated, but reliable nevertheless.

2. E. G. Rochow, An Introduction to the Chemistry of the Silicones, Chapman & Hall, London, 1946, 39, 53-58, 64-83Secondary source with information about synthesis of silicones and their use in industry. Slightly dated, but reliable nevertheless.

3. F. Weinhold, R. West, Organometallics, 2011, 30, 5815-5824Primary source explaining the phenomenon of silicon-oxygen bond stability. Featured in a well-known peer-reviewed journal recognised by the RSC.

4. M. Luo, B. Yan, Tetrahedron Letters, 2009, 50, 5208-5209Primary source containing information about use of octaphenylcyclotetrasiloxane, as well as an IR spectrum. Featured in a well-known peer-reviewed journal recognised by the RSC.

5. B. D. Craig, US Pat. WO2015034692 A1, 2015.A patent explaining the use of Silly Putty in industry. By definition, patents are peer-reviewed and reliable.

6. R. Cross, Am. J. Phys., 2012, 80, 870-875.Primary source containing information about physical properties of Silly Putty. Featured in a well-known peer-reviewed journal recognised by the RSC.

7. S. Jiang, S. Zha, L. Xia, R. Guan, J. Ad. Sci. Tech., 2015, 29, 641-656.

Primary source containing some details about diphenylsilanediol. Featured in a well-known peer-reviewed journal recognised by the RSC.

8. Silicones: Preparation, Properties and Performance, Dow Corning, 2005.https://www.dowcorning.com/content/publishedlit/01-3077.pdf [accessed February 2016]Secondary source summarising industrial synthesis routes and properties of silicones, mainly PDMS. Dow Corning is one of the top suppliers of silicone oil. The document contains references to over 30 primary sources, all featured in peer-reviewed journals recognised by the RSC.

9. World Silicones, Freedonia Group, 2011. http://www.freedoniagroup.com/brochure/27xx/2779smwe.pdf [accessed February 2016]Secondary source – a business report on the consumption of PDMS and other silicones.

10. IR spectrum: Silanediol, diphenyl-; Chemistry Webbook, National Institute of Standards and Technology, pre-1970.http://webbook.nist.gov/cgi/cbook.cgi?Name=diphenylsilanediol&Units=SI&cIR=on#IR-Spec [accessed February 2016]Secondary source – a database of IR spectra. NIST is a recognised institution.

Appendix

Figure 4: IR spectrum of diphenylsilanediol 4

Figure 5: IR spectrum of octaphenylcyclotetrasiloxane 5