Polymeric Materials. Inorganic polymer chemistry

Transcription

1 olymeric aterials Year 2 ptio odule Iorgaic polymer chemistry (ED)

2 By the ed of this course you should: earig bjectives i) uderstad the structural ad electroic features of metal complexes that lead to polymerisatio ii) be able to describe the mechaism of metal-catalysed Ziegler-atta ad rig opeig polymerisatios of alkees iii) uderstad the terms atactic, isotactic ad sydiotactic. iv) uderstad basic relatioships betwee polymer ad metallocee complex structure for Ziegler-atta catalysis v) uderstad the possible polymer structures from vi) uderstad the differece betwee eatiomorphic site cotrol ad chai ed cotrol for stereospecific polymerisatio. vii) uderstad the structures of commo iorgaic polymers, their preparatio ad applicatios. viii) uderstad the cocept of charge trasfer complexes. vix) describe the key structural ad electroic features of charge trasfer complexes that give rise to magetic behaviour. vx) uderstad how simple coordiatio polymer structures that ca be built from combiatios of metal ios ad orgaic likers vxi) briefly describe uses of coordiatio polymers Bibliography olymers, D. Walto ad. orimer, 2001, xford Chemistry rimers tallocees-a itroductio to sadwich complexes,. J. og, 1998, Blackwell Sciece Advaces i o-tallocee lefi olymerizatio Catalysis, V. C. Gibso ad S. T. Spitzmesser, Chem. ev. 2003, 103, olymer sciece with trasitio metals, I. aers, J. olym. Sci. art A: olym. Chem., 2002, 40, Iorgaic olymers, J. E. ark,.. Allcock ad. West, 1992, retice all. rgaometallic agets J. S. iller ad A. J. Epstei, Coord. Chem. ev., 2000, , tal-orgaic frameworks, S.. James, Chem. Soc. ev., 2003, 32, Egieerig coordiatio polymers towards applicatios, C. Jaiak, Dalto Tras., 2003, elated Courses Trasitio metals (AKDK) year 1 Alkee ad alkyes (JKT) year 1 acromolecules (DKS) year 1 tal-ligad ad metal-metal bodig (ED) year 2 rgaometallic chemistry (J) year 2 Iorgaic reactio mechaisms (SBD) year 2 adicals i sythesis (AF) year 3 Supramolecular chemistry (DKS) year 3 Iorgaic materials chemistry (ED) year 3 Solid state chemistry (KW) year 3 2

3 Itroductio olymers are ubiquitous; from the atural world (DA, proteis, atural rubber, cellulose, silk) to ma made (ylo, polyethylee, polyviylcholride, superglue etc.) tal complexes are of iterest i polymerisatio because: 1. They ca catalyse polymerisatio 2. Cotrol polymer chai legth ad structure 3. olymers cotaiig metals ca have useful properties (magetic, electrical, optical, catalytic) The topics this course will cover 1. The sythesis ad mechaism of orgaic polymers usig metal complex catalysts: Ziegler- atta catalysis: used to prepare polyethylee ad polypropylee. millios of tos a year. ig opeig polymerisatio catalysis: used to prepare desiger fuctioal polymers ad polymers with high mechaical stregth. 2. The sythesis ad applicatios of polymers cotaiig iorgaic atoms: ai group cotaiig polymers: polysiloxaes, polysilaes, polyphosphazees. Trasitio metal cotaiig polymers. 3. rgaic-iorgaic hybrid materials rgaometallic magets tal coordiatio polymers (metal-orgaic frameworks) 3

4 tal Catalysed eactios of Alkees Three mai classes are isomerisatio, polymerisatio (icludig oligomerisatio) ad metathesis. ere alkee polymerisatio ad rig opeig metathesis polymerisatio () will be described. Isomerisatio Drivig force(s) for reactio elative stability of alkees ad etropy tathesis elative stability of alkees ad etropy 4 7 ligomerisatio 3 C-C σ-bod formatio olymerisatio C-C σ-bod formatio ig opeig metathesis polymerisatio () elief of rig-strai 4

5 olymerisatio of alkees I 1955 Zeigler discovered that heterogeeous mixtures of Ti 4 ad AlEt 3 catalysed the polymerisatio of ethee to liear polyethylee at room temperature ad pressure. ater atta subsequetly developed this ad similar systems ad demostrated that propee could be polymerised stereospecifically to isotactic polypropylee. Aside: stereochemistry of polymers is described by the tacticity e.g. of propee Atactic: adjacet stereocetres have radom cofiguratio Isotactic: adjacet stereocetres have same cofiguratio Sydiotactic: adjacet stereocetres have opposite cofiguratio Stereoblock: But usig Ti 4 /AlEt 3 the tacticity is low ad polydispersity is high. 5

6 tallocees I the 1980 s Kamisky showed that early trasitio metal metallocee complexes i homogeeous solutio could polymerise α-alkees stereospecifically i the presece of a methyl alumioxae (A) cocatalyst. Al 3 2 (Al) methylalumioxae (A) = Ti, Zr, f ater Britziger showed that chiral metallocees could rapidly polymerise α- alkees with high stereospecificity (isotactic polymer) ad low polydispersity. 2 Si A = Zr, f asa-bridge metallocee C 2 symmetric Cocatalyst A is made up of a complex mixture of clusters cotaiig (Al) moieties Al Al A serves three key roles 1. Alkylatig aget for the formatio of - complexes 2. ewis base, formig catioic metallocee. (catioic polymerisatio) 3. emoves water from the alkee feedstock. A: catalyst usually about 1000:1 6

7 chaism of alkee polymerisatio Iitiatio A Basic propagatio mechaism (essetially same for metallocees ad Ti 4 /AlEt 3 ) Termiatio steps β- elimiatio 'Al' ' Al ' Chai trasfer The relative rates of iitiatio (k i ), propagatio (k p ) ad termiatio (k t ) determie if polymerisatio occurs ad polymer dispersity. 7

8 Stereospecific olymerisatio 2 Si A Isotactic Sydiotactic polymers Zr A C s symmetric These are examples of eatiomorphic site cotrol where the stereochemistry is determied by the structure of the metal complex. I other polymerisatios the growig polymer ca determie polymer stereochemistry. This is called Chaied cotrol (we will see a example later). 8

9 Why early metal polymerisatio catalysts work 1. Early metal complexes are electrophilic. apid bidig of alkee icreases k p 2. Group 4 complexes are d 0. Back-bodig from metal to alkee is weak lowerig the activatio eergy for isertio, k p ot reduced. 3. For metallocees k p >>> k t leads to every metallocee beig catalytically active So called sigle-site catalysis leads to low polydispersity if k i is > k p ay moomers ca be polymerised by metallocees but they are very sesitive to water (eed lot of A) ad are itolerat to some moomer polar fuctioal groups. Would be good because: But: e.g. ate-metal catalysts 1. ore kietically stable to hydrolysis (stroger -alkyl bod stregth) 2. Would ot eed lots of A (bad for the eviromet) 3. Tolerat to polar moomers 1. ate metals have strog metal-alkee back-bodig 2. Udergo rapid β- elimiatio 3. ate metals are soft ad complexes are ot usually electrophilic. All lead to k p k t. ate metals used i alkee dimerisatio (oligomerisatio). i((h) 3 ) 3 Et 2 Al I 1996 Brookhart reported the polymerisatio of ethee ad α alkees usig ickel ad palladium di-imie complexes ad a little A. CF 3 _ i Et 2 B CF 3 4 (BAr f 4 - ) 9

10 Why late metal polymerisatio catalysts work 1. Imie ligads are hard door ligads ad the complex is charged. BAr f aio is o-coordiatig. All these icrease metal electrophilicity. 2. Bulky -substituets prevet chai trasfer. 3. Chelatig di-imie ligad forces cis-dispositio of vacat site ad growig polymer chai. 4. Ether is a weak ligad ad ca easily be substituted for a alkee to iitiate polymerisatio. I cotrast to group 4 usig i ad d catalysts the polymers are highly brached. ewer Co ad Fe complexes give liear polyethylee. chaism is basically the same except rate of isomerisatio is competitive with rate of propagatio, which leads to brachig. 2 i 2 i 2 i 2 i 2 i 2 i 2 i 2 i Isomerisatio could be very useful for makig polyethylee from propee 2 i 2 i 2 i 2 i etc 2 i 2 i 2 i 2 i 2 i 2 i 2 i 2 i 2 i 10

11 ig peig tathesis olymerisatio () tathesis: the result of exchage, substitutio or exchage of atoms lefi (alkee) metathesis ig opeig metathesis polymerisatio tal alkylidee (carbee) complexes are cetral to all olefi metathesis processes. recatalysts 1 2 Schrock type carbee ( 1 ad 2 are C or ) Ti Ti Goodyear Ar o t Bu u h 3 h 3 h h = t Bu, C(CF 3 ) 2 Schrock Grubbs 11

12 mechaism etc. e.g. The polymers produced are very hard ad have high mechaical stregth. is also a easy method for the cotrolled formatio of fuctioalised ad coductig polymers. F 3 C F 3 C F 3 C F 3 C _ polyacetylee F 3 C CF 3 Also may cyclic diee moomers are used because they form a sigificat compoet of crude oil (i.e. they are cheap). 12

13 CF 3 F 3 C CF 3 Stereochemistry of elative orietatio of alkee ad alkylidee determies stereochemistry A closer look at orboree pposite Cofiguratio tras, isotactic polyorboree Tras isotactic polyorboree is oe of four possible stereoregular polymers. (cis ad tras isotactic ad cis ad tras sydiotactic) Stereochemistry is depedet o catalysts ad/or moomer. Ca get eatiomorphic site or chai ed cotrol. e.g. Chai ed cotrol. Icomig alkee is directed by polymer structure Ar CF 3 t o Bu t Bu t Bu Ar F 3 C tras-isotactic CF 3 t Bu o t Bu F 3 C CF 3 13

14 ode of Ziegler-atta ad Ziegler-atta catalysis ad are both examples of chai growth polymerisatio. Both are also livig. k i, k p >>> k t % coversio 14

15 Iorgaic olymers I additio to polymers that cotai oly carbo or other orgaic atoms i the chai, polymers cotaiig iorgaic atoms ca also be formed. These compouds are of iterest because they offer the opportuity to develop ew techological applicatios due to their reactivity, structure, ad physical properties. ay are ow kow. The most widely developed cotai silico as polysiloxaes ad polysilaes. Aother commo class are the polyphosphazees. Si Si olysiloxaes ost importat iorgaic polymers with regard to commercial applicatios. dical: prosthetics, catheters, cotact leses, drug delivery capsules. o medical: e.g. elastomers, adhesives, lubricats, water repellets, molds, cosmetics. reparatio Si 2 2 Si 2 2 Si ( 2 Si) ig opeig polymersiatio ( 2 Si) 3 or 4 cat. cat. = acid or base 2 Si Chlorie free route 2 Si(Ac) 2 -Ac 2 2 Si DS (polydimethylsiloxae) is the most widely used 15

16 olysilaes The polymer backboe cotais oly silico atoms. A silico chai allows electro delocalisatio of sigma electros. ay uses of polysilaes derive from the electro mobility. ost uses are i the electroics idustry as photoresists ad precursors to silico cotaiig materials. reparatio Two geeral methods Codesatio of dichlorosilaes with alkali metals 'Si 2 a, toluee > 100 o C ( ' Si) roduct distributio idicates radicals are importat itermediates Si Si ' Si Si a Si Si.- a - - Si. Si a Si Si - a ' Si 2 Si Si Si etc. ' ' ' ' ' ' x 2 Si Si Si Si ' ' Dehydrogeative couplig of silaes ' catalyst Si 2 (Si ' ) (-1) 2 Two proposed mechaisms Cp 2 Ti Si 2 Via itermediate silylee - 2 Si 3 Cp 2 Ti Si Cp 2 Ti Si Si 2-2 etc. Cp 2 Ti Si Si 2 16

17 σ-bod metathesis h 2 Si Si 2 h Feedback ito cycle Cp 2 ()Zr Si 3 h Cp 2 ()Zr Si 2 h Cp 2 ()Zr Si 2 h h 2 Si Si 3 h Cp 2 ()Zr Si 2 h 2 roperties σ-electro delocalisatio is preset i polysilaes that cotrasts orgaic polymers (e.g. polyacetylee) that have π-electro delocalisatio. hyscial properties arisig from delocalisatio icludig strog electroic absorptio, coductivity, photocoductivity ad photosesitivity. σ-electro delocalisatio occurs primarily because of the good overlap of Si-Si bods ad lower electroegativity of Si relative to carbo. The electroic descriptio of may atom materials ca be described usig bad theory. You will study bad theory i year 3, however briefly: descriptio of 2 descriptio of E σ* Atibodig E Atibodig σ* Bads icrease overlap σ* coductio bad Badgap σ Bodig Bodig σ e - s e - s σ valece bad 2 may E icreases with greater overlap For coductio to occur a bad must be partially filled. Ca do this by: 1. Addig electros to the coductio bad by reactio with a reducig aget. 2. emovig electros from the valece bad by reactio with a oxidisig aget. 3. romotig electros from the filled valece bad to the empty coductio bad usig heat or light. 17

18 I polysilaes the bad gap is such that m light ca excite electros from valece bad ito coductio bad. The bad gap is sesitive to: 1. Chai legth. Bad gap decreases ad absorptio wavelegth icreases (lower eergy). eaches limit at ~ 30 Si atoms. 2. Coformatio of polymer chai: tras coformatio leads to absorptio at lower wavelegths (better overlap). Si Si Si Si Si Si Si Si all-tras some-cis Coformatio depedet o temperature that leads to strog theormochromism (colour chage with temperature) olysilaes ca be used as photoresists. hν mask photoresist Si wafer exposed developed olyphosphazees argest class of iorgaic polymers because of ease of side group modificatio. > Backboe comprises alterate ad atoms. Side groups () ca be orgaic iorgaic or orgaometallic. thod of sythesis allows modificatio of side groups. Uses iclude sold electrolytes (batteries), advaced elastomers (elastic at low temperature). Aerospace ad automobile applicatios because of resistace to hydrocarbo solvets ad oils, ad flame resistace. Textiles. 18

19 - 5 3 or o C a 2 Ca also fuctioalise the - precursor prior to polymerisatio. F F F F F F hi F F F h F F eat F F F F F h chaism (rig opeig polymerisatio) eat - - Catioic chai propagatio. Bodig i chai Cis ad tras coformers exist ad barrier to rotatio about the - bod is very small. owever, the bodig i phosphazees is still ot well uderstood although theories have bee discussed sice Empirical observatios: hosphazees do ot exhibit alteratig bod legths but the bod legth is much less tha a sigle - bod (multiple bod character). This meas hypervalet. 19

20 olyphosphazees are ot coloured ad do ot coduct electricity that is characteristic of extesive cojugatio. σ-framework The problem is distributio of the remaiig valece electros. rigial Dewar proposed that hosphorus-3d orbitals are ivolved i bodig to give -- islads where 2p z overlap 3d orbitals. otatio about - bod is low eergy because 2p z ca retai overlap with 3d orbitals o rotatio. ore recet calculatios suggest ioic bodig appears to be domiat. 20

21 rgaometallic (molecular) agets ost magets comprise of metals cotaiig upaired electros i d ad f based orbitals. icroscopic magetism tal Io µ B (B = magetic field experieced by the io) Each metal io has a magetic momet µ the size of which depeds o the spi ad orbital agular momets. These idividual momets ca iteract (couple) with each other ad behave collectively i a magetic field. Types of BUK magetism Diamagetism All solids exhibit diamagetism. Atoms or molecules with closed shells of electros are diamagetic. aramagetism is observed for compouds that cotai upaired electros ad where the magetic dipoles do ot iteract with each other. Atiferromagetism ad Ferromagetism are observed for compouds that cotai upaired electros ad where there is a strog iteractio (couplig) betwee magetic dipoles below a characteristic temperature. Atiferromagets (momets atiparallel) (arrows are the magetic momets ot idividual electros) Ferromagetism (momets parallel) ost ati- ad ferromagets are comprised from codesed iorgaic solids such as metal ad metal oxides. This is because it is ecessary that momets are close 21

22 together to have a strog couplig. rgaic based magets were ot iitially cosidered likely because the momets would be too small ad too dilute. Above a critical temperature the thermal eergy overcomes the couplig iteractio betwee the momets ad the material becomes paramagetic. T < T C T > T C rgaometallic magets were first discovered i the 1980 s. They are described as charge trasfer complexes because reactio usually occurs betwee a reducig orgaometallic reaget ad a oxidizig orgaic compoud. Trasitio metal complexes iclude metallocees. = Cr,, Fe V Importat features of metallocees are they are relatively small ad ca have S > 0 (high spi desity) ad ligads ca be easily modified to allow structure-property relatioships to be examied. rgaic compouds are usually plaar ad cotai heteroatoms. C C C C C C TCE TCQ C C Importat features of the orgaic molecules are that they are relatively small givig high spi desity, oxidizig geeratig a radial aio, ad flat allowig packig i the solid state the gives good couplig betwee the spis. 22

23 Charge trasfer reactio Fe C C Fe C C C C C C reducig oxidizig Charge trasfer complex dxz, dyz dxy, dx 2 -y 2 dz 2 π* S = 0 S = 0 S = 0.5 S = 0.5 A electro is trasferred from a metallocee d-orbital to the orgaic π* orbital. Both catio ad aio have upaired electros ad ca couple. The solid state structure determies how the idividual magetic momets couple ad what type of magetic behaviour is observed. iear chais are formed. Iterchai couplig also very importat to give a bulk ferromaget. tallocee TCE Chai structure of [(C 5 5 ) 2 Fe][TCE] These compouds oly give T C below 5-10 K compared to 1073 K for iro metal. ai problem is couplig is still quite weak ad momets dilute compared to al iorgaic magets. owever a orgaometallic maget has bee discovered with T C = 400 K but its structure is ukow. A further disadvatage is that these materials decompose i air. V C C C 2 2 V(TCE) 2 (C 2 2 ) 0.5 C C The geeral field of molecular magetic materials is still at a early stage. There are may other metal-ligad combiatios i additio to metalloceebased examples that cotiue to be ivestigated. The attractio is the opportuity to desig materials from the bottom-up usig molecules that ca be modified ad processed easily. There is also the opportuity to sythesise materials that exhibit combiatios of properties that will form the basis of future techologies. 23

24 tal Coordiatio olymers These polymers are prepared by likig metal ios or metal clusters with orgaic likig uits. ost studies are cocered with the formatio of porous crystallie polymers. orous solids ca be used for catalysis, sesors, chromatography ad sorptio. c.f. zeolites odular approach Basic Idea tal ios rgaic likers 1-D polymer 2-D polymers 3-D polymers I priciple a vast umber of combiatios of ad 24

25 Examples of orgaic likers C C _ C C C C Examples of metal ios ad clusters C C C C C C C =Cr,, Fe C Cd 2 Z Z Z Z 25

26 Sythesis Sythesis of 3-D porous coordiatio polymers is complex because the product is crystallie (ad for characterisatio sigle-crystals have to be > (25 µm) 3 ). Every bod formed has to be cosistet with crystal growth (must be a corrective mechaism for bod formatio that is ot cosistet with crystal growth) Two commo methods 1. Cotrolled Diffusio Add reactats ito separate parts of vessel ad wait. Good for large sigle crystal growth, but slow for large quatities ad ot always reproducible. 2. ydrothermal thod Add all reactats together ito a hydrothermal bomb ad heat ad cool for a fixed period of time. ot always large eough sigle crystals but good for large quatities ad more reproducible. Compoud usually crystallise with solvet i pores. The problem before 1999 was that all attempts to remove solvet of crystallisatio resulted i collapse of the structure. 26

27 Example of a truly porous coordiatio polymer (there are ow may) Z( 3 ) 2 DF C Et 3 2 BDC Diffusio of Et 3 ito reactio mixture Crystals of Z 4 (BDC) 3.(DF) 8 (C 6 5 ) 300 o C Crystals of Z 4 (BDC) 3 ore sphere diameter 18.5 Å. (Big eough to accommodate molecules) tal moieties that are more complex tha simple ios, such as the cluster Z 4 are sometimes called secodary buildig uits SBU s. Z Z Z Z 27

28 By chagig the size of the iorgaic liker it is possible to desig a desired pore size. ote for structure 16 over 90% of the crystal volume is empty! 28

29 The formatio of porous structures ca be preveted by iterpeetratio of polymer frameworks. Iterpeetratio is where idividual frameworks lik together with o covalet bods betwee frameworks. These structures form due to o-covalet iteractios durig the crystal growig process ad because porous solids are thermodyamically ustable with respect to codesed solids. 29

30 roperties of coordiatio polymers Gas sorptio For applicatio of clea fuels gases such as 2 ad C 4 require storage ad access o demad. eavy gas cyliders uder large pressure are ot efficiet (or safe) ad materials are required that ca store large quatities of these gases ad release them whe eeded. Coordiatio polymers are beig ivestigated as storage media. e of the Z 4 (dicarboxylates) show above (structure 6) is able to sorb methae effectively at 36 atm (safe compared to 205 for a cylider). Chirality-Asymmetric catalysis Chiral orgaic likers ca be used to prepare chiral porous solids. These compouds are beig ivestigated for asymmetric catalytic applicatios. e.g. 2 C 2 C agetism ad sesors I may complexes Fe(II) (d 6 ) exhibits spi crossover (i.e. a trasitio from high to low spi or vice versa). Fe(II) spi crossover is very sesitive to ligad eviromet. Fe(II) low spi is diamagetic ad Fe(II) high spi is paramagetic. This chage is usually accompaied by a dramatic colour chage. Fe(II) coordiatio polymers have recetly bee sythesised that chage colour o absorptio of small alcohols. 30