Stephen Boothroyd, Rebecca Smith, Ezat Khosravi, Richard Thompson

Department of ChemistryDurham University

Catalytic Depolymerisation of Rubber

Contents Motivation

Metathesis Chemistry Basics

Breaking down polybutadienes by Cross-Metathesis

Test reaction on linear PB

Test reaction model cross-linked PB

Trials on SBR

Too good to be true?

Conclusions and outlook

Why?

Main use of rubber is in tyres

Millions of tyres are stockpiled each year due to difficulties in disposing of them

Difficulties due to the high crosslink density

Huge environmental pressure to make tyres reprocessable

without compromising on safety, durability, energy efficiency

The idea Rubber contains double bonds which are available for

further reaction

Use a cross metathesis reaction to break down the rubber network by reacting it with a small olefin

n

Cross Metathesis Rearranges carbon-carbon double bonds

Grubbs Catalyst - ruthenium based metal centre [M]

Reaction proceeds via a metallocyclobutaneintermediate

Number of chain ends is conserved!

Grubbs’ ruthenium catalysts• Most widely used for metathesis reactions

• High functional group tolerance

• Low sensitivity to air and moisture – scaleable!

• 2nd generation is more active than the 1st due to the N heterocyclic (NHC) ligand

1st generation 2nd generation

Experimental ApproachTo carry out cross metathesis reactions on models of

increasing complexity/realism:

1. Linear 1,4-polybutadiene

2. Cross-linked 1,4-polybutadiene

3. Tyre rubber (SBR sheet)

Polybutadiene (PBD)

Polymerise butadiene by 1,2- or 1,4-addition

1,4- PBD has the double bond in the polymer chain

Model material contains a mixture of repeat units.

1,2-PBD trans 1,4-PBDcis 1,4-PBD

Average Mw 280,000 92% 1,4; 8%, 1,2PBD

36% cis 1,4 linkages

Method (E. Khosravi, R.F. Smith)

Dissolve PBD and diester-olefin in DCM

Grubbs ruthenium catalyst (1 mol% per repeat unit)

Stir for 24 hours

2 ml ethyl vinyl ether added and stirred for another 2 hours

Precipitate in methanol

Filter solution

Remove solvent from filtrate & dry

Test reaction on linear PBD: results

Linear PBD can be broken down using the cross metathesis reaction + diester olefin

Changing the diester olefin has little effect- used dimethyl fumarate and dimethyl maleate in reactions

PBD can be broken down without any diester olefin

+?

+?

Test reaction on Model Cross-linked PBD Cross-link PBD with benzoyl peroxide

Generate elastic network (rubber behaviour)

Attempt catalysed breakdown…

10-1

100

101

102

103

100

101

102

103

104

105

106

Breakdown Products

Linear PBD

G, G

/ P

a

/ rad s-1

Crosslinked

Catalysed breakdown of model rubber

Rheology: Transform elastic solid to viscous liquid

GPC: Massive reduction in molecular weight (200 kg/mol to 2 kg/mol)

Both analyses: diester olefin has little effect on breakdown

G1 only

G1 + 1% DM diesterOriginal PBD

Mw / kg.mol-1 / rad.s-1

NMR Chemical Analysis of Breakdown Products

ciscis

cis

cis

trans

trans

trans trans

Cross-linked PBD and 1st

generation Grubbs’ catalyst

PBD

SBR

Styrene-butadiene rubber*

Copolymer of styrene and butadiene

Used in manufacturing tyres

Grubbs’ 2nd generation catalyst

n1,4-PBD

*Sample provided by Stephen Millington, ARTIS

SBR Reacted SBR with dimethyl maleate and Grubbs’ 2nd

generation catalyst at 40°C

SBR in DCM (control), 24 hours at 40°C

SBR broken down to rubber crumb by CM

Process also yields some soluble PBD of low mol wt.

Before reaction After reactionAfter 24 hours

Straight after reaction

After drying

After 24 hoursBefore reaction After ½ hour After reaction

Mass Loss Resulting following heptane swelling test on SBR

Sample % weight loss Error

SBR 11.2 0.2

G2 10% DM 40°C 25 4

G2 10% DM 25°C 16.9 0.9

G2 10% DF 20.5 0.8

G2, no diester 18.3 0.3

Checking for catalyst in rubber

PIXE Elemental analysis

Found 1 Grubbs’ catalyst : 700 butadiene repeat units

5 10 15 20

10

100

Br

Zn

Fe

Ru K

SBR sheet

fit

20HS

fit

Yie

ld

X-ray Energy / keV

Ru L

K

Catalysts work ‘too well’? Catalysts necessary, but diester olefin optional – why? Reaction must conserve chain ends…

So not ‘too good to be true’ – making molecular lassos?

Loop

formation

CM with 1,2

(vinyl) groups

Planned

reaction

Conclusions Cross metathesis reaction can break down PBD to

oligomers.

SBR can be broken down using Grubbs’ catalyst and diesterMain product = rubber crumb

Also get ~15% PBD oligomer

CM reaction has potential to be used to recycle rubber

Outlook / Future Directions Very promising route to PBD oligomer and/or crumb from rubber

Need to optimise in a ‘greener’ solvent

Repolymerise and establish reprocessed rubber properties.

Commercialise process – industrial partners wanted!

Smith, Boothroyd, Khosravi, Thompson, Green Chemistry. (ASAP) DOI: 10.1039/C5GC03075G

Free Access Until 11/5/16

Rheology

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3

Gʹ/Gʺ(Pa)

log Ang. Freq (rad/s)

Cross-linked PBD log G ʹ

Cross-linked PBD log Gʺ Product

log G ʹ

Product log Gʺ

Deformation and flow behaviourFrequency sweeps, 20 °CGʹ=storage modulus, Gʺ= loss modulus

Gel permeation chromatography (GPC)

Retention volume/ mL

Linear PBD before reaction

After reactionCross-linked PBD product

Linear PBD product

8.0 10.0 12.0 14.0 16.0 18.0

MALDI

Cross-linked PBD has been broken down using the cross metathesis reaction

Reaction Reference PBD grade/ molecular

weight(kDa)

Diester Type Grubbs’ Catalyst

generation

Double bond ratio of

polymer:diester

Outcomes

01 1.5 DF 1st 3:1 Major breakdown of PBD, mostly soluble in methanol

02 1.5 DF 2nd 3:1 Major breakdown of PBD, completely soluble in methanol

03 1.5 - 1st 1:0 Major breakdown of PBD but with large PDI

04 1.5 DF 2nd 3:1 Major breakdown of PBD, heating had no effect on the product

Reaction Reference PBD grade/ molecular

weight(kDa)

Diester Type Grubbs’ Catalyst generation Double bond ratio of

polymer:diester

Outcomes

05 420 DF 1st 3:1 Major breakdown of PBD, completely soluble in methanol

06 420 DF 2nd 3:1 Major breakdown of PBD, completely soluble in methanol

07 420 DF 1st 100:1 Major breakdown of PBD, mostly soluble in methanol

08 420 DF 2nd 100:1 Major breakdown of PBD, mostly soluble in methanol

09 420 DF 1st 200:1 Major breakdown of PBD, little difference to 07

10 420 DM 1st 3:1 Major breakdown of PBD, mostly soluble in methanol

11 420 DM 2nd 3:1 Major breakdown of PBD, mostly soluble in methanol

12 420 - 1st 1:0 Major breakdown of PBD, completely soluble

13 Cross-linked DF 1st 100:1 Broken down to same extent as linear PBD

14 Cross-linked DM 1st 100:1 Broken down to same extent as linear PBD

15 Cross-linked - 1st 1:0 Broken down to same extent as linear PBD

16RT Rubber crumb DM 1st 10:1 Appeared unchanged, Ru chemically bonded

16H Rubber crumb DM 1st 10:1 Appeared unchanged, Ru chemically bonded

17RT Rubber crumb - - 1:0 Appeared unchanged

17H Rubber crumb - - 1:0 Appeared unchanged

18RT Rubber crumb DM 1st 10:1 Appeared unchanged, Ru chemically bonded

18H Rubber crumb DM 1st 10:1 Appeared unchanged, Ru chemically bonded

19 Rubber crumb DM 2nd 10:1 Swelling tests show large weight loss

20RT SBR DM 2nd 10:1 Crumb product

20H SBR DM 2nd 10:1 Crumb product large weight loss in swelling test

21RT SBR - - 1:0 Swelled during reaction

21H SBR - - 1:0 Swelled during reaction

22 SBR DF 2nd 10:1 Crumb product, Ru chemically bonded, large wt loss in swelling

test

23 SBR - 2nd 1:0 Crumb product

24 420 DM 1st 3:1 Completely broken down within 2 hrs

25 420 DM 2nd 3:1 Almost completely broken down within 2 hrs

26 SBR DM 2nd 10:1 Crumb product after 21/2 hrs at RT