Lecture 3 - Cellulose modification.pdf

8/18/2019 Lecture 3 - Cellulose modification.pdf

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Cellulose: chemical modification

CHEM-E2140

Eero Kontturi

3rd November 2015


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Outline

(1) Chemical modification of cellulose – motivation

(2) Background: terminology, challenges(3) Esterification of cellulose

Inorganic esters: - xanthogenate

- carbamate

Organic esters: - formate

- acetate

(4) Etherification of cellulose: - alkyl ethers

- hydroxyalkyl ethers

- carboxymethyl cellulose

(5) Regioselectivity in chemical modification of cellulose


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Motivation for cellulose modification

• Preparation of substances that have different properties from cellulose,yet they are of renewable origin and biodegradable

- One of the most important properties is that most cellulose esters

and ethers are thermoplastic (cellulose is not)

• Modified cellulose, i.e., cellulose derivatives often possess properties that

are not easily achieved with totally synthetic polymers

• (With nanocellulose) modify the surface of nanocelluose to achieve better

compatibility with its environment (composites etc.)


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Basic concepts

• The idea of chemical modification of cellulose is to introduce

functional groups in the cellulose backbone

O

O

HO

OH

O

OH

n

O

O

RO

OR

O

OR

n

• Usually achieved by substituting the protons in the hydroxyl groups

of cellulose to a varying extent


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Basic concepts

Degree of substitution (DS)

Quality which measures the average amount of substituted hydroxylgroups in an anhydroglucose unit

Cellulose acetate with DS=2Carboxymethyl cellulose

with DS=0.5

n/2


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Basic concepts

Regioselectivity: which OH group/groups are modified


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Basic concepts

(1) Homogeneous modification• Cellulose is dissolved and individualized cellulose chains are modified

in a homogeneous solution

→ Uniform, homogeneous modification

(2) Heterogeneous modification

• Fibres, microfibrils, nanocrystals etc. are modified in a heterogeneous

suspension

→ Usually results in surface modification (not necessarily)


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Challenges

• The fundamental challenge in chemical modification of cellulose is thatcellulose is relatively inert and does not automatically obey the common

rules of organic chemistry

(For example, cellulose hydroxyl groups are alcohols but they do not form

esters with carboxylic acids under normal conditions)

• Regioselectivity is often difficult to achieve


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Esterification of cellulose


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Commercial cellulose esters

Klemm et al. Angew. Chem. Int. Ed. 2005, 44, 3358.


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Inorganic esters:

- cellulose xanthogenate

- cellulose carbamate

- cellulose sulphate- cellulose nitrate


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Cellulose xanthogenate

Cell-OH → Cell-O-

Cell-O- + CS2 → Cell-O-CSS- Na+

• Half ester, achieved by a reaction between cellulose alkoxy ion (Cell-O-)

and carbon disulfide (CS2) in alkaline medium

• Cellulose xanthogenate is water-soluble because of the polyelectrolyte

(charged) nature it introduces to the cellulose chain

• The reaction is important in practice because of its use in viscose process

- cellulose xanthogenate is produced from native cellulose

- cellulose xanthogenate is dissolved- the dissolved xanthogenate is regenerated in acid solution,

enabling controlled regenaration of cellulose into fibres and films


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Cell-OH → Cell-O-

Cell-O- + CS2 → Cell-O-CSS- Na+

Chemistry of xanthogenation is relatively complex:

• ca. 70% of CS2 input is converted into cellulose xanthogenate

• the rest is consumed by mainly formation of inorganic sulfidicproducts


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Cell-OH → Cell-O-Cell-O- + CS2 → Cell-O-CSS

- Na+

Xanthogenated cellulose material DS at C-2/C-3 DS at C-6

Fiber xanthogenate

(DS 0.61)

0.38 0.17

Viscose, non-ripened (DS 0.58) 0.34 0.24

Viscose, moderately ripened (DS 0.49) 0.16 0.32

Viscose, extensively ripened (DS 0.28) 0 0.32

Klemm et al. Comprehensive Cellulose Chemistry Vol. 2,

Wiley-VCH, 1998.

Ripening: phase of the viscose process where the DS of xanthogenate is reduced to

the level that is desired for the fibre spinning process.


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Cellulose carbamate

• High temperature reaction (~140°): processed above the melting point of

urea

• Catalyzed by metal salts, particularly zinc sulphate is used

• Urea forms isocyanic acid which is the actual reagent with cellulose:

H2N NH2

O

NH C O

140°C

+ NH3


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• Cellulose carbamate with DS 0.2-0.3 can be dissolved in aqueous NaOH

• Basis for the CarbaCell process:

• Aimed at substituting the environmentally hazardous viscose process

• Enabled by the effortless conversion of carbamate into cellulose inalkali

• No commercial applications as of yet

Cellulose carbamate


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Cellulose sulphate

• Can be prepared in a large variety of systems, usually containing either

SO3 or sulphuric acid

• Water soluble at above DS 0.2-0.3

• Preparation is generally accompanied by severe chain degradation due

to acid hydrolysis

• No commercial applications as of yet


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Cellulose nitrate

• Traditionally produced in a ternary system: HNO3/H2SO4/H2O

• Nitrogen contents of commercial cellulose nitrates range from 10.5-13.6%

Nitrogen content vs. DS


Wiley-VCH, 1998.


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Cellulose nitrate

The chemistry of traditional nitration is relatively complex:


Wiley-VCH, 1998.


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Cellulose nitrate

Nitrogen content vs. reaction time on nitration of spruce sulphite pulp


Wiley-VCH, 1998.


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Cellulose nitrate

• Celluloid (combs, hair ornaments, ping pong balls)

• Explosives (nitrogen content above 12.6%)

• Filters, membranes

• Component in lacquers


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Organic esters:

- Cellulose formate

- Cellulose acetate


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Cellulose formate

O

O

OH

OHHO

O

O

O

OO

O

OO

• Among the only organic esterifications that proceed spontaneously with

the free acid itself

• Cellulose formate is unstable: cellulose formate with DS 2.0-2.5 isdecomposed to cellulose and formic acid in 10 h in boiling water

HCOOH

H2O


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Cellulose formate

Philipp et al. Cellulose Chem. Technol. 1990, 24, 667.

Effect of catalyst on the formylation of cellulose

• Very high DS values of cellulose formate can be achieved


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Cellulose acetate

• Cellulose acetylation proceeds with acetic anhydride and a suitable

catalyst in water-free conditions


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Cellulose acetate – preparation

Heterogeneous acetylation (2 different methods):(1) ”Fibrous acetylation”: native cellulosic fibre is acetylated and no clear

changes in gross morphology of the fibres are observed

(2) Solution process: native cellulosic fibre is acetylated and as the

acetylation proceeds, the acetylated cellulose chains on the surface are

solubilized into the solution

NOTE: The solution process (2) is occasionally called homogeneous

acetylation, but in reality it is a heterogeneous process that may eventually

lead into a homogeneous product (cellulose acetate solution).

NOTE: All industrial procedures are initially heterogeneous processes.


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Schematic view on the

solution process: acetylated

cellulose chains are

released to the solution from

the cellulose microfibril

surface.


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A c

e t y l a t i on

Cross sectional views of

Valonia microfibrils upon

heterogeneous solution

acetylation. Electronmicroscopy and schematic

images.

Sassi and Chanzy Cellulose 1995, 2, 111.

First cellulose triacetate (CTA) is


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Cellulose acetate

First, cellulose triacetate (CTA) is

formed after which it is

deacetylated in homogeneous

conditions.


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Cellulose acetate – deacetylation


Wiley-VCH, 1998.

Dimethylamine

Hexamethylene-

diamine

Aliphatic

amines are

selectivetowards C-2

/C-3 in

deacetylation.

Medium: DMSO/Water/aliphatic diamine

→ Regioselectivity of deacetylation depends on the

reaction medium


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Cellulose acetate – solubility


Wiley-VCH, 1998.


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Cellulose acetate – applications


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Cellulose ethers

- methyl cellulose

- carboxymethyl cellulose- hydroxyethyl cellulose

- hydroxypropyl cellulose


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Commercial cellulose ethers


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Methyl cellulose – preparation

• Conventional preparation by Williamson reaction with gaseous or liquidchloroform (SN2 type nucleophilic substitution)

• 40% NaOH used in the industrial procedure (heterogeneous reaction)

• DS 1.5-2.0 are produced commercially


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Methyl cellulose – preparation


Wiley-VCH, 1998.

70°C

90°C

80°C

70°

C

80°C

90°C

Heterogeneous methylation of alkali cellulose with excess CH3Cl

• Fast initial stage in methylation

• Levelling off at just below DS 2


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Methyl cellulose – solubility

DS dependence of solubility of methyl cellulose (and ethyl cellulose)


Wiley-VCH, 1998.


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Methyl cellulose – solubility


Wiley-VCH, 1998.

At DS 1.7-2.3 the solubility of methyl cellulose is temperature sensitive at

ca. 50-60°C: it forms gels above a critical temperature and the gelationis reversible along the hysteresis loop.


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Methyl cellulose – applications

Some applications of methyl cellulose


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Carboxymethyl cellulose

• The most important ionic cellulose derivative

• Generally produced by a substitution reaction of monochloroacetic acid to

alkoxy cellulose


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Carboxymethyl celluloseCarboxymethyl cellulose - preparation

• Requires 20-30% NaOH concentration

• 1 mol NaOH consumed per 1 mol CMC substitution (main reaction)

• 2 mol NaOH consumed per 1 mol sodium glycolate (side reaction)

• Temperature 50-70°C• Exothermic process

• Heterogeneous process in water/isopropanol (or water/t-butanol)


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Carboxymethyl celluloseCarboxymethyl cellulose - preparation

• Commercial grades possess DS values 0.4-0.8

• CMC is water-soluble when DS>0.4


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Carboxymethyl cellulose - solubility


Wiley-VCH, 1998.

• Aqueous CMC solution does not usually represent a complete dissolution

down to the molecular level• Considerable part of the solution could be separated from the solution by

centrifugation, indicating poorly dissolved aggregates etc.

Higher DS

→ generally less

insoluble fragments

However, depends on

starting material andpreparation process


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Information from CPKelco

Carboxymethyl cellulose – applications

99.5 %


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Hydroxyethyl cellulose

Hydroxyethyl cellulose (HEC)

• A range of reactions occurs when HEC is prepared with ethylene oxide in

water • The hydroxyalkyl side chain can “grow” further from the first substitution


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Quantification of substitution:

DS: as usual, i.e., number of substituted hydroxyl groups in cellulose per

anhydroglucose unit

MS: molecular substitution, denoting the average number of alkylene oxide

molecules added per anhydroglucose unit


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Arisz et al. Cellulose 1996, 3, 45.

• MS/DS ratio represents the average length of the HEC side chains

• With high substitutions, MS/DS reaches a plateau


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Hydroxyethyl cellulose – preparation

• Heterogeneous system

• NaOH/cellulose between 0.3:1 to 1:1

• H2O/cellulose between 1.2:1 to 3.5:1

• Temperature: 30-80°C

• MS (DS) determined by ethylene oxide input


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• Not thermoplastic: decomposes below 100°C

• Soluble in water above MS 1

Major applications:


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Hydroxypropyl cellulose

• Thermoplastic (can be extruded at 160°

)

• Soluble in cold water at MS 4

• Gelling in water at 40°C, precipitation at 45°C


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Regioselective cellulose modification


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Background

• Generally, cellulose hydroxyl groups react in the order O6>O2>O3

• Reactivity of different hydroxyl groups can be tuned by reaction conditions

but they are rarely exclusive

• Regioselective synthesis applies various pathways to achieve nearly

complete regioselectivity of certain OH group / groups

• Regioselectively prepared cellulose derivatives yield information on the

structure-property relationship of polysaccharides and the function of the

different hydroxyls on cellulose


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Background

Terminology


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Pathways to regioselectivity

• Regioselective reactions with cellulose nearly always require

homogeneous reaction conditions where cellulose is completely dissolved

• Solvents applied: dimethylacetamide/LiCl, tert-butyl ammonium fluoride /

dimethylsulfoxide (TBAF/DMSO)


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Pathways to regioselectivity

Koschella et al. Macromol. Symp. 2006, 244, 59.


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Protecting group: trityl

One of the most popular protective groups for regioselective modification istriphenylmethyl (trityl):


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6-mono-O-methyl cellulose

Kondo et al. Carbohydr. Res. 1993, 238, 231.

Two different

protecting groups:

trityl and

1-propenyl


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Protecting group: TDMS

Koschella et al. Macromol. Symp. 2006, 244, 59.

Fox et al. Biomacromolecules 2011, 12, 1956.

• Highly swollen yet heterogeneous medium exclusive for C-6

• Homogeneous → 2,6 selective

= Thexyldimethylsilyl


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3-mono-O-ethyl cellulose

Koschella et al. Polym. Bull. 2006, 57, 33.

• Soluble in water and DMSO


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Summary

• Important cellulose esters: cellulose xanthogenate, cellulose nitrate,

cellulose acetate

• Important cellulose ethers: methyl cellulose, carboxymethyl cellulose,

hydroxyethyl cellulose

• Regioselective cellulose modification is a modern trend; it is an important

scientific advance in cellulose modification

Lecture 3 - Cellulose modification.pdf

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