Applications of Telechelic Reversible Addition Fragmentation Chain

Chapter 37

Applications of Telechelic Reversible Addition Fragmentation Chain Transfer Polymers *

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch037

John Lai , Dave Egan, Raymond Hsu, Carole Lepilleur, Alex Lubnin, Anthony Pajerski, and Ronald Shea Noveon Inc., A Subsidiary of Lubrizol Corporation, 9911 Brecksville Road, Brecksville, OH 44141 *Corresponding author: [email protected]

Telechelic di(mono)hydroxyl and di(mono)carboxyl-RAFT agents were synthesized and transformed to other functional blocks. The block-containing macro-RAFT agents underwent radical polymerization to prepare different block copolymers. These block copolymers, as well as the functional RAFT agents found applications in toughening epoxy resins, associative-thickening, ab initio emulsion and surfactant free emulsion polymerization, polyurethane synthesis, cross-linked polyacrylic acid synthesis and alkoxy-silane moisture-cure resins synthesis.

We reported the synthesis of mono- and di-carboxyl-terminated RAFT agents including trithiocarbonates (1) 1, dithiocarbamates (2) 2 and xanthates (2)3.

The carboxyl group can be transformed to the hydroxyl group by esterification with excess ethylene glycol (10). © 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

547

548 Ζ = RS (trithiocarbonate) 4 -COOCH CH OH 2

R N (dithiocarbamate) δ

2

2

RO (xanthate) 6

R'

One of the hydroxyl-terminated RAFT agents 5a is a highly purified white solid.


ÇH3

S

/

—

V

8

2

ÇH

3

-COOCH CH OH

HOH CH COOC-

2

2

CH3

2

CH3

5a

Telechelic polymers are of great interest (3) because of their ability to perform many useful operations, including a. chain extension of short chains to long ones by means of bifunctional linking groups, b. formation of networks by use of multifunctional linking agents and c. formation of (poly)block copolymers by combination of telechelics with different backbones. As depicted in Scheme 1, telechelic di- (Z is difunctional) and mono-carboxyl- and hydroxyl-terminated polymers have been madefromthese functionalized RAFT agents.

Ζ—

u

S II

R initiator S

[

COX +

M

S || Ζ—"

S-f>M

—

R' X= OH, OCH CH OH 2

2

R 1 COX R'

M= monomer(s)

Scheme 1. Polymerization with functionalized RAFT agents

Due to the difference in their chain-transfer constants, RAFT agents with different Ζ groups behave differently. For simple alkyl Ζ derivatives (4,5,6, R=alkyl), trithiocarbonate has greater chain-transfer constant and is in general more efficient in making block copolymers through sequential polymerization than dithiocarbamate and xanthate (4). However, with the presence of the terminal functional groups, dithiocarbamate and xanthate 2,3,5 and 6 can also make block copolymers in a different manner. They can do so by introducing the first block through condensation with the carboxyl or hydroxyl groups, followed by radical polymerization to attach the second block of vinyl polymer. Alternatively, polymerization first followed by condensation reactions on the functional group can give rise to block polymers. Scheme 2 illustrates block copolymer formation from the carboxyl-terminated RAFT agent. This is a very versatile process since there are many hydroxyl-terminated species that are


549 available. Examples include hydroxyl-terminated polyethylene (PE), polydimethylsiloxane (PDMS), polyethylene glycol monoether (MPEG) and fluorohydrocarbon (Fx), etc. R Z

"-s

[—COOH +


R, R'= alkyl

R-OH

^

Ζ

IL-S

R"= PE, PDMS, MPEG, Fx, etc

-| R'

COOR"

•COOR"

Scheme 2. Block copolymer synthesis from carboxyl-terminated RAFT agent

In this way, the carboxyl-terminated RAFT agent becomes synthetically equivalent to α-halocarboxyl derivative popular in ATRP (5) for polymerizing vinyl monomers as shown in scheme 3.

-COX'

+

R"OH

-COOR"

R'

R*

X=CI, Br; X'= CI, Br, OH Η 0=η

R

2

^ Cu(l) / ligand

X—^| x

"

CH ^j 2

R

COOR"

'

Scheme 3. ATRP synthesis with a-halo ester

RAFT can be the more practical method in many industrial applications because it involves all organic materials, has no need to remove the metallic catalyst, works well with (meth)acrylic acid and does not generate the green or blue color associated with Cu(II). Although RAFT polymers have issues of their own such as imparted color in the polymers and unstable functional groups that can release sulfur odor, the complementary selection from different RAFT agents such as 4, 5 and 6 often times can alleviate those problems. A B A block copolymers can be constructed from a difunctional RAFT agent such as l a or l b with a mono-functional block, or from a monofunctional RAFT agent with a difunctional block.


550 ÇH

3

S

CH


ÇH

S _S_iL_ _

HOOC3

1a

CH

Ç 3 H

3

-COOH 3

fi

fi

-H

HOOCC9H5

Ç 3 H

\ -

\ /

-COOH CH 2

5

2a

There are other differences among these RAFT agents. Trithiocarbonates and the polymers made from them are usually yellow in color. They are less stable toward water and amines (6) and are not suitable for aqueous polymerizations (7). Dithithocarbamates, on the other hand, are usually white solids and the polymers made from them are very light-colored and stable. Emulsion or dispersion polymers are totally white and are usually free from sulfur odor. Examples of the possible condensations with functionalized RAFT agents and polymers include reaction of the carboxyl group with epoxide, or esterification of the carboxyl group with alcohol. The hydroxyl group can react with isocyanate, or condense with a carboxyl group to form an ester. This article describes some of the processes and their use in different applications, including toughener for epoxy resins (8), associative thickener for latex paint (9), thermoplastic polyurethane and water-borne polyurethane dispersion (10), ab initio emulsion and surfactant-free emulsion polymers, cross-linked polyacrylic acid and moisture-cure coating. It provides a general scheme for their utilities, but by no means is a detailed study.

Toughener For Epoxy Resins Epoxy resin is often brittle and needs to be toughened for many applications. One of the most common commercial tougheners for epoxy resin is carboxyl-terminated poly(butadiene/acrylonitrile)(CTBN) (77). CTBN is made through recombination of a propagating poly(butadiene/acrylonitrile) radical initiated by a carboxyl-terminated initiator such as 4,4'-azobis(4-cyanovaleric acid) (ACVA). CTBN is usually a very viscous liquid with rather low molecular weight (Mn~3-4K). The expensive initiator takes up more than 10% of the toughener's weight. The functionality of CTBN is usually in the 1.8-1.9 range. The presence of double bonds also makes them susceptible to oxidation. There has been effort (12) to use carboxyl-terminated polyacrylates as epoxy tougheners. Although showing some effectiveness, these polyacrylates still need high dosage of ACVA and mercaptan chain-transfer agent. Scheme 4 represents the technique typically used to toughen an epoxy resin with a carboxyl-terminated polymer. Initially an epoxy adduct is prepared by employing an excess of epoxy to carboxyl equivalents. Ultimately a curing agent is used to achieve a cross-linked epoxy network.


551 HOOC—Polymer—COOH

+

R-

a. CTBN

excess

b. Polymer^ polyacrylates Ο

OH R-

•CH -02

JL

OH

ο •Polymer-

JL

Ο—CH 2

R-

adduct


+

A

A curing agent

Cross-linked epoxy network with polymer anchor Scheme 4. Toughening epoxy resins with caroxyl-termlnated polymers

Telechelic di-carboxyl-terminated polyacrylates are prepared from l a and 2a with bisphenol A diglycidyl ether as solvent. MALDI mass spectra (2,3) and HPLC with evaporative light-scattering detector (13) show that the functionalities are essentially completely transferred to the polymer chain ends. After being cross-linked with curing agent, the polyacrylate with Μη 10-20K phase-separates from the epoxy resin and shows good properties as toughener. Since the initiator is only used in a few mole% in relation to l a or 2a during radical polymerization, there is no drop-off in toughening effectiveness if A C V A is replaced with a non-carboxyl initiator such as AIBN. Scheme 5 illustrates telechelic dicarboxyl-terminated polyacrylates 7 and 8 synthesized with bisphenol A diglycidylether as solvent in 40% solid. The condensation between the epoxy and the carboxyl groups is performed in situ. The adduct then is ready to be cured with a curing agent. Associative Thickener for Latex Paint Rheological modifiers are extremely important additives in industrial and architectural coating systems (14). Different thickeners are added to latex paints to enhance their rheological properties. Traditional non-associative thickener such as hydroxyethyl cellulose works primarily by thickening the aqueous phase. Use of non-associative thickener alone is usually not sufficient due to the problems of roller spatter, poor leveling, water sensitivity and storage stability, etc. A second associative thickeners such as hydrophobically-modified ethoxylated urethanes (HEUR) or hydrophobically-modified alkali-swellable


552

1a

+

2

η

ÇH

S II

HC

η,

c-

3

CH -^(-COOHI OOR CH

OOR

2

3


8

Scheme 5. synthesis of telechelic dicarboxyl acrylates

O

R—NH—IL•(0CH CH )x~[0C(=0)NH-R .NH-C(=0)-(0CH2CH )x]—O,

2

2

2

HEUR ÇH

R= long chain alkyl ÇH

3

COOH

-NH—R

COOEt

3

C00(CH CH 0)n-R 2

2

HASE R= long chain alkyl

emulsions (HASE) is also needed (15) to provide the correct rheology for the paint systems. The interaction of the hydrophobic part of the thickener molecule with components of the paint such as die polymer binder molecule, pigment and extenders overcomes the aforementioned problems. We believe that a water-soluble (dispersible) telechelic polyacrylate with a, ω long chain alkyl groups may combine the better features of both HEUR and HASE by providing the associative-thickening performance from the endhydrophobes, with a polymer structure similar to that of the latex and lower cost to produce. The dicarboxyl-terminated dithiocarbamate 2a reacted with a longchain alkyl alcohol such as 1-hexadecanol or various Brij® PEG ether alcohols to provide the ester 2b with long chain alkyl at both ends. Subsequent polymerization with (meth)acrylates containing acid functions, followed by neutralization yields water soluble, or dispersible polyacrylates with telechelic hydrophobic groups.


553 ÇH, 2a

+

R(OCH CH ) OH 2

2

S

R(OCH CH )nOOC-

n

2

COO(CH CH 0)nR

2

2

2

R=long chain alkyl n=0-50

2b

H C=] 2

COOR* R'= H, H/Alkyl

-L

HC 2

s

ÇH

3

5-fi CH )-4--COO(CH CH 0)R C H COOR' 2

2

-+H C 2

2

2

5

9


Scheme 6. Synthesis of associative thickener

The dicarboxyl-terminated trithiocarbonate l a yields similar A B A hydrophobe-hydrophile-hydrophobe block copolymers in the same manner. Limited evaluation of these block copolymers for low and high shear viscosities demonstrate that they do show associative thickening properties by providing thixotropic behaviors.

Emulsion Polymer a. Ah Initio Emulsion Polymer Among different types of controlled radical polymerizations in an aq. emulsion, RAFT hold the best promise (16) to overcome challenges such as colloidal stability, molecular weight control and rate retardation. There have been many efforts reported in the literature, including using fluoro-containing xanthates (17), miniemulsions (18), seeded polymerization (18), water-soluble initiator (19) and other special method (18). We would like to report some successful results with dihydroxyl-terminated dithiocarbamate 5a as the RAFT agent to control emulsion polymerization. We believe that the emulsion polymerization works because the growing oligomeric radicals and the final polymers made from 5a can stay in the micelles from the help of the two hydrophilic terminal-hydroxyl groups. The molecular weight control and the polydispersity are essentially the same as in solution polymerization. Monomers conversions are very high, as in solution polymerization, with no retardation observed. Ab Initio emulsion polymerization, where monomers, 5a, surfactant and initiator are simply mixed with water and polymerized at elevated temperature, works well with different monomer combinations such as alkyl acrylate / styrene /(acrylic acid) and alkyl acrylate/alkyl methacrylate/acrylic acid to obtain fairly clean latex with little coagulum. Using polymer seeds can also give extremely clean emulsion with 5a (20). High molecular weight polymers (Μη -100K) with narrow polydispersity (Pd

I

».

2

H(CH )

C

-f 2 v > — 4r H C H

3

«a +

2

?

3

L

H

a

S - H CH24nj---COOCH2CH20H| COX CH 1

0

J2

3

X= H, alkyl, NHMe2CH2COCH3, etc —-H C

S


2

HO-RVR^OH + OCN-R-NCO^

)J-S—polyacrylate-polyurethane - H

2

C

2

—

ΛΛ

—•

11

Scheme 9. Synthesis of acrylate/urethane copolymer

Similarly the copolymer 11 can also be synthesized from 5a, or its combination with other polyols by reacting with the diisocyanate first to preform the polyurethane-RAFT agent, before polymerization with acrylate monomers. H(CH ) 3

H C==j 2

6a

2

+

HO-RVR -OH + OCN-R-NCO

•

COX »»

11

Scheme 10. Alternative synthesis of acrylate/urethane copolymer

Polyacrylates can be the side blocks if a mono-hydroxyl dithiocarbamate such as 5b is used as the end-caps of a polyurethane RAFT-prepolymer, followed by the controlled radical polymerization to obtain the copolymer, according to the following scheme:

5

Me N

L

ÇH 1

S

2

3

COOCH CH OH 2

2

2

+ HO-R7R -OH +

OCN-R-NCO

L 3

5b H

Applications of Telechelic Reversible Addition Fragmentation Chain

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