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The synthesis of block and graft copolymers of cellulose and its derivatives
The Synthesis of Block and Graft Copolymers of Cellulose and its Derivatives R. J. CERESA The mechanical rupture of polymer chains into polymer flee radicals is one of the basic processes for synthesizing linear block copolymers. Three mechanicochemical syntheses have been applied to cellulose and its derivatives. These are: (1) the mastication of methyl-, ethyl- and benzylcelluloses, cellulose acetate and starch with polymerizable vinyl monomers: (2) the vapour-phase swelling of cellulose acetate with acrylonitrile monomer; and (3) the freezing and thawing of starch-acrylonitrile emulsions. The isolation and characterization of the block copolymeric fractions for specific systems is presented.
OF THE methods developed at this College for the synthesis of linear and grafted block copolymers three have been found suitable for the copolymeric modification of cellulose and its derivatives. Full experimental data for each method of synthesis have been published ~-° and therefore an outline only of the experimental conditions will be given for each general system. The three processes to be considered are: (1) the mastication of cellulose derivatives with a polymerizable vinyl monomer; (2) the vapour phase swelling of cellulose derivatives with polymerizable monomers; and (3) the freezing of starch monomer emulsions. MASTICATION
BLOCK
COPOLYMERIZATION
It has been shown 7-1° that in general polymers in a visco-elastic state when subjected to mechanical working, such as occurs during mastication, extrusion or milling, undergo main-chain scission to form polymeric free radicals: P--P > P . + P . The active free-electron sites are therefore in terminal chain positions. The fate of these polymeric radicals depends upon the conditions in operation during their formation. Thus, if oxygen is present, polymeric peroxides or hydroperoxides are formed, as in the case of the mastication of natural rubber in the presence of air: R.+O2
~,RO2"
> ROOH
If extensive hydrogen abstraction occurs as the primary polymeric freeradical reaction reactive sites along the backbone of the polymer are formed which may lead to branching and eventual gelation, particularly if the main chain contains unsaturated olefinic bonds 11,1~. When mastication is carried out in the presence of a polymerizable monomer present in sufficient concentration to yield a visco-elastic material, and in the absence of a high concentration of atmospheric oxygen, the primary reaction of the polymeric free radicals is the initiation of block ¢opolymerization13,14: P . + n M > P--M,. 213
R. J. CERESA If the termination step of the polymerization is combination, disproportionation or transfer to monomer, essentially linear block copolymers are formed. Transfer to polymer, however, produces free-radical sites randomly placed along the polymer backbone and therefore under these conditions complex 'block-grafts' are formed. Transfer reactions, if they occur, result in the formation of homopolymer fractions: PM~-+M
>PM,+M.
>M~.
Mechanical scission of the block-copolymerized monomer is an additional source of homopolymer. It was found that cellulose esters and ethers when plasticized with vinyl monomers and masticated under blockcopolymerization conditions yielded substantial fractions of block copolymer with relatively little homopolymer of the block-copolymerized monomer (Table 1), Table I. The masticationof cellulose derivativeswith vinyl monomers Polymer A
Monomer B
B (%)
Mastication
Conventional percentage
(rain) Methyl cellulose Ethyl cellulose Benzyl cellulose Cellulose acetate Starch
Acrylonitrile 23.7 Methyl methacrylate 38-5
Composition Poly A Poly B Block AB
25
89
29.0
30
93
49-9
4-9
45-2
20
87
42-9
1. I
56
25
95
39.6
14.4
20
98
35-4 t 6.0
71-0
I
Styrene 35-1 Vinyl acetate 25.0 Methyl methacrylate 20-2,
46.0 58-6
M e t h y l cellulose--acrylonitrile
The mastication of a high-molecular weight-polymer with acrylonitrile monomer leads in general to the formation of 'pseudo' crosslinked block copolymerslS,1L The aggregation of the polyacrylonitrile chains of the block-copolymer fractions results in the formation of swollen gels when the polymerization products are extracted with solvents for the initial polymer. Thus with the system: methyl cellulose--acrylonitrile the hot aqueous extraction of the mastication pr.oduct removed unchanged methyl cellulose from the highly swollen but insoluble block-copolymer fraction. The latter fraction was soluble in dimethyl formamide and the solution when fractionally precipitated with methanol gave only one fraction which was precipitated over a narrow range. The addition of free methyl cellulose to this solution, before precipitation, yielded two clear-cut fractions without appreciable coprecipitation (Table 2). Table 2.
The fractional precipitation of methyl cellulose-polyacrylonitrile block copolymer Solution
1st fraction
Block copolymer Block copolymer+methyl cellulose
-51,2%cont.0-2%N 214
2nd fraetion
99"6 % cont. 8"05% N 48-6%cont. 7.90% N
COPOLYMERS OF CELLULOSE AND ITS DERIVATIVES The absence of homopolymeric polyacrylonitrile from the blockcopolymer fractions was confirmed by the ammonium thiocyanate extraction method TM 1~. The composition of the block-copolymer fraction was, therefore, 69.6 per cent of methyl cellulose and 30"4 per cent of polyacrylonitrile.
Ethyl cellulose-methyl methacrylate The products from the mastication experiments were dissolved in acetone and fractionally precipitated with methanol (Table 3).
Table 3. The fractional precipitation of ethyl cellulose-methyl methacrylate block copolymer Composition Fraction
I
It III tvt
We~ht (%) 4"8 25"3 29"9 40"0
Poly(methylmethacrylate)* (%)
Ethyl cellulose (%) 0"5 20-1 46"3 99-5
99"5 79 "9 53 "7 0"5
*Poly(methyl methacrylate) contents were determined bY infra-red spectrography. "tThis fraction was not precipitated by methanol from acetone solution and it was recovered by solvent evaOoratio~l at low temperature under v a c u u m .
Block-copolymer fraction II had a limiting viscosity number, ['7], in benzene of 252, whilst block-copolymer fraction III had ~7=165. The higher value corresponded to a larger average chain length of the poly(methyl methacrylate) segments, which is in agreement with the earlier precipitation of the fraction. Thus the differences in solution behaviour between the block-copolymer fractions II and III were a reflection of the differences in composition, relative chain lengths and segment distributions.
Benzyl cellulose-styrene Fractional precipitation from benzene solutions with methanol gave three distinct fractions (Table 4).
Table 4. The fractional precipitation of benzyl cellulose-styrene block copolymers Fraction
I ti IlI
Weight (%) 1-1 56.0 42.9
Composition
Benzyl cellulose (%)
Polystyrene* (%)
-47.5 98-0
98.5 52,5 2.0
*Polystyrene content me as ur e d b y infra-red adsorption.
From Table 4 it can be seen that from this system a single blockcopolymer fraction was isolated with an approximate composition of 1:1. Osmotic pressure measurements in benzene solutions of the block-copolymer fraction II gave an average molecular weight value of (1"6+0-2)x 10L 215
R. J. CERESA
Cellulose acetate-vinyl acetate In this system Soxhlet extraction with methanol removed homopolymeric vinyl acetate. The residue, after extraction to constant weight, was reprecipitated from benzene solution with methanol with a recovery in excess of 98"5 per cent. No coprecipitation occurred when up to 15 per cent of poly(vinyl acetate) was added to the benzene solutions before precipitation with methanol. The Soxhlet extraction was therefore considered to be complete. The residual mixture of cellulose acetate and block copolymer, after reprecipitation, was fractionated from benzene solution by methanol addition. Two clear-cut fractions were obtained (Table 5). Table
5.
Fraction
I II IlIt
The fractional precipitation of cellulose acetate-vinyl acetate block copolymers '
Composition
Weight %
Cellulose acetate
Poly(vinyl acetate)*
39"6 46-0 14- 4
97 87.9 --
-12.1 99- 5
(%)
(%)
*From infra-red sveetrogravhy. "t'Methanolextraction.
The osmotic molecular weight of the block-copolymer fraction II was measured as (2"3+0.22)× 105 and the limiting viscosity number, [,/], in benzene as 135.
Starch-methyl methacrylate The visco-elastic state cannot be achieved by the plasticization of starch with vinyl monomers. However, the polymeric chains are easily degraded mechanically b y frictional forces 17 to give free radicals which will initiate block copo!ymerization. The application of rotational shearing forces to starch-monomer mixtures in the fully filled mastication chamber gave rise to sufficient mechanical degradation to initiate the required block copolymerization. In mastication products containing more than 20 per cent of vinyl polymer, aqueous extraction of the residual starch fraction is only partially due to the low degree of swelling of the products in boiling water. Soxhlet extraction of the vinyl homopolymer, in this case poly(methyl methacrylate), is, as a rule, practically complete. 6'0 per cent of poly(methyl methacrylate) was recovered by Soxhlet extraction with benzene and the remaining fractions in the residue were separated by methanol precipitation from aqueous dimethyl formamide solutions 1" (Table 6). VAPOUR-PHASE SWELLING COPOLYMERIZATION
BLOCK
This synthesis of block copolymers was first used with poly(methyl methacrylate) as the base polymer, but it has since been extended to a range of high-molecular-weight and lightly crosslinked polymersL The mechanism and experimental techniques of this synthesis have recently been published 18 in full. The essential stages are: (1) vapour-phase swelling of 216
C O P O L Y M E R S O F CELLULOSE A N D ITS D E R I V A T I V E S
the high-molecular-weight polymer by the free-radical-polymerizable monomer; (2) rupture of carbon--carbon, or other weaker linkages, in the polymer by forces set up when a critical degree of swelling is attained; (3) block copolymerization of the monomer in the swollen polymer by the polymeric free radicals produced in situ; and (4) removal of unpolymerized monomer. Table 6 Composition Weight
Fraction
I
(%)
Starch*
~ Poly(methyl methacrylate)f
(%) 99"0 85"0
35.4 58.6 6"0
I
II
Iii,
(%)
i
21"1 97.5
*Determined from the iodine value. "t'Determined from infra-red spectrographY. SBenzene extraction.
A sample of cellulose acetate with an acetate content of 37' 1 per cent was found to swell in the vapour of acrylonitrile at a measurable rate at 30°C. The characteristic increase in the rate of swelling, which was an essential feature of the process, occurred after 40 minutes swelling at a monomer uptake of 15 per cent. Moulded specimens of the polymer swollen to 15 per cent and higher extents were insoluble in chloroform at 15°C, owing to the presence of the block-copolymerized acrylonitrile. Soxhlet extraction with chloroform removed both unpolymerized monomer and residual cellulose acetate. No block polymerization occurred until the critical degree of swelling was attained (Table 7). Table 7 Composition Swelling
Monomer
(min)
(%)
10 20 30 40 50 60
6.8 10-9 14.6 15-1 19-4 24"6
Polymerization
(%)
I
Cellulose acetate
Block copolymer
(%)
nil nil nil 1"6 3-8 13 "4
100 100 100 86 "2 80.6 64-1
(%)
i i
13 "8 19.4 35-9
The degree of swelling of the block-copolymer fractions in chloroform decreased with increasing times of initial swelling in the monomer vapour. Thus at 40 min the 'pseudo-gel"1~-1~ had a swelling index in chloroform in excess of 60 g of solvent per gramme of block copolymer. At 50 min the gel swelling index was reduced to 19 and at 60 min to 13. This trend was associated with the increasing polyacrylonitrile contents and with the shorter chain lengths of the cellulose acetate segments. 217
R. J. CERESA F R E E Z I N G OF P O L Y M E R - - M O N O M E R E M U L S I O N S Berlin and Penskaya 19'2° have shown that the freezing and defreezing of starch solutions is accompanied by free-radical degradation processes. The mechanical forces associated with the phase transitions and growth of ice crystals are sufficient to cause rupture of the starch macromolecules. When emulsions of starch with free-radical-polymerizable monomers are subjected to repeated freezing to - 2 0 0 ° C and subsequent thawing to room temperature, block copolymerization occurs, provided that oxygen has been removed by degassing. For these experiments acrylonitrile was used as the monomer owing to the ease of separating the insoluble block copolymer fraction. The results for this system are summarized in Table 8.
Table 8 Starch: Acrylonitrile in 10% emulsion*
No. offreezing cycles
Percentage block copolymerization
1:1
5 10 15 20 25 5 10 15 20 25 5 10 15 20 25
3-5 6.8 8.2 13-4 26-8 4-9 11.2 13.7 21.7 38.9 2.4 8.9 11.7 26-3 27-4
1:3
3:1
*The emulsionwas stabilizedwith 0'25 per.cent of a blockcoI~olymerof starch and methylmethacrylate synthesized by the masticationmethod. CONCLUSION In the three methods of synthesizing block polymers of cellulose and its derivatives which have been described, the basic principle is the mechanical scission of polymer chains in the macromolecule to give polymeric radicals which initiate the block copolymerization of a vinyl monomer. The energy required to rupture the molecules has, in the instances reported, been supplied by mechanical action, solvent swelling forces and aqueous freezing forces. It can therefore be postulated that any process which causes the rupture of cellulose molecules should be capable of being adapted to the synthesis of block copolymers. Ultrasonic, gamma, X-ray and sensitized ultra-violet irradiation, milling, grinding, high speed stirring and shaking are all basic methods which it should be possible, to adapt, and in fact several of these have been successfully adapted, to block copolymerization techniques for cellulose and its derivatives.
National College o~ Rubber Technology, Holloway Road, London, N.7 (Received 30th November, 1960) 218
C O P O L Y M E R S O F CELLULOSE A N D ITS D E R I V A T I V E S REFERENCES 1 ANGIER, D. J., CERESA, R. J. and WATSON, W. F. Chem. & Ind. 1958, p. 593 2 CERESA, R. J. and WATSON, W . F. J. appl. Polym. Sci. 1959, 1, 101 3 ANGIER, D. J., CERESA, R. J. and WATSON, W. F. I.R.I. Trans. 1958, 34, 8 4 ANGIER, D. J., CERESA, R. J. and WATSON, W. F. J. Polym. Sci. 1959, 34, 699 CERES^, R. J. Polymer, 1960, 1 (3), 397 6 CERESA, R. J. Polymer, 1960, 1 (1), 72 r ANGIER, D. J. and WATSON, W. F. I.R.1. Trans. 1957, 33, 22 8 CERESA, R. J. I.R.I. Trans. 1960, 36, 45 9 CERESA, R. J. and WATSON, W. F. I.R.I. Trans. 1959, 35, 19 70 CERESA, R. J. Trans. Plast. Inst. Lond. In press 11 ANGIER, D. J. and WATSON, W. F. J. Polym. Sci. 1955, 18, 129 12 ANGIER, D. J. and WATSON, W. F. J. Polym. Sci. 1956, 20, 235 13 ANGIER, D. J., FARLIE, F. O. and WATSON, W. F. I.R.I. Trans. 1958, 34, 8 14 CERES^, P-. J. I.U.P.A.C. Symposium on Macromolecules, Moscow, June 1960 ~'~ CERES^, R. J. Polymer, 1960, 1 (4), 477 6 CERESA, R. J. Polymer, 1960, 1 (4), 488 lr CERES^, R. J. and WATSON, W. F. I.U.P.A.C. Symposium on Macromolecules, Nottingham, September 1958 18 CERESA, R. J. and GRAY, M. A. 10th Canadian High Polymer Forum, Montreal, September 1960 ~9 BERLIN, A. A. and PENSKAYA, E. A. Dokl. Akad. N a u k S.S.S.R. 1956, 120, 585 2 0 BERLIN, A. A., PENSKAYA, E. A. and VOLKOVA, G. I. I.U.P.A.C. Symposium on Macromolecules, Moscow, June 1960
219
OF THE methods developed at this College for the synthesis of linear and grafted block copolymers three have been found suitable for the copolymeric modification of cellulose and its derivatives. Full experimental data for each method of synthesis have been published ~-° and therefore an outline only of the experimental conditions will be given for each general system. The three processes to be considered are: (1) the mastication of cellulose derivatives with a polymerizable vinyl monomer; (2) the vapour phase swelling of cellulose derivatives with polymerizable monomers; and (3) the freezing of starch monomer emulsions. MASTICATION
BLOCK
COPOLYMERIZATION
It has been shown 7-1° that in general polymers in a visco-elastic state when subjected to mechanical working, such as occurs during mastication, extrusion or milling, undergo main-chain scission to form polymeric free radicals: P--P > P . + P . The active free-electron sites are therefore in terminal chain positions. The fate of these polymeric radicals depends upon the conditions in operation during their formation. Thus, if oxygen is present, polymeric peroxides or hydroperoxides are formed, as in the case of the mastication of natural rubber in the presence of air: R.+O2
~,RO2"
> ROOH
If extensive hydrogen abstraction occurs as the primary polymeric freeradical reaction reactive sites along the backbone of the polymer are formed which may lead to branching and eventual gelation, particularly if the main chain contains unsaturated olefinic bonds 11,1~. When mastication is carried out in the presence of a polymerizable monomer present in sufficient concentration to yield a visco-elastic material, and in the absence of a high concentration of atmospheric oxygen, the primary reaction of the polymeric free radicals is the initiation of block ¢opolymerization13,14: P . + n M > P--M,. 213
R. J. CERESA If the termination step of the polymerization is combination, disproportionation or transfer to monomer, essentially linear block copolymers are formed. Transfer to polymer, however, produces free-radical sites randomly placed along the polymer backbone and therefore under these conditions complex 'block-grafts' are formed. Transfer reactions, if they occur, result in the formation of homopolymer fractions: PM~-+M
>PM,+M.
>M~.
Mechanical scission of the block-copolymerized monomer is an additional source of homopolymer. It was found that cellulose esters and ethers when plasticized with vinyl monomers and masticated under blockcopolymerization conditions yielded substantial fractions of block copolymer with relatively little homopolymer of the block-copolymerized monomer (Table 1), Table I. The masticationof cellulose derivativeswith vinyl monomers Polymer A
Monomer B
B (%)
Mastication
Conventional percentage
(rain) Methyl cellulose Ethyl cellulose Benzyl cellulose Cellulose acetate Starch
Acrylonitrile 23.7 Methyl methacrylate 38-5
Composition Poly A Poly B Block AB
25
89
29.0
30
93
49-9
4-9
45-2
20
87
42-9
1. I
56
25
95
39.6
14.4
20
98
35-4 t 6.0
71-0
I
Styrene 35-1 Vinyl acetate 25.0 Methyl methacrylate 20-2,
46.0 58-6
M e t h y l cellulose--acrylonitrile
The mastication of a high-molecular weight-polymer with acrylonitrile monomer leads in general to the formation of 'pseudo' crosslinked block copolymerslS,1L The aggregation of the polyacrylonitrile chains of the block-copolymer fractions results in the formation of swollen gels when the polymerization products are extracted with solvents for the initial polymer. Thus with the system: methyl cellulose--acrylonitrile the hot aqueous extraction of the mastication pr.oduct removed unchanged methyl cellulose from the highly swollen but insoluble block-copolymer fraction. The latter fraction was soluble in dimethyl formamide and the solution when fractionally precipitated with methanol gave only one fraction which was precipitated over a narrow range. The addition of free methyl cellulose to this solution, before precipitation, yielded two clear-cut fractions without appreciable coprecipitation (Table 2). Table 2.
The fractional precipitation of methyl cellulose-polyacrylonitrile block copolymer Solution
1st fraction
Block copolymer Block copolymer+methyl cellulose
-51,2%cont.0-2%N 214
2nd fraetion
99"6 % cont. 8"05% N 48-6%cont. 7.90% N
COPOLYMERS OF CELLULOSE AND ITS DERIVATIVES The absence of homopolymeric polyacrylonitrile from the blockcopolymer fractions was confirmed by the ammonium thiocyanate extraction method TM 1~. The composition of the block-copolymer fraction was, therefore, 69.6 per cent of methyl cellulose and 30"4 per cent of polyacrylonitrile.
Ethyl cellulose-methyl methacrylate The products from the mastication experiments were dissolved in acetone and fractionally precipitated with methanol (Table 3).
Table 3. The fractional precipitation of ethyl cellulose-methyl methacrylate block copolymer Composition Fraction
I
It III tvt
We~ht (%) 4"8 25"3 29"9 40"0
Poly(methylmethacrylate)* (%)
Ethyl cellulose (%) 0"5 20-1 46"3 99-5
99"5 79 "9 53 "7 0"5
*Poly(methyl methacrylate) contents were determined bY infra-red spectrography. "tThis fraction was not precipitated by methanol from acetone solution and it was recovered by solvent evaOoratio~l at low temperature under v a c u u m .
Block-copolymer fraction II had a limiting viscosity number, ['7], in benzene of 252, whilst block-copolymer fraction III had ~7=165. The higher value corresponded to a larger average chain length of the poly(methyl methacrylate) segments, which is in agreement with the earlier precipitation of the fraction. Thus the differences in solution behaviour between the block-copolymer fractions II and III were a reflection of the differences in composition, relative chain lengths and segment distributions.
Benzyl cellulose-styrene Fractional precipitation from benzene solutions with methanol gave three distinct fractions (Table 4).
Table 4. The fractional precipitation of benzyl cellulose-styrene block copolymers Fraction
I ti IlI
Weight (%) 1-1 56.0 42.9
Composition
Benzyl cellulose (%)
Polystyrene* (%)
-47.5 98-0
98.5 52,5 2.0
*Polystyrene content me as ur e d b y infra-red adsorption.
From Table 4 it can be seen that from this system a single blockcopolymer fraction was isolated with an approximate composition of 1:1. Osmotic pressure measurements in benzene solutions of the block-copolymer fraction II gave an average molecular weight value of (1"6+0-2)x 10L 215
R. J. CERESA
Cellulose acetate-vinyl acetate In this system Soxhlet extraction with methanol removed homopolymeric vinyl acetate. The residue, after extraction to constant weight, was reprecipitated from benzene solution with methanol with a recovery in excess of 98"5 per cent. No coprecipitation occurred when up to 15 per cent of poly(vinyl acetate) was added to the benzene solutions before precipitation with methanol. The Soxhlet extraction was therefore considered to be complete. The residual mixture of cellulose acetate and block copolymer, after reprecipitation, was fractionated from benzene solution by methanol addition. Two clear-cut fractions were obtained (Table 5). Table
5.
Fraction
I II IlIt
The fractional precipitation of cellulose acetate-vinyl acetate block copolymers '
Composition
Weight %
Cellulose acetate
Poly(vinyl acetate)*
39"6 46-0 14- 4
97 87.9 --
-12.1 99- 5
(%)
(%)
*From infra-red sveetrogravhy. "t'Methanolextraction.
The osmotic molecular weight of the block-copolymer fraction II was measured as (2"3+0.22)× 105 and the limiting viscosity number, [,/], in benzene as 135.
Starch-methyl methacrylate The visco-elastic state cannot be achieved by the plasticization of starch with vinyl monomers. However, the polymeric chains are easily degraded mechanically b y frictional forces 17 to give free radicals which will initiate block copo!ymerization. The application of rotational shearing forces to starch-monomer mixtures in the fully filled mastication chamber gave rise to sufficient mechanical degradation to initiate the required block copolymerization. In mastication products containing more than 20 per cent of vinyl polymer, aqueous extraction of the residual starch fraction is only partially due to the low degree of swelling of the products in boiling water. Soxhlet extraction of the vinyl homopolymer, in this case poly(methyl methacrylate), is, as a rule, practically complete. 6'0 per cent of poly(methyl methacrylate) was recovered by Soxhlet extraction with benzene and the remaining fractions in the residue were separated by methanol precipitation from aqueous dimethyl formamide solutions 1" (Table 6). VAPOUR-PHASE SWELLING COPOLYMERIZATION
BLOCK
This synthesis of block copolymers was first used with poly(methyl methacrylate) as the base polymer, but it has since been extended to a range of high-molecular-weight and lightly crosslinked polymersL The mechanism and experimental techniques of this synthesis have recently been published 18 in full. The essential stages are: (1) vapour-phase swelling of 216
C O P O L Y M E R S O F CELLULOSE A N D ITS D E R I V A T I V E S
the high-molecular-weight polymer by the free-radical-polymerizable monomer; (2) rupture of carbon--carbon, or other weaker linkages, in the polymer by forces set up when a critical degree of swelling is attained; (3) block copolymerization of the monomer in the swollen polymer by the polymeric free radicals produced in situ; and (4) removal of unpolymerized monomer. Table 6 Composition Weight
Fraction
I
(%)
Starch*
~ Poly(methyl methacrylate)f
(%) 99"0 85"0
35.4 58.6 6"0
I
II
Iii,
(%)
i
21"1 97.5
*Determined from the iodine value. "t'Determined from infra-red spectrographY. SBenzene extraction.
A sample of cellulose acetate with an acetate content of 37' 1 per cent was found to swell in the vapour of acrylonitrile at a measurable rate at 30°C. The characteristic increase in the rate of swelling, which was an essential feature of the process, occurred after 40 minutes swelling at a monomer uptake of 15 per cent. Moulded specimens of the polymer swollen to 15 per cent and higher extents were insoluble in chloroform at 15°C, owing to the presence of the block-copolymerized acrylonitrile. Soxhlet extraction with chloroform removed both unpolymerized monomer and residual cellulose acetate. No block polymerization occurred until the critical degree of swelling was attained (Table 7). Table 7 Composition Swelling
Monomer
(min)
(%)
10 20 30 40 50 60
6.8 10-9 14.6 15-1 19-4 24"6
Polymerization
(%)
I
Cellulose acetate
Block copolymer
(%)
nil nil nil 1"6 3-8 13 "4
100 100 100 86 "2 80.6 64-1
(%)
i i
13 "8 19.4 35-9
The degree of swelling of the block-copolymer fractions in chloroform decreased with increasing times of initial swelling in the monomer vapour. Thus at 40 min the 'pseudo-gel"1~-1~ had a swelling index in chloroform in excess of 60 g of solvent per gramme of block copolymer. At 50 min the gel swelling index was reduced to 19 and at 60 min to 13. This trend was associated with the increasing polyacrylonitrile contents and with the shorter chain lengths of the cellulose acetate segments. 217
R. J. CERESA F R E E Z I N G OF P O L Y M E R - - M O N O M E R E M U L S I O N S Berlin and Penskaya 19'2° have shown that the freezing and defreezing of starch solutions is accompanied by free-radical degradation processes. The mechanical forces associated with the phase transitions and growth of ice crystals are sufficient to cause rupture of the starch macromolecules. When emulsions of starch with free-radical-polymerizable monomers are subjected to repeated freezing to - 2 0 0 ° C and subsequent thawing to room temperature, block copolymerization occurs, provided that oxygen has been removed by degassing. For these experiments acrylonitrile was used as the monomer owing to the ease of separating the insoluble block copolymer fraction. The results for this system are summarized in Table 8.
Table 8 Starch: Acrylonitrile in 10% emulsion*
No. offreezing cycles
Percentage block copolymerization
1:1
5 10 15 20 25 5 10 15 20 25 5 10 15 20 25
3-5 6.8 8.2 13-4 26-8 4-9 11.2 13.7 21.7 38.9 2.4 8.9 11.7 26-3 27-4
1:3
3:1
*The emulsionwas stabilizedwith 0'25 per.cent of a blockcoI~olymerof starch and methylmethacrylate synthesized by the masticationmethod. CONCLUSION In the three methods of synthesizing block polymers of cellulose and its derivatives which have been described, the basic principle is the mechanical scission of polymer chains in the macromolecule to give polymeric radicals which initiate the block copolymerization of a vinyl monomer. The energy required to rupture the molecules has, in the instances reported, been supplied by mechanical action, solvent swelling forces and aqueous freezing forces. It can therefore be postulated that any process which causes the rupture of cellulose molecules should be capable of being adapted to the synthesis of block copolymers. Ultrasonic, gamma, X-ray and sensitized ultra-violet irradiation, milling, grinding, high speed stirring and shaking are all basic methods which it should be possible, to adapt, and in fact several of these have been successfully adapted, to block copolymerization techniques for cellulose and its derivatives.
National College o~ Rubber Technology, Holloway Road, London, N.7 (Received 30th November, 1960) 218
C O P O L Y M E R S O F CELLULOSE A N D ITS D E R I V A T I V E S REFERENCES 1 ANGIER, D. J., CERESA, R. J. and WATSON, W. F. Chem. & Ind. 1958, p. 593 2 CERESA, R. J. and WATSON, W . F. J. appl. Polym. Sci. 1959, 1, 101 3 ANGIER, D. J., CERESA, R. J. and WATSON, W. F. I.R.I. Trans. 1958, 34, 8 4 ANGIER, D. J., CERESA, R. J. and WATSON, W. F. J. Polym. Sci. 1959, 34, 699 CERES^, R. J. Polymer, 1960, 1 (3), 397 6 CERESA, R. J. Polymer, 1960, 1 (1), 72 r ANGIER, D. J. and WATSON, W. F. I.R.1. Trans. 1957, 33, 22 8 CERESA, R. J. I.R.I. Trans. 1960, 36, 45 9 CERESA, R. J. and WATSON, W. F. I.R.I. Trans. 1959, 35, 19 70 CERESA, R. J. Trans. Plast. Inst. Lond. In press 11 ANGIER, D. J. and WATSON, W. F. J. Polym. Sci. 1955, 18, 129 12 ANGIER, D. J. and WATSON, W. F. J. Polym. Sci. 1956, 20, 235 13 ANGIER, D. J., FARLIE, F. O. and WATSON, W. F. I.R.I. Trans. 1958, 34, 8 14 CERES^, P-. J. I.U.P.A.C. Symposium on Macromolecules, Moscow, June 1960 ~'~ CERES^, R. J. Polymer, 1960, 1 (4), 477 6 CERESA, R. J. Polymer, 1960, 1 (4), 488 lr CERES^, R. J. and WATSON, W. F. I.U.P.A.C. Symposium on Macromolecules, Nottingham, September 1958 18 CERESA, R. J. and GRAY, M. A. 10th Canadian High Polymer Forum, Montreal, September 1960 ~9 BERLIN, A. A. and PENSKAYA, E. A. Dokl. Akad. N a u k S.S.S.R. 1956, 120, 585 2 0 BERLIN, A. A., PENSKAYA, E. A. and VOLKOVA, G. I. I.U.P.A.C. Symposium on Macromolecules, Moscow, June 1960
219